What is the principle of conservation of momentum?

1. When two (or more) bodies act on each other and no external force acts on them, the total momentum of all the bodies is equal to the initial total momentum.
2. During any collision, the momentum is conserved but the kinetic energy is not.
3. This is because, during the collision, a part of kinetic energy is converted into heat and sound energies.
4. When two (or more) bodies act on each other, their total momentum remains constant provided no external force acts on them. This statement is called the principle of conservation of momentum. $Totalmomentumbeforecollision=Totalmomentumaftercollision$