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**1. Homework Statement**

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?

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?

## Homework Equations

T=I[itex]\alpha[/itex]=Fr

## 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)MR

^{2}

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

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

^{2}

Substituting the values, I'll have

alpha = (225)(2.5)/(1/2)(650/9.8)(2.5)

^{2}

alpha = 2.71 rad/s

^{2}

The solution in my note was

alpha = 2g(Torque)/WR

^{2}

which results to 3.54rad/s

^{2}

Am I missing something? Or is everything just right?