Final answer:
The motion of planets around the sun does not involve doing any work because the force of gravity acting between them is perpendicular to the direction of motion.
Step-by-step explanation:
The motion of planets around the sun does not involve doing any work because work is defined as the transfer of energy that occurs when a force acts on an object and causes it to move in the direction of the force. However, in the case of planets orbiting the sun, the force of gravity acting between them is perpendicular to the direction of motion. This means that the force does not cause any displacement along its line of action, and therefore, no work is done.
For example, consider a planet in circular orbit around the sun. The gravitational force exerted by the sun on the planet is directed toward the sun, while the planet's motion is perpendicular to this force. As a result, the work done by the gravitational force is zero.
Another way to understand this is that the motion of the planet is sustained by the gravitational force, which is a conservative force. Conservative forces do no work in a closed loop, and the motion of planets around the sun can be considered as a closed loop.