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Homework Help: How does the earth exert gravitational pull on the sun

  1. Mar 4, 2013 #1
    1. The problem statement, all variables and given/known data

    My physics teacher today said that the earth exerts the same amount of gravitational pull on sun;as the sun exerts on the earth.
    Is this really true?
    2. Relevant equations

    There is no equation.

    3. The attempt at a solution
    I am very confused with this. How can the earth's gravitational pull equal that of a massive body like the sun?
    Can somebody explain this to me please?
  2. jcsd
  3. Mar 4, 2013 #2
    Think about Newton's third law.
  4. Mar 4, 2013 #3
    Why don't you work out the 2 forces and compare them?
  5. Mar 4, 2013 #4
    The forces are equal but acting on different masses.
  6. Mar 4, 2013 #5


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    The force between two bodies is.. GmM/r2

    where m and M are the two masses and r the distance between them. Note how the force depends on both masses not just that of one of them.

    The acceleration due to gravity is only dependent on the mass of one body..

    F = GmMr2
    F = mg
    gm = GmMr2
    mass cancels
    g = GMr2

    So g on the surface of the earth or sun depends only on the mass of that body not the mass of both.

    The acceleration due to gravity on the surface of each is roughly..

    Earth = 9.81 m/s2
    Sun = 275 m/s2
  7. Mar 4, 2013 #6
    A little typo there. And G is gravitational constant.
  8. Mar 4, 2013 #7


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    Oi who's been stealing my / I'm sure they were there earlier :-)
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