Question

What is the torque that the Sun's gravity exerts on Neptune? Explain why your value is what it is.

Answer 1

The torque exerted by the sun's gravity on Neptune is zero. Because gravitational force is a central force that acts along the line joining the gravitational force and the line joining them as a result both force vector and position vector are parallel to each other. When the force acting on the object and the position vector is zero the torque is zero. This can demonstrated as below

The gravitational force vector is

The position vector joining Neptune and sun is

The torque is defined as

This is the answer.