Universal Gravitational Force

Newton's Laws of Universal Gravitational Forces: 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 dependence 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
distance is not the only variable affecting the magnitude of
a gravitational force.

But Newton's law of
universal gravitation extends gravity beyond earth. Newton's
law of universal gravitation is about the
**universality** of gravity. **ALL** objects attract
each other with a force of gravitational attraction. Gravity
is universal. 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 centers.

** ** The idea that all object experince the force of gravity, that the force that pulls an apple to the Earth is the same force that holds the moon in orbit. The
orbit of the Moon about the Earth is a consequence of the
gravitational
force, because the acceleration due to gravity could change the velocity
of the
Moon in just such a way that it followed an orbit
around the earth.

This can be illustrated with the thought experiment shown in the
following figure. Suppose we fire a cannon horizontally from a high
mountain;
the projectile will eventually fall to earth, as indicated by the
shortest
trajectory in the figure, because of the gravitational
force directed toward the center of the Earth and the associated
acceleration.
(Remember that an acceleration is a
change in velocity and that velocity is a vector, so it has both a
magnitude
and a direction. Thus, an acceleration occurs if either or both the
magnitude
and the direction of the velocity change.)

But as we increase the muzzle velocity for our imaginary cannon, the
projectile
will travel further and further before returning to earth.
Finally, Newton reasoned that if the cannon projected the cannon ball
with
exactly the right velocity, the projectile would travel completely
around the
Earth, always falling in the gravitational field but never reaching the
Earth,
which is curving away at the same rate that the projectile falls.
That is,
*the cannon ball would have
been put into orbit around the Earth.*
Newton concluded that the orbit of the
Moon was of exactly the same nature: the Moon continuously "fell" in
its path
around the Earth because of the acceleration due to gravity, thus
producing its
orbit.
Explaination and Diagram provided by AstroWiki