This problem is a classic example of how to apply physical principles of friction, force and pressure as well as basic integration into physics.

Friction producing a torque, rather than acting at a single radius, is applied across the face of an entire rotating disc. As a result we have to set up a function for the torque at a single radius, then integrate that function across the entire face of the disc.

This problem comes up in calculus based physics courses as well as mechanics courses.