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Homework Help: Moment of inertia

  1. Nov 10, 2012 #1
    What is the differece between the two formulas numbered 3 and 6 in the paint document?

    And what types of questions would I use eq. 3 and eq. 6?

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  2. jcsd
  3. Nov 10, 2012 #2


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    The moment is force * perpendicular distance. Summing gives ∫F(x)x.dx. When F is gravitational and the mass at offset x is y=y(x), that's ∫gyx.dx
    The moment of inertia is the factor converting rotation rate, ω, to angular momentum (sometimes called moment of momentum). An element at distance x from the axis is moving at speed xω so has linear momentum xω.dm. The moment of that is x2ω.dm. So this leads to ∫yx2.dx.
    (Sorry, just realised I've swapped x and y c.w. the link you posted. Can't be bothered to swap back.)
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