A grindstone in the shape of a solid disk

A grindstone in the shape of a solid disk with diameter 0.520 m, and a mass of 50.0 kg is rotating at 850 rev/min, you press an ax against the rim with a normal force of 160 N (Fig. 10.43), and the grindstone comes to rest in 7.50 s. Find the coefficient of friction between the ax and the grindstone. You can ignore friction in the bearings.