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Homework Help: Ratio of net gravitational pull on moon

  1. Jan 15, 2008 #1
    [SOLVED] Ratio of net gravitational pull on moon

    A new moon is almost exactly in line between the earth and the sun. A full moon is on the opposite side of the earth from the sun. What is the ratio of the net gravitational force on the moon when it is new to when it is full?

    I tried calculating the gravitational pull on the moon by the earth and sun when the moon is in each of the two positions. I used the masses of the three bodies, the radii, and the distances between them, but I was halted in my calculations because I couldn't figure out if the distance given in the textbook between the sun and the earth was measured from the center of the bodies or not. And anyway, I feel like I'm taking the wrong way around, and that the answer should be something simple. I would really appreciate some help.
  2. jcsd
  3. Jan 15, 2008 #2


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    The value of the distance from the sun to the Earth should be as measured from their centres. Your approach is correct. You need to calculate the gravitational force from the Earth and the Sun in each position then work out the net force. Once you have the net force you can find the ratio of the net forces.
  4. Jan 15, 2008 #3
    Thanks! I was able to solve the problem!
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