Question

A planet is moving around the Sun in a circular orbit of circumference $C$. The work done on the planet by the gravitational force $F$ of the Sun is $kFC$, then the value of $k$ is

Answer 1

As the planet is moving in circular orbit, so the direction of the gravitational force is radial while displacement is tangential. Force $F$ and displacement $d$ are perpendicular to each other.

Work done $W=Fd\cos \theta$ where $\theta$ is the angle between $F$ and $d.$

So work done will be zero as here $\theta$ is ${90}^o$.

$W=0=kFC$

$⟹$ $k=0$