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Homework Help: Physics Problem Involving Gravitational Force on an object based on R

  1. Nov 20, 2013 #1
    1. The problem statement, all variables and given/known data

    At what distance r inside the Earth is the gravitational force due to Earth the same as at
    the distance r = 4RE (where the latter distance is clearly outside Earth)? Assume uniform
    density of Earth.

    2. Relevant equations

    conservation of energy? TME = GPE + KE

    3. The attempt at a solution

    At first I thought I could do conservation of energy and set both GPE's equal to each other and solve for a new Radius, but this is not working out well. Is there something I need to do involving the density of Earth and the mass? I really have no idea. Thanks for any help!
  2. jcsd
  3. Nov 20, 2013 #2


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    Gold Member

    Energy conservation doesn't appear relevant to me... I'd go with Gauss.
  4. Nov 20, 2013 #3
    Hmm...I haven't learned Gauss's Law though. This wants Gravitational Force and Gravitational Potential not Electric Force and EPE, so I'm not sure how I'd apply Gauss (after googling, I saw it was a theroem for electric potential). How would you go about using Gauss's Law?
  5. Nov 20, 2013 #4


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    Gold Member

  6. Nov 20, 2013 #5
    That was perfect! Thank you!
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