collision

Collision is an event in which two or more bodies exert forces on each other for a relatively short time, resulting in changes to their velocities and possibly internal states such as deformation, heat, or sound. In classical mechanics, collisions are described with impulse and momentum exchange. In an isolated system with negligible external forces, the total linear momentum before and after the collision is conserved.

Kinetic energy may also be conserved, but only in elastic collisions. Inelastic collisions involve deformation or other dissipation, so kinetic energy is not conserved. A perfectly inelastic collision is one in which the objects stick together after impact, sharing a common velocity. The coefficient of restitution, e, quantifies elasticity: e is 1 for a perfectly elastic collision, 0 for a perfectly inelastic collision, and between 0 and 1 for partially elastic collisions.

Collisions can be analyzed in the center-of-mass frame, where total momentum is zero, and results can be transformed to other inertial frames. Real-world collisions vary in degree of elasticity: billiard balls offer a close approximation to elastic interactions, car crashes are highly inelastic, and collisions between gas molecules are typically near-elastic at standard conditions.

Applications span engineering, safety design, astrophysics, and particle physics. Collision theory explains reaction rates in chemistry in terms of reactant encounters, while in computational physics collisions are simulated in molecular dynamics and rigid-body dynamics. In planetary science and astrophysics, collisions influence formation processes and dynamical evolution of systems.