Answer to Question #118919 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.
Suppose the momentum of the ball has also a y-component before it collides elastically with the wall. After the collision the x-component of the ball's momentum changes but its y-component does not. If then the momentum of the ball before the collision is P=Pxi+Pyj, what is its momentum after the collision.

Answer 1

Since the velocity vector changes its direction to the opposite after the collision with the wall, the x-component of the momentum will change its direction as well:

$$\vec{P}=-P_x\textbf{i}+P_y\textbf{j}.$$