Gravity is More Than a Name
Universal Gravitation
Nearly every child knows of the word gravity. Gravity is the name associated with the mishaps of the milk
spilled from the breakfast table to the kitchen floor and the
youngster who topples to the pavement as the grand finale of the first bicycle
ride. Gravity is the name associated with the reason for "what goes up,
must come down," whether it be the baseball hit in the neighbourhood
sandlot game or the child happily jumping on the backyard mini-trampoline. We
all know of the word gravity - it is the thing that
causes objects to fall to Earth. Yet the role of physics is to do more than to
associate words with phenomenon. The role of physics is to explain phenomenon
in terms of underlying principles. The goal is to explain phenomenon in terms
of principles that are so universal that they are capable of explaining more
than a single phenomenon but a wealth of phenomenon in a consistent manner.
Thus, a student's conception of gravity must grow in sophistication to the
point that it becomes more than a mere name associated with falling phenomenon.
Gravity must be understood in terms of its cause, its source, and its
far-reaching implications on the structure and the motion of the objects in the
universe.
Certainly gravity is a force that exists between the Earth
and the objects that are near it. As you stand upon the Earth, you experience
this force. We have become accustomed to calling it the force of
gravity and have even represented it by the
symbol Fgrav. Most students of physics progress at least to this level of
sophistication concerning the notion of gravity. This same force of gravity
acts upon our bodies as we jump upwards from the Earth. As we rise upwards
after our jump, the force of gravity slows us down. And as we fall back to
Earth after reaching the peak of our motion, the force of gravity speeds us up.
In this sense, the force gravity causes an acceleration of our bodies during
this brief trip away from the earth's surface and back. In fact, many students
of physics have become accustomed to referring to the actual acceleration of
such an object as the acceleration of gravity. Not to be confused with the force of gravity (Fgrav), the acceleration of gravity (g) is the acceleration experienced by an object when the only force
acting upon it is the force of gravity. On and near Earth's surface, the value
for the acceleration of gravity is approximately 9.8 m/s/s. It is the same
acceleration value for all objects, regardless of their mass (and assuming that
the only significant force is gravity). Many students of physics progress this
far in their understanding of the notion of gravity.
Here we will build on this understanding of gravitation, making an attempt
to understand the nature of this force. Many questions will be asked: How and
by whom was gravity discovered? What is the cause of this force that we 
refer to with the name of gravity? What variables affect the actual
value of the force of gravity? Why does the force of gravity acting upon an object
depend upon the location of the object relative to the Earth? How does gravity
affect objects that are far beyond the surface of the Earth? How far-reaching
is gravity's influence? And is the force of gravity that attracts my body to
the Earth related to the force of gravity between the planets and the Sun?
These are the questions that will be pursued. And if you can successfully
answer them, then the sophistication of your understanding has extended beyond
the point of merely associating the name "gravity" with falling
phenomenon.