# Easy rotational problem

[SOLVED] Easy rotational problem

## Homework Statement

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

## 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?

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Doc Al
Mentor
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?
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.

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

Doc Al
Mentor
so I guess what i meant to say would the net force end up being 9.5 Nm?
The net torque is 9.5 Nm.

oops again...
Thanks.