A theorem on elastic collisions between ideal rigid bodies Available to Purchase

In an elastic collision between two rigid bodies of arbitrary shape, the component of relative velocity of the impact points along the impulse direction is exactly reversed. The two relative velocity components perpendicular to the impulse direction are usually changed in a nonsimple manner, as for collisions between smooth ellipsoids, but are sometimes simply not changed at all, as for a collision between smooth spheres, or do not exist, as for a collision between an ideal superball and a flat surface.