Originally Posted by toly
The physics of the collision is exactly the same if the racquet is at rest and the ball approaches at 50 mph or ball is at rest and the racquet approaches at 50 mph. The relative speed between them is relevant.
It cannot be the same. The initial momentum will be MV in one case and mv in the other, so if momentum is conserved, the final values will be different.
If I run into a wall, I will be hurt, but much less than if I stood still and the wall ran into me.