g-force
The measurement of g-force (or g-load) can be performed using an accelerometer which gives a measurement of an object's non-gravitational acceleration.[1] The unit of measure is informally but commonly known as the “gee” (symbol: g), Template:PronEng.
The gee is a non-SI unit that was first used in aeronautical and space engineering and is equal to one standard gravity (symbol: gn), which is defined as precisely 9.80665 meters per second per second (m/s2), or 980.665 gal (≈32.174 ft/s2).[2][3] The unit symbol for the gee, g, is usually lowercase and roman (upright)[2][4] (the same symbol as for the gram), but may also be uppercase roman G.[5]
The gee and its symbol, g (or G) should not be confused with the universal gravitational constant, (symbol G), which is a physical constant that fundamentally relates mass and gravitational attraction.
Nature of the measure
Acceleration is a phenomenon familiar to anyone who has ridden in an automobile, as it is the rate at which speed or velocity changes. Whenever a vehicle changes direction or speed, one feels lateral (side to side) and longitudinal (forward and backwards) forces. The value of one gee, 9.80665 meter per second per second, might be expressed in terms of m/s2 or in scientific literature as m s−2.
Acceleration and the gee can be expressed in more familiar terms: an acceleration of 1 g is a rate of change in velocity of approximately 35 km/hr for each second that elapses (22 mph per second). A high-performance automobile can brake (decelerate) at around 1 g. Accordingly, a high-performance automobile that is traveling at a speed of 35 km/hr can brake at 1 g to a stop in one second. An automobile traveling at three times this speed, 66 mph, can brake to a stop in about three seconds. The expression “1 g = 9.80665 m s−2 ” means that for every second that elapses, velocity changes 9.80665 meters per second (≡35.30394 km/hr). This rate of change in velocity can also be denoted as 9.80665 (meter per second) per second, or 9.80665 m/s2.
Gravitational and inertial acceleration
An accelerometer measures acceleration in one or more axis. It responds to only non gravitational acceleration.[6] If you orient a stationary, single-axis accelerometer so its measuring axis is horizontal, its output will show zero gee. Yet, if you rotate the accelerometer 90° so its axis points upwards, it will read +1 g upwards even though still stationary. If you mount the accelerometer in an automobile with its axis aligned forward with the vehicle’s direction of travel, and drive down the road at a constant speed, it will read 0 g. Yet, if you hit the brakes, it will read about −0.9 g. Accelerometers respond equally to gravity and inertial acceleration.
The connection between inertial and gravitational acceleration is profound. Albert Einstein showed in his 1916 paper on general theory of relativity that gravitational and inertial accelerations are identical and indistinguishable. According to general relativity, the force of gravity is a consequence of the curvature of spacetime. Consequently, a stationary object on earth’s surface is perpetually being accelerated by earth’s surface upwards through spacetime at 1 g, which causes all stationary objects on the earth’s surface to generate a force—weight—downwards that is proportional to their mass. A one‑kilogram mass on the earth’s surface has a weight of between 9.76 newtons (N) to 9.83 N (see also Mass versus weight ). In 1901, the International Bureau of Weights and Measures (known also by its French-language intials “BIPM”) set the value of standard gravity, gn, at 9.80665 m/s2. This established the standard force (weight) of a one-kilogram mass as being 9.80665 N. The avoirdupois pound, the unit of mass still used with U.S. customary units, has a standard weight of one pound-force.
The effect of gravity means that all stationary masses generate a downward force of about 9.8 newtons per kilogram and all stationary accelerometers aligned with earth’s barycenter indicate that they are being accelerated upwards at about 9.8 m/s2. Unless one is an astronaut, there are only two ways to make a three-axis accelerometer output zero‑g on all three axes: drop it, or put it into a ballistic trajectory. Some notable amusement park rides can provide several seconds at near-zero g. Riding NASA’s “Vomit Comet” provides near-zero g for about 25 seconds at a time.
A single-axis accelerometer mounted in an airplane with its measurement axis oriented vertically reads +1 g when the plane is parked. When flying at a stable altitude (or at a constant rate of climb or descent), the accelerometer will continue to indicate 1 g. Under such conditions, the downward force acting upon the pilot’s body is the normal value of about 9.8 newtons per kilogram (N/kg) (one pound-force per pound). If the pilot pulls back on the stick until the accelerometer indicates 2 g, his weight (the force acting downwards on him) will double to 19.6 N/kg. A spring-based weighing scale, for the duration of a 2 g pitch-up maneuver, would reveal that his weight has truly doubled; a pilot who normally weighs 160 pounds would momentarily weigh 320 pounds.
