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Gravity force probelm

  1. Oct 23, 2010 #1
    1. The problem statement, all variables and given/known data
    When the Earth, Moon, and Sun form a right triangle, with the Moon located at the right angle, as shown in the figure , the Moon is in its third-quarter phase. (The Earth is viewed here from above its North Pole.)
    Find the magnitude of the net force exerted on the Sun.
    Find the direction of the net force exerted on the Sun. Give the direction relative to the line connecting the Sun and the Moon.

    2. Relevant equations
    [tex]\theta[/tex]= inverse trig (whatever necessary)
    Mass sun=1.99E30
    Mass earth= 5.98E24
    mass moon= 7.35E22
    radius sun to moon=1.50E11
    radius moon to earth=3.85E8
    3. The attempt at a solution
    F1(Sun-Moon)=4.336E20 found by using gravity force eq.
    F2(sun earth)= 3.53E22, however this is wrong because it should be 3.58E22, so that means the distance between the sun and the earth at this time would have to be 1.49E11, which doesn't make since because this should be the hypotenuse, right???. So when using the sencond value for F2, and using hte pythagorean theory I get Fnet=3.57E22, which is the right answer.
    NOW to find the direction, i thought you can use the cos inverse *F1/F2, but this turns out to be the wrong answer I have no idea what else to do. Please help!
  2. jcsd
  3. Nov 7, 2010 #2
    I would've done cos-1 also. Did you ever find out the correct answer? I had this exact problem, only I was tasked with finding Fnet exerted on the moon. I found my angle by taking tan-1 of F3/F1, where my value for F3 was 1.9814E20, the force of earth-moon. F1 is obviously sun-moon, only the value I calculated was 4.3438E20. I got the correct answer.
  4. Nov 7, 2010 #3
    yes finding the net force on the moon was easy, but still never found out for the sun
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