<P> Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and early 17th centuries . In his famous (though possibly apocryphal) experiment dropping balls from the Tower of Pisa, and later with careful measurements of balls rolling down inclines, Galileo showed that gravitational acceleration is the same for all objects . This was a major departure from Aristotle's belief that heavier objects have a higher gravitational acceleration . Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere . Galileo's work set the stage for the formulation of Newton's theory of gravity . </P> <P> In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse - square law of universal gravitation . In his own words, "I deduced that the forces which keep the planets in their orbs must (be) reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly ." The equation is the following: </P> <P> F = G m 1 m 2 r 2 (\ displaystyle F = G (\ frac (m_ (1) m_ (2)) (r ^ (2))) \) </P> <P> Where F is the force, m and m are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant . </P>

Where is the force of gravity considered to be located