Momentum has a special property that, in a closed system (one in which no external forces act on the colliding/exploding bodies) it is always conserved, even in collisions and separations caused by explosive forces.

Kinetic energy is not conserved in collisions if they are inelastic (which most collisions are, only 'ideal' collisions - such as those considered in theory between gas molecules - conserve kinetic energy).

Because momentum is conserved it can be used to calculate an unknown velocity following a collision or a separation if all the other masses and velocities are known.

The sum of the momentum before the collision must equal the sum of the momentum after the collision.

- The symbol used for initial velocity is usually 'u'.

- The symbol for final velocity is usually 'v'

- Subscripts are used to distinguish between objects that are colliding.

- The sign is important to determine the direction the objects are travelling in.

See the example below for how the conservation of momentum can be used to find out unknown values after a collision: