^External force

1. Any force acting outside the system selected is called external force.
2. Only external force can accelerate a system.
3. Force internal or external is a matter of selection of system. e.g. if we consider a system of earth & sun, then the gravitational force on earth by sun & that by sun on earth are internal, but if we consider the system earth only then the same gravitational force on earth becomes external force & accelerates earth around the sun.