Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor
    Homework Helper

    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


    User Avatar
    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.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook