12.6 - Earth’s composition and g
The force of gravity differs slightly at different locations on the Earth. These variations
mean that g, the acceleration due to the force of gravity, also differs by location. Why do
the force of gravity and g vary?
First, the surface of the Earth can be slightly below sea level (in Death Valley, for
example), and it can be more than 8000 meters above it (on peaks such as Everest and
K2). Compared to an object at sea level, an object at the summit of Everest is 0.14%
farther from the center of the Earth. This greater distance to the Earth’s center means
less force (Newton’s law of gravitation), which in turn reduces acceleration (Newton’s
second law). If you summit Mt. Everest and jump with joy, the force of gravity will
accelerate you toward the ground about 0.03 m/s^2 slower than if you were jumping at
sea level.
Second, the Earth has a paunch of sorts: It bulges at the equator and slims down at the
poles, making its radius at the equator about 21 km greater than at the poles. This is
shown in an exaggerated form in the illustration for Concept 3. The bulge is caused by
the Earth’s rotation and the fact that it is not entirely rigid. This bulge means that at the
equator, an object is farther from the Earth’s center than it would be at the poles and,
again, greater distance means less force and less acceleration.
Finally, the density of the planet also varies. The Earth consists of a jumble of rocks,
minerals, metals and water. It is denser in some regions than in others. The presence of
materials such as iron that are denser than the average increases the local gravitational
force by a slight amount. Geologists use gravity gradiometers to measure the Earth’s
density. Variations in the density can be used to prospect for oil or to analyze seismic
faults.
Value of g
about 9.80 m/s^2 at sea level
Value for g varies:
Due to altitude
Value for g varies:
Because the Earth is not a perfect
sphereValue for g varies:
Because the Earth is not uniformly
dense