Does anybody know how to find the coefficient of kinetic friction w/ the given information?

Question

A grindstone in the shape of a solid disk with diameter 0.520 m and a mass of 50.0 kg is rotating at 800 rev/min. You press an ax against the rim with a normal force of 160 N and the grindstone comes to rest in 8.50 s. Find the coefficient of kinetic friction between the ax and the grindstone.
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Answer 1

N=Normal Force=160 N
F=Friction Force
E=kinetic Energy of disk
w=angular speed of disk=800*2*3.14/60=83.7 rad/s
R=Radius of disk=0.520/2=0.26 m
M=mass of disk =50kg

E=w^2 * R^2 * M /2
Friction work = kinetic Energy of disk = F * d
E=F * vt =F * wR t
F=E/(wR t)
F=(w^2 * R^2 * M /2)/(wR t)
F=wRM/(2t)

coefficient of kinetic friction =F/N
coefficient of kinetic friction =wRM/(2tN)
coefficient of kinetic friction =83.7* 0.26 * 50 / (2 * 8.5 * 160)
coefficient of kinetic friction =0.4