How do I calculate the total torque exerted on a rotating disk?

[Sheneron]

[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?

 

[Doc Al]

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.

 

[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?

 

[Doc Al]

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.

 

[Sheneron]

oops again...
Thanks.