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Homework Help: Gravitation (Preliminary Reference Earth Model)

  1. Nov 13, 2008 #1
    1. The problem statement, all variables and given/known data

    Assuming that we know the density of earth as a function of radius, we have to derive the equation for gravity as the function of radius.

    2. Relevant equations

    3. The attempt at a solution

    lets us choose density as [tex]\rho[/tex](r).
    I know that within a hollow sphere there is no gravity.
    We can assume sum of several hollow spheres with thickness dr, where r is the radius of the shell from center, and then we can integrate it.
    I know its an easy question we used to do in undergraduate study, but I am missing something and i cant figure out how it was done? Please help!
  2. jcsd
  3. Nov 15, 2008 #2


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    Hint: what is the volume of a hollow sphere, radius=r, thickness=dr?

    Given the volume and density, what would be it's mass?
  4. Nov 16, 2008 #3
    Thanks Redbelly98!

    I did this problem. However I am having another problem in an associated problem with this question. I am working it out, however if I get stuck then i would ask you to help me.

    Thanks once again!
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