Answer the following :
Comets move around the sun in highly elliptical orbits. The gravitational force on the comet due to the sun is not normal to the comet's velocity in general. Yet the work done by the gravitational force over every complete orbit of the comet is zero. Why?

Explanation

For a conservative force work done over a path is minus of change in potential energy. Over a complete orbit, there is no change in potential energy.