Question

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?

Answer 1

The gravitational force is a conservative force. Because the work performed by a conservative force on a closed path is zero, the gravitational force does no work over the complete orbit of a comet.