Gravity in detail
Gravity, g g is the symbol for the average acceleration produced by gravity
at the Earth's surface (sea level). The actual acceleration of
gravity varies from place to place, depending on latitude,
altitude, and local geology. The symbol g is often used informally
as a unit of acceleration. By agreement among physicists, the
standard acceleration of gravity g_{n} is defined to be
exactly 9.80665 meters per second per second (m s^{2)}, or
about 32.174 05 feet per second per second. At latitude p, a
conventional value of the acceleration of gravity at sea level is
given by the International Gravity Formula, g = 978.0495 [1
+ 0.0052892 sin^{2}(p)  0.0000073 sin^{2}
(2p)] centimetres per second per second (cm s^{2}).
The symbol g was used as a unit first in aeronautical and space
engineering, where it is important to limit the accelerations
experienced by the crew members of aircraft and spaceships: the "g
forces," as they are called. This use became familiar through the
space programs, and now a variety of accelerations are measured in
g's. Note that g is also the symbol for the gram. The Gal unit is used in making measurements of local variations in the acceleration of gravity g. Variations in the acceleration of Earth’s gravity (i.e. gravity anomalies) are typically measured in milligals (mGal). One Gal is approximately 0.0010197g, so a milligal (or mGal) is a very small acceleration, of about 10^{6} g (or 10^{–5} m s^{2}). The mean Earth gravity is about 981 000 mGal (the wellknown 9.81 m/s^{2}), varies from 978,100 mGal to 983,200 mGal from Equator to pole due to the Earth's flattening and rotation. Variations, due to density inhomogeneities, mountain ridges, etc., range from tens to hundreds of milligals. The unit is named after the Italian astronomer and natural philosopher Galileo Galilei, who lived from 1564 until 1642. Galileo proved that all objects at the Earth's surface experience the same gravitational acceleration. (*Note: to avoid confusion with the symbol for the gallon, and to conform to the usual metric style, the symbol for this unit should be Gal rather than gal). Eötvös (E) The Eötvös unit is used in geophysics to
measure the rate of change, or gradient in the acceleration of
gravity with horizontal distance. One Eötvös
equals 10^{7} Gal per metre or 10^{4} Gal per
kilometer. In proper SI units, the Eötvös unit
equals 10^{9} per second squared (s^{2}). The
largest component is the vertical gravity gradient, being about
3000 E on Earth (gravity changes by
3.10^{6}m/s^{2} per metre of elevation). The
horizontal components are approximately half this size; mixed
gradients are below 100 E for the normal field.
Gravitygradient anomalies can be much larger and reach 1000
E in mountainous area. This unit is named after the
Hungarian physicist Roland von Eötvös who lived from 1848
until 1919. Hertz is the SI unit of frequency, equal to one cycle per second. The hertz is used to measure the rates of events that happen periodically in a fixed and definite cycle. Multiples of the hertz are common: the frequencies of radio and television waves are measured in kilohertz (kHz), megahertz (MHz), or even gigahertz (GHz), and the frequencies of light waves in terahertz (THz). The unit is named after the German physicist Heinrich Rudolf Hertz (18571894), who proved in 1887 that energy is transmitted through a vacuum by electromagnetic waves. Units Conversion g ≈ 9.8 m s^{2} 
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