Most macroscopic interactions in the real world involve some of the initial kinetic energy of the system being converted to sound, light, or deformation (Figure 12). When deformation occurs, some of the initial kinetic energy of the system is converted to heat because friction acts on objects in almost all situations.

An inelastic collision is one in which the total kinetic energy of the system is greater before than after the collision (even though the momentum of the system is conserved).

Inelastic collisions are of two types. When the colliding objects stick together after the collision, as happens when a meteorite collides with the Earth, the collision is called perfectly inelastic. When the colliding objects do not stick together, but some kinetic energy is lost, as in the case of a rubber ball colliding with a hard surface, the collision is called inelastic (with no modifying adverb).