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!