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Homework Help: Comparing gravitational acc at surface of 2 diff planets

  1. Dec 23, 2008 #1
    1. The problem statement, all variables and given/known data
    Planet X has five times the diameter and
    seven times the mass of the earth.
    What is the ratio gX : ge of gravitational
    acceleration at the surface of planet X to the
    gravitational acceleration at the surface of the

    Well, I used the equation:
    [tex]\frac{Fplanetx}{FplanetE}[/tex]= [tex]\frac{Mx}{Me}[/tex] * ([tex]\frac{Rx})({Re}[/tex])2
    i decided to use variables:
    Mx= 7Me
    Rx= ((2Re)*5)/2= 5
    plugging in:
    i got 175...but none of the answers have that big of a number
    I would really appreciate it if someone could help me figure out what I did wrong.
  2. jcsd
  3. Dec 23, 2008 #2


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    Homework Helper

    You should get
    [tex]\frac{g_x}{g_E}= \frac{7GM_E}{25R_E^2} \times \frac{R_E^2}{GM_E}[/tex]
  4. Dec 23, 2008 #3
    ohh i see what i did wrong.. Thanks for your help! :)
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