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Homework Help: Flywheel and Torque

  1. Nov 15, 2012 #1
    1. The problem statement, all variables and given/known data
    Flywheels are large, massive wheels used to store energy. They can be spun up slowly, then the wheel's energy can be released quickly to accomplish a task that demands high power. An industrial flywheel has a 1.5 m diameter and a mass of 270 kg. Its maximum angular velocity is 1500 rpm. How much torque does the flywheel exert on the machine?

    2. Relevant equations
    to find Wf I used (1/2 K ) = 1/2 I wf ^ 2

    then Wf=Wo+alpha(t)

    then T for torque=I alpha

    3. The attempt at a solution

    to find Wf I used (1/2 K ) = 1/2 I wf ^ 2

    =(1/2 (9.4*10^5 (this is what the amount of energy stored) = 1/2 (70.3125) <--This number is I=1/2mr^2 (wf^2)

    I rearranged this to find Wf
    Wf^2= .5(9.4*10^5) / .5 (70.3125)

    wf= sqrt 470,000 / 35.15
    = sqrt 13371.26
    wf = 115.63

    then i used wf=wo+alphat to find the angular acceleration
    alpha=wf / wo + t
    alpha=115.63 / 157 rad/s+2.1 s
    alpha = .7267

    then torque=I alpha
    =70.3125 (.7267)
    T=51.09 N*M

    It was wrong and I have no idea what I'm doing wrong
  2. jcsd
  3. Nov 15, 2012 #2


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    The flywheel is turning at a constant speed. How much energy does the flywheel have in this condition?
  4. Nov 15, 2012 #3


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    The problem needs to indicate some rate of angular acceleration or deceleration such as change in angular velocity over some period of time, otherwise, if there's no change in angular velocity, then as SteamKing mentioned, there's no torque.
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