Talk:Fictitious force

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[edit] The physical use for taking account of inertia

I can't think of any physical use for the concept of "fictitious force", or any problem which is simplifies or makes possible. Can you? Does the page provide any? (William M. Connolley 17:20, 11 Feb 2005 (UTC))

The current article on centrifugal force describes the usefulness of the formula for centrifugal manifestation of inertia in a rotating reference frame. The current article on the coriolis effect describes the usefullness of the formula for the coriolis manifestation of inertia in a rotating reference frame. The formulas given in these articels are used by physicists and engineers all over the world.
(William M. Connolley 12:12, 12 Feb 2005 (UTC)) Well sure - I'm a meteorologist, I use a formula with the coriolis force in it, and so do the climate models. But thats not the question: the question is, is the use of "fictitious force" a useful concept? In metorology, not at all. Its just in the equations as a force like any other.
In corresponding with you I will no longer use the expression 'fictitious force'. In dynamics there is no necessity for the use of the expression 'fictitious force'. From now on I will consistently use the expression 'manifestation of inertia'. I prefer to call it manifestation of inertia, because that simplifies thinking about the dynamics, whereas the expression 'fictitious force' complicates thinking about the dynamics. --Cleon Teunissen 09:29, 12 Feb 2005 (UTC)
(William M. Connolley 12:12, 12 Feb 2005 (UTC)) Well... that sounds like you are coming rather close to my POV that the concept "fictiitious force" isn't very useful.
Well, by rephrasing I have done away with some apparent distance. My paradigm here is the example of the electric car that recharges its battery system when switched to braking. At first the car and planet Earth have a relative velocity with respect to each other, and afterwards their relative velocity is zero. The cars battery system has been recharged; the manifestation of inertia has been doing work.
So why not simply call manifestation of inertia a force? It's doing work, isn't it?
I am reluctant to put manifestation of inertia in the category of forces, because inertia can only manifest itself in response to a force; manifestation of inertia is never present independently. As long as no force is acting on an object, the object's velocity remains the same, the inertia is there, ready to manifest itself should some force start pushing in some direction.
Manifestation of inertia has unique properties that justify, in my opinion, that manifestation of inertia is seen as a category in itself. --Cleon Teunissen 14:18, 12 Feb 2005 (UTC)
Well sure - I'm a meteorologist, I use a formula with the coriolis force in it, [...] Its just in the equations as a force like any other.(William M. Connolley 12:12, 12 Feb 2005 (UTC))
Yes, in those equations it is a force like any other. The model that is used works with a stationary Earth, so the model needs to be tweaked in order to produce exactly the same outcomes as a model that uses a rotating Earth. In the rotating Earth model no tweaking is necessary, for in the rotating Earth model the inertial effects that will occur flow automatically from the standard laws of motion.
The stationary Earth model must be tweaked by introducing a coriolis force.
The stationary Earth model is a fictitious model, and just by itself it is wrong. By introducing exactly the right fictitious force, this wrongness is cancelled completely. This is analogous to the repair job on the Hubble space telescope. The main mirror was wrong. They discovered exactly how it was wrong, and then they replaced the perfect secondary mirror with a mirror with a wrongness that exactly cancelled the main mirror's wrongness. --Cleon Teunissen 14:18, 12 Feb 2005 (UTC)

[edit] All force laws refer to the inertial reference frame

In physics, it is recognized that the inertial reference frames have something in common that the rotating reference frames do not share. It is possible to formulate a set of laws of motion that is valid in all inertial references frames. For example, the force law for centripetal force is valid in all inertial refecence frames. To perform a calculation in a rotating reference frame, corrective laws must be introduced to get correct calculation results. The coriolis force law is an example of such an corrective force law.
Each rotating reference frame needs its own specific correction.
This frame-specific correction refers to the inertial reference frame.
That is why the formula for the coriolis force has the factor \vec \omega in it: the corrective force law refers to the inertial reference frame.

Non-physicists sometimes claim that rotating reference frames are indistinguishable, that among the rotating reference frames there is no preferred frame. Yet each time these non-physicists perform a calculation involving a coriolis effect, they are referring to the non-rotating reference frame.

In relativistic phyisics, it is recognized that it is not possible to formulate laws of motion that are valid in rotating reference frames. Instead, algorithms are devised that specify exactly what corrective laws need to be introduced in each specific rotating frame in order to get correct calculation results. --Cleon Teunissen 01:08, 13 Feb 2005 (UTC)

Earlier I wrote:
According to relativistic phyiscs, the same physics is going on in all references frames. Or, phrased in another way: according to relativistic physics, laws exist (and they are found by physicists) that are invariant under transformation, they are valid in all reference frames.
--Cleon Teunissen 08:01, 12 Feb 2005 (UTC)
I retract the 'phrased another way' part. Theoretically it is correct, but it refers to physics at relativistic rotation rates. The physics inside a hollow cylinder rotating at a speed very close to the speed of light will display gravitomagnetic forces. The mathematics of general relativity can handle hollow cylinders rotating at a speed very close to the speed of light, but they do not occur in nature. I feel a discussion of laws of motion should be limited to what occurs in nature. --Cleon Teunissen 02:59, 13 Feb 2005 (UTC)

[edit] Misplaced?

This paragraph seems out of place; is there a good reason for it to be in the article? ‣ᓛᖁ♀ᑐ 05:25, 21 September 2005 (UTC)

Within physics, there is no obvious use for the term "fictional", or even any precise definition. It is not clear that this characterisation is particularly useful, and many deny that forces are "fictitious" or "imaginary" in any real sense.
The request for comment on this article drew my attention here. I don't like this statement for several reasons, primarily because its too short and gives an impression of concensus. I suspect it might be an attempt to point out that the use of the term fictitious force is debated among scientists. A fictitious force, however, is well-defined as an apparent force. Whether the concept is useful in getting across the properties of forces and Newton's law is a subject of debate, although my impression is that most of us use it in teaching introductory physics, whether we use it afterwards varies . Serway and Jewett's, Physics for Scientists and Engineers, has a wonderful explanation of ficticious forces, and puts "centrifugual force" . Halliday, Resnick and Walker, Fundamentals of Physics {the other very popular, at least in the US, intro text}, on the other hand, doesn't use the term at all, and uses centripical force to refer to the external force keeping whatever object in circular motion. Salsb 11:42, 21 September 2005 (UTC)

[edit] Validity

William M. Connolley, your first edit to this article included the statement "Simply characterising a force as fictitious because it arises from a change of reference is not reasonable." What is your basis for this claim? ‣ᓛᖁ♀ᑐ 06:10, 21 September 2005 (UTC)

Because that is an entirely arbitrary choice of definition. William M. Connolley 08:17, 21 September 2005 (UTC)
Do you have a source that supports this? ‣ᓛᖁ♀ᑐ 08:20, 21 September 2005 (UTC)
I disagree with the idea that it is arbitrary; a real force in classical mechanics requires an interaction. On the other hand since not everyone likes using the idea of a fictitious force, do you have any source that claims it is not reasonable? Salsb 11:47, 21 September 2005 (UTC)

[edit] From PNA/Physics

[edit] Revert (2)

Eequot reverted the article, sans discussion. I've reverted it back. The Eequor/CT version is very wordy but not very useful. It is very fuzzy over something rather basic: whether gravity is to be considered fictitious. There is, as far as I can see, no real use for the concept fictitious force in physics: arguing about whether forces are fictitious or not seems to belong to not-physics. William M. Connolley 08:17, 21 September 2005 (UTC).

I'm very concerned that you feel comfortable reverting eight months of editing, especially when so much detail is removed. It is also not basic at all whether gravity is fictitious, which may be why it appears fuzzy. How solid do you consider your understanding of general relativity? ‣ᓛᖁ♀ᑐ 08:27, 21 September 2005 (UTC)
I actually think the reverted version is a better starting point. However, I disagree with your assesment of the concept of fictitious force, since it is used often in both Newtonian physics and in gravity. But I question the claim that gravity is considered fictitious in Newtonian physics (yes, even with as little GR as I know, I know that in GR it is considered fictitious) . I have never heard this before, nor could I find it after a quick glance through a few intro and mechanics books. In Newtonian physics, the key point in making a force fictitious is not the mass-dependance, but that it apparently comes without an interaction with another body. I think the claim about gravity being fictitious in Newtonian physics should be removed, since it is erroneous, and some short exposition on why gravity is fictitious in GR would be good. Salsb 11:57, 21 September 2005 (UTC)
I agree. One could mention the radiation emitted by an accelerated charge as an example here. An electrical charge falling in the gravitational field of a planet will radiate em-waves. But a charge won't radiate just because you are observing it from an accelerating frame. The equivalence principle isn't violated here, because the emission of em radiation depends on how you impose the boundary conditions at infinity. Count Iblis 12:46, 21 September 2005 (UTC)

I'm not aware of any physics that fruitfully uses "fictitious force" as a useful concept (except, arguably, in GR. But calling gravity fictitious is against most peoples intuition and not what they are thinking of when they hear "fictitious force"). There are however numerous examples - meterology being obvious - where treating "fictitious" forces such as coriolis as real forces is useful.

As to my understanding of GR, I don't consider it solid at all. But the GR bit was put in there by EMS, whose understanding *is* good, as far as I'm able to tell.

As to reverting 8 months editing... the point was that the article had developed into a mess of words and unclarity. More isn't necessarily better. If you (or anyone) can indeed find some useful physics being done around the "fictitious" idea, then do please include it.William M. Connolley 13:07, 21 September 2005 (UTC).

I agree completely with reverting as the article did look like a mess. However, as for useful physics being done with fictitious forces, the concept is usefully in distinguishing between force from interactions, versus from accelerating reference frames. For some introductory references, I can refer you to Ch 6 of Physics for Scientists and Engineers, by Serway and Jewett, or Ch 8 of Kleppner and Kolenkow's An Introduction to Mechanics, or in Landau and Lifshitz's Mechanics, where the term fictitious isn't used as such, rather (in the translation I have) "inertia forces" is in quotes when refering to the effect of rotating frames, and there's exposition discussing the rotation as having the same effect as a force, as opposed to being a force. Then in derivations of brownian motion {including if I recall correctly Einstein's original work}, one often refers to ficticious forces, and in a different context, the radial quantum solution to an S-wave has a potential which is often referred to as a quantum fictitious potential, leading to a quantum fictitious force. {There probably are also examples beyond those that pop into my head right now and references beyond those within a couple steps from my desk } So the idea that fictitious forces are somehow not used in physics to distinguish from effects do to changing reference frames from real forces is incorrect Salsb 13:30, 21 September 2005 (UTC)
It's not just for meterologists. Afterall, how else are stranded astronauts supposed to decide which direction to throw their wrench to get back to the shuttle? Cause I hear that NASA lets that happen a lot.
Also, I agree that saying Newtonian gravity is a fictitious force is totally sketchy. If rewritten, the normal force discussion might be helpful to this article, but Eequor's version danced around too much. — Laura Scudder | Talk 14:16, 21 September 2005 (UTC)

Let me state some points very clearly:

  1. Fictitious forces appear in many basic physics textbooks as a calculational method, and they are useful in many cases. See, for example:
    • Daniel Kleppner, Robert Kolenkow (1973). An Introduction To Mechanics. McGraw-Hill Science/Engineering/Math. ISBN 0070350485. 
  2. "Fictitious force" is absolutely not a philisophical label, it is rather a well-defined term that is dependent on the particular model being used. Thus:
    • In Newtonian Mechanics, gravity in a force.
    • In General Relativity, it is not a force, but it is sometimes useful to treat it as one in certain reference frames; it is then a "fictitious force."

