# Easy rotational problem

1. Apr 4, 2008

### Sheneron

[SOLVED] Easy rotational problem

1. The problem statement, all variables and given/known data
A string is wound around a uniform disk of radius 0.25m and mass 8.00kg. The end of the string is pulled with a constant force of 50.0 N, and there is a frictional torque acting at the axle of 3.00 Nm.
A) Find the total torque exerted on the disk

3. The attempt at a solution
I think I have it solved... just wanted to make sure and I have one question. It says that the frictional torque is acting on the axle, do I still use the radius r=0.25 to find the torque for that part? Here is what I did:

$$\Sigma \tau = \Sigma Fd$$
$$\Sigma \tau = (50)(0.25) + (-3)(0.25)$$
$$\Sigma \tau = 11.75 Nm$$

So is that how I would find the net torque?

2. Apr 4, 2008

### Staff: Mentor

Careful. For friction they gave you the torque, not the force. So you don't have to use any radius to find the frictional torque.

3. Apr 4, 2008

### Sheneron

Oh yeah I just realized that. so I guess what i meant to say would the net force end up being 9.5 Nm?

4. Apr 4, 2008

### Staff: Mentor

The net torque is 9.5 Nm.

5. Apr 5, 2008

### Sheneron

oops again...
Thanks.