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Homework Help: Gravitation and the Principle of Superposition

  1. Nov 12, 2007 #1
    1. The problem statement, all variables and given/known data
    How far from Earth must a space prove be along a line toward the Sun, so that that the Sun's gravitational pull on the probe balances the Earth's pull

    2. Relevant equations

    3. The attempt at a solution
    d1=distance from probe to Earth
    distance from sun to earth = 1.50x10^11 m
    distance from the sun to probe = 1.5x10^11 m - d1
    mass of earth = 5.98x10^24 kg = m2
    mass of sun = 1.99x10^30 kg = m3
    m1 = mass of probe

    I set the force on the probe from earth equal to the force on the probe from the sun to get:
    (Gm2m1)/d1^2 = (Gm1m3)/(d1^2-(3.0x10^11)d1+2.25x10^22)

    When I solve for d1, I do not get the answer which is 2.6x10^5 Km. It could easily be a math error, but the fact that I have to use such tedious calculations makes me wonder if I am doing the problem correctly, especially as the mass of sun, mass of earth, distance from sun to earth are not given.
  2. jcsd
  3. Nov 12, 2007 #2
    what is that equation you are using for the probe-sun distance?
  4. Nov 12, 2007 #3
    just F=Gm1m2/D^2

    I plugged in 1.5x10^11 - d1 into D^2
  5. Nov 12, 2007 #4
    i see. i would suggest you make sure you solve the equation carefully as to not make arithmetic errors. i'm pretty sure that's all this is.
  6. Nov 12, 2007 #5
    I do not believe that you are using the equations properly.

    I suggest using R_1 to represent the distance between the Earth and satellite and R_2 to represent the distance between the Sun and the satellite.

    You seem to be using 1 au-d1 to represent something here. But 1au is not the distance between the SATELLITE and anything; it is the distance between the Earth and Sun only.

    Do you see what I mean? You should end up with two unknowns.

  7. Nov 12, 2007 #6
    he's doing it correctly actually. the distance between the sun and the probe can be expressed as a difference of the earth-sun distance and the earth-probe distance.
  8. Nov 12, 2007 #7
    I'll double-check. Thanks for the help fliinghier :)
  9. Nov 12, 2007 #8
    Yes I see now. He is using two unknowns, and he eliminated one of them already.

    Kudos! It usually takes me two steps to acomplish what you have done in one! I will keep your method in mind for future problems.

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