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This article is about a measure of force or acceleration. For other uses, see G force (disambiguation).
g-force (also G-force, g-load) is a measurement of an object\'s acceleration expressed in g\'s. It may also informally refer to the reaction force resulting from an acceleration, with the causing acceleration expressed in g\'s. The g (pronounced /ˈdʒiː/) is a non-SI unit equal to the nominal acceleration due to gravity on Earth at sea level, defined as 9.80665 m/s2 (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.
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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).
While accelerations are often calculated relative to the Earth, g-force measures an object\'s acceleration in an inertial reference frame. Thus, if one is given an object\'s acceleration relative to the Earth, one must subtract off the acceleration of the Earth\'s reference frame relative to free-fall. The latter amount is, in most cases, approximately 1 g.
As acceleration is a vector quantity, this subtraction must be vector subtraction. However, if all the accelerations are in parallel directions, one can substitute scalar subtraction. Thus, in a simplified scenario where accelerations are assumed to act only upwards (positive) or downwards (negative), calculating g-force simply amounts to subtracting the acceleration (relative to the Earth) due to Earth\'s gravity (1 g in the downwards direction) from the object\'s acceleration relative to Earth. Since we are taking downward acceleration as negative, this is equivalent to adding 1 g. So, for example:
More generally, an object\'s acceleration may act in any direction (not just vertically), so in a fuller treatment the vector calculation must be used.
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 a 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 = v2/r, where a is acceleration, v is velocity 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).
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 resonance 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.
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 subject\'s body, which limits the maximum g-forces that can be tolerated.
In aircraft in particular, g-forces are often towards the feet, which forces blood away from the head; this causes problems with the eyes and brain in particular. As g-forces increase Brownout/greyout can occur, where the vision loses hue. If g-force is increased further tunnel vision will appear, and then at still higher g, loss of vision, while consciousness is maintained, this is termed "blacking out". Beyond this point losing consciousness will occur, also sometimes known as g-loc (loc stands for loss of consciousness). While tolerance varies, a typical person can handle about 5 g (49m/s²) before g-loc\'ing, 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 (88 m/s²) sustained (for a period of time) or more (see High-G training).
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/s² to -30 m/s²) range. The vision goes red and is also referred to as a red out. This is probably because capillaries in the eyes swell or burst under the increased blood pressure.
Humans can survive about 20 to 35 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.
The human body is considerably more able to survive g-forces that are perpendicular to the spine. In general when the g-force pushes the body backwards (colloquially known as \'eyeballs in\'NASA Physiological Acceleration Systems) a much higher tolerance is shown than when g-force 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
Elert, Glenn. Acceleration. The Physics Hypertextbook. Retrieved on 2007-01-21. (2003) "Are Amusement Park Thrill Rides Lethal?". Popular Mechanics (August 2003). Hearst Communications, Inc.. Retrieved on 2007-01-21.
Voluntarily:
Colonel John Stapp in 1954 sustained 46.2 g in a rocket sled, while conducting research on the effects of human deceleration.
Spark, Nick, The Story of John Paul Stapp, <http://www.ejectionsite.com/stapp.htm>
Voshell, Martin (2004), High Acceleration and the Human Body, <http://csel.eng.ohio-state.edu/voshell/gforce.pdf>
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.Anton Sukup (1977). David PURLEY Silverstone crash. Retrieved on July 31, 2006.
Indy Car driver, Kenny Bräck crashed on lap 188 of the 2003 race at Texas Motor Speedway. Bräck and Tomas Scheckter touched wheels, sending Bräck into the air at 200+ mph, hitting a steel support beam for the catch fencing. According to Bräcks\' site his car recorded 214 g. Bräck, Kenny, 2003 Season, <http://www.kennybrack.com/pages/personal-info/2003.html>
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