We need a complete article, and the long version was a better starting point than this crappy little stub. I'm going to revert, and then we can work on it. -- SCZenz 15:16, 21 September 2005 (UTC)

I totally agree with SCZenz's points above with respect to gravity and general relativity. Those are the points that must be made in the article.
At first glance, the new edits look fairly good. The GR part needs work however, since the new edit keeps refering to "gravity" when it should be refering to gravitation. (See talk:gravitation.) --EMS | Talk 16:53, 21 September 2005 (UTC)
I totally disagree with the intro to the current article, and indeed most of the article. "The two most common fictitious forces are the Coriolis force and the centrifugal force," - is this true? Isn't gravity a fictitious force? Or... is it? The current version of the article completely wimps out of clarifying this, and instead goes in for vast amounts of vague wurble. Its also quite unsatisfactory to assert that gravity's fictitiousness varies according to theory. Gravity is a real thing in the world, whatever its nature. SCZ asserts that FF is "well defined" and his version of the article defines it as "The term fictitious force refers to a calculational tool in physics". Spiffy. Does gravity fit that definition? I don't think so. As for "its a useful concept in physics" I see that asserted in the talk pages but its notably absent from the article itself. William M. Connolley 21:31, 21 September 2005 (UTC).
William, I have been absolutely clear. I'll try saying everything a different way.
  1. "Ficitious" doesn't mean that the action of an apparent force isn't really there, it just means it isn't really a force. Maybe it's a confusion between the definition of the term "fictitious force" and the usual English meaning of "fictitious" that is giving you trouble...?
    The confusion between the two meanings is indeed unhelpful. I don't think "it isn't really a force" has any meaning.
  2. A "fictitious force" is "a term in Newton's Second Law, in a particular non-inertial reference frame, that is not actually a force but is treated as one to make a calculation easier." That is a precise definition. Yes, it is model-dependent. That's why it's a calculational tool and not a statement about the true nature of a force.
    If that is indeed the precise definition, then it should be in the article, not languishing on the talk page. But... have you just made that up or can you source it to somewhere reliable?
  3. Gravity is a force in Newtonian mechanics. It is not a force in General Relativity. I'm sorry, that's just how it is. It's how those theories work. That is not a statement that gravitation exists in one theory and not the other; rather, in General Relativity, gravitation arises due to the curvature of spacetime, and affects the equivalent of a "straight line" (a geodesic) that particle will follow. (I know that's heavy--if anyone hasn't seen the idea before I can recommend any number of good general-info books on relativity.)
    This won't do. Gravity is a real thing in the real world. The thing called gravity in the models called NM or GR is another thing.
  4. It sometimes happens in GR problems that you pick a non-inertial frame of reference and do a problem there. Strange as it is, in GR, standing on the surface of the earth is non-inertial (even neglecting rotation)--and, in that frame in GR, gravity appears as a fictitious force.
    Fine. I have no problem with that. Note that your rephrased version is still a bit iffy over whether gravity is fictitious in GR or not.
  5. You are right that more examples could be given in the article, and more references. I will work to provide these. But they exist, I promise.
    What do you mean, more examples? There are none. I'll believe them when I see them.
  6. The article has examples, which I think would be helpful to introductory students working with fictitious forces. It is not so well-geared to explaining the idea to laypeople, and this should be fixed also.
By all means, keep asking questions and I'll keep answering. But there's nothing fundamentally incorrect about what's there. You can't change the definition of a scientific term on Wikipedia because you don't like it. -- SCZenz 22:01, 21 September 2005 (UTC)
I'd be much happier if you could provide a reliable, sourced, definition of FF. You haven't done that yet. Maybe you can. William M. Connolley 22:37, 21 September 2005 (UTC).
William, I do not appreciate the tone of your response.
Really? How interesting. I didn't much like your "crappy little stub" either. William M. Connolley 08:40, 22 September 2005 (UTC).
Although Wikipedia doesn't grant special status to "experts" in a field, and requires them to cite sources like anyone else, I am in fact an expert in Physics (albeit of the lowliest kind of expert). Thus when I write about an elementary physics topic from memory, it is quite likely that I am (approximately) correct. I am not citing details because I have been at work the entire time since I became aware of the debate on this article, and my mechanics textbooks are at home--plus I'm busy.
Well if you're too busy to do a decent job, don't produce a botch and then feel the need for a clean-up tag. But do please produce a source for your clear definition. William M. Connolley 08:40, 22 September 2005 (UTC).
I have been clarifying the definition, from memory, based on details I remember when you raise objections. But if I were to go through and cite every sentence, with ample quotes from sources, it would still say more or less what it says now. -- SCZenz 00:35, 22 September 2005 (UTC)
Your difficulties with gravity in GR seem to come from an incomplete understanding of GR--which is perfectly reasonable, as it's really quite an obscure subject. But if you don't completely understand it, wouldn't it be better to give others who have studied the subject the benefit of the doubt?
And you can cut the condescending crap too. This article isn't about GR, its about "fictitious force". My intuition was that calling gravity fictitious was odd. But I defered to EMS, who inserted (into the short version, which unlike your version wasn't stuffed full of wurble) the fact that under GR gravity is fictitious. That was fine by me. Your version however lacks clarity: it can't decide whether gravity-in-GR is fictitious or not. I lack kmowledge of GR, so can't fix that: but you and EMS (and anyone else with your "complete" understanding of the subject) ought to decide whether gravity-in-GR is fictitious sometimes or always. And you ought to provide a reliable citation for that.
Yes, acceleration due to gravitation is a real thing in the real world--but since we don't actually measure forces (only accelerations), we do not have to explain gravity as a force. Sometimes we do (Newton) and sometimes we don't (GR)--if we don't, then it may show up as a fictitious force. To have gravity appear as a fictitious force in a calculation is not a claim that gravity is fictitious, and this debate can't get anywhere if you insist that it is. -- SCZenz 00:35, 22 September 2005 (UTC)
Oh, and one more question... What do you mean there aren't any examples in the article? The examples include:
I admit they are currently poorly written, and I will contribute to fixing them... but there are examples. -- SCZenz 01:26, 22 September 2005 (UTC)
All they are examples of, are examples of "fictitious" forces. You've completely missed the point. They aren't examples of where *using* the concept of fictitious force is of any use at all. William M. Connolley 08:40, 22 September 2005 (UTC).

William, this is likely my last try at some sort of dialogue. You're being rude,

Oh really? So what was "crappy little stub"? Was that polite? I suggest you take the beam out of your own eye before looking at other peoples motes. William M. Connolley 15:41, 22 September 2005 (UTC).

and my obligation to address your legitimate concerns while being snapped at is pretty small, don't you think? I'm not going to speed up the usual pace of fixing problems in physics articles just because you're insistent that you don't like what's here now. We have very few physics editors, and we do our best.

This version (which I didn't write) is badly written, and I may have picked the wrong one to revert to.

I know you didn't write it. You reverted to it, so you're responsible for it being there. Yes you did pick the wrong one to revert to, so the obvious thing would be to fix that mistake. William M. Connolley 15:41, 22 September 2005 (UTC).

However, I think examples are useful to have, so I plan on going through and fixing up what's here. It's true that I have finite time, so the article may be a work-in-progress for a few days. That's not unusual, in my experience. What I don't want to do is throw everything out, or have an article that claims this is all nonsense just because nobody can instantaneously cite every sentence.

As for an example of "applying" a fictitious force, warship gun tables routinely correct for the coriolis force. If you fire a projectile for miles, it can miss by (I think) something on the order of 10's of meters. Yes, I can provide a citation for that fact (except the amount you miss by, which I'm not sure of, but I know it's substantial). And yes, I will put it in the article, along with other examples (some of which, like modleing gases, are already on this talk page). The fact that I have not done so yet is not a useful thing to point out, so please don't. -- SCZenz 13:15, 22 September 2005 (UTC)

You are still missing the point. The warship gun tables use the coriolis force, but make no use of it being fictional. I know of no examples where the idea that the force is *fictional* is of the slightest use. And clearly you don't either. William M. Connolley 15:41, 22 September 2005 (UTC).
See [1]. ‣ᓛᖁ♀ᑐ 15:50, 22 September 2005 (UTC)
I'm not quite sure how I'm making my point quite so unintelligible when it seems clear enough to me. To try again: your link is irrelevant. It merely asserts that the coriolis force is fictional. It does not *use* the "fictional-ness" for any purpose. If you dropped fictional-ness entirely from that link your understanding of the coriolis force, or physics in general, would not be changed. A better scienceworld link is this one: http://scienceworld.wolfram.com/physics/FictitiousForce.html. Note how it totally fails to define fictitious force, and simply gives examples. Note that gravity is not one of those examples. This is why my version said Within physics, there is no obvious use for the term "fictional", or even any precise definition. It is not clear that this characterisation is particularly useful, and many deny that forces are "fictitious" or "imaginary" in any real sense.. The challenge (so far un-taken-up) is for someone to find an example where using the concept "fictional" actually helps. In the case of the coriolis force, precisely the reverse is true: forgetting entirely about its fictionality and treating it as a force like any other is invariably the most useful way of thinking of it. William M. Connolley 17:02, 22 September 2005 (UTC).
The difficulty is that the coriolis force is a NOT a force, and I have provided several references in this talk, and in the article, where this fact is discussed.
Fair enough. Unfortunately they are all off-line and I don't have access to them. For myself, I still don't accept this viewpoint. William M. Connolley 18:08, 22 September 2005 (UTC).
The utility of the concept of a fictitious force -- in Newtonian physics, as opposed to GR where gravity is due to curvature issue -- is that we can be in a noninertial reference frame {typically a rotating frame but not necessarily} , and yet use Newton's laws by treating the effects of rotation exactly as a force. Hence, fictitious because it does not arise from an interaction, but behaves like a force in Newton's laws Salsb 17:37, 22 September 2005 (UTC)
OK, but I still think you're missing my point. Take the coriolis force. We can be in a rotating frame - say on the sfc of the earth - and yes the equations balance when you add in the coriolis force. Thats all agreed. But what isn't obvious at all is that calling it fictitious helps at all. What would go wrong if you called it a real force?
The problem is that you'd have to redefine what a force is. If you were to say that a force is anything that causes an acceleration regardless of reference frame {as opposed to in noninertial frames only}, then you wouldn't use a fictitious force at all, unfortunately, you would then be considering forces from interactions and "forces" due to coordinate transforms identically. This would be both bizarre and confusing when you deal with different problems. A;lthough if you stay in the same rotating reference frame perpetually {probably you do in climate modelling} this wouldn't be a big deal, except for the lose of physical intution, as a force could seem to spring up magically without an interaction. Not that this is particularly relevant to the article, since we're supposed to be reflecting how things are used, not how we think they should be used. Salsb 18:59, 22 September 2005 (UTC)
The distinction is important because fictitious forces "act" differently from real forces on objects of different masses. Suppose we have a large, frictionless box containing a number of balls of various masses. If the box is accelerated, the balls will remain in their locations until the end of the box catches up to them. To an observer inside the box, it will appear that the balls are all accelerating at the same rate. Since F=ma (Newton's second law), this would imply a different force is acting on each ball. If the balls were all acted upon by a single, real force, they would accelerate at rates inversely proportional to their masses. ‣ᓛᖁ♀ᑐ 17:33, 22 September 2005 (UTC)
Yes yes, this is splendid, the point you are missing here is that this is exactly like gravity. Since we appear to be agreed that gravity-in-newtonian *isn't* fictitious, your attempted definition fails. William M. Connolley 18:08, 22 September 2005 (UTC).
That there is an acceleration which is mass-dependent is a consequence, not a definition, i.e. the examples all have mass-dependent accelerations, but not all forces with mass-dependent accelerations are fictitious. Salsb 18:59, 22 September 2005 (UTC)
The Scienceworld link is "under construction," so I don't think that we can draw any conclusions from the fact that they don't define the term. It has no text at all! -- SCZenz 17:34, 22 September 2005 (UTC)
Yes. Precisely. Scienceworld came to write that article and realised (this is my interpreation of course) that they could find no source or definition of fictitious force, and so wisely they didn't define it.
William, "fictional" is a precisely defined notion based on the mathematics of Newtonian mechanics, and the concept of an inertial reference frame. I wrote a section on this, which is in the article--do you have any comment on it? The whole point of fictitious forces is that you do treat them as real forces, in the reference frame you're using, for the problem you're doing. But they do, in fact, arise from the fact that the reference frame is non-inertial, and the term that physicists use for this is fictitious force. Maybe it isn't the best term, but that's not something to be decided on Wikipedia. The article can only explain how the term is actually used. -- SCZenz 17:34, 22 September 2005 (UTC)
If you mean the section maths/general definition *this isn't a definition*. Its a derivation, which is quite different.
Actually, to quote myself paraphrasing Kleppner and Kolenkow:
"Now we define
\bold{F}_{fictitious} = - m \frac{d^2\bold{X}}{dt^2}"
The rest of the section is providing context for that definition. -- SCZenz 18:17, 22 September 2005 (UTC)
OK, you're right there - I read through too quickly. I agree: within newtonian mechanics this provides a consistent definition. I maintain my other objections. William M. Connolley 18:34, 22 September 2005 (UTC).
I also apologize for referring to previous version of the article as "crappy." It was a shorthand for the POV issues and lack of content, which I didn't think of as a big deal because I didn't realize the debate was going to become so heated. But I still shouldn't have said it. If you would reciprocate by asking for clarification when something is unclear, rather than assuming that I am wrong/lying about the content of my education in physics, I would be appreciative. -- SCZenz 17:34, 22 September 2005 (UTC)
OK, then I too apologise for being brusque and snappish. I have no issues with your education. If I've said stuff that was interpretable that way, I apologise again.

[edit] Cleanup

William has raised one very good point. This article is poorly-written, and it may be confusing to people who don't know already what it's talking about. I've worked on the intro, but that's it so far. I'll do more, but hopefully I can get some help. -- SCZenz 01:54, 22 September 2005 (UTC)

Yes. The article is very poorly written. Why was why reverting to this version was a very bad idea. William M. Connolley 08:40, 22 September 2005 (UTC).
I agree. I think the examples should probably just be blanked and started over again. Salsb 12:33, 22 September 2005 (UTC)
The examples are valuable; they do make the concept easier to understand, even if they could be better. If one could be cleaned up and made concise (preferably regarding centrifugal force, as that seems most obviously fictitious), and placed before the formulas begin, it would be very helpful to laypeople. ‣ᓛᖁ♀ᑐ 12:51, 22 September 2005 (UTC)
I disagree with blanking them. I think they can be fixed, but seriously rewritten of course. -- SCZenz 13:15, 22 September 2005 (UTC)
I also just added a section on fictitious forces as they arise from rotating references frames. I followed Kleppner, since the section above did, but the derivation is essentially identical in Fetter and Landau, along with the admonishments, about the forces not being real, and presumeably in any other mechanics text, although I only cited the ones I have on had to consult. I had to hurry at the end, since I'm supposed to be in a meeting one minute ago, but if someone could make the equations look nicer, I would apprecitate it. Salsb 12:33, 22 September 2005 (UTC)
I think we should be careful about making this article too much more technical than it now is. We need to have examples of applications and real-world effects, and move them to the top. -- SCZenz 13:15, 22 September 2005 (UTC)
I agree, I think its pretty much at the limits of technicality. I put it in mostly to make it obvious -- to the mathematically able -- where the fictitious forces arise, and since there was a request for more citations, as this is a concept found in essentially every intro mechanics book. It might be fruitful to move the technical parts to the end, so readers get the examples then the math. Salsb 13:58, 22 September 2005 (UTC)
I went ahead and reorganized the sections. It still needs ample work, but thats all I can do for now Salsb 14:18, 22 September 2005 (UTC)
I have rewritten "acceleration in a straight line" to be a useful general illustration of the concept. Does it serve this purpose? Are there other comments? -- SCZenz 18:05, 22 September 2005 (UTC)

Since we have rewritten all of the article except the relativity section, I've moved the cleanup tags to just that section. Does someone want to rewrite it? If not, I will likely replace it with something much shorter in a few days' time. -- SCZenz 21:38, 23 September 2005 (UTC)

[edit] relationship to mass

I have given the article more of a reading, and it is missing a very essential point: Fictitious forces act in direct proportion to the mass of the object. This of course is due to their presense being due to the acceleration of the observer.

It is a very essential point since that is what Einstein noticed in defining his original equivalence principle: that the action of gravity is identical to the action of being accelerated in a rocket ship, with the strength of the force being directly proportional to the mass of the object being operated on.

Note that for the "real" forces, the strength of the action is independent of the mass of the object being operated on. --EMS | Talk 18:55, 22 September 2005 (UTC)

I object to this point, at least if I understand it right. The fact that gravity, like fictitious forces, is proportional to mass is suggestive of the ideas that led Einstein to general relativity. However, this does not change the fact that, in Newtonian Mechanics, gravity is a real force. Nobody teaching introductory physics ever draws their free-body diagrams without gravity. -- SCZenz 19:04, 22 September 2005 (UTC)
As written this is not true. Whether or not the force depends on the mass of the object is irrelevant to it being real or fictitious. It so happens that fictitious forces from rotating coordinates, for example, depend on mass, but that isn't due to their being fictitious as such. Salsb 19:31, 22 September 2005 (UTC)
The point is that the reason for the "force" is that the observer is being accelerated. Think about it a bit: If you are being accelerated, the objects in inertial motion will in your frmae of reference will be seen to accelerate. If you say that this acceleration is due a force on those objects, then that force must be proportional to the mass of the object. So any force which acts in proportion to the mass of the object is automatically suspected of being "fictitious".
As for Newtonian physics and gravity in particular: Do note that there is no way to explain gravitation without a force being involved given the implicit Newtonian assumptions of a flat spacetime and inertial motion being motion at a constant rate with repect to a Cartesian coordinate system. Of course Einstein chose to toss those assumptions out of the window, but the result is a very, very different and non-intuitive view of spacetime. However, the point remains that in Newtonian physics the force of gravity is necessary to explain things as they are object on a massive body.
Finally, I should make it clear that this article is under no obligation to go into any depth about general relativity and in fact should not do more than mention the connection through gravity and let that be that. In fact, it is a legitimate question as to whether general relativity and gravity as a fictitious force should be mentioned here at all. I like mentioning it, but it does seem to put a strain on this article. --EMS | Talk 03:29, 23 September 2005 (UTC)

[edit] Remaining objections

I am starting a new section, as the other one was waaay too long and I've lost track of it all. I will try to copy in William's most recent comments and work on addressing them. If I've left anything out, William, can you put it in again? Thanks! -- SCZenz 19:00, 22 September 2005 (UTC)

[edit] No problem within Newtonian mechanics

OK, you're right there - I read through too quickly. I agree: within newtonian mechanics this provides a consistent definition. I maintain my other objections. William M. Connolley 18:34, 22 September 2005 (UTC).

Ok, I claim there is also a consistent definition in generally relativity, that is also mathematical. Unfortunately, writing it down would be too complex, especially since forces and reference frames are handled differently in GR. (Plus it would take me many hours of reading to understand it, I suspect.) I can certainly find a book that goes over this, and cite it, however. -- SCZenz 19:00, 22 September 2005 (UTC)
I'd like to note, though, that there is not consistent mathematical definition between newtonian mechanics and GR, again because reference frames, forces, and equations of motion are handled differently in the two theories. The definitions would be conceptually equivalent, however. The question of whether gravity is a fictitious force in GR arises from the fact that gravity is not a true force in GR. (iIn non-inertial reference frames, there is no force of gravity, although obviously objects still fall into each other.) -- SCZenz 19:00, 22 September 2005 (UTC)

[edit] ScienceWorld blank page

The Scienceworld link is "under construction," so I don't think that we can draw any conclusions from the fact that they don't define the term. It has no text at all! -- SCZenz 17:34, 22 September 2005 (UTC)

Yes. Precisely. Scienceworld came to write that article and realised (this is my interpreation of course) that they could find no source or definition of fictitious force, and so wisely they didn't define it. -- William M. Connolley
I don't think you can ascribe that motivation. Most likely they just haven't gotten to it yet. -- SCZenz 19:00, 22 September 2005 (UTC)

[edit] What's wrong if you call them real forces?

OK, but I still think you're missing my point. Take the coriolis force. We can be in a rotating frame - say on the sfc of the earth - and yes the equations balance when you add in the coriolis force. Thats all agreed. But what isn't obvious at all is that calling it fictitious helps at all. What would go wrong if you called it a real force? -- William M. Connolley

In Newtonian mechanics, there are a few fundamental forces: things like electromagnetism, contact forces, and gravity. These are all you need to explain all motion (within the capabilities of the theory) as long as you use an inertial reference frame. It is helpful to differentiate between these forces, and the additional effects (which you can treat as forces) that arise in non-inertial frames. The term used for this is "fictitious force." Even if we agreed that this definition wasn't necessary, Wikipedia can only report what scientific terminology is used, not change it. -- SCZenz 19:00, 22 September 2005 (UTC)
Things that would go wrong are quantities that depend on accelerations in inertial frames, such as the radiation emitted by an accelerated electrical charge. One can put this right by imposing the right kind of boundary conditions at infinity for the electromagnetic fields, though. Another effect worth mentioning in this article is the unruh effect. Count Iblis 21:37, 22 September 2005 (UTC)
I don't think either of those are very satisfactory. Also, SCZ objected violently to my Within physics, there is no obvious use for the term "fictional", or even any precise definition. It is not clear that this characterisation is particularly useful, and many deny that forces are "fictitious" or "imaginary" in any real sense.. Thinking about this, I have conceeded too readily the point about "precise definition". In newtonian mech; yes. In GR, possibly (though, err, its too complex to actually explain, it seems). But in *physics*? No. There is no definition that allows you to say: "hmmm... I'm measuring a force. I wonder if its fictitious or not? Oh, the defn is...". Or is there? If there is, please present it. William M. Connolley 15:05, 23 September 2005 (UTC).
Fictitious force as a concept is the same in either GR or Newtonian Mechanics: it's an apparent force that would not be present in an inertial reference frame, but is present in a particular non-inertial frame. But it doesn't, and needn't, have any meaning to say a particular force is fictitious independant of which model of the universe you're using. Gravity is the key example of this: if gravity is a force in GR, it's fictitious, but gravity is not fictitious in Newtonian Mechanics. Why not? Because the notion of an inertial reference frame is different in the two models. We're classifying apparent forces in relation to the theory we're using, not in relation to some absolute quality they have. -- SCZenz 18:05, 23 September 2005 (UTC)
Well, one can define what an inertial frame is, although there are some issues here having to do with Mach's principle. So, you know how to relate observed accelerations to the accelerations that would be observed in an inertial frame. Also note that forces that are proportional to mass cannot be detected except for the acceleration they cause. E.g. Earth's gravity (ignoring tidal effects) can be measured either by measuring the gravitational acceleration of falling objects, or by the effects of other forces that counteract it. So, you'll never have to worry about whether or not a weighing scale is indicating a real force or not. It's always real, because what you see is the result of normal forces which have an electromagnetic origin. The difference between fictional forces and gravity is that gravity only acts locally and thus gives rise to tidal forces.Count Iblis 16:17, 23 September 2005 (UTC)
I would be inclined to agree that gravity is always real, from intuition. But EMS clearly says that it isn't, in GR; the article is a bit vague, and says it sometimes (err, no, it says: gravity may appear as a fictitious force - what is this supposed to mean?). So are you asserting that gravity is always real, then? William M. Connolley 16:41, 23 September 2005 (UTC).
In GR gravity generated by massive bodies is real but it isn't a force. It is related to the curvature of space-time. Fictitious forces that act like gravity can be transformed away by a coordinate transformation. These transformations cannot transform a nonzero curvature away to zero. I guess that what matters is what you can objectively detect and how much relevant information that contains. If you see an object accelerating toward you in an inertial frame then that means that at the location of the object some physical effect is causing that acceleration (interaction with the gravitational filed or whatever). But if you are accelerating yourself while making this observation, then you must first subtract your own acceleration. If you don't then the force you attribute as acting on the body is the real force plus the fictional force. This fictional force is actually due to the physical effects that are accelerating you, they have nothing to do with the body you are observing.Count Iblis 17:20, 23 September 2005 (UTC)
What I meant by "may appear as a fictitious force" is that if it appears to be a force at all in GR, it is fictitious. I'll clarify that. -- SCZenz 18:05, 23 September 2005 (UTC)

I don't think the idea of "fictitious force" is a particularly useful one in general relativity. In GR, there are inertial and non-inertial frames. The inertial ones are freely falling, which is different from classical mechanics, where inertial frames are determined relative to a universal system of Cartesian coordinates.

The concept is principally useful in Newtonian mechanics, where there exists a simple, universally defined inertial Cartesian coordinate system in which the equations of motion are \ddot{x}_i=0 in the absence of external forces. In non-inertial frames, other terms appear. You can call these fictitious forces, because try as you might with strain gauges, you'll never measure them, as opposed to, say the Coulomb force. One property that the fictitious force shares with gravity (and indeed, a property that is crucial in GR) is that both terms are proportional to a particle's mass.

Now I don't understand why this page is so incredibly long. It seems to me that this topic would be better served by a punchy article with a couple of simple examples (especially rotating frames of reference), rather than a long, rambling and likely error-ridden treatise. –Joke137 21:59, 23 September 2005 (UTC)

The GR stuff needs to be fixed, ideally by someone who knows more than me. Would you like to do it? The rest I and a couple of others have rewritten in the past couple of days, in the face of rather intense pressure to justify even the existence of the concept. Are there specific changes you are suggesting, or is it ok? -- SCZenz 00:33, 24 September 2005 (UTC)

[edit] "It is extremely rare for space-time geometry to rotate significantly with respect to the universe as a whole,"

Does it at all? Or are you refering to frame-dragging here, or is there some other case? GangofOne 00:34, 23 September 2005 (UTC)

[edit] Good move (removing relativity stuff)

Even though I would like a mention of relativity to be in here, I approve of the deletion of the section on it. I had not had much time to look it over, but once I did, I realized the SCZenz was right to want it out. All that is needed here is a short blurb on gravity being "fictitious" under the equivalence principle of general relativity. A few extra sentences to somewhat explain that is advisable, but discussion of the technical details of GR (such as the description of geodesics) does not belong here.

For now, my feeling is that it is best left out until this article has had a chance to stabilize and mature some more. Then it can be brought in as an aside. However, the editors of this page can and should be willing to veto any such addition (even if it is mine) if in their opinion it does not "fit". --EMS | Talk 22:39, 27 September 2005 (UTC)

[edit] Just saw the current introduction

Now that I have looked at that, all that is needed is already in this article. Indeed, this is a very well-written paragraph on it. I can quibble with the parenthetical remark at the end noting the gravity is "real" in Newtonian physics, but I will edit that when I can. As I see it, the use of the "force" of gravity is required in Newtonian physics, independent of whether that force is fictitious or not. --EMS | Talk 22:46, 27 September 2005 (UTC)

Heh. I'm glad you like it. You can quibble about the perentheticla remark, but it's important for clairty to people who don't know a lot of physics. Also I think your quibbling would be wrong. ;) But if you want to argue it, let's take it to user_talk pages, rather than risk fanning any more debate on this article. -- SCZenz 04:14, 28 September 2005 (UTC)
I am going to edit that stuff at some point, to tighten it up and make it technically correct (or at least more correct). A major issue is how to do this in a way that enhances your excellent work instead of muddying it up again. The existing text communicates well without being buzzwordy, and this is essential to the success of this article. So you have left me with a interesting puzzle to work through. --EMS | Talk 16:16, 28 September 2005 (UTC)

[edit] Modifications done

I have put my changes in. I think that I have succeeded in explaining how GR turns gravity into a fictitious force in the main narrative, while having put some needed elaboration into the footnotes. So hopefully readers will be able to get the gist of what is going on in the view of general relativity. --EMS | Talk 23:01, 29 September 2005 (UTC)

My only concern now is that we have two sets of footnotes (yours, and the references), with identical labelling. ;) Not sure what to do with that. -- SCZenz 23:05, 29 September 2005 (UTC)
The comments can be combined with the paper/book reference in the references. That's not my style, but sometimes you see this in scientific articles.Count Iblis 00:20, 30 September 2005 (UTC)
Hmm... Not my style either. And, generally, not Wikipedia's. Let's think about it, or failing that leave it and hope nobody notices.. ;) -- SCZenz 00:25, 30 September 2005 (UTC)
I did notice and correct that. I am not sure that I like the corrections, as that is not my style either. (I would like the letters to be in square brackets.) However, this at least this resolves the conflict. Unfortunately, footnoting is not robustly supported in Wikipedia at this time. I will keep an eye out for a better way to do this, but for now I think that this the best I can do. --EMS | Talk 03:42, 30 September 2005 (UTC)

