# Energy, momentum and CM

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.

## Answers and Replies

dextercioby
Science Advisor
Homework Helper
What is the definition of the center of mass for a relativistic system of point particles...?

Daniel.

1/M (x1m1 + x2m2) ?

It should be the "zero momentum" frame.

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?

dextercioby
Science Advisor
Homework Helper
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.