A ball striking a wall at an angle keeps one momentum component unchanged and reverses:

1. Choosing axes along and across the wall separates the problem into two independent parts. 2. The wall can only push perpendicular to itself, so only that component changes. 3. The second ball continues with the same momentum component in the direction along the wall. 4. It reverses its component perpendicular to the wall. 5. Keeping in mind the proportionality between velocity and momentum makes the sketch straightforward. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 8 'Linear Momentum and Collisions', sections 8.1-8.7_