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  1. #1
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    Question Maths/astronomy help!

    For a star of constant density, calculate the fraction of the total mass inside a core of radius R/4, if the radius of the star is R.

    I have no idea.

    Thanks for any help.
    Last edited by emmaastronomy; April 27th 2009 at 10:05 AM.
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  2. #2
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    Quote Originally Posted by emmaastronomy View Post
    For a star of constant density, calculate the fraction of the radius R/4, if the radius of the star is R.

    I have no idea.

    Thanks for any help.
    This question is incomplete. Please re-check it and then post the complete question.
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  3. #3
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    Quote Originally Posted by emmaastronomy View Post
    For a star of constant density, calculate the fraction of the radius R/4, if the radius of the star is R.

    I have no idea.

    Thanks for any help.
    Sorry, but I have no idea what "calculate the fraction of the radius R/4" means. Since you mention density, do you mean calculate the mass out to radius R/4 or the fraction of the total mass that is? Do you know the formula for volume of a cube? Mass is just density times volume.
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  4. #4
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    Quote Originally Posted by HallsofIvy View Post
    Sorry, but I have no idea what "calculate the fraction of the radius R/4" means. Since you mention density, do you mean calculate the mass out to radius R/4 or the fraction of the total mass that is? Do you know the formula for volume of a cube? Mass is just density times volume.
    Yes you are right. It should read For a star of constant density, calculate the fraction of the total mass inside the core of radius R/4, if the radius of the star is R.
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  5. #5
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    Quote Originally Posted by emmaastronomy View Post
    Yes you are right. It should read For a star of constant density, calculate the fraction of the total mass inside the core of radius R/4, if the radius of the star is R.
    Start with the facts that mass = density times volume and that the volume of a sphere is \frac{4}{3} \pi r^3.
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    Quote Originally Posted by mr fantastic View Post
    Start with the facts that mass = density times volume and that the volume of a sphere is \frac{4}{3} \pi r^3.
    So to find the mass of the core = (density x \frac{4}{3} \pi r^3 ) / 4

    Is that even right? And how could i get a fraction out of that without figures?
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  7. #7
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    Quote Originally Posted by emmaastronomy View Post
    So to find the mass of the core = (density x \frac{4}{3} \pi r^3 ) / 4

    Is that even right? And how could i get a fraction out of that without figures?
    You have to substitute r = R/4 to get the mass of the core.

    You have to substitute r = R to get the total mass of the star.

    To get the required percentage, you have to calculate (core mass)/(total mass) times 100.
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