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Moment of Inertia of a Disk

  1. Nov 18, 2013 #1
    I know that the moment of inertia of a disk about the axis perpendicular to the plane of the disk is

    I=1/2 MR^2

    and by the perpendicular axis theorem the moment of inertia about the other axis is

    I=1/4 MR^2

    I want to get the 2nd result with calculus. I can't get the right answer. What am I doing wrong?

    I know the moment of inertia of a rod is 1/12 MR^2
    I take the mass of a section of the disc to me pyr dx where y is the depth of the disc
    but when I integrate it gets really messy.

    What's the correct way to do this?

    http://individual.utoronto.ca/jeremyli/moi.jpg [Broken]
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Nov 18, 2013 #2
    I got it

    OH nevermind I got it.
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