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2D elastic collision in CM Frame, velocities antiparallel?

  1. Oct 17, 2016 #1
    1. The problem statement, all variables and given/known data
    Why are the Center of Momentum velocities of masses after a 2D elastic collision anti-parallel? (as in the following diagram)
    2. Relevant equations
    CM_Velocity = (∑m_i*v_i)/(∑m_i)

    3. The attempt at a solution
    This is not actually a problem I have to do. I am just looking for a proof of this fact. I have not been able to find one online, but have probably been looking in the wrong places!
  2. jcsd
  3. Oct 17, 2016 #2


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    Homework Helper

    What is the velocity of the CM in the centre-of-mass frame of reference? What does it mean for the colliding particles, how are their velocities related?
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