[edit] Comment

I actually quite like the introductory paragraph now. One important point not made in the article is that fictitious forces not only arise from using non-inertial frames of reference, they also arise from using curvilinear coordinates (e.g. the centripedal force in polar coordinates). This is an important distinction: curvilinear coordinates are not a non-inertial frame of reference, since they are not time dependent. The effect of using a curvilinear coordinates is to obtain fictitious forces proportional to velocity squared, whereas accelerated rectilinear coordinates give forces independent of velocity. –Joke137 02:09, 28 September 2005 (UTC)

Could you give a simple example? I don't get it. The derivations given are all vector equations that nowhere mention Cartesian coordinates. GangofOne 03:10, 28 September 2005 (UTC)
Good point. The textbook I used didn't talk about fictitious forces arising from curvilinear coordinates, and their definition doesn't admit them. In fact, this idea doesn't make sense to me--centripedal force arises from a rotating reference frame, not from the polar coordinates!--but I could be wrong. -- SCZenz 04:10, 28 September 2005 (UTC)

Also, a smaller comment: it would be nice to also mention that these are sometimes called "pseudo-forces". Or does anyone have an introductory textbook that they can check? I think that is what mine called them. –Joke137 02:09, 28 September 2005 (UTC)

And rotating curvilinear coord. lead to pseudo-fictitious-forces, I guess. GangofOne 03:10, 28 September 2005 (UTC)
Nonono, I think they're saying that pseudo-force is a synonym for fictitious force. -- SCZenz 04:10, 28 September 2005 (UTC)

Sorry for the slow response. Here is my point. If you have a simple equation of motion \ddot{x}_i=0 and change to curvilinear coordinates y, then the transformed equation of motion is the second total derivative d2x(y,t) / dt2 = 0, given by

\ddot{x}_i=\sum_j\frac{\partial x_i}{\partial y_j}\ddot{y}_j+\sum_{jk}\frac{\partial x_i}{\partial y_j\partial y_k}\dot{y}_j\dot{y}_k+2\sum_j\frac{\partial x_i}{\partial y_j\partial t}\dot{y_j}+\frac{\partial^2 x_i}{\partial t^2}=0

or

\ddot{y}_\ell+\sum_{ijk}\frac{\partial y_\ell}{\partial x_i}\frac{\partial x_i}{\partial y_j\partial y_k}\dot{y}_j\dot{y}_k+2\sum_{ij}\frac{\partial y_\ell}{\partial x_i}\frac{\partial x_i}{\partial y_j\partial t}\dot{y_j}+\sum_i\frac{\partial y_\ell}{\partial x_i}\frac{\partial^2 x_i}{\partial t^2}=0

The first term corresponds to the acceleration in the new coordinates. If x(y) doesn't depend on t, then the last two terms vanish. The second term is the v2/r term that produces the centrifugal force: this comes from curvilinear coordinates, not from a rotating reference frame. The last term is a fictional force from being in an accelerated frame of reference. My point is that it is a bit misleading to say that the centrifugal force comes from being in a rotating reference frame: you can actually think about it as a calculational trick useful for writing F = ma in cylindrical coordinates. –Joke137 18:00, 2 October 2005 (UTC)

In fact, this is kind of like the geodesic equation in GR, which is

\ddot{x}^i+\Gamma^i{}_{jk}\dot{x}^j\dot{x}^k=0.

Joke137 18:02, 2 October 2005 (UTC)

I can't think of any way of clairfying this point to the lay reader. In fact, I'm still not 100% sure I understand it myself. You claim that, if you specify some coordinate system so that the path of a particle following a straight line is no longer "straight" (e.g. polar coordinates), then terms that look like fictitious forces arise to keep the particle on its original path... is that right? -- SCZenz 18:56, 2 October 2005 (UTC)
We could take a roller coaster as an example.... Count Iblis 20:56, 2 October 2005 (UTC)
I may like GR, but the last thing that I would want to do is to bring the geodesic equations of GR into this. If the observer is spinning, his view of events is such that the use of centrifugal force is needed. If he is not spinning but instead is using curvilinear coordinates: Well yes there is a problem if he considers his "directions" to be "radial" and "tangential". However, those are not the kinds of directions that Newton was refering to. In the end the deviations from "in the same direction" seen are not due to a force of any kind. Instead it is due to the fact that his coordinate system is not rectilinear to begin with. That is a whole different ball of wax, and while the geodesic equations do explain that quite elegantly, it is not in my opinion germane to this article. --EMS | Talk 22:16, 2 October 2005 (UTC)
I think I'm gonna back EMS on this one. Objects need not persist in straight-line motion if you define "straight line" in a screwy way. Whereas centrifugal force arises in a rotating reference frame independant of the coordinates chosen for that frame. We have now cited a number of mechanics textbooks that define fictitious forces in terms of non-inertial coordinates, whereas I have yet to see one that defines them in terms of changes to non-rectilinear coordinates. Yes, I would believe there is one somewhere. And yes, I know the two notions are easy to interchange (and impossible to separate???) in GR. But we can keep them separate in Newtonian Mechanics, and limit this article to that. (Bringing in GR has caused us a lot of trouble before!) -- SCZenz 23:05, 2 October 2005 (UTC)

[edit] Well done

I have not looked at this article for ages, but I am very impressed with the way it has turned out. My compliments to the authors. -- ALoan (Talk) 13:58, 28 November 2005 (UTC)

[edit] Acceleration in a straight line

In case 2, the observer being attached to the box, he does'nt move in respect to the box i.e. he is at rest in the box as everything else which is fixed to the box, except the passenger who is lousy attached. This passenger moves in respect to the observer, first by being accelerated backward, secondly by being desaccelerated till he stop i.e. becomes at rest again. Thus for the observer, two forces have acted on the passenger, but not at the same moment, for in this case, the passenger would not have moved at all (case of seat in plain steel). Thus for the observer, these two forces are real, not imaginary. If the first was not real, there would not be a real acceleration, and thus no desacceleration, but these two are real. One force cannot be real and the other not! Consequently, for this observer, no force is fictitious, perhaps mysterious, but real. So where is the fictitious force?--24.202.163.194 03:27, 2 January 2006 (UTC)

Beware: fictitious does not mean imaginary. A fictitious force describes an effect that really does happen when you view the system from an accelerated frame. In this context "fictitious" is simply a technical term meaning that the force is caused by the choice of coordinate system rather than by interaction between objects. It does not imply that there is anything wrong with the force. Henning Makholm 10:33, 2 January 2006 (UTC)

By coordinate system do you mean like the rectangular coordinates, the cylindrical polar coordinates and the spherical polar coordinates?--24.202.163.194 02:34, 3 January 2006 (UTC)

Not as such. "Coordinate system" in this context was used synonymously with "frame of reference". Henning Makholm 13:32, 3 January 2006 (UTC)

Thus, if I understand what you are saying, if we use an accelerated reference frame, we have to intoduce what is called a fictitious force, and if we use another reference frame ( I suppose you mean an inertial one), we don't have to use a fictitious force at all. Is it that? --24.202.163.194 15:04, 3 January 2006 (UTC)

Yes, that is it. (An "accelerating" and "non-intertial" frame is pretty much the same thing). Henning Makholm 15:14, 3 January 2006 (UTC)
Another way to see what is a fictitious force and what is not:[2]
--Aïki 06:14, 17 January 2006 (UTC)

[edit] Earth, planet and accelerated frame

In the introduction, it is said that the surface of the Earth is a rotating frame of reference. and at the end, that an observer on the surface of a planet (thus in general, which means the Earth too, and include the rotating as well than the non rotating planets) is in a accelerate frame.

What I understand from that, is that the rotating frame of the Earth is the accelerate frame of 'a planet'. If it is so, we are talking here of a rotating planet, not a planet having no rotation at all.

Is everybody here understand the same thing or understand it in another way? --24.202.163.194 15:33, 3 January 2006 (UTC)

[edit] Centrifugal force: 2 citations

1- 'A great deal of confusion has arisen regarding the term 'centrifugal force'. This force is not a real force, at least in classical mechanics, and is not present if we refer to a fixed coordinate system in space. We can, however, treat a rotating coordinate system as if it were fixed by introducing the centrifugal and coriolis forces. Thus a particle moving in a circle has no centrifugal force acting on it, but only a force toward the center which produces its centripetal acceleration. However, if we consider a coordinate system rotating with the particle, in this system the particle is at rest, and the force toward the center is balanced by the centrifugal force.'

Source: Mechanics, by Keith R. Symon, University of Wisconsin, Addison-Wesley publishing company, inc.


2- 'Centrifugal force is the inertial (or fictitious) force radially outward on objects when they are view from a rotating frame of reference. The force arises solely from choosing an accelerated frame, and 'disappears' when the problem is viewed from a stationnary frame. ... Remember: 'In a non-rotating frame of reference, there is no such thing as centrifugal force.' '

Source: Physics - A new introductory course, Particles and Newtonian Mechanics, by A. P. French and A. M. Hudson, by the Science Teaching Center at the Massachusetts Institute of Technology. --24.202.163.194 01:38, 4 January 2006 (UTC)

[edit] Euler force

Does anyone have a citation for the term Euler force? Salsb 19:20, 27 January 2006 (UTC)

added to article GangofOne 19:42, 27 January 2006 (UTC)

[edit] Physlets by Brian Fiedler

I have added the following two links to the external links section: Motion over a flat surface Java physlet by Brian Fiedler Motion over a parabolic surface Java physlet by Brian Fiedler

The following GIF-animations illustrate the same thing as the 'motion over a flat surface' physlet:
Inertial motion as seen from non-rotating perspective
Inertial motion as seen from rotating perspective
6 images with vectors depicting centrifugal term and coriolis term.
--Cleonis | Talk 11:53, 23 February 2006 (UTC)

[edit] Object dropped a equator example: isn't Coriolis zero at the equator?

This article has an example: "Neglecting air resistance, an object dropped from a 50 m high tower at the equator will fall 7.7 mm eastward of the spot below where it was dropped because of the Coriolis force.[1]" Whereas Coriolis effect states: "...in the equatorial region the coriolis parameter is small, and exactly zero on the equator." subasd 07:25, 22 April 2006 (UTC)

I now removed that example, because the fall eastward doesn't have to do with Coriolis, which is indeed zero at the equator.[3]subasd 07:43, 22 April 2006 (UTC)

Um, did you notice that that fact was cited from a well-known physics textbook? I'll double-check on monday (the book is in my office) if it makes you feel better. -- SCZenz 08:28, 22 April 2006 (UTC)
Your reference doesn't say the coriolis force is zero at the equator, as far as I can tell. -- SCZenz 08:31, 22 April 2006 (UTC)
In the CE article, that bit is talking about the CE in the plane tangent to the surface as a result of motion in that plane. So, that excludes things falling vertically. At the equator, since CE is perp to the motion and the rot axis, any CE will be perp to the plane and therefore the Coriolis parameter for Met equations (but not the CE) is zero William M. Connolley 08:42, 22 April 2006 (UTC)
Very well, maybe that should be clarified in the CE article then. And SCZenz, my reference (pretty much the first google result) does say "The Coriolis force is zero right at the equator." subasd 08:51, 22 April 2006 (UTC)
You're absolutely right, it does say so and I missed it (likely because I was tired). However, I do think you should be a bit careful removing explicitly cited facts because the first google result contradicts them. -- SCZenz 17:38, 22 April 2006 (UTC)


William is correct. If the distance to the rotational axis isn't stationary, then you have a corriolis force. This is all very elementary stuff, no need to look up anything, just use your brains! If the motion is along the surface of the Earth, then the corriolis force is zero at the equator. Count Iblis 12:31, 22 April 2006 (UTC)

No, not quite! If the motion is along the sfc, but not || to the rot axis, the CF isn't zero, but it is perp to the sfc (and hence disappears from the 2D equs) William M. Connolley 14:13, 22 April 2006 (UTC)
Yes, of course! I guess my brains weren't working properly :). An extreme case of this is mentioned in the article: A star viewed from the rotating spacecraft which I put in some time ago, so I should have known! Count Iblis 16:09, 22 April 2006 (UTC)

[edit] this article is misleading

Problem point: it claims that " There are two ways of analyzing the problem", and next it forces the choice between the use of real forces in an inertial frame, and pseudo forces in a more handy frame - thus denying the way it's analyzed in classical mechanics. Probably that author doesn't know better; but as it stands, this article is making propaganda for the use of pseudo forces, and is thus not conform to Wikipedia standards. Harald88 10:37, 22 April 2006 (UTC)

