# Homework Help: Stopping a rotating disk with an axe

1. Mar 16, 2005

### jaymode

My problem:
A grindstone in the shape of a solid disk with diameter=0.480m and a mass of m=50.0kg is rotating at omega = 820rev/min. You press an ax against the rim with a normal force of F=190N View Figure , and the grindstone comes to rest in 8.00s.

I need to find the coefficient of friction between the ax and the grindstone.

well the stuff i have some far is that the moment of Inertia is 1/2MR^2. That is really all that I have I just need a hint or something on how to proceed...if someone could help me out.

Last edited: Mar 16, 2005
2. Mar 17, 2005

### Galileo

The axe exerts a constant torque $\tau = I\alpha$ on the disk, combine this with the 'equation of motion' for the angular velocity: $\omega(t)=\omega_0+\alpha t$.

3. Mar 17, 2005

### jaymode

Galileo, thank you. That really helped me figure the problem out.