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Gravitational force equation help

  1. Jan 22, 2009 #1
    How high above the surface of the earth should a rocket be
    in order to have 1/100 of its normal weight? Express your answer
    in units of earth radii.

    Im not sure were to start with this one. I know that the moon is 60 times as far away as the core of earth to the surface.
  2. jcsd
  3. Jan 22, 2009 #2
    Use the gravitational force equation

    [tex]F = G\frac{Mm}{r^2}[/tex]


    F = mg

    Setting the two equations equal to each other provides the acceleration due to gravity as a function of the distance, r, from the center of the earth.
  4. Jan 22, 2009 #3


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    Homework Helper

    You'll need this formula


    Where M is the mass of the Earth and m is the mass of the object.
  5. Jan 22, 2009 #4
    how do you get G or calculate the mass of the earth?And which one of you is right? Yor saying two diffrent things.
  6. Jan 22, 2009 #5
    Mass of the earth and G are found in any physics book; usually in an appendix.
  7. Jan 22, 2009 #6
    i found the mass of earth 5.9742*10to the 24 power and the radius but how do i know the mass of the rocket? Or G?
  8. Jan 22, 2009 #7
  9. Jan 22, 2009 #8
    The mass of the rocket, m, is not required. When setting F = mg equal to the gravitational force equation the m on each side cancels.

    G is the universal gravitational constant. You should be able to find the value in the physics book or on line easily.
  10. Jan 22, 2009 #9
    You already have the answers you need, G is a well known physical constant, get it from a textbook or find it online. And as for the mass of the rocket, equate the equations given by chrisk and you will discover why it is irrelevent.
  11. Jan 22, 2009 #10
    1 6.67300 × 10-11 m3 kg-1 s-2 i hope i made it look right. So what do i put in for Kg and s and m?
  12. Jan 22, 2009 #11
    The appropiate units will cancel. Make sure the units for the earth radius is in meters.
  13. Jan 22, 2009 #12
    i got 24485606.14 thats not right is it?
    Last edited: Jan 22, 2009
  14. Jan 22, 2009 #13
    I have not done the calculation but express the value in earth radii then subtract one earth radius from this value to give the height above the earth surface.
  15. Jan 22, 2009 #14
    What i did to get G is 6.67300 × 10-11
  16. Jan 22, 2009 #15
    i got 383.9 kilometers above earth.
  17. Jan 22, 2009 #16
    Thats not right, and you still haven't expressed it in units of earth radii.
  18. Jan 22, 2009 #17
    im confused
  19. Jan 22, 2009 #18
    i looked up the real answer from a website and it said the answer is 57,402 and 9 earth unit radii.
  20. Jan 22, 2009 #19
    That is the correct answer, if you still don't understand try posting your workings and I'll show you where you have gone wrong.
  21. Jan 22, 2009 #20
    first i did F= G*M/r2
    G=6.67300 × 10-11
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