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Homework Help: Moments of Inertia

  1. Nov 6, 2006 #1
    Hi, I'm in the middle of programming an inertia system and am only really just starting to appreciate what the heck inertia is :) I have been taught inertia, but I haven't actually applied it in a real situation (all my exams and tutorials have resulted in formulae giving answers like 2/7Ma^2).

    So I've written a function to find inertia of a sphere and I've plugged some numbers in. I'm a bit confused over what formulae to use for starters.

    I can sorta see that inertia of the spheres centre might be useless? given that they're totally symetrical? Am I off with this?

    For inertia of a sphere about the diamater I'm using the equation 2/5 Ma^2. Is this appropriate?

    Also I've plugged in numbers (like I said). I've never got a numerical answer for a system before so I'd appreciate some guidance as to wether or not I'm correct.

    sphere at 0,0,0
    radius 2
    mass 1

    rotation about point on diameter

    2/5 Ma^2
    0.4 * 1 * 4
    MI = 1.6 ?
  2. jcsd
  3. Nov 6, 2006 #2


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    This should be useful: http://hyperphysics.phy-astr.gsu.edu/Hbase/isph.html" [Broken].
    Last edited by a moderator: May 2, 2017
  4. Nov 8, 2006 #3
    sorry to bump an old thread, but I've done some other bits and pieces and have some results that I'd like to ask about.

    I took my mass and added a point mass 1kg to it. If m is at the 'point' of inertia the resulting answer is: 1.6kgm sq

    if m is at the opposite side of the sphere from the point of inertia the resulting answer is: 17.6kgm sq

    firstly should it be this way round? i can't help but think that the bigger number should be for the higher resistance

    also, what is the meaning of what i have found? at the end of the day, what do these numbers actually mean?
  5. Nov 10, 2006 #4
    Ok after reading through another book I've found equations defining inertia of cylinders and cubeoids. These are given as inertia in x, y and z dimensions. Basically...this wasn't what I was expecting. Is a bodies inertia usually provided as a value in X, Y, Z dimensions?

    Does inertia have nothing to do with where on the body a force to move it would act?
  6. Nov 11, 2006 #5


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    It has nothing to do with any dimensions. Moment of inertia is calculated with respect to some axis, typically x, y and z.
  7. Nov 11, 2006 #6
    thanks for the responce!...but i would asume that the width/depth/height are the values that affect its inertia in each axis? for example the inertia of a cube along the x-axis is
    \frac{1}{12} m ( b^2 + c^2)

    where m = mass, and b and c are the length of the cube along y and z axis.

    also, if inertia is calculated through XYZ axis, how is the inertia along a non axis-aligned vector found? interpollation? do you even need to find this? To be honest i've never used inertia in anything (i've just "found" the inertia) so i'm finding it hard to learn.
    Last edited: Nov 11, 2006
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