Energy, momentum and CM

1. Nov 16, 2006

Tonyt88

So I have this problem:
A mass m travels to the right at 4c/5, and another mass m is at rest.

They ask fro the energy and momentum of each particle, so I have:

Energy of traveling: 5/3 mc^2

Energy of rest: mc^2

Momentum of traveling: 4mc/3

Momentum of rest: 0

Then they ask for the velocity of the CM of the system:

Which I have 2c/5, but then some people are telling me c/2.

But anyway, I have to solve for the velocities of the particles in the CM frame, so how I do go from there.

2. Nov 16, 2006

dextercioby

What is the definition of the center of mass for a relativistic system of point particles...?

Daniel.

3. Nov 16, 2006

Tonyt88

1/M (x1m1 + x2m2) ?

4. Nov 16, 2006

tim_lou

It should be the "zero momentum" frame.

5. Nov 16, 2006

Tonyt88

Then I really don't know what to say, but I would like to know, if you have the value for the velocity at the CM (which I think is either 2c/5 or c/2), how do I go from there to solve for the resultant velocities?

6. Nov 17, 2006

dextercioby

You just need the CM's velocity. You've been hinted that in the CM frame the total momentum of the particles is zero.

Daniel.