I disagree. First of all this article is about fictitious force. So, most of the article is about how to make use of this technique. Second, the fictitious force method is part of the curriculum for first year students when they learn classical mechanics. In high school, teachers will usually avoid the concept of fictitious force. However, high school physics education is so low in standard, especially in the US, that we shouldn't pay attention to that here. Count Iblis 12:22, 22 April 2006 (UTC)
Your point one is a faulty argument: an article about a technique should be focussed on it, but not make faulty claims about competing techniques. That's definitely a NPOV violation (your point two didn't address the subject matter). Harald88 10:33, 23 April 2006 (UTC)
In the introduction it is clearly mentioned that sometimes it is useful to use this technique. And it is also clear from the very defefinition of fictitious force that it is just a mathematical technique. So, you don't need to use it, but then you need to work in an inertial reference frame. This is all such elementary stuff that it is hard to see how anyone could be misled.
It's elementary stuff that your claim is erroneous. That approach may sometimes come handy, but working in an inertial reference frame if one doesn't use fictitious forces isn't the only alternative. Instead, the standard alternative is to simply map to a rotating frame, based on an inertial frame - without exchanging potential and kinetic energies. Harald88 13:59, 23 April 2006 (UTC)
B.t.w., perhaps you should take a look at maths articles here. The article on differentiation suggests that to find the N-th derivative you must differentiate a function N-times. It fails to explain that you only need Log[N]/Log[2] steps. That makes a huge difference if you want to compute the billion-th derivative of some function. Is this misleading, POV? Count Iblis 12:42, 23 April 2006 (UTC)
Nice article! But I don't see the claim that you must use that method to find the answer. And exactly that is the issue here; if that article would similarly deny the existance of other methods, it would indeed be misleading (false claims are worse than just "POV"). Harald88 13:59, 23 April 2006 (UTC)
This article doesn't deny alternative methods either. In fact, it is completely trivial to see that you can do without fictitious forces. Count Iblis 17:27, 23 April 2006 (UTC)
I now changed the sentence in order to make your above claim true. Remains that next the statement that Neither viewpoint is more "correct" in any sense recognized by physicists isn't encyclopedic: has an opinion poll about this been published? Idf so, where? Almost certainly the majority of physicists has a contrary opinion. Harald88 17:52, 23 April 2006 (UTC)
Physicists generally recognize the difference between calculational tools and truth claims. One could cite a wealth of physics textbooks that use both methods, and exhort the student (as long as he is careful) to use the one most convenient to the problem at hand. This is not controversial. -- SCZenz 03:45, 24 April 2006 (UTC)
In order to keep such a claim in wikipedia, it must be possible to corroborate it with citations. Harald88 18:04, 24 April 2006 (UTC)
Uncontroversial statements that appear in an overwhelming number of sources usually don't need to be cited. — Laura Scudder ☎ 18:25, 24 April 2006 (UTC)
This one certainly is controversial, as the whole subject is controversial: looking at on the web published comments by academics, apparently many physicists are opposed to the use of fictitious forces. With Google you can readily find sites such as http://regentsprep.org/Regents/physics/phys06/bcentrif/centrif.htm. Some go to extremes in their aversion against that concept wanting to ban it (http://www.physicsclassroom.com/Class/circles/U6L1d.html and http://www.antonine-education.co.uk/Physics_GCSE/Unit_3/Topic_2/topic_2.htm), while others go to extremes in their defense of it, claiming that it's a "real" force. Thus the claim as appears in this article needs at least corroboration, and most probably it needs correction. Harald88 20:17, 24 April 2006 (UTC)
These are all web sites, and they are all designed for teaching; they certainly oversimplify. I see no reason to conclude, for example, that someone who tells a high school student that "there is no such thing as centrifugal force" actually means that centrifugal forces aren't useful for solving problems if you know how to handle them. You are only presenting evidence that people like to teach about fictitious forces in different ways. That's reasonable, since fictitious forces can be philisophically troublesome in certain ways; that's why physicsts don't go in for philosophy much. ;) -- SCZenz 20:48, 24 April 2006 (UTC)
No problem. Now please provide a reference that backs up the claim about physicists in general, or we must change it to make it encyclopedic (no OR). Harald88 20:54, 24 April 2006 (UTC)
Kleppner and Kollenkow, page 340: "By introducing non-inertial systems we can simplify many problems; from this point of view, the use of noninertial systems represents one more computational tool." Now your turn: provide evidence that this statement is controversial among physicists (i.e. not among people who make websites). -- SCZenz 20:58, 24 April 2006 (UTC)
Thanks; note that no-one challenges that it can be used as a tool. Please back up that "Neither viewpoint is more "correct" in any sense recognized by physicists", or modify it such that it corresponds to your source. And do I correctly hear you claim that those web courses are not given by physicists? Harald88 21:23, 24 April 2006 (UTC)
I can find many sources of physicists suggesting different frames be used interchangably; you have yet to present any of physicists claiming that one frame is more correct than another. None of the webpages you cited were created by physicists as far as I can tell; they are high-school-level or corporate created sites. -- SCZenz 21:30, 24 April 2006 (UTC)
See Twin paradox for the claim that inertial frames are special. And so far you have provided nothing that backs up your above counter claim. If it can't be backed up by decent sources, it is unsuited for Wikipdia. Harald88 11:40, 25 April 2006 (UTC)
I'll thank you to stop lecturing me on WP:NOR; we've been disputing whether a statement was original research, not what OR is or whether it's bad. Your understanding of the claim in twin paradox is incorrect: special relativity, like Newtonian mechanics, must have modifications made to make calculations in a non-inertial frame; the paradox arises because calculations are made in a non-inertial frame as if it were inertial. (Of course inertial frames are unique in that they have no fictitious forces, but that doesn't support your claim that people don't use them or don't think of them as equally correct.) -- SCZenz 23:00, 25 April 2006 (UTC)

As far as I know there was no dispute nor any "lecturing"; of course, a dispute will arise if someone here insists on including his/her unsupported POV as fact in this article. And you will have a hard time to find even one ~single physicist who agrees that a frame in which all stars have superluminal velocities is "equally correct" as frames in which they all move at less than c. In general we don't agree with that. Harald88 20:28, 26 April 2006 (UTC)

I don't think I really follow exactly what you dislike about that statement. I mean, the discussion above that is merely comparing the experience of two observers. How can anyone say that one observer is more correct than another? It seems to me like the very experimental nature of science itself forces physicists to accept both observers' experiences on equal footing. — Laura Scudder ☎ 22:53, 24 April 2006 (UTC)


See above. Harald88 11:40, 25 April 2006 (UTC)
I think that Harald's point has more to do with the fact that fictitious forces are not due to any "real" interactions of the physical system and merely an artifact of the choice of coordinates. We discussed this here some time ago. But in this article this isn't very relevant, because we focus on the use of the fictitious force technique. You could say the same thing about virtual particles being an artifact of perturbation theory.... Count Iblis 01:17, 25 April 2006 (UTC)
Yes indeed; and that makes unfounded statements about physicist's opinions of what point of view is or is not equally correct off-topic and thus soapbox, apart of being unencyclopedic. I'll now simply "be bold" and correct it, as too much time is wasted for a minor correction. Harald88 11:40, 25 April 2006 (UTC)
Your minor correction, aside from reflecting your POV, is rather misleading. Physicists commonly use the inertial frame for certain problems, and commonly use other frames for other problems. I'll see if I can't change it to something that's both accurate and won't annoy you, but it's frankly beyond me why people have so many philisophical issues with this article. -- SCZenz 22:50, 25 April 2006 (UTC).
That's fine of course. Note that your remark that "so many people" have issues with this article" is a real surprise of me for I understood from earlier in this conversation that all this was "non-controversial". In any case, we all agree that when presented as a calculation tool, it's indeed non-controversial. Regards, Harald88 20:28, 26 April 2006 (UTC)
If you like, I can refer you to some introductory university-level texts that discuss this technique in some detail. You did remind me to restore the emphasis that fictitious forces are a tool or technique—as such, there's really no such thing as propaganda for them since people can either use them or not and they'll get the same answer. Fictitious forces are used a lot in climatology, because a reference frame fixed to the surface of the earth is rotating once every 24 hours, as we discuss above. -- SCZenz 17:50, 22 April 2006 (UTC)
Thanks but I don't need to be instructed about that technique, it's pretty straightforward. And there is no need to introduce fictituous forces when mapping to a rotating frame; I can refer you to an introductory university mechanics book by Alonso&Finn on that. Harald88 10:33, 23 April 2006 (UTC)
Perhaps you can state clearly for us which sentences in the article are wrong or misleading, or write down what information we should add? -- SCZenz 17:23, 23 April 2006 (UTC)
I did at the start, and corrected it; but I haven't checked the whole article yet. Harald88 17:52, 23 April 2006 (UTC)

[edit] What's in a Name?

We observe an aircraft looping the loop and infer that 'real' forces are present, and can quantify them from the trajectory curvature, etc.. On board the aircraft, we can measure the acceleration directly. Contrary to the fundamentals of empirical science, we view the inference as 'real' and the measurement as 'fictitious'. Maybe the barf bag is fictitious also, and we are left with a not so fictitious mess to clean up.

If we combine the forces acting on bodies within the aircraft with those acting on the aircraft itself, and resolve them into inertial axes, divide by the mass, integrate twice, taking account of the change in direction of the forces over the integration period, and then resolve back, we get the correct motion within the moving frame of reference. But this is an extremely tedious process. It is far simpler to work in the non-inertial frame and introduce the additional terms. In so doing, force no longer equals rate of change of momentum, so we must introduce additional terms, derived from the known motion of the frame of reference, basically to recover Newton's Second Law in its simplest form.

In fact, all we are saying in introducing 'fictitous forces' is that the body moving in an accelerating frame of reference, itself has the acceleration of the frame of reference. The two accelerations must be added vectorially, and resolved to the moving frame for the motion to be predicted correctly. Resolving the acceleration of the moving frame from inertial axes into itself gives rise to the so-called 'fictitious' forces.

In short 'fictitious forces' are 'real' forces acting on a moving frame of reference which are resolved into the moving frame. Somewhere in the process of resolution they apparently left 'reality' behind.

Gordon Vigurs 09:56, 9 July 2006 (UTC)

You wrote "In fact, all we are saying in introducing 'fictitous forces' is that the body moving in an accelerating frame of reference, itself has the acceleration of the frame of reference."
That however is (indeed!) not fictitious at all! There is no necessity when doing that to introduce fictitious forces, and consequently most(?) textbooks don't do that. For the distinction, see the article centrifugal force. Harald88 20:59, 10 November 2006 (UTC)
All I am saying is 'fictitious forces' furnish a simple means of dealing with motions within accelerating frames of reference, nothing more. They are every bit as 'real' as their resolutions into inertial axes, indeed they are the same forces.
For example, circular motion may be interpreted as two orthogonal harmonically-varying motions (with respect to an inertial frame) in quadrature with each other, but it is far easier to deal with an apparently constant 'centrifugal acceleration' if we are interested in finding out if the main spar will break or not. By using the 'fictitous' centrifugal force, we reduce a rather cumbersome dynamics problem to one of simple statics. Gordon Vigurs 20:27, 25 November 2006 (UTC)

[edit] still misleading statements

Checking the situtaion now, I immediately hit on an erroneous statement: "fictious forces could be felt easily by humans, as they are on a spinning carousel."

No fictitious forces can be felt; only true forces can be felt. Fictitious forces are calculation aids for rotating frames. One can't feel what doesn't exist!

What one feels in a spinning carousel is the contact force from the carousel which pushes against the body as it deviates the person from an inertial trajectory. Harald88 20:55, 10 November 2006 (UTC)

But a person's perception is that they are being pushed outward for some reason, and they have to hold on to avoid flying off. -- SCZenz 21:45, 10 November 2006 (UTC)
Sure. According to mechanics theory, that perception of a mystical distant force without a source is erroneous: instead of being pushed outward they are being accelerated inward by the caroussel, as already the second sentence of this article points out. Due to that acceleration, they themselves are pushing back against the caroussel with a real local (and centrifugal) contact force. Claiming that the fictitious force is "real" implies denying the acceleration, thus contradicting the intro and mechanics theory -- one can't have it both ways. Harald88 09:16, 11 November 2006 (UTC)
I claim that the intention of the sentence is that what people "feel" is what they "perceive themselves to feel"... perhaps the sentence needs to be clarified. In fact it is in some cases impossible in principle to distinguish (without outside reference points) whether one is perceiving a fictitious force or a real force. -- SCZenz 09:23, 11 November 2006 (UTC)
There is no disagreement that the contact force that people feel is real, and that is now also emphasized in the Centrifugal force article. People may perceive the real cause as what it is according to science ("absolute" acceleration), or what it looks like from a near-sighted perception (a mysterious force from an unknown source).
Anyway, according to both classical mechanics and modern science the acceleration is "real" -- so that fictitious forces can be imagined but not be felt. Harald88 15:59, 11 November 2006 (UTC)

Actually, true forces can't be felt either, see e.g. here. What you feel are always deformations in your body. If your body doesn't deform, then there is nothing you can notice. If you are accelerated in a homogeneous gravitational field at a million g's you will still be weightless. You need to have non homegeneous forces acting on your body, such as a normal force that acts on the boundary of an object. Then that body will deform, stresses will build up in that body until the normal force is transferred through the body. So, on a closer examination, all forces are fictitious. Indeed you can formulate classical mechanics entirely without introducing forces at all. Count Iblis 23:00, 12 November 2006 (UTC)

In case of the spinning carousel, if the contact force is replaced by a long rang force, e.g. a strong magnetic field (in an experiment scientists have succeeded in letting frogs float in strong magnetic fields), you wouldn't feel anything, even if accelerated by a million g's. Count Iblis 23:06, 12 November 2006 (UTC)

[edit] the chain from cause to effect as the criterium

I copy and paste from above:

I claim that the intention of the sentence is that what people "feel" is what they "perceive themselves to feel"... perhaps the sentence needs to be clarified. In fact it is in some cases impossible in principle to distinguish (without outside reference points) whether one is perceiving a fictitious force or a real force. -- SCZenz 09:23, 11 November 2006 (UTC)

I think a distinction must be maintained between what is felt and what supposition one is tempted to make. Sitting in a soft seat in a car that is accelerating hard, the tempting supposition is that one is being pushed into the seat. From a physics point of view, the seat is doing the pushing. The tempting supposition is one in which cause and effect are reversed. Likewise, in the case of the carroussel, the tempting supposition is to reverse cause and effect.

