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Homework Help: Torque and Acceleration question

  1. Nov 7, 2008 #1
    1. The problem statement, all variables and given/known data
    A solid wheel (I = 1/2 MR^2) initially rotating at 5 rad/s comes to a stop after 8 rotations.
    a) Angular acceleration?
    b) Tangential accelration at the rim?
    c) If the mass is 2 kg and the radius is 1.5 m, find the net torque.
    d) Using values form c, how much energy was required to bring the wheel to a stop?

    2. Relevant equations

    3. The attempt at a solution
    a) -24.9 rad/s^2

    b) -37.4 m/s^2

    c) -56 Nm

    d) I am not sure how to find the energy used.
  2. jcsd
  3. Nov 7, 2008 #2


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    part(a) is incorrect and that error propagates through to (b) and (c)...perhaps you should show me how you got part (a)...
  4. Nov 7, 2008 #3
    For part a I used (w for omega for tangential velocity, a for alpha for angular acceleration )

    w final ^2 - w initial^2 = 2a delta angle

    (0 - (50 rad /s)^2 ) / (2 (50.3rad))

    = -24.9 rad /s^2
  5. Nov 7, 2008 #4


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    Your original post said the w_initial=5rad/s not 50...if it is 50 then your original answer is correct.

    As for the energy required to stop the wheel; it should be the same as the 'work done by the torque' over an angular distance of 8*2pi...does that help?
  6. Nov 7, 2008 #5
    Sorry for the typo!

    So the energy used will be 8* 2pi (-56.0Nm) ?
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