Collisions

A collision refers to an interaction between two or more bodies that come into contact for a short time. In classical mechanics, collisions are analyzed in terms of conservation laws: momentum is generally conserved in isolated systems, and kinetic energy may be conserved depending on elasticity.

Elastic collision: kinetic energy and momentum are conserved; objects rebound with no net loss of kinetic energy in the center-of-mass frame; examples include idealized billiard-ball collisions and many gas-molecule interactions.

Inelastic collision: momentum conserved, kinetic energy not conserved; some energy is transformed into deformation, heat, or sound; many real-world impacts are inelastic.

Perfectly inelastic collision: a special case in which objects stick together after impact; final velocity is determined by total mass and momentum; the collision entails maximal kinetic-energy loss.

Contexts: Macroscopic collisions involve everyday objects like car crashes and sports equipment; microscopic collisions occur between gas molecules, driving diffusion; celestial collisions alter orbits and can lead to planetary formation or disruption.

In engineering and simulations, collision detection and resolution are essential. Impulse-based methods apply instantaneous impulses to enforce non-penetration and produce physically plausible responses. The coefficient of restitution describes bounciness, ranging from 0 to 1.