Every game will have a number of Straight Edges and Flat Surfaces. Although not a very exciting part of game play, they nonetheless exhibit a few important physics principles.

The physics:

When a ball approaches and strikes a flat surface, it will rebound off that surface at the same angle with which it struck it, assuming there was no side-spin on the ball.

The ball rebounds due to the action-reaction forces bewteeen the ball and these surfaces, thus causing a change in momentum (mostly a change in direction, to a lesser extent a speed change).

The ball will always rebound off any dead (no slingshot, flipper, pop bumper, etc) object with less speed than it hit due to the inelastic collision.