<P> By itself, the law of conservation of momentum is not enough to determine the motion of particles after a collision . Another property of the motion, kinetic energy, must be known . This is not necessarily conserved . If it is conserved, the collision is called an elastic collision; if not, it is an inelastic collision . </P> <P> An elastic collision is one in which no kinetic energy is absorbed in the collision . Perfectly elastic "collisions" can occur when the objects do not touch each other, as for example in atomic or nuclear scattering where electric repulsion keeps them apart . A slingshot maneuver of a satellite around a planet can also be viewed as a perfectly elastic collision . A collision between two pool balls is a good example of an almost totally elastic collision, due to their high rigidity, but when bodies come in contact there is always some dissipation . </P> <P> A head - on elastic collision between two bodies can be represented by velocities in one dimension, along a line passing through the bodies . If the velocities are u and u before the collision and v and v after, the equations expressing conservation of momentum and kinetic energy are: </P> <Dl> <Dd> m 1 u 1 + m 2 u 2 = m 1 v 1 + m 2 v 2 1 2 m 1 u 1 2 + 1 2 m 2 u 2 2 = 1 2 m 1 v 1 2 + 1 2 m 2 v 2 2 . (\ displaystyle (\ begin (aligned) m_ (1) u_ (1) + m_ (2) u_ (2) & = m_ (1) v_ (1) + m_ (2) v_ (2) \ \ (\ tfrac (1) (2)) m_ (1) u_ (1) ^ (2) + (\ tfrac (1) (2)) m_ (2) u_ (2) ^ (2) & = (\ tfrac (1) (2)) m_ (1) v_ (1) ^ (2) + (\ tfrac (1) (2)) m_ (2) v_ (2) ^ (2) \,. \ end (aligned))) </Dd> </Dl>

Of a rigid object is equal to mass multiplied by velocity