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Planet X

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data
    An astronaut weighs 672 N on the Earth. What is her weight on planet X, which has a radius Rx = Rearth / 2.00 and a mass Mx = Mearth/9.00?
    I know that the radius of Planet X is 3189.05
    and that the mass of Planet X is 6.638x10^23

    2. Relevant equations
    so i know that i need to find the gravity for planet X found with this equation gx = GM/R^2 and also that the eequation to find her mass is F=Mm/r^2 (at least that is what i think)



    3. The attempt at a solution
    so i tried using that equation but it was wrong and then i figued out i prob needed the gravity but did not know exactly how to use it cause what do i use G for in the initial equation?
     
  2. jcsd
  3. Oct 13, 2008 #2

    nicksauce

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    It's not entirely clear to me what you did, so I'll start from the top. First of all, the formula for force due to gravity is F = GMm/r^2. So you have two equations[tex]W_{e}=\frac{GM_{e}m}{R_{e}^2}[/tex], and [tex]W_{x}=\frac{GM_{x}m}{R_{x}^2}[/tex]. Try taking a ratio of Wx and We and the answer should pop out.
     
  4. Oct 13, 2008 #3
    but how do i find out what G is for planet X? that is the part that is confusing me
     
  5. Oct 13, 2008 #4

    nicksauce

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    G is the same everywhere in the universe. It is a fundamental constant of nature.
     
  6. Oct 13, 2008 #5
    ok so G is not gravity then i am guessing. what is G exactly then? like what is its number
     
  7. Oct 13, 2008 #6

    nicksauce

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  8. Oct 13, 2008 #7
    YES! got the answer. thanks man. i actually have a few more and if you have time that would be awesome. i will post them now.
     
  9. Oct 13, 2008 #8

    nicksauce

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    No problemo.
     
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