The collision of a ball always involves some loss of energy.
For example, if a ball of mass m is dropped from a
height h1 onto a surface and it rebounds to a height h2 , then
the loss of energy is mg(h12h2). The energy loss can be
expressed in terms of the coefficient of restitution, e, defined
in the case of a rigid surface by e5v2 /v15Ah2 /h1, where
v1 is the incident speed of the ball and v2 is the rebound
speed. The coefficient of restitution ~COR! has been measured
for many objects and surfaces, but very little information
is available on the energy loss process itself or on the
force acting on a colliding ball