# Homework Help: Frictional force

1. Dec 13, 2011

### borobeauty66

1. The problem statement, all variables and given/known data
A rotating disk of radius 2.0m (the mass on the disk, assume is negliable) with 4 points of mass each spaced 90 degrees apart from each other, each weighing 19kg. Each is position 0.2m from the axle.
Inertia of the disk is 3.04 kg m^2
Angular Speed is 310 rad/s
Torque is initially at 0 Nm, as no force is acting upon it.

A frictional force is then applied to the rim, of 94 N. How long does it take to come to a complete stop?

2. Relevant equations

I assume F x r = torque
But after this, I'm not sure where to go.

3. The attempt at a solution

Not sure where to begin

2. Dec 13, 2011

### BruceW

You've got the right idea in your 'relevant equations' section. You can calculate the torque using the information given. That's the first step you should try.

3. Dec 13, 2011

### borobeauty66

Therefor torque = 94 N x 2.0 m = 188 N m.

Will I need inertia for the next step?

4. Dec 13, 2011

### LawrenceC

"Will I need inertia for the next step?"

It's a rotating object that is being stopped. What does that tell you?

5. Dec 13, 2011

### BruceW

This is correct.

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