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Homework Help: Moment of inertia of the sun

  1. Dec 16, 2011 #1
    I want to find the rotational energy of the sun.

    m=1.9891*10^30 kg
    r=6.955*10^5 km

    When Im using the formula for a sphere's moment of inertia: I=2/5*m*r2
    I'm getting 3.848671797*1047 km*m2

    Can i find the moment of inertia in another way? the moment of inertia needs to be in *1046 , to get the rotational energy right.

    Thank you.
  2. jcsd
  3. Dec 16, 2011 #2

    D H

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    Staff Emeritus
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    A uniform density sphere is a fairly bad model even for the earth, which is more or less solid. For a gas it is an incredibly lousy model. The sun's core contains about half of the total mass of the sun but only 1.5% of its total volume.

    Unless you want to get into astrophysics, you are going to need an empirical value. From http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html, I/MR2 for the sun is 0.059. Compare that for a uniform density sphere, where I/MR2=2/5 or 0.4.
  4. Dec 16, 2011 #3
    Thank you very much for the answer, that sorts it out. Much appreciated.
    I'm waiting with the astrophysics :)
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