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Homework Help: Mass Moment of inertia of a cylinder with 4 holes?

  1. Mar 12, 2012 #1
    1. The problem statement, all variables and given/known data

    Hi I was reviewing for my final and I came across this problem:

    Basically there is a 6in long cylinder with dia = 24in
    Given weight density: 490 lb/ft^3

    Each Hole is drilled symmetrically, each has 6in dia and equally space around a 10in dia circle concentric with the cylinder.

    This is the picture of the frontview of the cylinder: https://lh4.googleusercontent.com/-8m1r4npC7cg/T11OqGF6qhI/AAAAAAAAABY/4v2HCrEqhd4/s333/cyinder.png [Broken]

    Thanks for your time and consideration.
    Appreciate it!!!

    2. Relevant equations

    3. The attempt at a solution
    So what I would do is:
    -First get that weight density into mass density by dividing it by 32.2
    -Then find the mass of the cylinder without holes.
    -Find mass of each holes.
    -calculate the M of I of each hole about the center axis (parallel axis theorem)
    -calculate the M of I of the whole solid cylinder (no holes)
    -subtract the M of I of the holes from the whole cylinder

    Will I get the correct answer based on the technique above?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Mar 12, 2012 #2


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    Homework Helper

    That is a correct approach.

  4. Mar 12, 2012 #3
    Thanks and for the mass of the cylinder, the unit is slug right?

    Thanks again.
  5. Mar 12, 2012 #4


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    Homework Helper

    I am sorry, I am not familiar with imperial units. But Wikipedia says it is slug.
    http://en.wikipedia.org/wiki/Slug_(mass [Broken])

    Last edited by a moderator: May 5, 2017
  6. Apr 2, 2012 #5
    this only is the right way of solving it.
    Last edited by a moderator: May 5, 2017
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