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g (also gee, g-force or g-load) is a unit of acceleration defined as exactly 9.806 65 m/s², which is approximately equal to the acceleration due to gravity on the Earth's surface. This conventional value was established by the 3rd CGPM (1901, CR 70). The absolute total g-force is found by vector addition of the opposite of the actual acceleration (in the sense of rate of change of velocity) and a vector of 1 g downward for the ordinary gravity (or in space, the gravity there). For example, being accelerated upward with an acceleration of 1 g doubles the experienced gravity. Conversely, weightlessness means a zero g-force, which is the result in the case of acceleration just due to gravity ( freefall).
The symbol g is always written in lowercase, to distinguish it from the symbol G, the gravitational constant, which is always written in uppercase.
The value of g defined above is an average over the whole of the Earth's surface. It is sometimes written as gN or g0 to distinguish it from the local value of g that varies with position.
The actual acceleration of a body at the Earth's surface depends on the location at which it is measured, smaller at lower latitudes, for two reasons.
The first is that the rotation of the Earth imposes an additional acceleration on the body that opposes gravitational acceleration. The net downward force on the body is therefore offset by a centrifugal forceIn classical mechanics, centrifugal force is the experience of the inertia of an object moving in circular motion, causing it to move away from the center. It is often designated a " fictitious force". We know from Newton's first law of motion that a body that acts upwards, reducing its weight. This effect on its own would result in a range of values of g from 9.789 m/s² at the equator to 9.823 m/s² at the poles.
The second reason is the Earth's equatorial bulgeAn equatorial bulge is a planetological term which describes a bulge which a planet may have around its equator, distorting it into an oblate spheroid. The Earth has an equatorial bulge of 42. 72 km due to the centrifugal force of its rotation. That is, i, which causes objects at the equator to be further from the planet's centre than objects at the poles. Because the force due to gravitational attraction between two bodies (the Earth and the object being weighed) varies inversely with the square of the distance between them, objects at the equator experience a weaker gravitational pull than objects at the poles.
The combined result of these two effects is that g is 0.052 m/s² more, hence the weight of an object is 0.5% more, at the poles than at the equator. Thus massMass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. It is a central concept of classical mechanics and related subjects. Strictly speaking, there are two different quantities called mass Inertial mass and weightFor the 1994 album by the group Rollins Band, see Weight (album). Weight is the force exerted upon an object by virtue of its position in a gravitational field. In a constant gravitational field, such as the Earth's, this force is proportional to the obje not only have different units, but even on Earth, they are not quite proportional.
If the terrain is at sea level, we can estimate g, at a height h in the air above it:
where
The last term, 0.3086 mgal/m, is the free air correction: gravity decreases with height, at a rate which near the surface of the Earth is such that linear extrapolation would give zero gravity at a height of one half the radius of the Earth, i.e. the rate is 9.8m/s² per 3200 km.
For flat terrain above sea level a term is added, for the gravity due to the extra mass; for this purpose the extra mass can be approximated by an infinite horizontal slab, and we get 2πG times the mass per unit area, i.e. 0.000,042 mgal/(kg/m²) (the Bouguer correction). For a mean rock density of 2.67 kg/cm³ this gives 0.11 mgal/m. Combined with the free-air correction this means a reduction of gravity at the surface of ca. 0.20 mgal for every metre of elevation of the terrain. (The two effects would cancel at a surface rock density of 4/3 times the average density of the whole Earth.)
For the gravity below the surface we have to apply the free-air correction as well as a double Bouguer correction. With the infinite slab model this is because moving the point of observation below the slab changes the gravity due to it to its opposite. Alternatively, we can consider a spherically symmetrical Earth and subtract from the mass of the Earth that of the shell outside the point of observation, because that does not cause gravity inside. This gives the same result.
Local variations in both the terrain and the subsurface cause further variations; the gravitational geophysicalGeophysics the study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods. It includes the branches of: # Seismology ( earthquakes and ela methods are based on these: the small variations are measured, the effect of the topography and other known factors is subtracted, and from the resulting variations conclusions are drawn. See also physical geodesyDefinition Physical geodesy is the study of the physical properties of the gravity field of the Earth, the geopotential, with a view to their application in geodesy. Traditional geodetic instruments such as theodolites rely on the gravity field for orient and gravity anomalyGravity anomalies are widely used in geodesy and geophysics. A gravity anomaly is the difference between the observed gravity and its theoretical value, which is calculated at the surface of an global spheroid ( ellipsoid of Hayford or WGS84) by rather si.