# Torque: Flywheel problem

1. Nov 12, 2011

### XwyhyX

1. The problem statement, all variables and given/known data
A flywheel in the form of a uniform disk (I = ½ MR2) 5.0 ft in diameter, weighs 650 lb. What will be its angular acceleration if a net force of 225 ft-lb acts it upon? If the disk is rotating at 1200 rev/min, what torque is required to stop it in 30 minutes?

2. Relevant equations

T=I$\alpha$=Fr

3. The attempt at a solution

I need help in the first question, because apparently I arrive with a different answer from the solution that my teacher gave me. I just wanted to know if I got it wrong or my teacher forgot something

So using the definition of torque, I solve for alpha which is

alpha = FR/I

Defining I in the equation gives

alpha = FR/(1/2)MR2

Since the given is weight, I still have to define mass in the equation

alpha = FR/(1/2)(W/g)R2

Substituting the values, I'll have

alpha = (225)(2.5)/(1/2)(650/9.8)(2.5)2
alpha = 2.71 rad/s2

The solution in my note was

alpha = 2g(Torque)/WR2

which results to 3.54rad/s2

Am I missing something? Or is everything just right?

2. Nov 12, 2011

### Staff: Mentor

Two problems:
(1) Even though it says 'net force', what's really given is the net torque.
(2) Careful with units: g ≠ 9.8

3. Nov 12, 2011

### XwyhyX

Oh I see, one question though, How can I tell whether its net force or net torque?

4. Nov 12, 2011

### Staff: Mentor

The units given (ft-lb) are for torque, not force. Check the problem statement; I suspect there was a typo somewhere.

5. Nov 12, 2011

### XwyhyX

Thanks! I got my english units rambled up, I usually think of pounds as mass, so I was thinking of what the english unit is for force. Nevermind, I understand it now. :D