Conservation of Momentum

If all external forces on a system are zero, then equation (10.5) reduces to

$$\mbox{\bf p}_{tot} = const ~~~ \mbox{(isolated system)} .$$ (11.6)

A system of particles with no external forces acting on it is called isolated. Newton's third law thus tells us that the kinetic momentum of an isolated system doesn't change with time. This law is called the conservation of momentum.