Question

A planet revolves around the sun due to gravitational force.But torque on the planet due to gravitational force os zero.Why?

Answer 1

We know the expression for torque,

τ = r × F = r F sinθ

where θ is the angle between the line of action of force and radius vector.

Here we're given to comment on the torque on the planet due to gravitational force which is acting towards Sun. Well, the radius vector of the planet is also acting towards Sun.

So we get that θ = 0°, thus sin 0° = 0.

Hence τ = r F sin 0° = 0.