g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9.80665 m/s², which is approximately equal to the acceleration due to gravity on the Earth's surface. The International System of Units (abbreviated SI from the French language name Système International dUnités) is the modern form of the metric system. ... Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the gradient of the tangent to that point In physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of velocity. ... Gravity is a force of attraction that acts between bodies that have mass. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ...

The symbol g is properly written in lowercase and italic, to distinguish it from the symbol G, the gravitational constant, which is always written in uppercase and italic; and from g, the abbreviation for gram, which is not italicized. According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. ... The gram or gramme, symbol g, is a unit of mass. ...

This conventional value was established by the 3rd CGPM (1901, CR 70). The total acceleration 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 an acceleration of 1 g downward in an inertial reference frame. The Conférence générale des poids et mesures (General Conference on Weights and Measures or CGPM) is one of the three organizations established to maintain the SI system under the terms of the Metre Convention (1875). ... In physics and in vector calculus, a spatial vector is a concept characterized by a magnitude, which is a scalar, and a direction (which can be defined in a 3-dimensional space by the Euler angles). ... This article is about velocity in physics. ... An objects weight, henceforth called actual weight, is the downward force exerted upon it by the earths gravity. ... Astronauts on the International Space Station display an example of weightlessness Weightlessness is the experience (by people and objects) during freefall, of having no apparent weight. ...

The value of g defined above is an arbitrary midrange value on the Earth, approximately equal to the sea level acceleration of free fall at a geodetic latitude of about 45.5°; it is larger in magnitude than the average sea level acceleration on Earth, which is about 9.797 645 m/s². The standard acceleration of free fall is properly written as g_n (sometimes g₀) to distinguish it from the local value of g that varies with position. Latitude, sometimes denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ...

The units of acceleration due to gravity, meters per second squared, are interchangeable with newtons per kilogram. The quantity, 9.806 65, stays the same. These alternate units may be more helpful when considering problems involving pressure due to gravity, or weight. This article is about the SI unit of force. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... Pressure (symbol: p) is the force per unit area acting on a surface in a direction perpendicular to that surface. ... In the physical sciences, weight is the downward force exerted on matter as a result of gravity. ...

Variations of Earth's gravity

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. Latitude, sometimes denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ...

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 force 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 term centrifugal force is the reaction force exerted by an object moving in a circular path upon the object that is causing its circular motion, according to Newtons Third Law. ...

The second reason is the Earth's equatorial bulge (itself also caused by centrifugal force), 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. An 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 combined result of these two effects is that g is 0.052 m/s² more, hence the force due to gravity of an object is 0.5% more, at the poles than at the equator.

If the terrain is at sea level, we can estimate g:

where

g_φ = acceleration in m/s² at latitude φ

This is the International Gravity Formula 1967, the 1967 Geodetic Reference System Formula, Helmert's equation or Clairault's formula.

The first correction to this formula is the free air correction (FAC), which accounts for heights above sea level. 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 9.8 m/s² per 3200 km. Thus:

where

h = height in meters above sea level

For flat terrain above sea level a second 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. 4.2 × 10^-10 m³ s^-2 kg^-1 (0.000,042 mGal/(kg/m²)) (the Bouguer correction). For a mean rock density of 2.67 g/cm³ this gives 1.1 × 10^-6 s^-2 (0.11 mGal/m). Combined with the free-air correction this means a reduction of gravity at the surface of ca. 2 µm/s² (0.20 mGal) for every meter 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. In vector calculus, the divergence theorem, also known as Gauss theorem, Ostrogradskys theorem, or Ostrogradsky–Gauss theorem is a result that links the divergence of a vector field to the value of surface integrals of the flow defined by the field. ...

Local variations in both the terrain and the subsurface cause further variations; the gravitational geophysical 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 geodesy and gravity anomaly. Geophysics, the study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods. ... Definition 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. ... Gravity anomalies are widely used in geodesy and geophysics. ...

Calculated value of g

Given the law of universal gravitation, g is merely a collection of factors in that equation: It has been suggested that this article or section be merged into Gravity. ...

where g is the bracketed factor and thus:

We can plug in values of G and the mass and radius of the Earth to obtain the calculated value of g: Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... In classical geometry, a radius of a circle or sphere is any line segment with one endpoint on the circle (i. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ...

