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Homework Help: Rotational inertia

  1. Apr 19, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    I assume the energy stored is = 1/2 (I) (ω^2)

    I (moment of inertia) is MR^2 since it's a hoop? or is it a solid cylinder?

    do we need to convert the rpm (revolutions per minute) to radians per sec?
  2. jcsd
  3. Apr 19, 2015 #2
  4. Apr 19, 2015 #3
    You're on the right track, just realize that a disc is a cylinder with vanishing height.

    Otherwise yeah, try it out and report back on what you end up with. It'll be important to do a sanity check on the answer that you get - that'll tell you whether you got the right answer or not most of the time.
  5. Apr 19, 2015 #4
    No, that's a thin disk. A disk is a solid cylinder.

    As you defined it, ω is angular velocity, which is typically measured in radians per second.
    Last edited: Apr 19, 2015
  6. Apr 19, 2015 #5
    Sorry, I should have been explicit - I meant a solid cylinder.
  7. Apr 19, 2015 #6
    after doing all the math, answer came out to be 1.4 x 10^6 Js.
  8. Apr 19, 2015 #7
    So right away there's something wrong - the units should be J not J##\cdot##s, unless you meant Joules :P

    Does that answer make sense? In the context of the problem, does that seem reasonable?
  9. Apr 19, 2015 #8
    yes, i meant joules.
  10. Apr 19, 2015 #9


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    Seems too low. Check your conversion from revolutions to radians.
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