Answer to Question #118916 in Classical Mechanics for Lizwi

Question

A ball moving along the x axis bounces elastically off a horizontal wall that runs in along y-axis.
The magnitude of the momentum of the ball is the same before and after the collision.
If the x-component of the ball's momentum is Px before the collision with with the wall, what is it after collision?

Answer 1

The wall runs in along y-axis, therefore it cannot change the x-component of the ball's momentum.

The x-component of the ball's momentum after the collision with with the wall is the same as before the collision:

$$P_x'=P_x$$