Newton's Law of Universal Gravitation
Isaac Newton compared the acceleration of the moon to the
acceleration of objects on
earth. Believing that gravitational forces were responsible for each,
Newton was able to
draw an important conclusion about the depen
dence of gravity upon distance. This
comparison led him to conclude that the force of gravitational
attraction between the
Earth and other objects is inversely proportional to the distance
separating the earth's
center from the object's center. But distanc
e is not the only variable effecting the
magnitude of a gravitational force. In accord with Newton's
famous equation
F
net
= m*a
Newton knew that the force which caused the apple's acceleration
(gravity) must be
dependent upon the mass of the apple. And sin
ce the force acting to cause the apple's
downward acceleration also causes the earth's upward acceleration
(Newton's third law),
that force must also depend upon the mass of the earth. So for
Newton, the force of
gravity acting between the earth and any ot
her object is directly proportional to the mass
of the earth, directly proportional to the mass of the object, and
inversely proportional to
the square of the distance which separates the centers of the earth
and the object.

But Newton's law of universal g
ravitation extends gravity beyond earth. Newton's law of
universal gravitation is about the
universality
of gravity. Newton's place in the
Gravity
Hall of Fame
is not due to his discovery of gravity, but rather due to his
discovery that
gravitation is univ
ersal.
ALL
objects attract each other with a force of gravitational
attraction. This force of gravitational attraction is directly
dependent upon the masses of
both objects and inversely proportional to the square of the
distance which separates their
cent
ers. Newton's conclusion about the magnitude of gravitational
forces is summarized
symbolically as

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- Fall '10
- noris
- Mass, General Relativity, Isaac Newton