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Homework Help: Calculation of torque to stop a wheel.

  1. Jan 3, 2017 #1
    1. The problem statement, all variables and given/known data

    A wheel of moment of inertia 5×10-3kg-m2 is making 20 rev/s. Find the torque required to stop it in 10s is

    (A) 2π×10-2Nm (B) 2π×102Nm

    (C) 4π×10-2Nm (D) 4π×102Nm

    Answer - (A).

    2. Relevant equations

    ω=2πƒ (ƒ=frequency)
    Power (P) = Γω.
    P = W/t.
    W = ΔK.E. = (1/2)Iω2.

    3. The attempt at a solution

    The power delivered is equal to the rate of doing work which is equal to the change in rotational kinetic energy.
    On calculating, I get the final answer as 10-2πN.m. Kindly help.
  2. jcsd
  3. Jan 3, 2017 #2
    You are thinking in a very complicated way! Keep it simple! Use the rotational kinematic equations and the relation between torque and angular acceleration.
  4. Jan 3, 2017 #3


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    Science Advisor
    Homework Helper
    Gold Member

    Show your working. Note that the power isn't constant. Power = torque * angular velocity
    And the angular velocity is reducing.

    PS My internet connection is currently unreliable so sorry in advance if I "dissapear".
  5. Jan 3, 2017 #4
    Oh yes. The angular velocity isn't constant. I didn't pay attention to that. Thanks.
  6. Jan 3, 2017 #5
    Yeah! Turns out this was a simple question. Thanks a lot!
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