Then there is the thought experiment of a very large carroussel, with a capsule on it with no outside view. Let the carroussel be pulling 1 G. The passenger in the capsule feels the floor of the capsule pushing against him, and because of inertia his own weight exerts a force upon the floor of the capsule. By general agreement, inertia is not referred to as a force. Adding a vector for inertia in a diagram does not add information to the diagram, for inertia is omnipresent anyway.

The reason that "centrifugal force" and "coriolis force" are referred to as fictitious forces does not amount to a claim that inertia is fictitious. Inertia is real of course!
The reason for the label 'fictitious' is the direction of cause and effect that is attributed. The reactive force (the contact force) in the case of acceleration is due to inertia. Inertial reactive force is always a reaction to acceleration due to a force. That is the chain from cause to effect. --Cleonis | Talk 15:34, 12 November 2006 (UTC)

Thanks for the comment, I agree. The inertial reaction force in a carousel is determined by the carousel's acceleration. The way I see it, causally we have:
carousel rotation -> acceleration of body -> body resists with counter force F=-ma -> carousel pushes with F=ma
Harald88 17:37, 12 November 2006 (UTC)
Huh? The only reason the body accelerates in the first place is that the carroussel pushes it. If you make the push an (indirect) effect of the acceleration, you've got yourself a cyclic causal chain, which is generally frowned upon. Henning Makholm 23:45, 12 November 2006 (UTC)
I think this chain is non-cyclic:
carousel rotation -> carousel pushes with F=ma -> acceleration of body -> body resists with counter force F=-ma
Jeepien remarked the following: as a physics teacher, he would ask the biggest guy in the class to come forward and try to push hard against Jeepien. At first, Jeepien didn't put up any resistance. As a consequence, the big guy could not push hard; if something doesn't resist your push, then you can only exert a faint push on it. The more resistence Jeepien put up, the more force could be exerted upon Jeepien. I'd say something like that was on Harald's mind as he formulated his cause-to-effect chain. --Cleonis | Talk 00:32, 13 November 2006 (UTC)
Exactly, and I must confess that I only achieved a sharp perception of this thanks to discussions in Wikipdia. The caroussel pushes with F= m_body * a because the body is resisting with -F=-m_body*a. It's the body that determines how much the push force must be at given carousel acceleration. Without undergoing the body's inertial resistance the caroussel can't push it with F. However, for this discussion one may also summarize it as follows:
carousel rotation -> acceleration of body -> body resists resulting in a force/counter-force pair of F=ma.
These forces are real; they should not be confused with fictitious forces. Harald88 22:24, 13 November 2006 (UTC)
I would agree with your chain, if forced to write one down. In general, however, I am not convinced that such explicit accounting for causality is particularly helpful when discussing action and reaction. The Third Law of Newton is not a causal relation. It merely asserts the empirical fact that (non-fictitious) forces somehow always manage to occur in balanced pairs, and using it causally has an uncomfortably teleological feel to me. The conceptual status of "the reaction force" seems to get more confused the deeper you dive into the quantum origin of contact forces. If I understand correctly, modern physics prefers to speak instead of conservation of momentum, which is equivalent as far as Newtonian dynamics is concerned and appears in the modern physicist's worldview to be more fundamental than action-reaction pairs. Henning Makholm 01:19, 13 November 2006 (UTC)
A statement about inertia does not necessarily involve Newton's third law. One can also opt to take Newton's third law as a statement about the concept of physical interaction: for example stating that electrostatic force is an interaction precisely because it involves two particles interactng with each other. One can then assert that inertia falls outside the scope of Newton's third law, because it involves just one particle.
Inertia is one of the enigma's of physics. So, yeah, asserting a particular sequential order is on the edge. However, there can be no doubt that the existence of inertia must be assumed in order to formulate a theory of motion. Newton's first and second law are statements about inertia. --Cleonis | Talk 07:52, 13 November 2006 (UTC)
With my above causal chain I did not mean to suggest sequential order, as such implies delay times. A macroscopic description of a local inter-reaction doesn't have any, to my knowledge. Harald88 22:37, 13 November 2006 (UTC)
The original question was about the passenger of the carousel. The passenger of the carousel is correct in feeling that he is exerting a force upon the floor of the carousel; the reality of inertia. Since the carousel is much heavier than the passenger we opt to say that the floor started pushing; size matters. By contrast: let the carousel be a lightweight one, about as heavy as the passenger. Then the situation is more symmetrical. The carousel can push the passenger around, because of the carousel's inertia, the passenger can exert a force upon the carousel because of his own inertia. The carousel's leverage for altering the passengers momentum arises from its own inertia, and vice versa. For Newton's third law to hold good, interactions must be reciprocal (such as with electrostatic force) and inertia must be uniform throughout space. The law of conservation of momentum holds good only if inertia is uniform throughout space. Hence, asserting conservation of momentum is a statement about inertia. --Cleonis | Talk 15:53, 13 November 2006 (UTC)

[edit] The question which observer is preferred

I copy and paste from above:

How can anyone say that one observer is more correct than another? It seems to me like the very experimental nature of science itself forces physicists to accept both observers' experiences on equal footing. — Laura Scudder ? 22:53, 24 April 2006 (UTC)
I think that Harald's point has more to do with the fact that fictitious forces are not due to any "real" interactions of the physical system and merely an artifact of the choice of coordinates. [...] Count Iblis 01:17, 25 April 2006 (UTC)

I take it as generally agreed that the observer who has more information at his disposal (and puts it to use) is preferred. In the case of the Earth's motion around the Sun, we have the geocentric model of the Solar system and the heliocentric model. For several centuries now, we have the knowledge to see that the heliocentric model is superior. Anyone who adopts the geocentric model is disregarding a wealth of knowledge.

If motion is mapped with respect to an inertial frame of reference then there are no artifacts. The evergreen among the examples: a carousel with a passenger. The carousel is exerting a force upon the passenger, causing the passenter to move along a circular trajectory, and the passenger is exerting a force upon the carousel. (Generally the carousel is much heavier than the passenger, and then the motion of the carousel due to the force exerted by the passenger is imperceptible.)

For any object, the leverage enabling that object to exert a force upon another object comes from inertia. Important as inertia is, inertia is never specified in any diagram. Inertia never being specified is not a problem: inertia is always present, hence adding a vector for inertial reactive force in a diagram would not add information to the diagram.
When motion is mapped with respect to a non-inertial frame of reference, then vectors for the acceleration with respect to the inertial coordinate system are added. In the case of motion along a circular trajectory, the vector for the centrifugal force coincides with the vector for the inertial reactive force. As is emphasized in the centrifugal force article, what is referred to as the 'fictitious centrifugal force' is quite distinct from what is referred to as the 'inertial reaction force'.

The point of view that recognizes that the passenger and the carousel are exerting a force upon each other, and that the carousel and the passenger are both circling around their common center of mass, is the point of view that has the highest information content. That point of view is preferred.
The very information accumulating, deductive nature of science forces physicists to recognize that not all observer's experiences are on equal footing.

Inertia is still an enigma, but there are theories that seek to describe properties of inertia. The general theory of relativity asserts that inertia must be seen as a property of spacetime. According to general relativity, spacetime itself acts like a field, in the sense that spacetime itself is opposing change of velocity (with respect to space) of inertial mass. This assertion of general relativity can retroactively be taken as a corresponding axiom of newtonian dynamics. (This shows in what respect there is a continuity from newtonian dynamics to relativistic physics.)

Finding out which frames are inertial frames and which frames aren't is a process of gathering information. Observers who have gathered that information and put it to use are preferred. --Cleonis | Talk 23:09, 15 November 2006 (UTC)

"Preferring" one observer over another is, it seems to me, essentially POV. What an encyclopedia should do is explain how one the different observer's frames differ from each other, and how to do mechanics in each of them, without adding any artificial (and essentially non-scientific) value judgements between them. Henning Makholm 01:13, 16 November 2006 (UTC)
Making assessments is part of the endeavour of science. For example, in the period of the Copernican revolution, individual scientists had to make up their mind which research programme to follow, the ptolemaic programme or the copernican/keplerian/newtonian programme. Long after the scientific revolution has taken place, evidence has mounted, and the choice is obvious. However, during the actual transition, the evidence was inconclusive, and which research programme to follow was a judgement call. There can be no doubt that the scientists who made that judgement call were scientists indeed.
Making assessments as to what avenues are worthwile to explore is part and parcel of science. An academic who would be so foolish as to avoid judgement calls altogether has little opportunity to be scientifically productive.
Generally, ranking points of view on the basis of their information content is part of science. --Cleonis | Talk 05:54, 16 November 2006 (UTC)
In physics, we write down theories to determine measurable quantities. How can the superiority of one reference frame over another be determined if calculations in both give all the same measurements? It's philosophy that assigns value judgements like that; physics just seeks to answer concrete questions correctly. -- SCZenz 08:00, 16 November 2006 (UTC)
At the same time, there are certain very clear conveniences to setting up a basic theory in inertial reference frames. For example, when I jump up and down the entire universe moves from my perspective—it's clear that it's far saner to work in a frame where I'm not jumping than to try to treat the motion of the universe as arising from a physical mechanism. -- SCZenz 08:05, 16 November 2006 (UTC)
The use of fictitious forces goes even further than that. According to the Force article, "In physics, force is an influence that may cause a body to accelerate". An influence is an effect that results from something that causes it, and a fictitious force lacks one -- a fictitious force is an influence without a cause, or more precisely, the cause is first admitted but next denied to exist: the "pulling" is caused by no other physical entity. BTW, that is not the case with gravity, so that the article's statement that gravity is also a fictitous force is (I think) contested. Harald88 07:35, 17 November 2006 (UTC)
I don't really agree with the causality argument. Causality is very hard to define rigorously, because of invariance under time reversal. Causality is not something that is present in Newton's Laws, but it has to be put in by hand in the form of low entropy initial conditions.
All that you can say is that whenever there is an acceleration, there is a force such that F = m A. There is no cause and effect here at all. The problem is that when we write about this in English, words like "cause" etc. slip in. Language is not a very suitable way to describe the physical world and there is usually no alternative than to tolerate inaccuracies to prevent readers being confronted with an unreadably convulated piece of text. Physicists do this all the time when they write.
As I wrote in the previous section above, ordinary force is just as fictitious as "fictitious force". You can't measure a real force directly either (we can measure deformations, deformations etc.). What you can say, however, that a force in an inertial system is more directly related to observable effects like deformation of an object, than fictitious force. Count Iblis 13:33, 17 November 2006 (UTC)
First of all, this page is not meant to express our agreement or disagreement. Causality is certainly included in Newton's theory and classical mechanics. But if you can show that the Force article is erroneously sourced, please don't hesitate to correct it. Harald88 20:13, 17 November 2006 (UTC)
The purpose of theories is that the raw perceptions are ordered into overseeable structures. A point of view along the lines of "physics just seeks to answer concrete questions correctly" is very naive. I recommend reading the following two works by the historian of science Thomas Kuhn: 'The copernican Revolution' and 'The structure of scientific revolutions'. Thomas Kuhn adresses naive suppositions and describes what scientists actually do. (It is necessary to read the actual books, to my knowledge, the majority of reviews and commentaries misrepresent the work of Thomas Kuhn. I am pleased to see that the Synopsis section of the wikipedia article about Thomas Kuhn's work is pretty good.) --Cleonis | Talk 01:03, 17 November 2006 (UTC)

[edit] Removing derivations of equations

I would like to remove the derivations of the equations in the last two sections, and just keep the final result plus a few references to textbooks, preferably available on-line. (For example section 3.4 of this textbook on celestial mechanics for the rotating frame of reference.)

There main reason why I want to do this is that I think the mass of equations (even though they are near the end of the article) might scare some non-mathematical readers into thinking they will not understand the article, even though the rest of it in written in a way that should be accessible to most. For those readers who are interested in the deductions of the equations, I still think references are better than including the deductions, because a textbook can give much better context to the equations than Wikipedia can.

Anybody opposed?

--PeR 11:17, 11 February 2007 (UTC)

I am. I don't think your stated reason is a good one for removing useful information from Wikipedia. -- Matthew Woodcraft 14:07, 11 February 2007 (UTC)

OK. How about moving the derivations to articles entitled "Derivations of ..." (or similar) and linking to those? That way the information isn't removed from Wikipedia. --PeR 15:39, 11 February 2007 (UTC)
I'd have no objection to that. --Matthew Woodcraft 16:23, 11 February 2007 (UTC)

PeR, you mentioned section 3.4 of the celestial mechanics textbook in connection with rotating frames of reference. From what I can see, section 3.4 is about the derivation of general acceleration components for polar coordinate systems. You seem to be confusing the two topics. I can't actually find anything to do with rotating reference frames and fictitious forces in the entire book. There would be no reason for having a rotating reference frames chapter in a celestial mechanics book because it is not remotely relevant.

