Talk:Force

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Earlier talk archive at

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Contents 1 Units of measurement section 2 Rotation 3 Correct definition of force 4 Electrodynamics and force

[edit] Units of measurement section

IMO the units of measurement section is much too long, and gives disproportionate weight to silliness like the use of kg as a unit of force.--24.52.254.62 00:17, 4 November 2006 (UTC)

[edit] Rotation

As the article stands it says a force causes an object to rotate, and this is not accurate. No force is needed to keep an object spinning, such as in a gyroscope for instance. A force is needed to keep an object orbiting however. Both these motions however are called rotation. I'm not quite sure how to phrase it to make it accurate. Any suggestions? Roy Brumback 08:47, 6 November 2006 (UTC)

In my opinion, the first sentence of the article should be simple. If you apply a force on a body that is not rotating, it may start rotating. This is what the sentence says, and I think it's good. I would suggest discussing the point mentioned by Roy Brumback later in the article. Yevgeny Kats 15:55, 6 November 2006 (UTC)

I agree it should be simple, but not at the expense of accuracy. As it's written, it makes it sound like a force it necessary to keep it rotating, which is not true in the case of spin. Roy Brumback 00:36, 7 November 2006 (UTC)

I've made a stab. I think the key is to emphasise the difference between a point particle and an extended body. A point particle doesn't rotate. The points in the gyroscope keep spinning because of the centrepetal force being provided by the structure of the metal. A gyroscope needs forces to keep spinning, they are just internal stresses. A "see also" bit pointing to moment would be good too.Rpf 14:11, 8 November 2006 (UTC)

This must be one of the hardest articles in Wikipedia to write. There are two issues; firstly it is meaningless to talk of motion without indicating what datum we are using for reference, i.e. the idea of inertial frames is necessary before moving on to force. Secondly, whilst in Newtonian mechanics force is viewed as the 'cause' of motion, modern field theories treat it more as a derived quantity, so to cover both contexts it becomes both 'cause' and 'effect'. If we treat force as an 'effect' in Newtonian mechanics, we end up with a circular argument defining mass. I think we can sacrifice rigour and retrict ourselves to the Newtonian world view. Sorry I can't help more, you are doing a great job. Gordon Vigurs 08:18, 8 December 2006 (UTC)

[edit] Correct definition of force

It was mentioned in the discussion already several times but obviously it did not help. In the article there is no note saying that correct physical definition of force does not exist. It would be also very useful to write two paragraphs about forces in QTF and GTR.

The article already contained a paragraph:

It is very important to mention that it is impossible to define force. All attempts in history failed because of definitions in circles. This is a reason why modern physics theories don't operate with the forces as the source or symptom of interaction. General relativity uses a conception of curved spacetime and Quantum field theory talks about exchanging of intermediate particles like photons, W and Z bosons or gluons. Both theories don't need force. However, because it is easy to imagine forces, one can compute them from these theories. But we must not forget, that correct definition of this concept does not exist.

It was denoted to be silly and removed. Perhaps it is not the best way how to express the fact but something like this should be included. It shouldn't be a problem to cite it, something like this is written in most of theoretical physics textbooks.

Please, consider it. Miraceti 15:36, 19 December 2006 (UTC)

As an undergrad with lots of physics (but my major was chemistry), I'm not sure this is a very productive argument. Force is a primary thing. We're all familiar with it as a physical fact of life, as a push or pressure or material stress. What good does it do to say it "really" doesn't exist? Of course it "really" exists. It's as "real" as anything else in your life or your experience. If you fall off a tall building and get squished on the cement, is it any use to say that it's not "force" that's smashing you, but an exachange of virtual photons? Does it hurt less. Or maybe even some even stranger Pauli exclusion "interactions" that opperate between electrons trying to be jammed into the same space? So what? It walks like a duck, quacks like a duck. We believe in energy, do we not? We believe in space, do we not? We have an unusual property of both, which we can feel with our hands and feet and butts, which happens to be the spacial gradient of work and energy in potential fields (like gravity, in this case). We need a name for it. And by gosh, we have one. SBHarris 00:35, 23 December 2006 (UTC)

Is this article about our believes, or about physics? Nobody says we should delete this article. Force is an important term because we use it in our everyday lives. However, a high-quality encyclopedia should mention everything important which is related to such a term. And impossibility of its correct definition is very important (besides this, it is very interesting). Miraceti 21:02, 12 January 2007 (UTC)

I do not see that that there is any impossibility in correctly defining force. Force is the spacial gradient of the energy (the Lagrangian). Or, if you like, force is the differential change with time, in momentum. Force is defined indirectly, but experienced directly, which is what leads to need to define it AS an entity. I guess I still don't see what your problem here is. What is it you believe is less "real" about force than about time, space, energy, momentum, or any other primary quantity? SBHarris 02:02, 13 January 2007 (UTC)

. That is correct. However, what is V? V is an potential energy. How do you define the potential energy? Please, consider that V is not just an energy, it would be simple then. (Energy is a physical quantity which is conserved in system invariant with respect to time. But this not our case. We need a definition of the potential energy.) Can you define the potential energy without the term “force”? Force is not defined indirectly, force cannot be defined at all! All definitions of force are only intuitive.

I am really sorry, I cannot direct you to some literature. I am on a long term work travel and the only literature which I have here is The Good Soldier Švejk. I am pretty sure, you can find it also in The Feynman Lectures on Physics. Feynman knew what Newton had known. Miraceti 12:34, 14 January 2007 (UTC)

No, the correct formula uses the Hamiltonian or Lagrangian T-V, which is not only potential but also kinetic energy. Obviously force may be used to increase kinetic energy only, no potential involved. Though potential cannot be defined at a point, differences in potential are real, and appear in QM equations all the time, where nature shows that the wavefunction senses them directly. And DIFFERENCES in potentials are just as useful in the equation you give above that defines force. So there you are. Energy differences are real and definable separately, without resort to use of "force." Does this answer your question? There are many definitions of energy which don't involve force. It's hv. It's mc^2. And so on. SBHarris 20:44, 14 January 2007 (UTC)

Well, is correct when the total energy is conserved in the system. In general, it is really (generalized force).

But there is another problem. You have to be able to measure distances if you want to use an operator . Of course, you can measure distances in both, inertial and non-inertial frame of reference. In non-inertial frame of reference, you have to use "force" to be able use a real measuring instrument with finite rigidity (you cannot use light either, in such cases we would have to look at GR to explain why). In inertial frame of reference, you have to use "force" to be able to define the inertial frame of reference. Therefore "force" cannot be physical correctly defined in this way.

I hope, this helps a bit again. Probably, I should start from the end, from problems in inertial and non-inertial frames of reference. Sorry about that. Have you already tried to find it in Feynman?

BTW, this is not just a cosmetic problem. This is a fundamental problem which led to GR. Miraceti 23:05, 14 January 2007 (UTC)

[edit] Electrodynamics and force

I feel, a following statement is not correct:

With the development of quantum electrodynamics in mid 20 century it was realized that "force" is strictly a macroscopic concept which arises from conservation of momentum of interacting elementary particles. Thus currently known fundamental forces are not called forces but "fundamental interactions".

Quantum electrodynamics can talk only about electromagnetic forces. Of course, QFT involved two other interactions later but it did not involved gravity yet (and we don't know if it will ever do it - we don't know if gravitons really exist and how they behave). The best model of gravity we have, is GR. GR does not say anything about exchanging particles, it uses a curved spacetime - a totally different concept. Miraceti 21:19, 12 January 2007 (UTC)