Suppose two balls A and B of masses m and km are moving in the same direction with speeds We can find the coefficient of restitution Conservation of momentum gives If k*(2) – (1) gives Because e <=1, we must have We have
and
respectively, where
and 0 
in terms of
and
and use the expression for e to conditions onand
(1)is the coefficient of restitution between A and B, then
(2)
hence
and