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''g''-force (also '''g-load''') is a measurement of an object's unit equal to the Nominal Acceleration Due To Gravity On Earth At Sea Level , defined as 9.80665 m/s2, or 32.174 ft/s2. More precisely, ''g''-force measures the net effect of the Acceleration that an object actually experiences and the acceleration that Gravity is trying to impart to it, as explained further below. 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 from g, the symbol for Gram , which is not italicised. CONNECTION WITH FORCE Although actually a measurement of acceleration, the term ''g''-force is, as its name implies, popularly imagined to refer to the '' Force '' that an accelerating object "feels". These so-called "''g''-forces" are experienced, for example, by fighter jet pilots or riders on a roller coaster, and are caused by changes in speed and direction. For example, on a roller coaster high positive ''g''-forces are experienced on the car's path up the hills, where riders feel as if they weigh more than usual. This is reversed on the car's descent where lower ''g''-forces occur, causing the riders to feel lighter or even weightless. The relationship between force and acceleration stems from Newton's Second Law , ''F'' = ''ma'', where ''F'' is force, ''m'' is mass and ''a'' is acceleration. This equation shows that the larger an object's mass, the larger the force it experiences under the same acceleration. Thus, objects with different masses experiencing numerically identical "''g''-forces" will in fact be subject to forces of quite different magnitude. For this reason, ''g''-force cannot be considered to measure force in absolute terms. However, the interpretation of ''g''-force as a force can be partially rescued by noting that its numerical value is the ''ratio'' of the force "felt" by an object under the given acceleration to the force that the same object "feels" when resting stationary on the Earth's surface. For example, a person experiencing a ''g''-force of 3 ''g'' feels three times as heavy as normal. Because of the potential for confusion about whether ''g''-force measures acceleration or force, the term is considered by some to be a misnomer. Scientific usage prefers explicit reference to either acceleration or force, and use of the appropriate units (in the SI system, metres per second squared for acceleration, and newtons for force). CALCULATING ''G''-FORCES Unlike simple acceleration, ''g''-force is a measure of an object's acceleration relative to the local Gravitational acceleration Vector , rather than being compared to an Inertial Reference Frame . In other words, it is the (vector) difference between an object's actual acceleration and the acceleration that it would experience if it were falling freely. It is this difference, rather than the actual acceleration of the object, that gives rise to the feeling of force ("apparent weight"), and hence to the feeling of heaviness and lightness in high and low ''g''-force environments. For further details, including examples of conversion between acceleration and apparent weight force, see Apparent Weight . In a simplified scenario, where accelerations are assumed to act only downwards (positive) or upwards (negative), calculating this difference simply amounts to subtracting the object's actual acceleration from the gravitational acceleration. For an object on or near the Earth's surface, gravitational acceleration is for practical purposes equal to 1 ''g''. (For more precise measurements, the variation of Earth's Gravity with location and altitude must be taken into account.) So, for example:
More generally, an object's acceleration may act in any direction (not just vertically), so in a fuller treatment it must be considered as a vector quantity. The "difference" in acceleration that ''g''-force measures is found by Vector addition of the opposite of the actual acceleration and the local gravitational acceleration vector (about 1 ''g'' downward on or near the Earth's surface). In cases when the magnitude of the acceleration is relatively large compared to 1 ''g'', and/or is more-or-less horizontal, the effect of the Earth's gravity is sometimes ignored in everyday treatments. For example, if a person in an car accident decelerates from 30 m/s to rest in 0.2 seconds, then their deceleration is 150 m/s2, so one might say that they experience a ''g''-force of about 150/9.8 ''g'', or about 15.3 ''g''. Strictly speaking, due to the vector addition of the gravitational acceleration, the true ''g''-force has a slightly larger magnitude and is pointing slightly downwards (intuitively this is because the person is already experiencing 1 ''g'' just by sitting in the car). The ''g''-force experienced when cornering can be calculated from the Radial Acceleration formula, ''a'' = ''v''2/''r'', where ''a'' is acceleration, ''v'' is speed and ''r'' is the corner's Radius Of Curvature . For example, a racing car driver travelling at 50 m/s around a corner with radius of curvature 80 m undergoes an acceleration of 502/80 m/s2, or 31.25 m/s2. This equates to a ''g''-force of about 31.25/9.8 ''g'', or about 3.19 ''g'' (again, for the purposes of this example, ignoring the additional ''g''-force due to Earth's gravity). EXAMPLES OF USAGE
HUMAN TOLERANCE TO ''G''-FORCE Human tolerances depend on the magnitude of ''g''-force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body. The human body is flexible and deformable, particularly the softer tissues. A hard slap on the face may impose hundreds of ''g''-s locally but not produce any real damage: a constant 15 ''g''-s for a minute, however, may be deadly. When Vibration is experienced, relatively low peak ''g'' levels can be severely damaging if they are at the Resonant Frequency of organs and connective tissues. To some degree, ''g''-tolerance can be trainable; and there is also considerable variation in innate ability between individuals. Further some illnesses reduce ''g''-tolerance, particularly cardiovascular problems. Vertical axis g-force Aircraft in particular exert ''g''-force on the axis aligned with the spine. This causes significant variation in blood pressure along the length of the subjects body, which limits the maximum g-forces that can be tolerated. One often hears the term being applied to the limits that the human body can withstand without Losing Consciousness , sometimes referred to as "blacking out", or '' ''g''-loc '' (''loc'' stands for ''loss of consciousness''). A typical person can handle about 5 ''g'' (50m/s&2) before this occurs, but through the combination of special G-suit s 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/s&2) 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 m/s&2 to -30 m/s&2) range. The vision goes red and is also referred to as a Red Out . This is probably due to capillaries in the eyes swelling or bursting under the increased blood pressure. Humans can survive about 20 to 40 ''g'' instantaneously (for a very short period of time). Any exposure to around 100 ''g'' or more, even if momentary, is likely to be lethal, although the record is 179 ''g''. Horizontal axis g-force The human body is considerably more able to survive ''g''-forces that are perpendicular to the spine. In general when the acceleration pushes the body backwards (colloquially known as 'eyeballs in' NASA Physiological Acceleration Systems ) a much higher tolerance is shown than when acceleration is pushing the body forwards ('eyeballs out') since blood vessels in the retina appear more sensitive to that direction. Early experiments showed that untrained humans were able to tolerate 17 ''g'' eyeballs-in (compared to 12 ''g'' eyeballs-out) for several minutes without loss of consciousness or apparent long-term harm. NASA Technical note D-337, Centrifuge Study of Pilot Tolerance to Acceleration and the Effects of Acceleration on Pilot Performance , by Brent Y. Creer, Captain Harald A. Smedal, USN (MC),and Rodney C. Vtlfngrove HUMAN G-FORCE EXPERIENCE
Everyday g-forces
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Strongest g-forces survived by humans Voluntarily: Colonel John Stapp in 1954 sustained 46.2 ''g'' in a rocket sled, while conducting research on the effects of human deceleration. See Martin Voshell (2004), [http://csel.eng.ohio-state.edu/voshell/gforce.pdf 'High Acceleration and the Human Body' . Involuntarily: Formula One racing car driver David Purley survived an estimated 179.8 ''g'' in 1977 when he decelerated from 173 km/h (108 mph) to 0 in a distance of 66 cm (26 inches) after his throttle got stuck wide open and he hit a wall.6 SEE ALSO REFERENCES EXTERNAL LINKS
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