Journey Into Space: Gravity, Orbits, and Collisions
Space Astronomy and Space
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The Earth's Gravity

The strength of gravity is not the same at all places on the Earth. Three things determine the strength of gravity at any given place:
(1) the distance from the center of the Earth,
(2) the spin of the Earth, and
(3) the nearby sources of gravity variations, such as mountains or underground caverns.

Consider the distance from the center of the Earth. A house at the seashore is at a lower elevation than one in the mountains, which means that it is closer to the center of the Earth. The strength of gravity is stronger at the seaside house than the strength of gravity is at the mountain house.
The Earth's spin also produces an effect that can appear to reduce the strength of gravity. Known as the centrifugal effect, it is caused by the tendency of a body to move in a straight line unless acted upon by a force trying to change its path. The tendency of a body at the surface of the spinning Earth is to move outward in a straight line. At the same time, the Earth's gravitational force is pulling the body toward the center of the planet. Part of the Earth's gravitational force is reduced in changing the body's path from a straight line in space into the circle it actually follows as the Earth rotates, and this serves to lessen the body's weight.

An example is a body at the Earth's equator, where the centrifugal effect is greater than anywhere else on the surface of the planet. A body at the equator must travel nearly 24,000 miles (39,000 kilometers) during one rotation of the planet, but this distance and the centrifugal effect decrease as you move away from the equator and toward the poles. The result is that the weight of a body on the Earth's surface increases slightly as it moves away from the equator and toward the poles. This is because the Earth's gravitational force is slightly less at the equator than at the poles. A bag of sugar at the equator would weigh about 1/1,000 less than it would in Honolulu, Hawaii, which is closer to the North Pole.

Variations in gravity may also be caused by nearby concentrations of mass such as mountain ranges or underground deposits of materials. The pull of gravity is greater near large or dense concentrations of mass or deposits of dense materials, and it is weaker near underground caverns or deposits of light materials, such as oil. Looking for gravity variations with a gravimeter is an important way of searching for deposits of oil or minerals.
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