In other words, the sum of the momenta of all the objects in a closed system is constant. This is true regardless of the nature of the interactions between the objects.
For example, if two billiard balls collide, the total momentum of the two balls before the collision is equal to the total momentum of the two balls after the collision. This is true even if the collision is elastic (in which case the kinetic energy of the two balls is conserved) or inelastic (in which case the kinetic energy of the two balls is not conserved).
The law of conservation of momentum is a fundamental law of physics. It is used to analyze a wide variety of problems, including collisions, explosions, and rocket propulsion.
