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Homework Help: Angular Momentum of a flywheel problem

  1. Mar 8, 2008 #1
    1. The problem statement, all variables and given/known data

    The angular momentum of a flywheel having a rotational inertia of 0.140 kg m[tex]^{2}[/tex] about its central axis decreases from 3.00 to 0.800 kg m[tex]^{2}[/tex]/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?

    2. Relevant equations

    [tex]\tau[/tex] = I[tex]\alpha[/tex] ?

    3. 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?
    Last edited: Mar 8, 2008
  2. jcsd
  3. Mar 8, 2008 #2

    Doc Al

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    Staff: Mentor

    Find the angular acceleration, then use kinematics to find the angle.
  4. Mar 8, 2008 #3
    Well, I thought of that, but then how do I find angular velocity?
  5. Mar 8, 2008 #4


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    Staff Emeritus
    Science Advisor
    Gold Member

    Using kinematics as Doc Al said. You know that,

    [tex]\alpha = \frac{d^2\theta}{dt} = \frac{d\omega}{dt}[/tex]

    Can you take the next step?
  6. Mar 8, 2008 #5
    Uh.. I'm still not getting 20.4 rad, for some reason.
    Last edited: Mar 8, 2008
  7. Mar 8, 2008 #6


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    Staff Emeritus
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    Gold Member

    Perhaps if you showed us your working we could help you out.
  8. Mar 9, 2008 #7
    Answers are not important if ya 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 ya want .rest is easy.
    Last edited: Mar 9, 2008
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