Coefficients of restitution for a collision

Abstract The coefficient of restitution (COR) for a collision is usually a number between 0 and 1 depending on whether the collision is completely inelastic, or perfectly elastic or something in between. That is the usual situation for colliding spheres or a ball dropped on a horizontal surface. A different situation arises when a bat collides with a ball. A simple experiment is described where a rod is dropped on another rod at right angles. In that case, two different values of the COR can be defined.