What is the momentum of a ball at the exact point where it comes into contact with a wall?
Assume that the ball is so uniform that there is an instant of time when all particles of the ball have come to a zero velocity, fully compressed, and about to rebound. At that instant the momentum is 0. (I am ignoring the velocity that the ball imparts to the wall. Otherwise, there is some tiny momentum at the time of full compression and particles of the ball will not have 0 velocity till a small amount of rebound has begun.)
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