Talk:Motion graphs and derivatives

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Motion graphs and derivatives was a good article nominee, but did not meet the good article criteria at the time. There are suggestions below for improving the article. Once these are addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake. Reviewed version: November 29, 2006

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Should this be re-cast in terms of derivates rather than deltas? The delta notation is only correct for constant velocities over time. -Splash^talk 02:41, 12 December 2005 (UTC)

Hmm, the anonymous creator had deltas, but I think that extending it to differentials is better. Is there any way you can produce an SVG diagram for these? :) Titoxd^{(?!? - did you read this?)} 02:43, 12 December 2005 (UTC)

I wonder also if there is room in a "more advanced" section to talk about the second-deriv relationship (and the integral implications). That way, the whole calculus thing can be shunted into its own quarantined section without having to terrorise non-calculus aware individuals. I don't appear to have any SVG-making software at all, unfortunately. -Splash^talk 03:02, 12 December 2005 (UTC)

Although we do already have acceleration, velocity and displacement (vector). -Splash^talk 03:04, 12 December 2005 (UTC)

(edit conflict): I don't know from where I got the idea that you had it, then... o.O. But I would imagine that the intent of the original creator was to make it as simple as possible, so the calculus could be moved to a separate section without any harm being done. Titoxd^{(?!? - did you read this?)} 03:05, 12 December 2005 (UTC)

I think that the purpose was to have a good explanation in one place, instead of three, and then refer to other articles for more detailed information. Titoxd^{(?!? - did you read this?)} 03:08, 12 December 2005 (UTC)

Are we quite sure that the green line is in fact the derivative of the curve in the time range that both are drawn over? The gradient of the blue curve appears to go from -ve to +ve for example, which would need at least the opposite gradient on the green curve. Or am I just confused? -Splash^talk 03:40, 12 December 2005 (UTC)

Hmm. The velocity of the curve before it reaches the green tangent line is negative, and in the period before the line reaches the "tangent point", the acceleration is constant and negative. At some point slightly before that, the velocity begins to decrease less, so the acceleration is not constant, but it is still negative... so yeah, the green curve is right, but it seems that we're talking about jerk here, which we don't address in the article. Titoxd^{(?!? - did you read this?)} 03:47, 12 December 2005 (UTC)

No, no, I'm confused. It's not drawing the acceleration over time, for which it would be wrong since the gradient is +ve most of the time in that region. It is drawing the tangent to the curve at that point. Obviously enough. My fault, but I'll write a clearer caption. -Splash^talk 03:48, 12 December 2005 (UTC)

Oh, but since we have concavity, we do have jerk! :O But yeah... the caption seems to indicate that that's based on Δt, not dt, so it's better to change it. My fault. Titoxd^{(?!? - did you read this?)} 03:51, 12 December 2005 (UTC)

Contents 1 You horrible horrible vandals... 2 Impressed 3 Inaccurate arguments? 4 Error: Displacement vs. distance 5 Integrate displacement? 6 GA review

[edit] You horrible horrible vandals...

You decide to write this up JUST AFTER I finish PHY 210!!! 68.39.174.238 17:13, 14 December 2005 (UTC)

Meh. I used it to review for PHY 121... :P Titoxd^{(?!? - did you read this?)} 18:54, 14 December 2005 (UTC)

[edit] Impressed

Amazing what an article can look like in only two days, and a good example of WP:AFC actually working. This is a really neat article. - orioneight (talk) 18:24, 14 December 2005 (UTC)

[edit] Inaccurate arguments?

the beginning of this article seems to be stating that because the units of the slope are m/s, therefore the slope must be the velocity. i dont think this is quite correct, i dont think something can be said to be a certain property of an object just because it has the right units to be that property (compare Joule for energy with newton-metre for torque, they have the same dimensions of force x distance, but are different things). the units of the slope are m/s because it's the velocity, and the velocity of an object is defined as its change in position with respect to time. i think the simplicity of the argument as it is now (without calculus) is compromising its accuracy. the same goes for the acceleration of course. --Someones life 03:46, 15 December 2005 (UTC)

The energy-torque analogy is quite correct, and it should be reworded to indicate that velocity is defined to be the change in position over the change in time, but I can't think of a way to do it without sacrificing the simplicity that we tried to have while we wrote this page. I'll think about that one, and I'll try to come with something, but you're welcome to be bold if you think of something faster than I do. Titoxd^{(?!? - did you read this?)} 04:04, 15 December 2005 (UTC)

[edit] Error: Displacement vs. distance

"The process of determining the area under the curve, as described above, can give the displacement and change in velocity over particular time"

I believe this is incorrect. According to http://en.wikipedia.org/wiki/Displacement_%28vector%29 the only thing we can read from the area under the curve is distance, not displacement. Since the area under the curve says nothing about direction, travelled distance is the only thing that can be inferred.

An example may be an object making a circle. The area under the curve will be >0, distance will be >0, but displacement will be 0. 192.35.17.11 10:37, 15 December 2005 (UTC)

Correct. I'm fixing that now. Titoxd^{(?!? - did you read this?)} 17:21, 15 December 2005 (UTC)

Wait. According to my calculus textbook, the definite integral of a semi-sinusoidal curve gives the displacement of the object; the distance is given by applying the integral to the absolute value of the function. Titoxd^{(?!? - did you read this?)} 17:25, 15 December 2005 (UTC)

I'd agree with your textbook, but only as far as motion along a straight line is concerned. That does not hold true for motion in a curved line imho.192.35.17.11 15:43, 16 December 2005 (UTC)

Technically, velocity contains information about the direction (speed does not). The graph is for positive velocity, meaning that the body's position s(t) is increasing with time (the body is moving to the right along the s-axis). I think, we need to clarify this issue about area below the graph, and positive/negative velocity.(Igny 17:27, 15 December 2005 (UTC))

I've changed it to mention distance now. Titoxd^{(?!? - did you read this?)} 17:38, 15 December 2005 (UTC)

[edit] Integrate displacement?

Any formal name? Any uses? 203.218.86.162 01:57, 31 May 2006 (UTC)

The integral of displacement over time? Not that I'm aware of. Its units would be meter-seconds, and there's no physical quantity that springs to mind with such units. -Splash^talk 12:28, 31 May 2006 (UTC)

[edit] GA review

After reading this article (current version) I don't think it's quite up to the good article criteria. My suggestions for the article:

• I'm confused by the decision to bring up SI units. I'm not sure they're relevant to an abstract discussion of displacement and time, but even if they are they should be used consistently throughout the text and graphs, not just in a couple examples. I also think the use of units like m/s to obliquely refer to physical properties like velocity is not a good idea. This sort of thinking can be problematic for physics students -- consider that newton-meters are used for both torque and work, but these are not at all interchangeable, and only discussing the units obscures that fact.
• It could use more images that clearly correspond to parts of the text. There's no position vs. time graph, for instance, while the text discusses it.
• The informal organization of the article makes it hard to follow in some parts. For instance, doesn't the intro section also discuss variable rates of change w.r.t. instantaneous velocity? What does "The expressions given above" apply to, exactly? To make sure the discussion doesn't get too meandering, I think most of the content in the 0th section should be reorganized into a new section, and a new lead should be written. Make sure to clearly separate different concepts in different sections.
• The article's scope is too narrow. I think the article could benefit from a real-world example, and from discussion of kinematic graphs in physics and calculus education.

Twinxor t 09:39, 29 November 2006 (UTC)