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Eng Course- Introduction to Earth Gravity- Download Free PDF


Things on the earth’s surface – chairs, houses, people - stay on the earth unless
subjected to extreme forces. We all stay here due to the gravity of the earth. This has
been recognized for thousands of years. The earth also rotates on its polar axis once per
day. This has been recognized only for a few hundred years. This rotation also produces
a force that tends to throw things outward from the axis.
On of the objections to earth rotation was this outward force. It was ask “Why
don’t we all fly off the earth?”. To some extent we do. But the gravitational attraction
of the earth is about 300 times as strong as the rotational outward force, so we stay put.
However this force does have it’s effects. It causes the earth to change shape, being a
little larger at the equator than at the poles. (By about 1/300 of course.)
One effect of this history is the way “gravity” is defined in geodesy. “Gravity” is
taken as the observed acceleration. This is the sum of the two effects. Therefore you
must be careful when reading technical books and articles mentioning “gravity”. Iit is
important to know who is writing and which definition of “gravity” is used. Here we will
use the geodestist nomenclature:
Newtonian Gravitation The physics book attraction
Rotational Effects Centrifugal Acceleration
Gravity The sum of these two.
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