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Gravitational field strength problem- help needed

  1. Sep 13, 2011 #1
    gravitational field strength problem-urgent help needed

    1. The problem statement, all variables and given/known data
    Moon has a radius of 1740km, Mass of 7.35 x10^ 22kg, grav field strength of moon= 1.62Nkg^-1, distance from Earth to Moon is 380000km G=6.67x10^-11Nm^2kg^-2

    The Moon's gravitational pull on the Earth causes the ocean tides. Show that the gravitational pull of the Moon on the Earth's oceans is approximately 3 millionths of the gravitational pull of the Earth on its oceans.


    2. Relevant equations
    g = GM / r^2 g = gs R^2 / r^2 where R= radius of planet, r = distance

    3. The attempt at a solution
    g = gs R^2 / r^2
    1.62 = gs 1740x10^3 / 380000 x 10^3
    (1.62 x 380000x10^3) / 1740 x 10^3 = gs

    1/3000000= 3 x10^-6
    3 x10^-6 x 9.81 = gs ???
    Please help!
     
    Last edited by a moderator: Sep 13, 2011
  2. jcsd
  3. Sep 13, 2011 #2

    BruceW

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    Homework Helper

    Re: gravitational field strength problem-urgent help needed

    I don't think you've done it quite right, but you're not far off. You equations are a little confusing also.

    I would first try to calculate the gravitational field at earth due to the mass of the moon (as an equation). And then calculate the gravitational field at earth's surface due to the mass of the earth (as another equation). Then divide one by the other to get the ratio.
     
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