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The g-force or gravitational force equivalent is a Specific force (force per unit mass), expressed in units of standard gravity (symbol g or g0, not to be confused with "g", the symbol for grams). It is used for sustained that cause a perception of weight. For example, an object at rest on Earth's surface is subject to 1 g, equaling the conventional value of gravitational acceleration on Earth, about . More transient acceleration, accompanied with significant jerk, is called shock.
When the g-force is produced by the surface of one object being pushed by the surface of another object, the reaction force to this push produces an equal and opposite force for every unit of each object's mass. The types of forces involved are transmitted through objects by interior mechanical stresses. Gravitational acceleration is one cause of an object's acceleration in relation to free fall. G Force . Newton.dep.anl.gov. Retrieved on 14 October 2011.
The g-force experienced by an object is due to the vector sum of all gravitational and non-gravitational forces acting on an object's freedom to move. In practice, as noted, these are surface-contact forces between objects. Such forces cause stresses and strains on objects, since they must be transmitted from an object surface. Because of these strains, large g-forces may be destructive.
For example, a force of 1 g on an object sitting on the Earth's surface is caused by the mechanical force exerted in the upward direction by the ground, keeping the object from going into free fall. The upward contact force from the ground ensures that an object at rest on the Earth's surface is accelerating relative to the free-fall condition. (Free fall is the path that the object would follow when falling freely toward the Earth's center). Stress inside the object is ensured from the fact that the ground contact forces are transmitted only from the point of contact with the ground.
Objects allowed to free-fall in an inertial trajectory, under the influence of gravitation only, feel no g-force a condition known as weightlessness. Being in free fall in an inertial trajectory is colloquially called "zero- g", which is short for "zero g-force". Zero g-force conditions would occur inside an elevator falling freely toward the Earth's center (in vacuum), or (to good approximation) inside a spacecraft in Earth orbit. These are examples of coordinate acceleration (a change in velocity) without a sensation of weight.
In the absence of gravitational fields, or in directions at right angles to them, proper and coordinate accelerations are the same, and any coordinate acceleration must be produced by a corresponding g-force acceleration. An example of this is a rocket in free space: when the engines produce simple changes in velocity, those changes cause g-forces on the rocket and the passengers.
Measurement of g-force is typically achieved using an accelerometer (see discussion below in section #Measurement using an accelerometer). In certain cases, g-forces may be measured using suitably calibrated scales.
The reason for the minus sign is that the actual force (i.e., measured weight) on an object produced by a g-force is in the opposite direction to the sign of the g-force, since in physics, weight is not the force that produces the acceleration, but rather the equal-and-opposite reaction force to it. If the direction upward is taken as positive (the normal cartesian convention) then positive g-force (an acceleration vector that points upward) produces a force/weight on any mass, that acts downward (an example is positive-g acceleration of a rocket launch, producing downward weight). In the same way, a negative-g force is an acceleration vector downward (the negative direction on the y axis), and this acceleration downward produces a weight-force in a direction upward (thus pulling a pilot upward out of the seat, and forcing blood toward the head of a normally oriented pilot).
If a g-force (acceleration) is vertically upward and is applied by the ground (which is accelerating through space-time) or applied by the floor of an elevator to a standing person, most of the body experiences compressive stress which at any height, if multiplied by the area, is the related mechanical force, which is the product of the g-force and the supported mass (the mass above the level of support, including arms hanging down from above that level). At the same time, the arms themselves experience a tensile stress, which at any height, if multiplied by the area, is again the related mechanical force, which is the product of the g-force and the mass hanging below the point of mechanical support. The mechanical resistive force spreads from points of contact with the floor or supporting structure, and gradually decreases toward zero at the unsupported ends (the top in the case of support from below, such as a seat or the floor, the bottom for a hanging part of the body or object). With compressive force counted as negative tensile force, the rate of change of the tensile force in the direction of the g-force, per unit mass (the change between parts of the object such that the slice of the object between them has unit mass), is equal to the g-force plus the non-gravitational external forces on the slice, if any (counted positive in the direction opposite to the g-force).
For a given g-force the stresses are the same, regardless of whether this g-force is caused by mechanical resistance to gravity, or by a coordinate-acceleration (change in velocity) caused by a mechanical force, or by a combination of these. Hence, for people all mechanical forces feels exactly the same whether they cause coordinate acceleration or not. For objects likewise, the question of whether they can withstand the mechanical g-force without damage is the same for any type of g-force. For example, upward acceleration (e.g., increase of speed when going up or decrease of speed when going down) on Earth feels the same as being stationary on a celestial body with a higher surface gravity. Gravitation acting alone does not produce any g-force; g-force is only produced from mechanical pushes and pulls. For a free body (one that is free to move in space) such g-forces only arise as the "inertial" path that is the natural effect of gravitation, or the natural effect of the inertia of mass, is modified. Such modification may only arise from influences other than gravitation.
Examples of important situations involving g-forces include:
A classic example of negative g-force is in a fully inverted roller coaster which is accelerating (changing velocity) toward the ground. In this case, the roller coaster riders are accelerated toward the ground faster than gravity would accelerate them, and are thus pinned upside down in their seats. In this case, the mechanical force exerted by the seat causes the g-force by altering the path of the passenger downward in a way that differs from gravitational acceleration. The difference in downward motion, now faster than gravity would provide, is caused by the push of the seat, and it results in a g-force toward the ground.
All "coordinate accelerations" (or lack of them), are described by Newton's laws of motion as follows:
The second law of motion, the law of acceleration, states that meaning that a force F acting on a body is equal to the mass m of the body times its acceleration a.
