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Moment of Inertia, working backwards

  1. Oct 21, 2014 #1
    I've to work out b & d in this equasion.


    I know that the ratio between b & d in this case is 1.166 and im given the value of I.

    It is an engineering problem as the software will only allow the use of square/rectangular or round section but we are using H section and we are allowed to substitue a rectangle of the same h/w ratio and moment of inertia.

    This kind of thing was never what I was much good at although I believe it may be possible to work out but I dont know how.
  2. jcsd
  3. Oct 21, 2014 #2


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    Staff: Mentor

    Do you mean ##\frac{b}{d} = 1.166##? Then ##b = 1.166d##, put that into your equation for the moment of inertia, and solve for ##d##.
  4. Oct 21, 2014 #3
    Yes , I managed to figure it out a few min ago.

    Ixx = (D (1.116) * D ^ 3) / 12

    D^4 = I * 12 * 1.116

    D = 4√ I * 12 * 1.116
  5. Oct 21, 2014 #4


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    Science Advisor

    In your original post you used "d". In your response you used "D". It is a bad idea to switch symbols like that.
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