Originally Posted by bhupaes
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...
I am asking why the difference in speeds if analyzed with one at rest, then with the other at rest. If m is moving to the right with v towards M, isn't it the same as M moving to the left with v towards m? Then why is there a difference in the speed of the combined mass after the collision? (direction will of course be different).