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Homework Help: Moment of inertia

  1. Mar 10, 2015 #1
    1. The problem statement, all variables and given/known data
    There are four solid balls with their centres at the four corners of a square of side a. The mass of each sphere is m and radius is r. Find the moment of inertia of the system about one of the sides of the square.

    2. Relevant equations

    3. The attempt at a solution
    I= 2 (two balls) × [ 2/5 mr^2 (Icm) + ma^2 ] (just used parallel axis theorem)

    However, in the answer,
    I = 2 ( 2/5 mr^2 ) + 2×[ 2/5 mr^2 + ma^2 ]

    Why have they added 2 ( 2/5 mr^2 ) ??
  2. jcsd
  3. Mar 10, 2015 #2


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    Using the parallel axis theorem is good. I don't see it under 2. What does it look like and for what balls do you apply it ?
  4. Mar 10, 2015 #3


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    Consider the above two lines.
  5. Mar 13, 2015 #4
    Ok, right!!
    I got it, thanks. I didn't consider moi of the two balls along the axis.
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