Conservation of momentum states that the momentum of the body remains conserved if no external force is applied on it.
The state of rest or motion is not disturbed if external force is not applied on it. If external force is applied on the body the state will change with change in its momentum.
Suppose a body is moving in a straight line with a constant speed. The body will continue its motion and the momentum of the body will remain conserved until and unless some external force acts on it.
The change in momentum can be observed only when external force acts on the body. Internal force never brings change in the momentum of the body.
Examples of conservation of momentum:
Rocket – Gas system
Initially when the rocket is at rest the momentum of the system is zero. When rocket takes off it throws gas in opposite direction to its motion. The magnitude of the velocity of the gas in downward direction is equal to the magnitude of the velocity of rocket in the upward direction. The direction of velocity is in opposite direction therefore the net momentum again comes out to be zero and hence the conservation of momentum.