Angular Momentum of a flywheel problem

[Seraph404]

Homework Statement

The angular momentum of a flywheel having a rotational inertia of 0.140 kg m^{2} about its central axis decreases from 3.00 to 0.800 kg m^{2}/s in 1.50 s. a) What is the magnitude of the average torque acting on the flywheel about its central axis during this period? b) Assuming a constant angular acceleration, through what angle does the flywheel turn?

Homework Equations

\tau = I\alpha ?

The Attempt at a Solution

a) Part a is easy. Torque equals final momentum minus initial momentum over the time. The answer is -1.47 N

b) Part b is what I need help with. As a hint, what equations should I look at or try to combine?

 

[Doc Al]

Find the angular acceleration, then use kinematics to find the angle.

 

[Seraph404]

Well, I thought of that, but then how do I find angular velocity?

 

[Hootenanny]

Well, I thought of that, but then how do I find angular velocity?

Using kinematics as Doc Al said. You know that,

\alpha = \frac{d^2\theta}{dt} = \frac{d\omega}{dt}

Can you take the next step?

 

[Seraph404]

Uh.. I'm still not getting 20.4 rad, for some reason.

 

[Hootenanny]

Uh.. I'm still not getting 20.4 rad, for some reason.

Perhaps if you showed us your working we could help you out.

 

[physixguru]

Answers are not important if you know the correct approach.

I hope my great colleagues above would agree.

Now for the help part..

Torque is the rate of change of angular momentum.
Next, torque= M*I * alpha
alpha=omega/t or omega= alpha*t
next, omega * time= angular displacement.
finally... angular displcement is what you want .rest is easy.