A potter's wheel having a radius 0.55 m and a moment of inertia 12 kgm2 is rotating freely at 51 rev/min.

A potter's wheel having a radius 0.55 m and a moment of inertia 12 kg·m2 is rotating freely at 51 rev/min. The potter can stop the wheel in 7 s by pressing a wet rag against the rim and exerting a radially inward force of 67 N. Find the effective coefficient of kinetic friction between the wheel and the wet rag.