91影视

The attractive force between two objects by virtue of their mass is called gravitational force.

The concept of the gravitational force gives the net amount of work done.

As the gravitational force is regarded as a conservative force, then the work done is the difference in the potential energy of the two points.

Let the planet has moved in a conservative field of force from the point A to the point B in the path C, then the equation of the work done is expressed as:

$$\begin{gathered} W={鈭�}_{C}F.dr \\ =U\left(B\right)-U\left(A\right) \end{gathered}$$

Here, W is the work done, F is the force exerted, dr is the distance travelled, U(B) is the potential energy at point B and U(A) is the potential energy at point A.

As the orbit has been completed and as it is circular, then the planet will start and end both at the point A. Hence, the work done will be-

$$\begin{gathered} W=U\left(A\right)-U\left(A\right) \\ =0 \end{gathered}$$

Thus, the net work done on the planet by the star鈥檚 gravitational force in the circular orbit is .

In case of elliptical orbit, as the planet is moving closer to the star, it increases its speed and hence the work done is positive. In the same way, as the planet moves away from the star, it reduces its speed and hence does equal amount of negative work. So net work done in one complete revolution is zero.