Originally Posted by sureshs
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).
In a given coordinate system or frame of reference, of course there is a difference between a small mass and a big mass moving with a given velocity. I don't think anybody is disputing that. As I recall, this argument started because someone wanted to measure the angle of reflection of a ball when it hits a racquet, and that is a relative entity which only depends on the relative movement between the racquet face and the ball.