This seems to be at least part of the reason why you don't believe that the linear expansion of a radial vector can be real when the vector is rotating. You associate rotating frames of reference with fictitious forces, and hence you are wrongly associating polar coordinates with fictitious effects. David Tombe 11th February 2007 (203.87.176.3 16:48, 11 February 2007 (UTC))

Opposed
The derivations are at the end of the article, so I don't see the nonmathmatical being scared off - by the time they have read that far, they have got all they are going to get from the article. On the other hand, the mathematically or physically inclined reader needs the derivation to get a good grip on the topic. For that reader, the math makes more sense than the chatter. More than that, one of the advantages of Wiki is that you don't need a book - that is the point of the free encyclopedia and a big part of its convenience and value. And to add a minor point - the math is useful on discussion pages when issues come up. Brews ohare (talk) 23:13, 7 May 2008 (UTC)

Opposed, I agree with Brews ohare. Also, people who are scared of mathematical equations should seek psychiatric treatment for their "mathaphobia"  :) Count Iblis (talk) 00:27, 8 May 2008 (UTC)


Yes, I would agree with Brews ohare too. The maths equations in this particular case are extremely revealing. David Tombe (talk) 14:05, 16 May 2008 (UTC)

[edit] Real Centrifugal Force

Mr. Connolley, are you sure that your reversion wasn't simply a knee jerk reaction in relation to the edit war on the 'Centrifugal Force' page? The edits to the centrifugal force page that were the source of controversy were much more extensive than the small article that I inserted in this topic. I had already stopped reverting in the centrifugal force page as a reasonable discussion had opened up on the talk pages.

The essence of what I was saying was that in meteorology, the Coriolis force is always fictitious. Nobody is disputing this fact. I was merely pointing out that by its very nature, a rotating frame of reference will impose a circular motion artifact on all objects in the inertial frame. This artificial circular and tangential motion is catered for by the Coriolis force equation.

The situation regarding centrifugal force is not so simple. A rotating frame of reference cannot create a real radial effect. If a real radial effect occurs in the rotating frame of reference, then it must exist as a reality in any frame of reference.

Fictitious centrifugal force is said to act radially outwards on objects that are stationary in the rotating frame of reference. But these objects possess tangential motion in the inertial frame and the radial expansion is already real.

If this kind of centrifugal force is really an artifact, how can it throw somebody against the inside of the door of a car? In the centrifugal force article, this real centrifugal force is actually acknowledged under the title of 'Reactive Centrifugal Force'.

Was there really any reason for you to delete my insertion, or were you merely jumping on the bandwagon which began when Racecarr deleted my insertions in the Coriolis force article?

Rracecarr deleted my insertions in the Coriolis Force article for his own selfish reasons. He obviously sees himself as a guardian of Einstein's theories of relativity. I finally acknowledged that my insertions about real Coriolis force were original research and so I stopped the edit war. But that is no reason why you should have to delete a simple insertion pointing out that centrifugal force is often real. David Tombe 11th February 2007 (203.87.176.3 14:03, 11 February 2007 (UTC))

David, there is long discussion about this insertion on Talk:Centrifugal force. You are the only one in favor, while a number of editors have spoken against it. Repeatedly inserting the text serves no purpose, and causes annoyance to others. Please stop. --PeR 18:55, 11 February 2007 (UTC)

Mr. Connolley, the long discussion in the centrifugal force section is about a different insertion. But at any rate, the essence of that discussion comes down to a denial on their part, of the reality of two objects having a mutual radial acceleration, if they have a mutual tangential velocity. It is an undeniable fact, yet they seem to think that it can be screened out by viewing the situation from cartesian coordinates.

It was cheap of you to ask me to please stop making the insertion when you have already blocked me from doing so, and when I wrote to you asking you why. You have totally ducked the issue. It's a bit like somebody getting sacked from a job. When they ask why they were sacked, they receive the reply 'Please don't come back'.

You failed to address the question, 'how can a rotating frame of reference cause a radial outward acceleration as an artifact?' If you think about that question, you will see that my insertion into this article was valid. Instead you chose to play the numbers game and point out that I am outnumbered by a group of zealous people who keep a 24 hour vigil on the article. David Tombe 12th february 2007 (222.126.33.122 19:32, 11 February 2007 (UTC))

Mr. Connolley, I must apologize for my reply to you above. I have just noticed that it was actually Mr. PeR who wrote the letter that I was responding to. The position of his letter made me assume that it was your reply to my letter to you directly above it. The fact that it was Mr PeR who told me to stop making the insertion now makes it all the worse because he already knows fine well that I have stopped reverting on the Centrifugal force article, and he would have seen my letter to you querying the reversion on this article. David Tombe 12th February 2007 (222.126.33.122 20:10, 11 February 2007 (UTC))
David, I know that you have stopped making the insertion to Centrifugal force, but the text was inserted twice today into this (Fictitious force) article. If it wasn't you I apologize, but it was natural to assume since you have inserted that text into different articles at least a dozen times before from various IPs. In order to avoid further confusion, perhaps you could consider creating an account? --PeR 20:45, 11 February 2007 (UTC)

Are you replying as proxy for Mr. Connolley? The offending text in question points out that a rotating frame of reference can induce circular and tangential effects as artifacts (Coriolis Force), but that it cannot, by its very nature, produce radial effects. The underlying tangential velocity in the inertial frame causes the radial effects in a centrifuge.

I'm fascinated as to how this basic realization has managed to cause so much upset. I'd like to hear Mr. Connolley's views directly as to why he blocked me from making this insertion. David Tombe 12th February 2007 (222.126.33.122 10:09, 12 February 2007 (UTC))

I edit as myself, not via proxies. Please get yourself an account. Who is Mr Connolley? You were blocked, as I recall, for breaking 3RR. Please don't William M. Connolley 11:02, 12 February 2007 (UTC)

I assumed from your user name William M. Connolley that you are Mr. Connolley. I'm sure you don't edit via proxies, but clearly Mr. PeR presumed to be your proxy by replying to my original letter to you. Are you not going to reply to it yourself? We know the reasons why I was blocked. That doesn't need to be restated. My letter to you was asking you to reconsider the issues in the blocked insertion. You can read it at the beginning of this section. Are you willing to consider the fact that if a centrifuge causes radial acceleration, that this cannot be a fictitious artifact? Whereby Coriolis force is fictitious in all officially recognized situations, centrifugal force is often not fictitious at all. David Tombe 12th February 2007 (222.126.33.122 12:03, 12 February 2007 (UTC))

If you know you've been blocked for 3RR, why did you write I'd like to hear Mr. Connolley's views directly as to why he blocked me from making this insertion? Please get yourself an account, if you want to avoid irritating people. You might also avoid the meningless officially recognized situations terminology, unless there is some institute for certifying physics I'm unaware of William M. Connolley 12:31, 12 February 2007 (UTC)

Mr. Connolley, the officially recognized situations are such as in meteorology and oceanography, as opposed to in magnetism where it is not officially recognized. On the other point, I wasn't asking to hear your views as to why I was blocked. You twisted that one completely. Here's the quote from a few paragraphs up "My letter to you was asking you to reconsider the issues in the blocked insertion."

Are you willing to face up to the fact that centrifugal force in a centrifuge is not fictitious? And if so, can the insertion be reinstated? I don't see why this issue has to be irritating people. David Tombe 12th February 2007 (222.126.33.122 13:45, 12 February 2007 (UTC))

William - In talk:centrifugal force#Inertia, David cites the geodesic equation for radial motion in polar coordinates \frac{\mathrm{d}^2 r}{\mathrm{d}t^2} - r \left( \frac{\mathrm{d}\theta}{\mathrm{d}t} \right)^2 = 0, and says that
I don't adopt the Newtonian concept that this motion is explained by inertia.
This makes David's claim complete and total original research. It is kindly requested that you semiprotect this article and centrifugal force if David should try to edit either one again. --EMS | Talk 14:46, 12 February 2007 (UTC)

ems57fcva, that is a downright lie. I have never quoted that equation as being equal to zero. Check the two citations once again. The equation that I referenced contains the left hand side of the one that you have just quoted, but equates it to radial aceleration, and not to zero as you have alleged. One of these two terms is the centrifugal acceleration. If we equate the other term to Newton's inverse square law of gravity, then we have got the planetary orbital equation that leads to ellipses, hyperbolae, or parabolae. If the kinematical approach upsets you, then multiply it through by inertial mass and convert it into force.

My quote about inertia has got absolutely nothing to do with this basic textbook kinematics. Once again, you are twisting the facts in order to try to get my arguments excluded on the grounds of being contrary to wikipedia policy.

You basically haven't got a clue what you are talking about. David Tombe 12th February 2007 (202.69.162.228 16:16, 12 February 2007 (UTC))


To anon/David: I trust EMS on this. And please... get yourself an account if you intend to contribute seriously William M. Connolley 15:00, 12 February 2007 (UTC)

Mr. Connolley, that is one of the weakest arguments that I have ever heard. You trust EMS? EMS is a person who misrepresents the truth. EMS said that I used that equation above. EMS knows that I equated the terms to radial acceleration and not to zero.

If you trust EMS, then I can only conclude that either you haven't got a clue about fictitious forces, despite being an editor for this topic, or that you are merely closing ranks with a little clique.

What has having an account got to do with whether a contribution is serious or not? You are trying to insinuate that I am not serious about my arguments. Is that self delusion on your part to avoid you having to face up to the fact that rotational motion cannot create a radial artifact? David Tombe 12th February 2007 (202.69.162.228 16:25, 12 February 2007 (UTC))

ems57fcva, I have seen right through you now. Here is your profile [4]

You are hoping that some day your own original research will appear in wikipedia.

"I hope to see an article on Flat Background General Relativity here eventually, but in order to play it safe I will not be the one who starts it."

What I have been advocating is basic textbook applied mathematics but it contradicts your own original research. That is the grounds upon which you are trying to make the editors believe that what I am saying is original research. You want it excluded on that false premises because it contradicts your own original research. David Tombe 12th February 2007 (202.69.162.228 16:39, 12 February 2007 (UTC))

David - You would do well do read the No personal attacks policy. If this keeps up, I will ask William to protect these talk pages too! You have chosen not to accept that a radial coordinate acceleration in polar coordinates is an effect of inertial motion. That is where everyone else disagrees with you. I know that you cannot see what you have done, but I honestly assure you that this is a novel application of the geodesic equation given above and Newton's second law. Try getting this past a physics professor and see what he or she has to say. In the meantime please stop bugging us, or at least please learn how to be civil. --EMS | Talk 17:31, 12 February 2007 (UTC)

EMS, you say

"You have chosen not to accept that a radial coordinate acceleration in polar coordinates is an effect of inertial motion. That is where everyone else disagrees with you."

First of all, the radial acceleration is a reality irrespective of what coordinate system we use. Secondly, if the acceleration occurs due to the tangential velocity, then there is absolutely no need to bring the subject of inertia into the discussion. The only factor that gives rise to this centrifugal acceleration is the mutual tangential velocity, and we can solve the orbital equation without even bothering to multiply through by inertial mass. It's no good saying "Try getting this past a physics professor". Which particular physics professors are you talking about? What about an applied maths professor? David Tombe 13th February 2007 (124.217.34.54 01:17, 13 February 2007 (UTC))

[edit] Detection of non-inertial reference frame

I am unhappy with the "Detection of non-inertial reference frame" section. Its general disposition of setting up a strawman argument, attributed to some unidentified "philosophers", only to knock it down smugly seems to me to be unsalvageably non-encyclopedic. While such rhetoric is not uncommon in textbooks that aim for a informal touch here and there, an encyclopedia article deserves a rather more detached tone.

I tried briefly to trim the section down to make it less polemic and more matter-of-fact – but ended up with nothing, or at least no definite points that clearly deserve to be made early in an article about the fictitious force concept. If I understand the intent of the section correctly, its basic point is that one can know that one is in a non-inertial frame by observing that bodies move in ways that appear to require fictitious forces to explain. But even to make this point properly for readers who learned Newtonian gravity in school would seem to lead into a discussion of GR or at least the Equivalence Principle, and I don't think we want to go there as early in the article as this section. GR should not come before we have exhausted most of what there is to say about the subject in a Newtonian context.

I'm not quite bold enough to just delete the section unilaterally and wait for the outcry, so if somebody thinks this section has redeeming qualities, could they please speak up? –Henning Makholm 02:38, 4 March 2007 (UTC)

Agreed, what do you think of it now? Zedall 05:54, 13 August 2007 (UTC)

[edit] Are fictitious forces Newtonian?

Correct me if I am wrong, but my understanding is that in Newtonian Dynamics there is no such thing as fictitious forces. If we believe in an Absolute Space, as Newton did, we have frames of reference absolutely at rest or absolutely in motion (and also absolutely accelerating). When a body experiences a centrifugal force it is clear that there is a rotating frame of reference whose motion needs to be taken into account when transforming coordinates from a different frame of reference. But Newton wouldn't call this fictitious force; he would simply say that the frame of reference is moving (or rotating) with respect to absolute space.

The example of the passenger inside an accelerating car shows this very clearly: the first way of analyzing the problem says that the car is accelerating wrt absolute space and it pushes the passenger. This is Newton's view. The second interpretation using fictitious forces is post-newtonian.

The concept of fictitious force arises only when we reject absolute space and, as far as I know, Newton never used this term.

If the above is correct, then the heading "Newtonian examples of fictitious forces" in this article should be changed, because it is contradictory. I would suggest simply "Examples of fictitious forces". —The preceding unsigned comment was added by Dukeofalba (talk • contribs) 09:09, 4 May 2007 (UTC).

