Hi there!
Briefly stated, Sir Isaac Newton discovered the Law of Gravity in 1666
while trying to figure out why the moon orbited the Earth. He
theorized that it must be forces of the Earth that were keeping it
close, and that the Earth was attracting all objects.
It's actually a little more complex than that:
"Newton's great insight of 1666 was to imagine that the Earth's
gravity extended to the Moon, counterbalancing its centrifugal force.
From his law of centrifugal force and Kepler's third law of planetary
motion (see KEPLER, JOHANNES), Newton deduced that the centrifugal
(and hence centripetal) force of the Moon or of any planet must
decrease as the inverse square of its distance from the center of its
motion. For example, if the distance is doubled, the force becomes
one-fourth as much; if distance is trebled, the force becomes
one-ninth as much. This theory agreed with Newton's data to within
about 11%."
Newton, Sir Isaac - Gravitation
http://euler.ciens.ucv.ve/English/mathematics/newton.html
Though the famous legend surrounding Sir Issac and a certain fruit
tree is said in the previously noted page to be a myth, the University
of Tennessee's Astronomy Web Syllabus incorporates it into it's
excellent examination and explanation of Newton's Universal Law of
Gravitation:
"What Really Happened with the Apple?
Probably the more correct version of the story is that Newton, upon
observing an apple fall from a tree, began to think along the
following lines: The apple is accelerated, since its velocity changes
from zero as it is hanging on the tree and moves toward the ground.
Thus, by Newton's 2nd Law there must be a force that acts on the apple
to cause this acceleration. Let's call this force "gravity", and the
associated acceleration the "accleration due to gravity". Then imagine
the apple tree is twice as high. Again, we expect the apple to be
accelerated toward the ground, so this suggests that this force that
we call gravity reaches to the top of the tallest apple tree."
Sir Isaac Newton: The Universal Law of Gravitation
http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html
(The page also includes diagrams, equations, explanations of the
equations, and several "calculators".)
Regarding how he came to understand Gravity, Newton himself wrote in
1666:
"In the same year I began to think of gravity extending to the orb of
the Moon, 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
to answer pretty nearly."
HISTORY OF SCIENCE - 18. Newton
Prof. Fred L. Wilson - Rochester Institute of Technology
http://www.rit.edu/~flwstv/newton.html
At least one biographer, the University of Florida's Dr. Robert A.
Hatch, thinks this is a bit inaccurate:
"In 1666, as tradition has it, Newton observed the fall of an apple in
his garden at Woolsthorpe, later recalling, 'In the same year I began
to think of gravity extending to the orb of the Moon.' Newton's memory
was not accurate. In fact, all evidence suggests that the concept of
universal gravitation did not spring full-blown from Newton's head in
1666 but was nearly 20 years in gestation. Ironically, Robert Hooke
helped give it life. In November 1679, Hooke initiated an exchange of
letters that bore on the question of planetary motion. Although Newton
hastily broke off the correspondence, Hooke's letters provided a
conceptual link between central attraction and a force falling off
with the square of distance. Sometime in early 1680, Newton appears to
have quietly drawn his own conclusions."
Isaac Newton Biography
Dr. Robert A. Hatch, University of Florida
http://web.clas.ufl.edu/users/rhatch/pages/01-Courses/current-courses/08sr-newton.htm
A biography of Newton describes his discovery thusly:
"4. NEWTON'S LAW OF UNIVERSAL GRAVITATION:-
Newton struck upon his theory of gravitation at the age of 23. Like
most perfectionists Newton was cautious in his pronouncements, such
that, the publication of this theory was not to take place until many
years later.10
Newton's Law of Universal Gravitation has been described as follows:
It is a force between any two bodies and is "directly proportional to
the product of their masses and inversely proportional to the square
of the distance between them. ... The measure of the force of
gravitation on a given body on earth is the weight of that body."
While certain of Newton's theories have not stood the tests applied in
the 20th century, his law of universal gravitation has stood: "In the
general theory of relativity, gravitation is explained geometrically:
matter in its immediate neighborhood causes the curvature of the
four-dimensional space-time continuum."
The Scientist's List - Biographies - Sir Isaac Newton
http://www.blupete.com/Literature/Biographies/Science/Newton.htm#Gravitation
Some groups use Newton's discovery and his faith in discussions about
whether or not God exists, pointing to gravity as proof that God
exists:
"The law of gravity became Isaac Newton's best-known and most
important discovery. Isaac warned against using it to view the
universe as only some machine like a great clock. He said, "Gravity
explains the motions of the planets, but it cannot explain who set the
planets in motion. God governs all things and knows all that is or can
be done."
Isaac Newton and God's Law of Gravity
http://www.doesgodexist.org/NovDec01/IsaacNewtonAndGodsLawOfGravity.html
For more information about Sir Isaac Newton and his amazing
discoveries, you can read about him here:
Sir Isaac Newton - University of St. Andrews (Fairly comprehensive,
and includes pictures)
http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Newton.html
The Newton Project - a vast collection of Sir Isaac's writing,
available online
http://www.newtonproject.ic.ac.uk/index.html
I hope you find this information helpful!
--Missy
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