total momentum before an event = total momentum after the event
A ‘closed system’ is something that is not affected by external forces. This is called the principle of conservation of momentumThe principle that the total momentum of a system remains the same. When bodies collide, whatever momentum is lost by one body, the other gains in equal amounts.. Momentum is conserved in collisionWhen two objects meet and interact, eg two particles moving towards each other will collide. and explosionWhen parts of a system separate and move apart. For example, a supernova is an exploding star - the outer layers are thrown out into space in all directions..
Conservation of momentum explains why a gun or cannon recoils backwards when it is fired. When a cannon is fired, the cannon ball gains forward momentum and the cannon gains backward momentum. Before the cannon is fired (the ‘event’), the total momentum is zero. This is because neither object is moving. The total momentum of the cannon and the cannon ball after being fired is also zero, with the cannon and cannon ball moving in opposite directions.
