Role of friction during Collisions Normally, in questions of collisions, we tend to apply conservation of momentum (and in cases of elastic collisions, conservation of energy as well) and we usually ignore the frictional force acting. But what actually happens, when friction is to be accounted for, theoretically, and mathematically. (Say ù is coefficient of friction between the two colliding bodies.)
 A: Normally the friction will not do any work during a quick collision of two hard bodies, like billiard balls. The reason is that work is equal to force times displacement: if the surfaces of the balls which are touching momentarily do not slip relative to each other then the work is precisely zero! Nevertheless, it will have observable consequences: there will be a transfer of angular momentum between the balls leading to rotation. You might even be able to calculate exactly the collision of two finite-sized non-slippery perfectly-elastic billiard balls but it is not an easy problem.
A: With friction operating in the system between the two bodies while impact(elastic),mechanical energy conservation law still holds good because in elastic impact friction doesn't do work as it is instantaneous(somewhat impulsive).
Linear momentum may be conserved if there is no external force on two bodies but as friction(between colliding bodies) will be an internal force in the system so it will not change the total momentum of the system.
