Talk:Velocity

From Wikipedia, the free encyclopedia

	Physics Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
Start	This article has been rated as Start-Class on the assessment scale.
High	This article is on a subject of High importance within physics.
Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.

[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 ?

---

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] physics (velocity)

HELP

A car is traveling along the road at a moderate speed. One person describes the velocity of the car as 30 km/hr forward. Another person describes the car as moving 20 km/hr in reverse. Explain how both observers can be correct

Tha answer is ..... its your homework isnt it? (Think relative!)--Light current 06:18, 8 February 2006 (UTC)

Whoever said= Velocity is simply speed, I think you. A beautiful definition.

XunexXunex abcdefghijklmnopqrstuvwxyz

poopy...

[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).