Inelastic Collision
Definition
A collision where the objects stick together or deform — momentum is conserved, but some kinetic energy is lost as heat, sound, or permanent shape change.
Technical definition
A collision conserving total momentum but not total kinetic energy. In a perfectly inelastic collision the objects stick together (maximum kinetic energy loss). The ‘lost’ energy becomes internal energy — heat, sound, deformation.
Example
Two cars crashing and crumpling together: momentum is conserved (the wreckage moves at the predictable combined velocity), but kinetic energy is not — it went into bending metal.
Momentum is ALWAYS conserved in collisions of an isolated system — even inelastic ones. What changes is kinetic energy. Students often think ‘inelastic’ means momentum is lost too; it is not.