Human tolerance
Human tolerances depend on the magnitude of the g-force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body[7].
The human body is flexible and deformable, particularly the softer tissues. A hard slap on the face may briefly impose hundreds of g locally but not produce any real damage; a constant 16 g 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. In addition, some illnesses, particularly cardiovascular problems, reduce g-tolerance.
Vertical axis g-force
Aircraft, in particular, exert g-force along 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, 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 loss of consciousness will occur, 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, 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 g's, which drive blood to the head, is much lower. This limit is typically in the −2 to −3 g (−20 m/s² to −30 m/s²) range. The subject's vision turns red, 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 up to 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 179.8 g.[8]
Horizontal axis g-force
The human body is better at surviving g-forces that are perpendicular to the spine. In general when the acceleration is forwards, so that the g-force pushes the body backwards (colloquially known as "eyeballs in"[9]) a much higher tolerance is shown than when the acceleration is backwards, and the g-force is pushing the body forwards ("eyeballs out") since blood vessels in the retina appear more sensitive in the latter 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.[10]
Notable accelerations
| Value (or range) | |
| The gyro rotors in Gravity Probe B and the free-floating proof masses in the TRIAD I navigation satellite[12] |
0 g |
| Moon surface at equator | 0.1654 g |
| Earth surface, sea level – standard | 1 g |
| Saturn V moon rocket just after launch | 1.14 g |
| Space Shuttle, maximum during launch and reentry | 3 g |
| High-g roller coasters[13] | 3.5–5 g |
| Apollo 16 on reentry[14] | 7.19 g |
| Typical max. turn in an aerobatic plane or fighter jet turn | 9 g |
| Maximum for human on a rocket sled | 46.2 g |
| Sprint missile | 100 g |
| Brief human exposure survived in crash[8] | 180 g |
| Shock capability of mechanical wrist watches[15] |
5,000–7,500 g |
| Rating of electronics built into military artillery shells[16] | 15,500 g |
| 9 × 19 Parabellum handgun bullet (average along the length of the barrel)[17] |
31,000 g |
| 9 × 19 Parabellum handgun bullet, peak[18] | 190,000 g |
See also
References
- ^ Eshbach's Handbook of Engineering Fundamentals By Ovid W. Eshbach, Byron pg 9
- ^ a b ESA: GOCE, Basic Measurement Units
- ^ BIPM: Declaration on the unit of mass and on the definition of weight; conventional value of gn
- ^ NASA: Multiple G, Astronautix: Stapp, Honeywell: Accelerometers, Sensr LLC: GP1 Programmable Accelerometer, Farnell: accelometers, NASA: CONSTANTS AND EQUATIONS FOR CALCULATIONS, Jet Propulsion Laboratory: A Discussion of Various Measures of Altitude
- ^ Lyndon B. Johnson Space Center: ENVIRONMENTAL FACTORS: BIOMEDICAL RESULTS OF APOLLO, Section II, Chapter 5, Honywell: Model JTF, General Purpose Accelerometer
- ^ Cite error: The named reference
Eschbachwas invoked but never defined (see the help page). - ^ Beyond the Black Box: the Forensics of Airplane Crashes; George Bibel, John Hopkins University Press, 2008 (ISBN 0-8018-8631-7), p350
- ^ a b 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. Anton Sukup (1977). "David PURLEY Silverstone crash". Retrieved July 31.
{{cite web}}: Check date values in:|accessdate=(help); Unknown parameter|accessyear=ignored (|access-date=suggested) (help) - ^ NASA Physiological Acceleration Systems
- ^ 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
- ^ The Ejection Site: The Story of John Paul Stapp
- ^ Stanford University: Gravity Probe B, Payload & Spacecraft, and NASA: Investigation of Drag-Free Control Technology for Earth Science Constellation Missions. The TRIAD 1 satellite was a later, more advanced navigation satellite that was part of the U.S. Navy’s Transit, or NAVSAT system.
- ^ Beyond the Black Box: the Forensics of Airplane Crashes; George Bibel, John Hopkins University Press, 2008 (ISBN 0-8018-8631-7), p340
- ^ NASA: Table 2: Apollo Manned Space Flight Reentry G Levels
- ^ Omega FAQ, Ball Watch Technology
- ^ "L-3 Communication's IEC Awarded Contract with Raytheon for Common Air Launched Navigation System".
- ^ Assuming a 124 grain (8.04 gram) bullet, a muzzle velocity of 1,150 ft/s (350 m/s), and a 4‑inch (102 mm) barrel.
- ^ Assuming a 124 grain (8.04 gram) bullet, a peak pressure of 35,000 psi (241 MPa) and 100 pounds (440 N) of friction.
