Graphics Reference
In-Depth Information
was supposed to have conducted these early experiments by dropping balls
from the top of the Tower of Pisa. Unfortunately, this endearing story is more
than likely apocryphal. However, the accounts of his experiments that entailed
rolling a series of balls down slopes to demonstrate the effect of gravity aren't
fiction. He demonstrated that through the effects of gravity, objects of varying
weight fell and accelerated at the same rate, which supplanted the earlier
belief, promoted by Aristotle, that heavier objects fell faster than lighter ones.
The difference that may be observed in the rate at which different objects
fall to earth is due to air resistance. The air resistance on most objects is
negligible, so a cannonball and a small marble released from the same height
and at the same moment will hit the ground at the same time. A feather falls
to earth at a slower rate than a hammer, not because the feather is lighter
than the hammer but simply because of the different effects of air resistance
on both objects. An experiment carried out by astronauts on the moon
demonstrated that, when dropped at the same moment and from the same
height, a hammer and a feather hit the moon's surface at the same time. Since
there is no atmosphere on the moon, there can be no air resistance; therefore
no friction acted on the falling objects.
Newton's Theory of Gravitation
In 1687 Sir Isaac Newton published his groundbreaking work, the
Principia,
in which he presented his theories on the principles of universal gravitation,
meeting with immediate universal acclaim. Although there have been many
further developments in the field since their publication, Newton's theories
remain central to our understanding of the forces of gravity. In the
Principia
Newton set out the theories that explained the gravitational forces that
accounted for the moon's orbit around the earth and the planets' orbit around
the sun. Although some of the details of his theory have been replaced or
superseded by the work of Albert Einstein and others, Newton's general
theory still holds good to this day.
The general tenet of the theory states that every
point mass
(a theoretical point
with mass assigned to it) exerts a force that attracts every other point mass that
is oriented along a line intersecting both points. The theory goes on to state that
every object within the universe attracts every other object with a force that
is directly proportional to the objects' mass and inversely proportional to the
square of the distance between the objects. The greater the mass of an object,
the more force is exerted, and the closer the object, the more force is exerted.
Without the effect of gravitational forces, much of the material within the
universe would simply fly off in all directions. Indeed, the universe as we know
it could not have formed. Clouds of gas could not have coalesced and would
then not have collapsed to form stars, and without the formation of stars, the
heavier elements such as carbon (of which you and I are created) could not
have formed.