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How to solve this ratio/proportionality question?

  1. Nov 17, 2014 #1
    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. Assume that the distance from the Earth to the Moon is 380,000 km.
    Other given/worked out previously: mass of moon = 7.35 x10^22kg, surface gravitational field strength of the moon: 1.62Nkg^-1, radius of the moon = 1740km (3sf)

    g at the earth's surface is 9.8.
    g at moon's surface I worked out using the formula g = GM/r^2,g * mass of moon/ distance from moon to earth sqred = 3.40x10^-5 (3sf)

    My big problem is the ratios. How do I show that its 3/1000000 of the grav. pull of the Earth? I know that grav pull of the earth on oceans will be 9.8 and the moon on the ocean will be 3.40x10^-5, but I just simply do not know how to do physics ratio questions whatsoever. I am terrible at them... How do I approach ratio/ proportionality question such as this one? What is the process?
  2. jcsd
  3. Nov 17, 2014 #2
    You're nearly there, what do you get if you divide the moon's pull on the ocean, by the Earth's?
  4. Nov 17, 2014 #3
    Okay that was too easy. Thanks.
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