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Homework Help: Gravity problem

  1. Dec 10, 2009 #1
    1. The problem statement, all variables and given/known data
    Determine the distance from earth's centre where the force of gravity acting on a space probe is only 11% of the force acting on the same probe at the earth's surface. express your answer in terms of earth's radius,


    2. Relevant equations
    F = Gm1m2/R^2


    3. The attempt at a solution
    i tried plugging in the values but i cant seem to get the answer
     
    Last edited: Dec 10, 2009
  2. jcsd
  3. Dec 10, 2009 #2

    ideasrule

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    Write down two versions of the gravitation equation:

    Fs=Gm1m2/R^2
    Fp=Gm1m2/r^2

    where Fs is the force of gravity on Earth's surface and Fp is the force of gravity put in space. Fp/Fs = 0.11, as the question states. See what you can do with that.
     
  4. Dec 10, 2009 #3

    rl.bhat

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    The relevant equation given by you is for the force on the surface of the earth. If h is the distance of the probe from the earth's surface, what is its distance from the center of the earth? What is the force on it? take the ratio of these forces and equate it to the given value and solve for h.
     
  5. Dec 10, 2009 #4
    i don't understand

    h/h' = 0.11?

    then what can i do?
     
  6. Dec 10, 2009 #5

    rl.bhat

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    terms of R, what is h and h' for Fs and Fp?
     
  7. Dec 10, 2009 #6
    h = 1r
    h' = 2r
     
  8. Dec 10, 2009 #7

    rl.bhat

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    No.
    h is r and h' = r + H where H is the height of the probe from the surface of the earth. find H.
     
  9. Dec 10, 2009 #8
    so it would be r/r+h = 0.11?
    how do i solve with 2 unknowns?
     
  10. Dec 10, 2009 #9

    rl.bhat

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    You have to write h in terms of r.
     
  11. Dec 10, 2009 #10
    this is so confusing can you give me another hint?
     
  12. Dec 10, 2009 #11
    Did anyone get the answer so i can compare?
     
  13. Dec 10, 2009 #12
    give us the radius and mass of earth ur using
     
  14. Dec 10, 2009 #13
    The question doesn't give it
     
  15. Dec 10, 2009 #14

    rl.bhat

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    r/(r+h) = 0.11
    r = 0.11(r+h)
    Now simplify and find h.
     
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