# Collisions - elastic and inelastic

Collisions between objects are governed by laws of conservation of momentum and conservation of energy.

Conservation of momentum

Provided that there are no net external forces acting upon the objects, the sum of the momentum of all objects before a collision equals the sum of the momentum of all objects after the collision.

When a collision occurs in an isolated system (one where no net external forces acting upon the objects), the total momentum of the system of objects is always conserved.

If there are only two objects involved in the collision, then the momentum lost by one object equals the momentum gained by the other object.

Elastic collisions are collisions in which both momentum and kinetic energy are conserved.

The total system kinetic energy before the collision equals the total system kinetic energy after the collision. This type of collision only occurs in atomic and subatomic situations.

Inelastic collisions are those where total kinetic energy is not conserved.

There is usually a loss of kinetic energy in a 'real life' collision. This is usually due to transfer of some of the kinetic energy to heat and sound.