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Homework Help: Finding net gravitational force magnitude on moon

  1. Sep 13, 2011 #1
    1. The problem statement, all variables and given/known data
    The drawing (not to scale) shows one alignment of the sun, earth, and moon. The gravitational force that the sun exerts on the moon is perpendicular to the force that the earth exerts on the moon. The masses are: mass of sun=1.99 × 1030 kg, mass of earth=5.98 × 1024 kg, mass of moon=7.35 × 1022 kg. The distances shown in the drawing are rSM = 1.50 × 1011 m and rEM = 3.85 × 108 m. Determine the magnitude of the net gravitational force on the moon.

    2. Relevant equations

    3. The attempt at a solution
    Solved for Fsm, which equals to 4.3359444667x1064

    After finding the sum of these two forces, which is 8.68x10128, I decided to find the square root of this and ended up getting 2.946183972531247x1064.

    This, however, was proven incorrect.
  2. jcsd
  3. Sep 13, 2011 #2


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    Staff Emeritus
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    Gold Member

    The forces aren't in the same direction, so you can't just add their magnitudes together. You need to find their vector sum.

    EDIT: Also you should really include units in all of your calculations.
  4. Sep 14, 2011 #3
    Edit: did try finding it by vector sum, but did not worked as well. I'm not going to post what number I ended up with.
    It's that, or WileyPlus's system...
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