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## Homework Statement

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

Find the coefficient of kinetic friction between the ax and the grindstone.

## Homework Equations

## The Attempt at a Solution

The concept of torque has been giving me alot of problems, mostly UNDERSTANDING what the equation means, and when to use them.

So here is how I started out,

I=1/2MR^2

850rev/min= 85pi rad/3s

So my logic is I have the inertia, and I have enough information to solve for acceleration which means I can find the torque.

I solve for the acceleration

w

_{z}-w

_{oZ}/t=-11.9rad/s^2

Here is where I get a bit confused, where I was going with this problem is I was going to solve for the torque using Ʃτ

_{z}= Iα

_{z}

Then when I aquired the torque I was going to use

ƩFext=Ma

_{cm}

to find the coefficient of friction.

The solution manual sets Ʃτ

_{z= μkNR=Iαz, and solves for μk. How do we know when to stay at Ʃτz=Iaz or when we have to switch over to ƩFext=Macm? Thank you Higgenz}