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 Gravity is a force of attraction that exists between any two objects. There is a
force of gravity between the sun and the Earth, between the Earth and us, and
even between two marbles.
 Projectiles, satellites, planets, galaxies, and clusters of galaxies are all influenced
by Gravity.
 Gravity is the weakest of the four known forces of nature, yet the most dominant
force. Even though it's the weakest force, Gravity holds together entire solar
systems and galaxies!
 The law of universal gravitation says that every object attracts every other object
with a force that, for any two objects, is directly proportional to the mass of each
object and inversely proportional to the square of the distance between the two
objects.

 The gravitational formula is
 This is an example of the "inverse-square law": gravitational force varies as the
inverse square of the distance between the two objects. As a result, the effect of
Gravity falls rapidly as the distance increases between two objects.
 The best current estimate of G is that it equals about 6.67259 10-11 Newton-
square meter per square kilogram.
 The gravitational field for a planet, g, equals GM/Squared, where G is the mass
of the planet and R is the distance of the object from the center of the planet (the
planet's radius if the object is on the surface of the planet). This means that
Gravity is greater where a planet is more massive and where it has a smaller
radius. So even though Mars has only about 1/10th of the mass of the Earth, the
gravitational force on the surface of Mars is more than 1/10th that on the surface
of the Earth -- because Mars' surface is closer to the planet's surface!
 The force of attraction between you and the Earth is your weight.
 Gravity determines the "escape speed" for an object like a rocket. The stronger
the gravitational pull of the object, the larger the escape speed. The following
chart shows the escape speed for the sun, two planets, and the Earth's moon.

Sun 620 km/s
Jupiter 60.2 km/s

Earth 11.2 km/s
Moon 2.4 km/s

Gravity Fun Facts

 The first reliable measurement of G was made by Henry Cavendish in 1798! He
calculated that G was equal to 6.754 10-11 Newton-square meter per square
kilogram (compared to today's calculation of 6.67259).