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Physics an event in which two or more bodies or particles come together with a resulting change of direction and, normally, energy
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

Collision (physics)

Any interaction between particles, aggregates of particles, or rigid bodies in which they come near enough to exert a mutual influence, generally with exchange of energy. The term collision, as used in physics, does not necessarily imply actual contact.

In classical mechanics, collision problems are concerned with the relation of the magnitudes and directions of the velocities of colliding bodies after collision to the velocity vectors of the bodies before collision. When the only forces on the colliding bodies are those exerted by the bodies themselves, the principle of conservation of momentum states that the total momentum of the system is unchanged in the collision process. This result is particularly useful when the forces between the colliding bodies act only during the instant of collision. The velocities can then change only during the collision process, which takes place in a short time interval. Under these conditions the forces can be treated as impulsive forces, the effects of which can be expressed in terms of an experimental parameter known as the coefficient of restitution. See Conservation of momentum, Impact

The study of collisions of molecules, atoms, and nuclear particles is an important field of physics. Here the object is usually to obtain information about the forces acting between the particles. The velocities of the particles are measured before and after collision. Although quantum mechanics instead of classical mechanics should be used to describe the motion of the particles, many of the conclusions of classical collision theory are valid. See Scattering experiments (atoms and molecules), Scattering experiments (nuclei)

Collisions can be classed as elastic and inelastic. In an elastic collision, mechanical energy is conserved; that is, the total kinetic energy of the system of particles after collision equals the total kinetic energy before collision. For inelastic collisions, however, the total kinetic energy after collision is different from the initial total kinetic energy.

In classical mechanics the total mechanical energy after an inelastic collision is ordinarily less than the initial total mechanical energy, and the mechanical energy which is lost is converted into heat. However, an inelastic collision in which the total energy after collision is greater than the initial total energy sometimes can occur in classical mechanics. For example, a collision can cause an explosion which converts chemical energy into mechanical energy. In molecular, atomic, and nuclear systems, which are governed by quantum mechanics, the energy levels of the particles can be changed during collisions. Thus these inelastic collisions can involve either a gain or a loss in mechanical energy.

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


An interaction resulting from the close approach of two or more bodies, particles, or systems of particles, and confined to a relatively short time interval during which the motion of at least one of the particles or systems changes abruptly.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


When two hosts transmit on a network at once causing their packets to corrupt each other.

See collision detection.


This article is provided by FOLDOC - Free Online Dictionary of Computing (

collision avoidance system

(1) See adaptive cruise control, semiautonomous vehicle and self-driving car.

(2) An in-vehicle safety system that applies the brakes when it detects a possible forward collision. Based on the current driving speed and the distance to the vehicle ahead (determined by radar), a collision avoidance system warns the driver first and intervenes if no human action is taken. Advanced systems can apply the brakes when heading into an oncoming car while turning the corner.

Cross Traffic Avoidance
A cross traffic system alerts the driver that a car or cyclist is about to cross in front. It may also stop the vehicle.

Collision Avoidance in Reverse
Safety devices when backing up a vehicle have matured. The most elementary "rear blind zone assist" is a set of sensors that cause audible beeps when getting close to an object. That evolved to a rear camera that enables drivers to see objects on screen that are directly behind the car. The camera was later augmented with a "cross traffic" or "cross path alert" system that warns the driver when a car is moving perpendicular to the rear. Advanced systems can also sense pedestrians and cyclists and may even be able to apply the brakes as in forward collision detection. See automotive safety systems.

Rear Cross Path Alert
When backing up in this 2017 Honda, cars coming into the driver's path are alerted by audio and visual alerts. In this example, beeps were heard along with flashing orange arrows (right side) as a car was approaching from behind on the passenger side.


(Carrier Sense Multiple Access/Collision Detection) The transmission method used in Ethernet networks. When Ethernet was designed in the 1970s, it was a shared medium. At any moment, only one frame from one station was transmitting in one direction (half duplex). See 10Base5 and 10Base2.

With CSMA/CD, if the network is busy when a station wants to transmit (carrier sense), the station waits a random number of microseconds before trying again. However, if two stations coincidentally transmit their frames at exactly the same time, their signals will collide. Both stations detect the collision and back off a random duration before retrying.

Today, collisions have been mostly eliminated, because shared Ethernet gave way to full-duplex, point-to-point channels between sender and receiver (see switched Ethernet). However, CSMA/CD provides compatibility for older shared Ethernet hubs that may still be in place. Ethernet is a data link protocol, and CSMA/CD is a MAC layer protocol (see MAC layer). See data link protocol, Ethernet and CSMA/CA.
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