Comets mover 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 orbit of the comet is zero. Why?

Gravitation topics include: Forces of nature, inertia, restitution coefficient, gravitational and gravity.

The coefficient of restitution (COR) is a unitless quantity that indicates the elasticity of a collision. It is the ratio of the final velocity to the initial velocity between two objects after their collision. The COR is denoted as "e" and its values range between 0 and 1.