Talk:Velocity

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Please update this rating as the article progresses, or if the rating is inaccurate. Click to show/hide comments. Please add to or update the comments to suggest improvements to the article. I note that in the calculation for velocity, that the term 's' is used to denote distance. Shouldn't it instead refer to 'd' for distance and leave 's' as referring to speed. In which case v = d / t Crabtreemp 23:57, 10 May 2007 (UTC) S represents displacement, speed can be denoted as the magnitude of 'v', 'd' as the total distance traveled--Cronholm144 19:39, 24 May 2007 (UTC)

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Contents 1 Position and Displacement 2 Velocity in curved space 3 Why is this article so bare? 4 Absurd!!! 5 Simpler Equation? 6 What Happened!? 7 wikipedia

[edit] Position and Displacement

The problem is not so much that "displacement" is a technical term but that it is actually incorrect to say that "velocity is the rate of change of displacement". Displacement is a change in position and so "change in displacement" divided by "change in time" becomes "change in the change of position" divided by time. This is a very common error that is advanced in many physics books as well as on many websites. It is something that can confuse students learning physics for the first time. For many professional physicists, the difference between a change in position and a change in displacement is something that they would consider trivial in that the context would often provide the necessary clarification. However the difference is important for proper concept formation at the foundational levels of learning in physics. I did try and correct this in August 2007 but with time the errors do seem to come creeping back. --Phillip (talk) 10:24, 23 December 2008 (UTC)

[edit] Velocity in curved space

Since we dont have straight lines in curved space, how is velocity then defined?--Light current 01:20, 17 January 2006 (UTC)

Since velocity has both magnitude and direction, it is not necessary to have straight lines, although this means there is an acceleration. The velocity would still be defined as the derivative of the position of a particle with respect to time. Or, if you like, the speed of the particle in a direction tangential to the curve the particle is travelling on. I hope that helps! --Someones life 18:32, 4 February 2006 (UTC)

---

Question: If a particle has a defined position at every time, must it necessarily also have a defined velocity? Consider a particle moving along a line, so its position along the line at time t is x(t). Suppose we define x(t) as follows:

         { 1   if t > 0
 x(t) =  {
         { 0   if t <= 0

If I remember my calculus correctly, x'(0) is undefined, while x(t) = 0. Does it therefore follow that at time t=0, the particle has a position and is moving but has no velocity? Would it be physically possible (i.e. compatible with the laws of physics as we currently understand them) for a particle with that behaviour to actually exist? -- SJK

In classical (non-quantum) mechanics a particle with mass cannot make such an instantaneous jump in position. It implies infinite acceleration which implies infinite force. So this case is not physically possible in classical mechanics (assuming zero-mass particles are not physically possible). -- Eob

Eob: What about in quantum mechanics? IIRC, quantum mechanics predicts instantaneous jumps in position (consider e.g. the Bohr model of the atom). And it has a zero-mass particle, the photon... -- SJK

I was not sure about quantum mechanics which was why I explicitly restricted my comments to classical mechanics. But now that I consider it more I would hazard that the question that was posed is not meaningful in quantum mechanics because you can never know x(t) exactly. As for the The Bohr Model, it has been superceeded by a model of the atom surrounded by orbitals which are standing waves of the wave function, so I am not sure it is relevant. --Eob

the derivative of the step function x(t) you wrote above is the Dirac delta function. -- User:RAE

SJK: in Physics, if you can define exactly a position, you cannot define exactly a velocity. And vice versa. That's Heisenberg Uncertainty Principle: http://zebu.uoregon.edu/~imamura/208/jan27/hup.html.

A "velocity" is mathematical concept, not a physical concept. It's a distance divided by a time when the time is close to zero. It's a limit in mathematical terminology. "The laws of physics" and "mathematics" are not compatible. By the way, isn't that question some of your homework, mmh ?

Your function is not defined at zero, therefore x'(t) at t=0 is meaningless.

---

I am in browse mode tonight... but at some point a mention will have to made of tangent spaces and tie the discussion back to differential geometry.

[edit] Why is this article so bare?

I got a report to do, and I figured I'd get the info off of Wikipedia, and there is practically no history of velocity, like who thought of it and whatnot. Could someone fix it? —The preceding unsigned comment was added by 69.26.30.35 (talk) 01:01, 14 February 2007 (UTC).

[edit] Absurd!!!

It's absolutely absurd that this article doesn't cover common units, abbreviations etc. What kind of reference work has equations without references!!! // FrankB 01:13, 14 August 2008 (UTC)

[edit] Simpler Equation?

I'm debating whether to put a simpler equation for calculating velocity as an addition to this article: d/t (distance divided by time). What do you think? "It's over 9000!" (talk) 06:10, 16 June 2009 (UTC)

[edit] What Happened!?

Who put, at the top of this page, a lowercase definition of velocity? And at least figure out the answer. Can someone please revert this change. Thank you.

Bab (talk) 01:53, 4 January 2009 (UTC)

May I add that the user is a IP address, and this vandlism has been done before. I repeat my revert cry.

Bab (talk) 01:59, 4 January 2009 (UTC)

[edit] wikipedia

dear wikipedia, i am in grammer school and i am asking you to pleese simplify some of the articals i do not understand them thank you, jzme