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Rotational motion -- Energy stored in a flywheel

  1. Dec 16, 2014 #1
    1. The problem statement, all variables and given/known data
    Cylindrical shape pulley ( m = 6 kg, R = 0.18 m) is rotating at a frequency f = 10 s-1. Due to constant torque it stops. Calculate the work done by the breaking force.

    2. Relevant equations
    w = 2πf

    3. The attempt at a solution

    I can calculate the angular velocity:
    w = 2πf = 62.8 rad/s

    I don't know what to do now.
     
  2. jcsd
  3. Dec 16, 2014 #2

    berkeman

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    Staff: Mentor

    (I added to your thread title for you...)

    How much energy is stored in that spinning flywheel? What is its moment of inertia?
     
  4. Dec 16, 2014 #3
    I don't know, it is not given. I think I'm suppose to find some kind of ratio.
     
  5. Dec 16, 2014 #4

    berkeman

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    Staff: Mentor

    Calculate it. Look up the formula for the moment of inertia for a cylinder in your textbook or use Google or Wikipedia...
     
  6. Dec 16, 2014 #5
    Here is moment of inertia for a cylinder:

    I = 1/2 MR2 = 1/2 * 6 kg * 0.18 m = 0.54 kg/m
     
  7. Dec 16, 2014 #6

    berkeman

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    Staff: Mentor

    And what is the energy stored in that rotating cylinder? Once you have the energy stored in the rotating cylinder, how much work would it take to slow it down and stop it?
     
  8. Dec 16, 2014 #7
    Energy stored in a rotating cylinder:

    Erotational = ½Iω2 = 1065 J

    So in order to stop this cylinder one must do 1065 J work?
     
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