According to the Principle of Conservation of Momentum, the expulsion of hot gases through the rear of rockets or air crafts will produce a large momentum in the opposite direction. The forward thrust of a jet engine enables the aircraft to move forward while the upthrust of a rocket engine enables the rocket to propel upwards..
In a collision between two objects or in an explosion, the total momentum of the objects in the system remains unchanged. The Principle of Conservation of Momentum states that in a collision, the total momentum in the system is conserved provided no external forces are acting.
Total momentum before a collision =
Total momentum after a collision
When snooker ball A strikes snooker ball B, the momentum is transferred from snooker ball A to ball B. The momentum of snooker ball A decreases as its velocity slows down. Thus, the momentum of snooker ball B increases as it gains velocity. The total momentum of the snooker balls do not change but is conserved.
In an explosion such as firing a shotgun, the total momentum of the shotgun and bullet before the explosion is zero since is no motion. After the shotgun is fired, the bullet moves forward with high velocity whereas the shotgun will recoil with a much lower velocity. The total momentum of the bullet and the shotgun is zero since they move in opposite directions. The forward momentum of the bullet is positive whereas the backward momentum of the gun is negative.