Collisions 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, you can calculate the final velocity of this second object after it has collided. To obtain an exact answer however, we must find out what type of collision takes place, whether it is elastic or not elastic. The type of collision is characterized by what is called the coefficient of restitution. This quantity is approximately constant for a collision between two given objects, and can be determined experimentally. If the relative velocities of the two objects are the same before and after impact, the coefficient is equal to 1, and the collision is elastic. In practice, however, such perfectly elastic collisions occur only on an atomic scale; most collisions are therefor not elastic, with a coefficient of restitution of less than 1.