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Homework Help: Question on gravitation(low-altitude satellite)

  1. Oct 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A low-altitude satellite near the earth's surface has a speed of 7.90 km s-1. The radius of the earth is about 4 times that of the moon and the ratio of the average density of the earth to that of the moon is about 5:4. The speed of a low-altitde satellite near the moon's surface would be

    A. 1.77km s-1
    B. 2.21km s-1
    C. 2.47km s-1
    D. 3.57km s-1
    E. 4.42km s-1

    2. Relevant equations
    v2 = GM/r

    3. The attempt at a solution
    i got the ans of 3.12km s-1
    the ratio of the mass of earth to moon would be 80:1
    n the ratio of radius of earth to moon is 4:1
    so i guess it's 7.92/20 ?

    Pls help, i dunno what i did wrong :(
    Thz so much
  2. jcsd
  3. Oct 24, 2009 #2
    Look at your equation again. What variable does 7.9^2/20 represent?
  4. Oct 24, 2009 #3
    7.9 is the speed of satellite near earth's surface, and 20 is the ratio of the mass and radius combining, (1/80)*4
  5. Oct 25, 2009 #4
    Not the individual terms, what variable does your answer aka 7.9^2/20 = 3.12 represent? Look carefully; is it the speed of the satellite?
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