Overview

A collision is an event in which two or more bodies exert forces on each other for a relatively brief time as they come into contact or interact at short range. In physics the term applies broadly, from macroscopic impacts such as car crashes and billiard balls to microscopic scattering of atoms, molecules or subatomic particles. Collisions are central to understanding motion, energy transfer and material response when objects meet.

Fundamental principles

Two conservation principles commonly govern collisions in isolated systems: conservation of momentum and conservation of energy. The total linear momentum of an isolated system is conserved during any collision. Mechanical energy, however, is conserved only in elastic collisions; in other collisions some kinetic energy is converted into other forms such as heat, sound, or permanent deformation.

Types of collisions

  • Elastic collisions: Kinetic energy and momentum are conserved. Ideal elastic collisions are approximations useful for hard, elastic bodies and many molecular collisions.
  • Inelastic collisions: Momentum is conserved but some kinetic energy is transformed into internal energy, heat or sound.
  • Perfectly (completely) inelastic collisions: A limiting case where colliding objects stick together and move with a common velocity after impact.

History and development

Concepts behind collisions emerged as part of classical mechanics during the 17th and 18th centuries as scientists formalized laws of motion and momentum. Later developments introduced idealized parameters such as the coefficient of restitution to quantify how 'bouncy' an impact is, and modern continuum and materials science describe how energy and forces distribute during collisions in real materials.

Examples and applications

Everyday and scientific examples abound: vehicle collisions inform safety design and accident reconstruction; billiards illustrate conservation laws in two dimensions; particle collisions in accelerators reveal subatomic structure; molecular collisions determine gas behavior and reaction rates. Engineers and designers use collision analysis to predict damage, design crumple zones, and improve protective equipment.

Notable distinctions and facts

  • Impulse is the integral of force over the short collision time and equals the change in momentum of a body.
  • Analysis often uses the center-of-mass frame to simplify calculations by removing overall motion.
  • At very small scales, 'collisions' are described by quantum scattering theory rather than classical contact mechanics.

Understanding collisions requires combining kinematics, dynamics, material behavior and, in many contexts, statistical or quantum mechanics. The idealized categories—elastic, inelastic, perfectly inelastic—help organize real situations and guide both theoretical analysis and practical design.