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Homework Help: Moment of inertia (disk with off center hole)

  1. Mar 3, 2009 #1
    I have a disk with some thickness to it and I need its moment of inertia.
    So this is the formula with r1=0

    Now there is a "circular hole of diameter 'd' at a distance of 'r' from the geometric center of the disk." So I'm thinking that I should subtract the MoI of the hole from the disk. Here is a picture if you need it.

    http://img99.imageshack.us/img99/782/50817033.jpg [Broken]

    I could use the same MoI equation as I did for the disk on the hole but that doesn't account for the hole being off centered. Any ideas??

    I need to eventually equate this to a small mass rotating around a center point, so if there is a more direct approach please tell me. Thanks in advance for the help.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 3, 2009 #2


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    Welcome to PF!

    Hi Imperitor ! Welcome to PF! :smile:
    That's right! :smile:

    And then use the parallel axis theorem to find the moment of inertia about an axis not through the centre of mass. :wink:
  4. Mar 3, 2009 #3
    Thanks. I really should have known that...

    I'm really enjoying this forum. It's a great resource for an engineering student.
  5. Mar 3, 2009 #4
    Wait a minute... hit another snag. To use that theorem I need to know what the mass of the missing hole is. I only know the mass of the disk as is (with the hole in it). Also the first formula I gave could only be used if I knew what the mass of the disk was with the hole filled in. This is driving me nuts. I can't even start working on the question I'm doing without getting this MoI.
  6. Mar 4, 2009 #5


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    just got up… :zzz:
    Easy-peasy …

    divide the given mass by the area, to give you the density :wink:

    then multiply by each area separately! :smile:
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