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Physical collision

Dynamics

In physics, collision means the action of bodies striking or coming together (touching). Collisions involve forces (there is a change in velocity). Collisions can be elastic, meaning they conserve energy and momentum, inelastic, meaning they conserve momentum, or totally inelastic (or plastic), meaning they conserve momentum and the two objects stick together.

The magnitude of the velocity difference at impact is called the closing speed.

The field of dynamics is concerned with moving and colliding objects.

Billiards

In billiards, collisions play an important role. Because the collisions between billiard balls are almost perfectly elastic, and the balls roll on a low-friction surface, their predictable behaviour is often used to illustrate Newton's laws of motion.

Traffic

In traffic such a collision can be between two vehicles, a vehicle and a person, a vehicle and an object, two persons or a person and an object (and more if an animal is involved). It is an accident or even a disaster. At level crossings sometimes a train collides with a vehicle or person. Due to the speed and weight of a train it needs a long distance to stop, typically longer than the train driver can see ahead. When a train collides with a car this is more likely to be deadly for the people in the car than for those in the train, because the train has more mass and momentum.

See also: Car accident

Attacks by means of a deliberate collision

Attacks by means of a deliberate collision can be:

• with the body: striking, punching, kicking
• with a weapon, without throwing: striking with a club, or (to attack a fly) flyswatter
• throwing or shooting a projectile that does not carry explosives:
  • catapult
  • bullet
  • kinetic missile ("kinetic kill engagement", "body-to-body hit"), see also anti-ballistic missile, Missile Defense Agency

Others

• A "collision" applied for construction work, is hitting a nail with a hammer, etc., and for cleaning a carpet-beater.
• In baseball a ball is hit with a baseball bat, in racquet sports also a ball or other object is hit, in bowling a ball hits a target.
• Supercolliders slam molecules, atoms, and subatomic particles together.
• Collision detection for use in computer games, computational geometry, AI, physical simulations etc.
• Bumper cars are designed to collide for fun.

See also: crater, impact event, space debris

Telecommunications

In telecommunication, the term collision has the following meanings:

1. In a data transmission system, the situation that occurs when two or more demands are made simultaneously on equipment that can handle only one at any given instant.
2. In a computer, the situation that occurs when an attempt is made to store simultaneously two different data items at a given address that can hold only one of the items.

Source: from Federal Standard 1037C and from MIL-STD-188

See also: CSMA-CA