## Relative Velocity/Energy Question

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Nlelith
Posts: 85
Joined: Fri Dec 05, 2008 12:18 pm UTC

### Relative Velocity/Energy Question

Alright, something recently occurred to me.

You have a system consisting of two point masses m1 and m2. m1 is moving towards m2 with constant speed v (with m2 stationary). Now, if I'm not mistaken, you can equivalently say that m2 is moving towards m1 with speed v. My question is, what expression correctly describes the total kinetic energy of the system: 1/2m1v2 or 1/2m2v2? Or can they both be considered correct in some way? Maybe I'm just messing up my relative velocities.

Just a note: this is not a homework problem, just something I started wondering about.

hackthat
Posts: 25
Joined: Sat Dec 20, 2008 6:41 am UTC

### Re: Relative Velocity/Energy Question

The kinetic energy in a system depends on what frame you are viewing it in.
In one frame the KE is 1/2 m1 V^2 and in the other 1/2 m2 V^2.
This isn't any kind of contradiction, Physics works in both frames exactly how you would expect it to. (try imagining the masses bouncing off one another or interacting)

danpilon54
Posts: 322
Joined: Fri Jul 20, 2007 12:10 am UTC

### Re: Relative Velocity/Energy Question

I am pretty sure energy is only conserved in a given reference frame. If you change reference frames, the energy need not be the same.
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