1 Conservation of linear momentum: If the net external force acting on system of objects is zero, the vector sum of the momenta of the objects will remain constant. In collisions and explosions, the vector sum of the momenta just before the event equals the vector sum of the momenta just after the event. The vector sum of the momenta of the objects involved does not change during the collision or explosion. Thus when two bodies of masses m1 and m2 collide, total momentum before impact = total momentum after impact m1u1 +m2u2 = m1v1 +m2v2, (26) where u1 and u2 are the velocities before impact, and v1 and v2 are the velocities after impact. A perfectly elastic collision is one in which the sum of the translational kinetic energies of the objects is not changed during the collision. In the case of two bodies,
