# Homework Help: Determine torque required to accelerate the flywheel

1. May 29, 2017

### em3

1. The problem statement, all variables and given/known data
A solid disk of diameter equal to 500 mm and a mass of 60 kg is to be used as a flywheel in an energy recovery device. Determine the torque required to accelerate the rotation of the flywheel, about its axis, from rest to 250 rpm in 6 revolutions.

2. Relevant equations

τ=Iα

3. The attempt at a solution

r = 0.25

I = m *r2
I= 60 * 0.252
I = 3.75

3.75 * 26.17993
=98

I'm not really sure what to do, I can't get the correct answer.

2. May 29, 2017

### scottdave

It helps if you use units when doing your calculations. Then you might realize that 26.17 rad/sec is the angular velocity (equivalent to 250 rpm), not acceleration. It took 6 seconds to get up to this speed. How can you use this information to get angular acceleration?

3. May 29, 2017

### Staff: Mentor

Is that the correct formula for the moment of inertia of a disk?
What are the units?
What were you trying to calculate there? Looks like you might be multiplying the angular velocity by your moment of inertia, but since there are no units shown we can't be sure.

The kinematic formulas that apply to linear motion can be applied to rotational motion if you substitute the analogous terms. So:

mass ⇔ moment of inertia,
velocity ⇔ angular velocity,
acceleration ⇔ angular acceleration,
force ⇔ torque,
distance ⇔ angular distance.

If you relate the rotational motion terms to those of linear motion you're given a mass (moment of inertia) , a final speed (angular velocity), and the distance covered (number of revolutions).

Sort out your moment of inertia by finding the correct formula for a solid cylinder (disk), then look at your SUVAT equations to see if you can find the required torque from what you've been given.