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Homework Help: Proofing Moment of Inertia Solid Sphere

  1. Mar 31, 2013 #1
    1. The problem statement, all variables and given/known data
    Can i use "pyramid" method to derive the equation of Moment Inertia solid sphere?
    The pyramid is such we slice a watermelon.
    Sorry for my bad english.

    2. Relevant equations

    2/5 MR^2

    3. The attempt at a solution
  2. jcsd
  3. Mar 31, 2013 #2
    Hello chocophysuny,
    Welcome to Physics Forums,

    Have you tried it? You must show what you have tried to get help on PF (forum rules).

  4. Apr 1, 2013 #3


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    It doesn't seem to me that it would be a useful approach. It does not simplify anything. You need to slice it into elements that have an easily calculated MI, such as discs perpendicular to the axis of rotation.
  5. Apr 2, 2013 #4
    But my lecture say this method is easy enough to solve.
    And, actually i confused to find the element of volume and element of MI.

    So, in your opinion, this is not solvable?
  6. Apr 2, 2013 #5
    Yes, I've tried it. But i didn't find the solution.
    I'm confused with the element of volume and element of pyramid

    "To find MI from the element, we need element of volume and element of MI, isn't it?"
  7. Apr 2, 2013 #6
    I'm sorry, I didn't understand what you meant there.
  8. Apr 2, 2013 #7


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    Maybe I misunderstand the proposed method. From your watermelon analogy, I assumed it involves slicing the sphere into thin wedges using planes through the axis of rotation. If so, I think you would then need to cut each wedge into laminae, either rectangular ones parallel to the axis or triangular ones orthogonal to the axis.
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