coefficient of kinetic friction?

Question

a potters wheel (a thick stone disk of radius 0.500m and a mass of 125kg) is freely rotating at 50.0 rev/minute. the potter can stop the wheel in 6.00seconds by pressing a wet rag against the rim and exerting a radially inward force of 68.0N. find the effective coefficient of kinetic friction between the wheel and the rag.

Answer 1

for the wheel I=m*r^2/2
125*0.25/2

I=15.6

we need alpha
50 rev/minute is
50*2*3.14/60 rad/sec
5.23 rad/sec

0=5.23-alpha*6
alpha=5.23/6
alpha=0.872


Torque=I*alpha
Torque=15.6*0.872
Torque=13.6
Torque also is
Torque=F*r
F=Torque/r
=27.2 N
F=normal force time u
27.2=68*u
u=27.2/68
u=0.4

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