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Moment of inertia question

  1. Jun 13, 2009 #1
    can somebody please tell what i am doing wrong with this moment of area question???

    question and working in the attached doc

    thanks alot
     

    Attached Files:

  2. jcsd
  3. Jun 15, 2009 #2

    ideasrule

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    Homework Helper

    To calculate the second moment about the x-axis, you have to choose a horizontal element dA, because only a horizontal element has a constant y. I don't know why you chose vertical strips and used their centers as the y value, but your method is not right.

    To calculate Ix, express dA in terms of y and you'll get dA=(sqrt(ay)-y)dy. Integrate y^2 dA from 0 to a and you'll get the correct answer, a^4/28. I have no idea how your lecturer got to his equation for Ix, but it's probably unnecessarily complicated because the method I just described leads to the right answer very quickly.

    To calculate Iy, choose vertical elements and express dA in terms of x. Then integrate x^2 dA from 0 to a.
     
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