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I Forces of gravitation of moon and earth + Newtons 3rd law

  1. Oct 7, 2016 #1
    In relation to two objects with a gravitational field on them (i.e. a planet and its moon), would there be two pairs of partner forces (the force of gravity exerted from each object and the resulting partner force from each force of gravity)? For example with the earth and moon:

    1. Force of gravity exerted on the moon by the earth = - Force exerted on earth by moon
    2. Force of gravity exerted on the earth by the moon = - Force exerted on moon by earth

    Or is the force of gravity exerted by the moon on earth = - force of gravity exerted by the earth on moon? My intuition would say my first answer is correct?
  2. jcsd
  3. Oct 7, 2016 #2


    Staff: Mentor

    This one.

    The gravitational interaction between the earth and moon results in one pair of forces.
  4. Oct 7, 2016 #3
    They would have to be equal though? Isn't the force exerted by the moon's gravitation on the earth going to be less than the force exerted by the earth's gravitation on the moon (since gravity is less on the moon)?
  5. Oct 8, 2016 #4


    Staff: Mentor

    Equal magnitude, opposite direction. Yes.

    Work it out. What does Newton's law of gravitation say?
  6. Oct 8, 2016 #5
    Thanks! I didn't know that G is the same between any two objects. That was what was confusing me!
  7. Oct 8, 2016 #6
    But the earth is more massive than the moon and the force is based on the product of the masses.
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