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A question on Shell Theorem

  1. Oct 4, 2013 #1
    Imagine all the mass of the earth is in a shell of a thickness of R(earth)/2. So if the object is inside the shell or outside the shell, I know I can apply the shell theorem to solve the gravitational force acting on it. But, what if the object is IN the shell, in another words R(earth)/2<r<R(earth), how do we calculate the gravitational force?
  2. jcsd
  3. Oct 4, 2013 #2


    Staff: Mentor

    so its like you have two shells. The object is outside one shell and inside the other. Apply the shell theorem to each one and what do you get.
  4. Oct 4, 2013 #3
    Oh! So we only care about the smaller shell with radius= R/2. The distance from mass m to the center is r so basically we have the same formula as for an object outside the bigger shell= (GMm)/r^2 (only the r is different is the two cases)?
  5. Oct 4, 2013 #4


    User Avatar
    Science Advisor
    Gold Member

    M is also different. After all, you don't care about the part of the shell above the object.
  6. Oct 4, 2013 #5
    Oh yes! I really forget the M. Thank you very much!
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