Calculate the work done by the torque.

Explanation

Consider a rigid body rotating about a fixed axis. Figure showa a point P on the body rotating about an axis perpendicular to the plane fo the page. A tangential force F is applied on the body.

It produces a small displacement ds on teh body. The work done (dw) by the force is
`dw=Fds`
As teh distance, the angle of rotation `d theta` and radius r are related by the expression,
`ds = r d theta`
The expression for work done now becomes,
`d w = F d s, d w= Fr d theta`
The term (Fr) is the torque `tau` produced by the force on the body.
`d w = tau d theta`
The expression gives the work done by the external torque `tau`, which acts on the body rotating about a mixed axis through an angle `d theta`.