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Energy, momentum and CM

  1. Nov 16, 2006 #1
    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. jcsd
  3. Nov 16, 2006 #2


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

  4. Nov 16, 2006 #3
    1/M (x1m1 + x2m2) ?
  5. Nov 16, 2006 #4
    It should be the "zero momentum" frame.
  6. Nov 16, 2006 #5
    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?
  7. Nov 17, 2006 #6


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

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