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Rotational Motion Problem - 9

  1. Dec 16, 2013 #1
    Hi friends,
    Please help me in solving this problem, I'll appreciate the help.

    The problem is as:

    https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-ash3/q71/s720x720/1467427_1461727597387679_1141225220_n.jpg

    Attempt -

    https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-ash3/q71/s720x720/1472752_1461727710721001_1917071548_n.jpg

    Thank you all in advance.
     
  2. jcsd
  3. Dec 16, 2013 #2

    haruspex

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    You treated the rod furthest from the axis as a point mass at its centre.
    Also, I think you have confused the mass per rod with the total mass.
     
  4. Dec 17, 2013 #3
    So what will with the farthest rod Should I take its moment of inertia as ml2/12(about center) + m(l√3/2)2

    If I do so,

    The total M.I. is found to be (3/2) ml2

    Which gives radius of giration as l√(3/2)

    But the answer is l/√2
     
  5. Dec 17, 2013 #4

    haruspex

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    Yes.
    That's the other error I mentioned. If each rod has mass m then the entire structure has mass 3m. If its radius of gyration is k then its moment of inertia will be 3mk2.
     
  6. Dec 17, 2013 #5
    Well, I did a mistake in reading the question also. I thought that I have to calculate the moment of inertia of the system about the axis parallel to the plane of the structure.

    If I have to find the M.I. of the system about an axis parallel, So what would the M.I of the farthest rod?
    In that case should I treat it as a point mass situated at the center of the rod?
     
  7. Dec 17, 2013 #6

    haruspex

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    Yes.
     
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