Originally Posted by sureshs
Mass 2 is resting sets one of the variables to 0. That is not what you are saying. You are saying relative velocities are all that matter. That is like replacing vx1 by vx1-vx2.
Say you have a small mass m moving horizontally to the right with speed v and colliding completely inelastically with a larger mass M. The two together will then move to the right with speed mv/(m+M). If m was stationary and M moved to the left with speed v, the two will move to the left with speed Mv/(m+M).
1. In first case momentum before collision was mv. After collision momentum is
(M+m)(mv/(m+M))=mv. So, mv=mv and everything is fine with law of conservation of momentum.
2. In second case momentum before collision was Mv. After collision momentum is
(M+m)(Mv/(m+M))=Mv. Again everything is fine and Mv=Mv.