Dear Student,Law of conservation of linear momentum :- when there is no any external force applied on a system of particles then , momentum of system of particles will be conserved or you can say that if external force is zero. Then momentum of system of particles in intially = momentum of system of particles in finally. Mathematically, From Newton's second law, Fext = dP/dt If Fext = 0, ∫ dP =∫ 0.dt Means dP = 0 e.g., Pi = Pf Let two bodies A and B of masses mâ‚ and mâ‚‚ are moving with velocities uâ‚ and uâ‚‚ , after collision velocities of bodies are vâ‚ and vâ‚‚ respectively. Then, initial momentum of bodies ,Pi= mâ‚uâ‚ + mâ‚‚uâ‚‚ Final momentum of bodies, Pf = mâ‚vâ‚ + mâ‚‚vâ‚‚ Because external force doesn't apply here, so, linear momentum will be conserved . e.g., Pi = Pf mâ‚uâ‚ + mâ‚‚uâ‚‚ = mâ‚vâ‚ + mâ‚‚vâ‚‚ Regards