A collision is an isolated event in which two or more bodies (colliding bodies) exert relatively strong forces on each other for a relatively short time.


Collisions involve forces (there is a change in velocity). Collisions can be elastic, meaning they conserve energy and momentum, inelastic, meaning they conserve momentum but not energy, 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.


In billiards, collisions play an important role. Because the collisions between billiard balls are nearly elastic, and the balls roll on a surface that produces low-rolling friction, their behavior is often used to illustrate Newton's laws of motion. After a low-friction collision of a moving ball with a stationary one of equal mass, the angle between the directions of the two balls is 90 degrees. This appears to be an important fact that many professional billiard players take into account.

Consider an elastic collision in 2 dimensions of any 2 masses m1 and m2, with respective initial velocities v1 in the x-direction, and v2 = 0, and final velocities V1 and V2.

Conservation of momentum: m1v1 = m1V1+m2V2.

Conservation of energy for elastic collision: 1/2m1|v1|2 = 1/2m1|V1|2+1/2m2|V2|2

Now consider the case m1 = m2, we then obtain v1=V1+V2 and |v1|2 = |V1|2+|V2|2

Using the dot product, |v1|2 = v1•v1 = |V1|2+|V2|2+2V1•V2

So V1•V2 = 0, so they are perpendicular.


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 velocity and mass 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.


Types of attack by means of a deliberate collision include:

An attacking collision with a distant object can be achieved by throwing or launching a projectile.

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