Question

A: Two bodies having mass m1 and m2 collide elastically the net momentum of the system conserves whereas the individual momentums of the particles changes. R: During elastic collision, by Newton's third law of motion force on m1 is exactly equal and opposite to force on m2.

Consider an isolated system of n interacting particles. The mutual forces between pairs of particles in the system cause changes in momenta of the individual particles. By third law, the mutual forces between any pair of particles are equal and opposite. By second law, the change in momenta for any pair of particles are Ft and -Ft. Thus, the momentum changes cancel in pairs and total momentum of the system remains constant. This leads to a fundamental principle of physics called the law of conservation of linear momentum. This law states that the total linear momentum of an isolated system of interacting particles is conserved. The recoil of a gun on firing, explosion of a bomb into different fragments due to internal forces, the working of rockets and jet planes, etc.; can be explained on basis of momentum conservation.