  • The Coriolis force is a ficticious force that appears in Newtonian dynamics. Newtonian in this example probably refers to Newtonian physics, not the old virgin who worked at the mint. But I recall Newton doing the spinning bucket experiment (at least in thought). Which involves a ficticious centrifugal force.WilyD 12:36, 4 May 2007 (UTC)

Sure, Newton mentioned these and similar experiments. But the point is, however, that his interpretation of these experiments did not include the concept of fictitious force. Therefore, the word "Newtonian" should not be used in this case. It is not a physical, but a historical remark.

It is a fact that the term Newtonian, as commonly used today, does not imply a restriction to ideas formulated (or believed) by Isaac Newton himself. It refers more generally to the physical worldview that reigned in physics from the time of Galileo and Newton until it was superseded by (special and general) relativity. Your contention that the word should only be used to refer to what Newton himself thought, is not supported by modern usage. –Henning Makholm 08:31, 5 May 2007 (UTC)

Sorry, but that is not the point I am trying to make. As you correctly say, Newtonian means pre-relativitistic physics, not only what Sir Isaac thought -- although if you look up "Newtonian" in this Wikipedia, it reads: "Newtonian refers to the scientific work of Isaac Newton". The point I am trying to get across is that the Classical, Newtonian or pre-Relativistic view did not include fictitious forces. Fictitious forces correspond to the modern view. (It is not strictly post-Relativistic either, because some authors like Mach and Bishop Berkeley hinted at it, but these were not mainstream philosophers at that time.) Therefore, fictitious forces and Newtonian (or Classical) mechanics ARE MUTUALLY EXCLUSIVE. I believe that this point should be made clear in this article, so deleting the word "Newtonian" from the above mentioned heading is but one step, the next one should be to add an explanatory paragraph.

You are wrong. There is no conflict between fictitious forces and classical mechanics. They are not "mutually exclusive" in any sense. Sorry, and thanks for playing. –Henning Makholm 22:50, 5 May 2007 (UTC)

In a scientific discussion it is essential to provide evidence, or at least, to show your logical process of deduction. Therefore, I would like to ask you to substantiate any of your categorical assertions. For example, it would be appreciated if you could provide one single reference of an author speaking of "fictitious forces" before 1905 - the date of publication of the theory of relativity. Lacking that, I would kindly ask you to stop interfering in this serious debate.

"Before 1905" is not relevant. The relevant point is that the fictitious forces arise within the Newtonian model of time and space, which continues to be a useful model also after the publication of the theory of relativity. –Henning Makholm 12:12, 6 May 2007 (UTC)
Indeed "before 1905" is not relevant in itself. What is relevant, is that in Newtonian theory any fictitious cause is rejected for a physical description of mechanics. And of course, in Newtonian mechanics gravity is not fictitious. It's a hallmark of Newtonian mechanics to ascribe physical causes to physical phenomena. The introduction of a rotating frame of reference as being in rest with the resulting unphysical description of nature is not just "post-Newtonian", it is pertinently anti-Newtonian. Suggesting that anti-Newtonian concepts belong to Newtonian theory is misguided and readers should be spared such a misrepresentation. Harald88 13:04, 6 May 2007 (UTC)
That is just nonsense. The examples in the article are Newtonian, because they are about forces that arise in accelerating frames that live inside ordinary Newtonian spacetime. Since they are completely deductive consequences of inertial-frame Newtonian mechanics (no new physics at all is involved, it's just a purely mathematical re-framing of the same theory), it makes no sense to refer to them as "anti-Newtonian". –Henning Makholm 13:18, 6 May 2007 (UTC)
I agree. I think that this sort of discussion also happened when people first started to use negative numbers, imaginary and complex numbers, etc. etc.... Count Iblis 15:16, 6 May 2007 (UTC)
I agree with Harald88: the concept is anti-Newtonian. Speaking of "Newtonian fictitious forces" is just like speaking of "Relativistic aether"; it's an oxymoron. So is the "Newtonian spacetime", that someone has mentioned in this discussion, since in Classical Mechanics space and time are two completely separate concepts. In Newtonian mechanics time has the property of being absolute and the need for the idea of fictitious force arises only when we reject absolute space. Therefore this idea is fundamentally anti-Newtonian. It seems that some people confuse the physical system with the interpretation we make of it. A system can behave within Classical limits if there are no close-to-speed-of-light velocities, and yet our interpretation can be completely non-classical. Of course Nature does not change one bit its behavior, what changes is our interpretation of the phenomena.
Far from being "anti-Newtonian", the concept of fictitious forces is at its core a Newtonian one; it does not work well except within Newtonian mechanics. In Newtonian mechanics, fictitious forces give a complete and exact answer to the problem of describing motion relative to non-intertial space coordinates. On the other hand, within relativity they can only be approximate, and they are only helpful at all in precisely those low-speed cases where the Newtonian theory is also a useful approximation. For example, according to special relativity an observer that rides on the outside edge of a spinning centrifuge experiences less time than an observer that rides the centrifuge closer to its center. There is no way to fix up a rotating Lorentzian space to do that simply by adding extra fictitious force; time cannot be influenced by forces in SR. In Newtonian mechanics, on the other hand, fictitious forces provide a complete and exact explanation of all the ways in which observations in a rotating coordinate system differ from observations in an inertial system. Fictitious forces are, if possible, even farther removed from general relativity which embraces the use of non-uniform coordinates as its basic substrate; a non-uniform coordinate system does not need any special correction in GR, because the core theory is born to deal with it.
Also, Newtonian mechanics does reject the concept of absolute space; this rejection is not particular to Einstein's theories. Sir Isaac himself may have believed in absolute space, but the theory that modern physicists apply his name to does not. –Henning Makholm 20:17, 6 May 2007 (UTC)

[edit] GR - gravity not equivalent to acceleration + Why not "reaction force" versus "fictitious force"? + "non-contact" forces

GR - gravity, acceleration

With GR, the intensity of a gravitational field results in fixed time dialation, while acceleration results in a constantly changing time dialation, so there is a difference, but it requires an "outside" observer. For example, a person on earth could observe a clock in a non-orbiting rocket that is rising vertically (along earth's orbital path around the sun is good enough for "vertical") at a constant speed As the rocket increases it's distance from the earth, the intensity of the gravitational field is reduced, and the rocket's clock appears to speed up. In the second phase of the experiement, the rocket starts to accelerate holding it's altitude (following a path orbiting the earth), and the rocket's clock will steadily slow down as the speed of the rocket increases.

"reaction force"

For forces that are the result of inerital resistance to an applied force, why not call these reaction forces? Centrifugal force would be an exampleof a reaction force. Using the accelerating car example, the seat applies a force to the person in the car accelerating the person, and the person's body applies an equal and opposing "reactive" force against the seat, increasing the pressure and causing deformation at the point of contact.

"non-contact forces"

Other forces don't require physical contact. Gravity is one of these and is relative to mass. Electrical forces are relative to charge. Should electrical forces be considered "fictitious"? What about the nuclear strong and weak forces?

Jeffareid 21:18, 12 September 2007 (UTC)

1. You appear to be making the common mistake of thinking that coordinates have physical significance. If I understand you correctly, by "time dilation" you mean the ratio between coordinate time and proper time. However, this ratio is not a physical thing, but purely results from your choice of coordinate time - GR allows you to use any smooth and monotonic coordinate time you care to. If you have a metric where you and your neighbors have each have constant dilation, you can transform it to one in which everybody's time dilation change simply by applying a nonlinear substitution to the time coordinate. And it is a fundamental tenet of GR that descriptions that are related by (metric-preserving) coordinate transformation are physically identical. Your "outside observer" would have to observe not only phenomena in the physical world, but also differences in expression that (thus claims GR) have no physical reality.
2. The best reason not to call fictitious forces reaction forces is that "reaction force" already has a different meaning, namely the opposite end of an action-reaction pair according to Newton's third law. Sometimes people do call fictitious forces "reaction forces", which often results in confusion and misunderstanding - see the archives of Talk:centrifugal force for plenty of examples.
3. I don't get your point. Physical contact or the lack of it is not a defining feature for fictitious forces - in fact the very question of whether a force acts locally or at a distance makes sense only for real forces (which obey Newton's third law and always occur in pairs, such that you can ask whether the two sides of the pair are at the same place). A fictitious force involves only a single object and its relation to your coordinate system, so there is neither anything that can "contact" nor anything that can be "at a distance". The observation that hints (*) that gravity may be thought of as a fictitious force is not that it acts at a distance, but that the acceleration it causes a body to have (in the absence of other forces) does not depend on the mass (or any other intrinsic properties) of the body. Electromagnetic or nuclear forces do not have this property.
(*). This hint is of course not sufficient to deduce that gravity must be a fictitious force. A much stronger argument is the fact that the idea happens to work and agree with observation when worked out in detail, namely GR. –Henning Makholm 21:09, 13 September 2007 (UTC)

[edit] Recommended reading

I think we should add http://xkcd.com/123/ ;-) --Itub 15:10, 20 September 2007 (UTC)

It's been added to centrifugal force several times. Consensus seems to be that it does not add real encyclopedic value to the article. (Being hysterically funny is not an encyclopedic value). –Henning Makholm 18:03, 22 September 2007 (UTC)
I know, I was (mostly) joking. However, I think this comic has the best answer to the perpetual questions that pop up here from people who think that fictitious forces "do not exist". Very educational. ;-) --Itub 15:58, 24 September 2007 (UTC)
I think the comic would be an excellent illustration, either to this article, or to the section entitled "confusions and misconceptions" at centrifugal force. Unfortunately it is licensed under Creative Commons Non-Commercial, while Wikipedia requires Creative Commons Share-Alike (I think). Anybody feel like writing the artist and asking him to re-license it? --PeR 10:51, 25 September 2007 (UTC)
I understand that the artist makes his living partly from merchandise sales. Any Wikipedia-acceptable license must entail a blanket permission for anyone to print and sell t-shirts (or whatever) with the comic alone (i.e., not necessarily in an encyclopedia context). Though that particular strip is not (as far as I can see) currently being merchandised, it is still one of the more popular Xkcd strips, and it seems rather unlikely that the artist would agree to letting go of it in that way. –Henning Makholm 08:28, 29 September 2007 (UTC)
The obvious motivation for the artist would be that Wikipedia would link to xkcd, thus driving traffic to the site, increasing t-shirt sales. Anyone else making t-shirts with that comic under the CC license would also have to credit the original artist, so it would probably increase, rather than decrease, his sales. --PeR 09:35, 29 September 2007 (UTC)
Speculating probably won't help, we would have to ask the author. But first, do we really have a consensus for including the strip in this article (or in centrifugal force? --Itub 17:21, 29 September 2007 (UTC)
opposed
The article is fun, but unhelpful. Brews ohare (talk) 14:35, 11 May 2008 (UTC)

[edit] Removal of addendum

I've deleted the addendum by 97.66.28.156 (Talk) about fictitious force. It appears to contradict the discussion that precedes it, which is entirely accurate. In my view, "fictitious force" is a technical term, as defined in the article, and clearly present in the noninertial frame of the accelerating car. The addendum confuses it with a "feeling" by the car passenger. That is not a correct interpretation. Brews ohare (talk) 15:31, 14 May 2008 (UTC)

[edit] New category

I've now created a new category, Category:Fictitious forces, to include all articles about fictitious forces. Question: should gravity be in this category? -- The Anome (talk) 11:54, 15 May 2008 (UTC)

I would say yes. Because it occurs as a fictitious force in a prominent scientific theory. Taemyr (talk) 12:12, 15 May 2008 (UTC)
I've posted a link to here from Talk:Gravitation. Personally I wouldn't include it in the category. I don't know general relativity well enough to make a definitive statement, but graivty is certainly very different from the other current members of the category. Perhaps the category description could include a link to Introduction to general relativity? --PeR (talk) 13:18, 15 May 2008 (UTC)
I agree with Per. Gravity is not a fictitious force as you cannot transform it away globally using a coordinate transformation. This fact actually led Einstein to the idea that gravity generated by mass is a curvature in space-time which you cannot get rid off by coordinate transformations. Count Iblis (talk) 15:28, 15 May 2008 (UTC)
Curved spacetime is a coordinate transformation. You might want to say that you can not transform it away with a coordinate transformation that is independent of the objects that the force act on. Taemyr (talk) 16:02, 15 May 2008 (UTC)
Gravity is described by the Riemann tensor. If some of its components are nonzero in one frame then no matter what coordinate transformations you perform, you cannot get all the components of that tensor to become zero. Physically this corresponds to the fact that an observer in a free-falling box can still detect the effects of the gravitational field via tidal effects.
It seems to me that fictitious force must be defined as those forces that are purely an artifact of the chosen coordinate system. But that's equivalent to saying that a fictitious force field can be globally transformed away using a single coordinate transformation. Count Iblis (talk) 16:18, 15 May 2008 (UTC)
Some questions, what do the above argument look like if we assume a uniform gravity field? And do we always take tidal forces into account when constructing our models? Taemyr (talk) 16:48, 15 May 2008 (UTC)
I do not think adding gravity to Category:Fictitious forces will enhance understanding, nor will anyone look for it there. As for whether gravity is a fictitious force in the context of general relativity, the treatment of that question on Wikipedia will be defined by what the sources say, not by discussions like the one above. -- SCZenz (talk) 17:13, 15 May 2008 (UTC)
I agree: it looks like gravity definitely shouldn't be in the category, based on the discussion above. -- The Anome (talk) 20:56, 15 May 2008 (UTC)