A Sample Rotational Energy Problem

A grinding wheel in the form of a uniform solid disk is spun up to an angular velocity of wi. The wheel has a mass m and a radius r. A tool to be sharpened is pressed against the wheel with a normal force of N.

(a) If, after t seconds, the wheel's angular velocity is wf, what is the coefficient of friction between the tool and the grinding wheel? Assume no other torques act on the wheel.

(b) What is the average rate of kinetic energy loss over this time period?