The Law of Conservation of Momentum

When two or more objects exert forces on each other, their total momentum always stays the same. This is called the law of conservation of momentum, and it helps to explain why aircraft and rockets move.

A rocket engine sends out a high­speed jet of gas when it is fired. The rocket exerts a force on the gas and, according to Newton’s third law of motion, the gas reacts by exerting an equal and opposite force on the rocket. The jet of gas has momentum in one direction. The only way that the total momentum of the rocket and gas can remain the same is if the rocket gains the same momentum in the opposite direction. So, the rocket moves. The same conservation law applies to air­craft. The momentum of the gas rushing out of an aircraft’s jet engines is equal and opposite to the plane’s momentum.