In the elementary theory of collisions of two small particles, it is a well-known result that the momentum of the system as a whole is the same after impact as before. This is because the impulses on the two particles are taken to be equal and opposite, thereby implying zero total impulse on the system. It is not obvious that the same will be true after rolling snooker balls collide because, apart from the impulsive forces due to the impact, frictional forces also act on the balls. These are caused by the friction of the table when the balls are in a temporary sliding phase immediately following the impact.