# 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.)

• Friction is often not directly relevant. Collisions are perpendicular interactions. Friction is a parallel force. For instance, think of a billiard ball hitting another ball on the pool table - it may come in at an angle, but the momentum transfer during collision happens perpendicular to the contact plane. Do you have a specific scenario we can look at? – Steeven Nov 12 '19 at 9:25
• A ball strikes a plank(at rest) at an angle with some velocity, there is friction between the plank and the ball, how would the final velocities be affected, taking friction into account. – sergeantkickass Nov 12 '19 at 9:28
• @Qmechanic♦ I don't think friction between two colliding bodies have any effect on energy conservation as friction will not do any work – RunMachine_Kohli Nov 12 '19 at 9:42
• @Steeven When they come at an angle, friction would play a role as the relative velocity between the points of contact of the balls would have a tangential component--so the friction would act to reduce the magnitude of that relative velocity. So, there would be a momentum transfer along the tangential direction. And, if the coefficient of kinetic friction is close to unity, this would be an effect of the same order as the effect of the normal forces. Correct me if I am mistaken. – Dvij Mankad Nov 12 '19 at 9:48
• Are you restricting it to friction between the colliding bodies or can it include kinetic friction between the bodies and their environment before and after the collision? – Bob D Nov 12 '19 at 9:56