<P> When a soft object strikes a harder object, most of the energy available for the post-collision velocity will be stored in the soft object . The COR will depend on how efficient the soft object is at storing the energy in compression without losing it to heat and plastic deformation . A rubber ball will bounce better off concrete than a glass ball, but the COR of glass - on - glass is a lot higher than rubber - on - rubber because some of the energy in rubber is lost to heat when it is compressed . When a rubber ball collides with a glass ball, the COR will depend entirely on the rubber . For this reason, determining the COR of a material when there is not identical material for collision is best done by using a much harder material . </P> <P> Since there is no perfectly rigid material, hard materials such as metals and ceramics have their COR theoretically determined by considering the collision between identical spheres . In practice, a 2 - ball Newton's cradle may be employed but such a set up is not conducive to quickly testing samples . </P> <P> The Leeb rebound hardness test is the only commonly - available test related to determining the COR . It uses a tip of tungsten carbide, one of the hardest substances available, dropped onto test samples from a specific height . But the shape of the tip, the velocity of impact, and the tungsten carbide are all variables that affect the result that is expressed in terms of 1000 * COR . It does not give an objective COR for the material that is independent from the test . </P> <P> The COR is not a material property because it changes with the shape of the material and the specifics of the collision, but it can be predicted from material properties and the velocity of impact when the specifics of the collision are simplified . To avoid the complications of rotational and frictional losses, we can consider the ideal case of an identical pair of spherical objects, colliding so that their centers of mass and relative velocity are all in - line . </P>

Relationship between coefficient of restitution and kinetic energy