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Mass Moment of inertia of a cylinder with 4 holes

  1. Mar 11, 2012 #1
    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
    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?
    Is there any mistake?

    Thanks for your time and consideration.
    Appreciate it!!!
  2. jcsd
  3. Mar 17, 2012 #2


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    Gold Member

    What you are given is a specific weight, 490 lb/ft^3, and I would suggest that you work with that value until the last step, converting to mass only in the last step. This will preserve a little bit of accuracy (less rounding).

    Otherwise, your procedure is fine; go to it!
  4. Mar 18, 2012 #3
    yes,,,, go ahead
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