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Moment of inertia for rectangular plate

  1. Apr 17, 2011 #1
    I've got a problem with this:
    Count moment of inertia for rectangular plate a x b, if you know that moment of inertia of thin rod is [tex] \frac{1}{12}ml^2 [/tex]. Do not use integrals!, others mathematical functions required (I can proof this moment by integrals, but this is not issue). I know that I have to use Steiner theory, but how? No integrals? :(
    Please help.
  2. jcsd
  3. Apr 17, 2011 #2
    Consider the plate as made of several paralel slices.
  4. Apr 18, 2011 #3
    Ok, I know that, but the problem is: how can I count it without integrals? I must sum up all the slices' distance from axis, but how am I supposed to do it without integrals?
  5. Apr 18, 2011 #4
    let us consider a rectangular plate to be a x b dimensions of total mass m

    let x axis be along the length direction and y be along width direction and origin be at the center of plate.
    consider it to have 'n' parallel slices (n being very large number) along x direction so that each slice a like a rod of mass m/n and length 'a'
    Moment of inertia of each rod along x axis, I = m(a^2)/(12n)
    Moment of inertia of plate along x aixs =Ix = n I = m(a^2)/(12)
    ( this is because u have n slices)
    similary if u repeat above exercise along y direction
    Moment of inertia of plate along y aixs = Iy = m(b^2)/(12)

    Moment of inertia of plate along z aixs = Iz = Ix + Iy
    = m(a^2)/(12) + m(b^2)/(12)
    = m( a^2 + b^2)/12

    note : we havent used any integrals..its just addition
    Last edited: Apr 18, 2011
  6. Apr 18, 2011 #5
    There is one mistake, I think.
    One slice has of course [tex] I = m(a^2)/(12n) [/tex] but not by this axis! the axis is in the centre of plate and every slice has [tex] I = m(a^2)/(12n) + \frac{m}{n} * r^2 [/tex] from Steiner theory when r ist distance from axis.
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