Conservation of Momentum

When a force causes an object to change its velocity, there is also a change in momentum. When two objects interact or collide, they exert equal and opposite forces on each other and so the momentum of each one changes. This is illustrated below where the blue ball loses momentum and the black ball gains momentum.

Image

Like the forces they exert on each other during the collision, the changes in momentum of the balls are equal in size and opposite in direction.

It follows that:

• the combined momentum of the balls is the same before and after they collide.

This is an example of the principle of conservation of momentum.

The principle of conservation of momentum states that:
When two or more objects interact the total momentum remains
constant provided that there is no external resultant force.

In the context of the colliding balls, an external resultant force could be due to friction or something hitting one of the balls. Either of these would result in a change in the total momentum.

To satisfy the conditions for conservation of momentum to apply, the only forces acting on the objects must be the ones between the objects themselves. This is why conservation of momentum is usually demonstrated on an air track or other low friction surface.