G-Force
Definition
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 gs. 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), defined as 9.80665 m/s2 (32.174 ft/s2). The symbol g is properly written in 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 italicised).
Scientific and Engineering Significance
Analysis of g-forces is 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 localised g-forces in the hundreds for a split second; a hard slap on the face may impose hundreds of g locally without producing any real damage. However, sustained g-forces above about 16 g for a minute can be deadly or lead to permanent injury.
Human Tolerance
There is considerable variation among individuals when it comes to g-force tolerance. Race car drivers have survived instantaneous accelerations of up to 214 g during accidents.
To some degree, g-tolerance can be trained, and there is also considerable variation in innate ability between individuals. In addition, certain 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.
Everyday and Recreational Examples
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. On a roller coaster, high positive g is experienced when the car’s path curves upwards, where riders feel heavier than usual. This reverses 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.
Newton’s Second Law
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. This means 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 a measure of force in absolute terms.
Variation Across Celestial Bodies
g-force varies on different planets and 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, and 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 – smaller than might be expected 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 ranges between 2x10^11 and 3x10^12 g.
Aerospace Applications
In the aerospace industry, the g is a convenient unit for specifying the maximum load factor that aircraft and spacecraft must be capable of withstanding. Light aircraft of the kind used in pilot training (utility category) must sustain a load factor of 4.4 g (43 m/s2, 141.5 ft/s2) with the undercarriage retracted. Airliners and other transport aircraft must be capable of 2.5 g. Military aircraft and pilots (especially fighter pilots) wearing g-suits can withstand more than 9 g.
Very short-term accelerations, measured in milliseconds, are usually referred to as shocks and are often measured in gs. The shock that a device or component must withstand may be specified in g. For example, mechanical wristwatches 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.
Automotive Applications
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 it was the hardest hit he had ever taken in a car.
IndyCar driver Kenny Brack crashed on lap 188 of the 2003 race at Texas Motor Speedway. Brack and Tomas Scheckter touched wheels, sending Brack into the air at over 200 mph, where his car hit a steel support beam for the catch fencing. According to Brack’s website, his car recorded 214 g.
G-Forces in Formula 1
Formula 1 drivers usually experience 5 g while braking, 2 g while accelerating, and 4 to 6 g while cornering. Every Formula 1 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 1 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 became stuck wide open and he hit a wall.