Collisions of Two Objects When two objects collide, their motions are changed as a result of the collision, as is shown when playing pool. There are several laws governing collisions, the principal one being the law of conservation of linear momentum, which says the total momentum of an isolated system is the table and the balls, and the law then implies that the total momentum of the balls just before they collide is the total momentum just after the collision. Therefore if the masses of two colliding objects are known, the velocity and the velocity of the other before the collision,