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Homework Help: Find the moment of inertia of a solid sphere.

  1. Jul 9, 2012 #1
    1. The problem statement, all variables and given/known data

    Beginning with Icm = Integral of r^2 dm from r1 to r2, find the moment of inertia of a solid sphere about any tangential axis.

    2. Relevant equations

    Icm = Integral of r^2 dm

    3. The attempt at a solution

    I set up the infinitesimally mass of an infinitesimally thin "shell" of the sphere:

    dm = 4ρπr2 dr

    And then solved for the moment of inertia:

    I = ∫r2dm

    = ∫r2(4ρπr2 dr)

    = 4ρπ∫r4 dr

    = (4/5)ρπr5

    And solving for ρ we get the following:

    ρ = M/((4/3)πr3).

    Substituting that into the previously solved equation for I, I get the following:

    I = (3/5)Mr3.

    What am I doing wrong? I know the formula involves a coefficient of 2/5, not 3/5, but I can't find my problem.

    Thank you in advance!
  2. jcsd
  3. Jul 9, 2012 #2
  4. Jul 9, 2012 #3
    I saw that, the only issue is I am supposed to start with the above formula. Icm = Integral of r^2 dm
  5. Jul 9, 2012 #4


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    I believe your problem is that you are misinterpreting the meaning of "r" in Icm = Integral of r^2 dm. It does not represent the distance from the mass element dm to the origin of your coordinate system. Rather it represents the perpendicular distance from dm to the axis of rotation. So, if the axis of rotation is the z-axis, then r is the distance from dm to the z-axis.

    That's why it's preferable to write r[itex]_{\bot}[/itex] as in the link that azizlwl gave.

    You can't find Icm of the sphere by doing a whole shell at a time, because different mass elements of the shell are at different distances from the axis of rotation.
  6. Jul 9, 2012 #5


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    The problem asks you to "find the moment of inertia of a solid sphere about any tangential axis."

    I assume that you are not allowed to use the parallel axis theorem for this.
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