If a comet is orbiting a star is there any work done? I understand that work is force * displacement, but the force must be in the direction of motion. In the case of a comet and star the star exerts a gravitational force on the comet, but this force is not in the direction of the motion of the star (as the motion is circular). Is this the correct logic? For the question in the image shown I understand that potential energy would be greatest at A and kinetic would be greatest at D, but I still am stumped on the work side of things.