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Homework Help: Quick Question about torque

  1. Apr 27, 2004 #1
    There's a question that asks: if a wheel with 1.56 m/rad radius which reached a final velocity of 200 rad/min(or 10/3 rad/s) were slowed down with a constant torque of 10,000 NM being applied, how long would it take to stop the wheel. I can't seem to find how torque is related to this problem in anyway. I can solve it by doing the following:

    Angular Displacement = (W^2-Wo^2)/(2*Angular Acceleration)
    = (11.11rad^2/s^2)/(4.44rad/s^2)
    = 2.5 rad
    Angular Displacement = (t/2)(Wo+W)
    =(2.5 rad) = (t/2)*(10/3rad/s)
    =(2.5 rad)/(3.33rad/s)
    = .75s = (t/2)
    t = 1.5 s

    But it may not be correct because I didn't use the radius or torque in that problem. Please respond with your ideas on how they could be applied to this problem.
  2. jcsd
  3. Apr 27, 2004 #2

    Chi Meson

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    Science Advisor
    Homework Helper

    for now: T= torque A= angular acceleration I= moment of inertia

    T=IA so A=T/I

    now use the angular acceleration in an angular kentmatics problem with final angular velocity =0
  4. Apr 27, 2004 #3

    Doc Al

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    Staff: Mentor

    You are for some reason assuming a value (of 2.22 rad/s^2) for the angular acceleration. Where did that come from?

    If you had the angular acceleration, you wouldn't bother calculating the angular displacement--you'd immediately calculate the time given that you know the Δω.
    That should give you a hint that something's wrong! :rolleyes:

    As Chi Meson explained, you need to apply Newton's 2nd law for rotation to find α, α = Τ/I.
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