The law of conservation of momentum can be stated as, "When two separate objects collide with each other in an isolated system, the total amount of momentum of the two objects prior to and after the collision would remain the same." It can be mathematically described as m1*?v1 = -(m2*?v2), where the negative sign before the right-hand term is used to indicate change in direction.
One of the most important laws of physics, the law of conservation of momentum, can also be expressed as "?m*v = constant", where "m" is mass of the objects and "v" is their respective velocity. This dictates that the net amount of momentum before and after a collision always remains the same. During collision, while one of the objects loses its momentum, the other object gains the exact same amount of momentum but in the opposite direction.
Another important term involved in this law is the "isolated system." Generally speaking, a system is a collection of two or more objects. An isolated system is such a system where the objects within the system are not subjected to any net external force that can alter the momentum of the entire system. In other words, when applying the law of conservation of momentum in day-to-day scenarios, the surrounding forces are neglected for simplifying the problem, and only the objects within the system are examined.