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More on Newton's Law of Universal Gravitation
http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html
Prehistoric man realized a long time ago that when objects are released
near the surface of the earth, they always fall down to the ground. The
Earth attracts masses near its surface to itself. Galileo (1564-1642)
pointed out that heavy and light objects fall toward the earth at the
same rate (so long as air resistance is the same for each). But it took
Sir Isaac Newton (in 1666) to realize that this force of attraction
between masses is UNIVERSAL! He proved that the force that causes,
for example, an apple to fall toward the ground is the same force that
causes the moon to fall around, or orbit, the earth. This universal force
also acts between the earth and the sun, or any other star and its
satellites. Each attracts the other. Sir Isaac Newton defined this
attraction mathematically. The force of attraction between two masses is
directly proportional to the product of
their masses and inversely proportional to the square of the distance
between their centers. This is all multiplied by a universal constant
G whose value was determined by Henry Cavendish in 1798.
The value of the universal gravitational constant G is:
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