This agrees approximately with the measured value of g. The difference may be attributed to several factors:

• The Earth is not homogeneous
• The Earth is not a perfect sphere
• The choice of a value for the radius of the Earth (an average value is used above)
• The normal measured g also includes the centrifugal force effects due to the rotation of the Earth

There are significant uncertainties in the values of G and of m₁ as used in this calculation. However, the value of g can be measured precisely and in fact, Henry Cavendish performed the reverse calculation to estimate the mass of the Earth. The term centrifugal force is the reaction force exerted by an object moving in a circular path upon the object that is causing its circular motion, according to Newtons Third Law. ... Henry Cavendish (October 10, 1731 - February 24, 1810) was a British scientist. ...

Usage of the unit

The g is used primarily in aerospace fields, where it is a convenient magnitude when discussing the loads on aircraft and spacecraft (and their pilots or passengers). For instance, most civilian aircraft are capable of being stressed to 4.33 g (42.5 m/s²; 139 ft/s²), which is considered a safe value. The g is also used in automotive engineering, mainly in relation to cornering forces and collision analysis. Look up aerospace in Wiktionary, the free dictionary. ... A Japan Airlines Boeing 747-400. ... Ariane 5 lifts off with the Rosetta space probe on March 2, 2004. ... cars go vroom vroom . ...

One often hears the term being applied to the limits that the human body can withstand without losing conciousness, sometimes referred to as "blacking out", or g-loc (loc stands for loss of consciousness). A typical person can handle about 5 g (50 m/s²) before this occurs, but through the combination of special g-suits and efforts to strain muscles —both of which act to force blood back into the brain— modern pilots can typically handle 9 g (90 m/s²) sustained (for a period of time) or more. Resistance to "negative" or upward gees which drive blood to the head, is much less. This limit is typically in the -2 to -3 g (-20 to -30 m/s²) range. The vision goes red and is also referred to as a "red-out". This is probably due to capillaries in the eyes bursting under the increased blood pressure. Humans can survive about 20 to 40 g instantaneously (for a very short period of time), and any exposure to around 100 g instantaneous or more is lethal. Fainting or syncope is a sudden (and generally momentary) loss of consciousness due to a lack of sufficient blood and oxygen reaching the brain. ... g induced Loss Of Consciousness is a condition where a person loses consciousness because g-forces move the blood away from the brain (black out) or move excess blood towards the brain (red out). ... A G-suit is worn by aviators and astronauts subject to high accelerations to prevent loss of consciousness, commonly called blackout or G-LOC (G-induced Loss Of Consciousness). ...

Human g-force experience

• Amusement park rides such as roller coasters typically do not expose the occupants to much more than about 2 g.
• A sky-diver in a stable free-fall experiences his full weight of 1 g. - (see terminal velocity)
• A scuba-diver experiences his full weight of 1 g.
• Astronauts in earth orbit experience 0 g.
• Passenger on planes on a parabolic trajectory experience 0 g. (see Vomit Comet)

Six Flags New England, an amusement park in Agawam, Massachusetts. ... The terminal velocity of an object falling towards the ground, in non-vacuum, is the speed at which the gravitational force pulling it downwards is equal and opposite to the atmospheric drag (also called air resistance) pushing it upwards. ... U.S. Space Shuttle astronaut Bruce McCandless II using a manned maneuvering unit (MMU) outside the Challenger in 1984. ... Earth orbit is an orbit around the planet Earth. ... A parabola A parabola (from the Greek: παραβολή) is a conic section generated by the intersection of a cone, and a plane tangent to the cone or parallel to some plane tangent to the cone. ... A trajectory is an imagined trace of positions followed by an object moving through space. ... Weightlessness inside the Vomit Comet The Vomit Comet was the nickname given to the aircraft used by NASAs Reduced Gravity Research Program. ...

Strongest g-forces survived by humans

Voluntarily: Colonel John Stapp in 1954 sustained 45.4 g in a rocket sled, while conducting research on the effects of human deceleration. John Stapp rides the rocket sled at Edwards Air Force Base. ...

Involuntarily: Formula One race car driver David Purley sustained 179.8 g in 1977 when he decelerated from 107 mph (172 km/h) to 0 in a distance of 26 inches (66 cm) after his throttle got stuck wide open and he hit a wall. David Purley was a Formula One driver from Britain. ...

Reference

International Association of Geodesy (1971) : Geodetic Reference System 1967. Publi. Spéc. n° 3 du Bulletin Géodésique, Paris.