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Homework Help: Understanding Newton's Law of Universal Gravitation

  1. Oct 14, 2007 #1
    "Consider the earth following its nearly circular orbit about the sun. The earth has a mass mearth=5.98x10^24kg and the sun has mass msu=1.99x10^30kg. They are separated, center to center, by r=93 million miles = 150 million km."

    What is the size of the gravitational force acting on the earth due to the sun?

    I'm still setting up the problem...

    I think I use the equation Fg=G(m1m2 / r2).

    r is the distance between the centers of the two objects.
    G is the gravitational constant (so I just leave that as G, right?)

    But, is the sun or the earth m1?
  2. jcsd
  3. Oct 14, 2007 #2
    Wait... I just realized how stupid of a question that is. Why do I do this when I'm so tired?
  4. Oct 14, 2007 #3
    K, so now for a better question!

    Fg=G(5.98x10^24kg x 1.99x10^30kg / 150,000,000 km)
    Fg=G(7.933 x 10^46)

    Am I on the right track? What do I do with the G?

    I know that the gravitational constant G has a value G=6.67x10^-11 N x m2/kg2

    If I plug the mass of the sun in as m2 it is...

    G=6.67x10^-11 N x 1.99x10^30kg/kg2

    but... now I'm lost. :(
  5. Oct 14, 2007 #4
    Fg=(6.67x10^-11 N x 1.99x10^30kg / kg2) (7.933 x 10^46)

  6. Oct 14, 2007 #5

    The formula for gravitation is F=GMm/R^2. Some people may prefer to rewrite it as F=GM1M2/R^2. Just need to take note that M1 or M is the primary mass and M2 or m is the secondary mass. In this question, the mass of Sun is taken to be M1 or M.

    Data used:
    Gravitational constant = 6.67x 10^-11 Nm^2kg^-2 = G
    Mass of Sun = 1.99x 10^30 kg = M
    Mass of Earth = 6.02x 10^24kg = m
    Distance between Sun and Earth = 1.5x 10^11m = R

    Force acting on Earth by Sun = GMm/R^2
    = 3.39X 10^22 N
  7. Oct 14, 2007 #6
    Thank you! :)
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