# g - Force

A physical force equivalent to one unit of gravity that is multiplied during rapid changes of direction or velocity. Drivers experience severe g-forces as they corner, accelerate and brake.
g force is a measurement of an object's acceleration expressed in g-s. It is proportional to the reaction force that an object experiences as a result of this acceleration or, more correctly, as a result of the net effect of this acceleration and the acceleration imparted by natural gravity.
g-force is not an absolute measurement of force and the term is considered a misnomer by some.
The g is a non-SI unit equal to the nominal acceleration of gravity on Earth at sea level (standard gravity), which is defined as 9.80665 m/s2 (32.174 ft/s2). The symbol g is properly written both lowercase and italic to distinguish it from the symbol G, the gravitational constant and g, the symbol for gram, a unit of mass, which is not italicized.

Analysis of g-forces are important in a variety of scientific and engineering fields, especially planetary science, astrophysics, rocket science, and the engineering of various machines such as fighter jets, race cars, and large engines.
Humans can tolerate localized g-forces in the 100s of g's for a split second, such a hard slap on the face may impose hundreds of g locally but not produce any real damage. However, sustained g-forces above about 16 g for a minute can be deadly or lead to permanent injury.

There is considerable variation among individuals when it comes to g-force tolerance, however. Race car drivers have survived instantaneous accelerations of up to 214 g during accidents.
To some degree, g-tolerance can be trainable, and there is also considerable variation in innate ability between individuals. In addition, some illnesses, particularly cardiovascular problems, reduce g-tolerance. In rocket sled experiments designed to test the effects of high acceleration on the human body, Colonel John Stapp in 1954 experienced 46.2 g for several seconds.

Usually, accelerations beyond 100 g, even if momentary, are fatal.
In everyday life, humans experience g-forces stronger than 1 g. A typical cough produces a momentary g-force of 3.5 g, while a sneeze results in about 2 g of acceleration. Roller coasters are usually designed not to exceed 3 g, although a few notable exceptions produce as much as 6.7 g. For example, on a roller coaster high positive g is experienced when the car's path curves upwards, where riders feel as if they weigh more than usual. This is reversed when the car's path curves downwards, and lower than normal g is felt, causing the riders to feel lighter or even weightless.

Slight increases in g-force are experienced in any moving machinery, such as cars, trains, planes, and elevators. Astronauts in orbit experience 0 g, called weightlessness.

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. That's mean that 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.

g-force varies on different planets or celestial bodies. When an object has a greater mass, it produces a higher gravitational field, resulting in higher g-forces. The g-force on the Moon is about 1/6 g, on Mars about 1/3 g. On the Martian satellite Deimos, only 13 km in diameter, the g-force is about 4/10,000ths of a g. In contrast, the surface of Jupiter experiences a g-force of about 2.5 g. This is smaller than it seems it should be because Jupiter's low density causes its surface to be very far from its primary concentration of mass at the core. On the surface of a neutron star, a degenerate star with a density similar to the atomic nucleus, the surface gravity is between 2×1011 and 3×1012 g's.

In the aerospace industry the g is a convenient unit for specifying the maximum load factor which aircraft and spacecraft must be capable of withstanding. Light aircraft of the kind used in pilot training (utility category) must be capable of sustaining load factor of 4.4g (43 m/s2, 141.5 ft/s2) with the undercarriage retracted. Airliners and other transport aircraft must be capable of 2.5g. Military aircraft and pilots (especially fighter pilots) with g-suits can withstand more than 9g.

Very short-term accelerations, measured in milliseconds, are usually referred to as shocks and are often measured in g's. The shock that a device or component is required to withstand may be specified in g. For example, mechanical wrist-watches might withstand 7 g, aerospace rated relays might withstand 50 g, and GPS/IMU units for military artillery shells need to withstand 15,500 g to survive the acceleration on firing.

In the automotive industry the g is mainly used in relation to cornering forces and impact analysis.

NASCAR Sprint Cup driver Jeff Gordon experienced the third-highest ranked g-force crash recorded by NASCAR at the 2006 Pennsylvania 500 race at Pocono Raceway, measuring an unprecedented 64 g. Gordon reported that at the time, it was the hardest hit he ever took in a car.

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 Bräck's site his car recorded 214 g.

Formula One drivers usually experience 5 g while braking, 2 g while accelerating, and 4 to 6 g while cornering. Every Formula One car has an ADR (Accident Data Recorder) device installed, which records speed and g-forces. According to the FIA, Robert Kubica of BMW Sauber experienced 75 g during his 2007 Canadian Grand Prix crash.

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 rest over a distance of 66 cm (26 inches) after his throttle got stuck wide open and he hit a wall.

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