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).
Going directly to your post #32...
This is a different problem, isn't it? The original problem assumed an elastic collision to simplify the calculation, which would be true if the energy lost in the collision is small (as I think it is). But in any case, even with totally inelastic collision, the relative speed between the two masses after the collision is zero in both of the cases you have mentioned. Not sure what I am missing...