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Homework Help: What is the moment of inertia of the flywheel?

  1. Apr 3, 2014 #1
    1. The problem statement, all variables and given/known data

    An energy storage system based on a flywheel (a rotating disk) can store a maximum of 4.4 MJ when the flywheel is rotating at 21,300 revolutions per minute. What is the moment of inertia of the flywheel?

    2. Relevant equations

    K= Ktranslational + Krotational
    Krot= (1/2)Iω^2

    3. The attempt at a solution

    I solved 21,300 rev/min to be equal to 2.23E3 rad/s
    I then tried to use this as angular acceleration in the Krot equation to solve for moment of inertia and got 1.77E-3 kgm^2, which was incorrect. I'm wondering if I need to set 4400J= Ktrans + Krot, to solve for I (moment of inertia) but if so I can't figure out how to solve for velocity in the Ktrans equation.
  2. jcsd
  3. Apr 3, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor


    Looks like you're off by a few powers of 10. Do that calculation over.

    No, you're on the right track.
  4. Apr 3, 2014 #3
    I tried again and got the same answer. I'm sure I have a stupid mistake somewhere
    Is it incorrect that 4.4MJ = 4400 J?
  5. Apr 3, 2014 #4
    4.4MJ = 4.4x10^6 J. 4400J is 4.4x10^3 J, or 4.4 kJ. :)
  6. Apr 3, 2014 #5
    WOW. I might need to take a break. Thank you for the correction help!
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