A planet is moving at constant speed in a circular orbit around a star. In one complete orbit, what is the net amount of work done on the planet by the star’s gravitational force: positive, negative, or zero? What if the planet’s orbit is an ellipse, so that the speed is not constant? Explain your answers.

Solution 11DQ Introduction We have to discuss about the total work done by the gravitational force of a star on the planet orbiting it. Step 1 Since the planet is moving with constant speed, the orbit is circular. Now the gravitational force will act directly from planet to the star, that is the gravitational force will always act radially inwards. Hence the direction of motion of the planet and the direction of the force are perpendicular to each other and hence the work done by the gravity will be zero.