Momentum is one of the important physical variables used in the quantitative description of dynamics. What makes it important is its usefulness, and what makes it useful is that in an isolated system it is conserved. The conditions for momentum conservation are: If the net external force acting on an object, or system of objects, is zero the total momentum of that object, or system of objects, is constant. The linear momentum of a physical body is defined as the product of its mass and its velocity, P=mv
When two freely moving bodies collide, momentum is “conserved”. That is, the momentum of the system before the collision is equal to the momentum of the systemafter collision. This Conservation of Momentum Law applies to elastic and inelastic collisions,regardless of the nature of the interaction force between the objects
