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Homework Help: How to devise moment of inertia formula of solid sphere?

  1. Oct 24, 2015 #1
    1. The problem statement, all variables and given/known data
    how to devide moment of inertia of solid sphere about its central axis?. Solid sphere has radius R, mass M.

    2. Relevant equations

    2/5 MR^2
    3. The attempt at a solution
    i try to imagine that solid sphere is a group of infinite disk. a disk have volume dv=πr2dx.
    I=16/15 πρR5
    I=4/5 (4/3πρR3)R2

    where my eror? i have spent 2 days to solve it but i am failed until now.

    Attached Files:

  2. jcsd
  3. Oct 24, 2015 #2


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    In your first integral, you introduce an extra factor r2. I assume this is related to the moment of inertia of a disk about its axis. Have you forgotten something there?
  4. Oct 24, 2015 #3
    i think this is just subtitution for dm=ρπr2dx. i'm confused
  5. Oct 24, 2015 #4


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    No, in the next line you have another r2 factor.
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