Hello, A very basic question, with a bit of confusion. a ball hitting a wall which is not moving with velocity v in an elastic collision has its momentum conserved, opposite velocity; -v. if the ball is to collide with a wall which has a velocity -v, equal speed towards the ball, what is the return velocity for the ball of mass m? Can we just say that the ball is moving relative to the wall and has a velocity of 2v who's momentum will be conserved? Or is there some other relationship? Thanks in advance, Alex M.