Constant Torque Find magnitude of friction force

[sarahjt1]

Homework Statement

A grinding wheel of radius 0.330m rotating on a frictionless axle is brought to rest by applying a constant friction force tangential to its rim. The constant torque produced by this force is 72.1 N*m. Find the magnitude of the friction force.

I'm honestly lost at even starting this problem but below are some equations that I THINK are relevant but I don't really know.

Homework Equations

torque=I alpha, where I = 1/2MR^2.

This is where I get confused because we are not given a mass....

The Attempt at a Solution

 

[sarahjt1]

Nevermind... silly me. Much simpler than I thought.