Elastic and Inelastic Collisions

Collisions are classified into two broad types based on whether kinetic energy is conserved.

An ELASTIC collision is one in which the total kinetic energy of the system (in addition to momentum) is conserved: $(KE)_{\text{initial}}=(KE)_{\text{final}}$. A perfectly elastic collision essentially never occurs for macroscopic, everyday objects; it is, however, an excellent approximation for interatomic or intermolecular collisions (as used later in the kinetic theory of gases).

An INELASTIC collision is one in which there IS a loss of kinetic energy during the collision, even though linear momentum is still conserved: $(KE)_{\text{initial}}>(KE)_{\text{final}}$. This lost kinetic energy is converted, via internal friction or vibrational motion of atoms, into heat (and sometimes sound/light). …