The third law of motion, the law of reciprocal actions, states that all forces occur in pairs, and these two forces are equal in magnitude and opposite in direction. Newton's third law of motion means that not only does gravity behave as a force acting downwards on, say, a rock held in your hand but also that the rock exerts a force on the Earth, equal in magnitude and opposite in direction.
In an airplane, the pilot's seat can be thought of as the hand holding the rock, the pilot as the rock. When flying straight and level at 1 g, the pilot is acted upon by the force of gravity. His weight (a downward force) is . In accordance with Newton's third law, the plane and the seat underneath the pilot provides an equal and opposite force pushing upwards with a force of 725 N. This mechanical force provides the 1.0 g upward proper acceleration on the pilot, even though this velocity in the upward direction does not change (this is similar to the situation of a person standing on the ground, where the ground provides this force and this g-force).
If the pilot were suddenly to pull back on the stick and make his plane accelerate upwards at 9.8 m/s2, the total g‑force on his body is 2 g, half of which comes from the seat pushing the pilot to resist gravity, and half from the seat pushing the pilot to cause his upward acceleration—a change in velocity which also is a proper acceleration because it also differs from a free fall trajectory. Considered in the frame of reference of the plane his body is now generating a force of downwards into his seat and the seat is simultaneously pushing upwards with an equal force of 1450 N.
Unopposed acceleration due to mechanical forces, and consequentially g-force, is experienced whenever anyone rides in a vehicle because it always causes a proper acceleration, and (in the absence of gravity) also always a coordinate acceleration (where velocity changes). Whenever the vehicle changes either direction or speed, the occupants feel lateral (side to side) or longitudinal (forward and backwards) forces produced by the mechanical push of their seats.
The expression means that for every second that elapses, velocity changes metres per second (). This rate of change in velocity can also be denoted as (metres per second) per second, or For example: An acceleration of 1 g equates to a rate of change in velocity of approximately for each second that elapses. Therefore, if an automobile is capable of braking at 1 g and is traveling at 35 km/h, it can brake to a standstill in one second and the driver will experience a deceleration of 1 g. The automobile traveling at three times this speed, , can brake to a standstill in three seconds.
In the case of an increase in speed from 0 to v with constant acceleration within a distance of s this acceleration is v2/(2 s).
Preparing an object for g-tolerance (not getting damaged when subjected to a high g-force) is called g-hardening. This may apply to, e.g., instruments in a projectile shot by a gun.
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-force levels can be severely damaging if they are at the resonance of organs or 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.
Positive, or "upward" g-force, drives blood downward to the feet of a seated or standing person (more naturally, the feet and body may be seen as being driven by the upward force of the floor and seat, upward around the blood). Resistance to positive g-force varies. A typical person can handle about (meaning some people might pass out when riding a higher-g roller coaster, which in some cases exceeds this point) before G-LOC, but through the combination of special and efforts to strain muscles—both of which act to force blood back into the brain—modern pilots can typically handle a sustained (see High-G training).
In aircraft particularly, vertical g-forces are often positive (force blood towards the feet and away from the head); this causes problems with the eyes and brain in particular. As positive vertical g-force is progressively increased (such as in a centrifuge) the following symptoms may be experienced:
Resistance to "negative" or "downward" g, which drives blood to the head, is much lower. This limit is typically in the range. This condition is sometimes referred to as red out where vision is literally reddened due to the blood-laden lower eyelid being pulled into the field of vision. Negative g-force is generally unpleasant and can cause damage. Blood vessels in the eyes or brain may swell or burst under the increased blood pressure, resulting in degraded sight or even blindness.
Early experiments showed that untrained humans were able to tolerate a range of accelerations depending on the time of exposure. This ranged from as much as for less than 10 seconds, to for 1 minute, and for 10 minutes for both eyeballs in and out. 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. Wingrove, figure 10 These forces were endured with cognitive facilities intact, as subjects were able to perform simple physical and communication tasks. The tests were determined not to cause long- or short-term harm although tolerance was quite subjective, with only the most motivated non-pilots capable of completing tests. 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 record for peak experimental horizontal g-force tolerance is held by acceleration pioneer John Stapp, in a series of rocket sled deceleration experiments culminating in a late 1954 test in which he was clocked in a little over a second from a land speed of Mach 0.9. He survived a peak "eyeballs-out" acceleration of 46.2 times the acceleration of gravity, and more than for 1.1 seconds, proving that the human body is capable of this. Stapp lived another 45 years to age 89 Fastest Man on Earth – John Paul Stapp . Ejection Site. Retrieved on 14 October 2011. without any ill effects.
The highest recorded g-force experienced by a human who survived was during the 2003 IndyCar Series finale at Texas Motor Speedway on 12 October 2003, in the 2003 Chevy 500 when the car driven by Kenny Bräck made wheel-to-wheel contact with Tomas Scheckter's car. This immediately resulted in Bräck's car impacting the catch fence that would record a peak of .
After a free fall from a height followed by deceleration over a distance during an impact, the shock on an object is · . For example, a stiff and compact object dropped from 1 m that impacts over a distance of 1 mm is subjected to a 1000 deceleration.
Jerk is the rate of change of acceleration. In SI units, jerk is expressed as m/s3; it can also be expressed in standard gravity per second (/s; 1 /s ≈ 9.81 m/s3).
| The gyro rotors in Gravity Probe B and the free-floating proof masses in the TRIAD I navigation satelliteStanford 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. | 0 g |
| A ride in the Vomit Comet (parabolic flight) | ≈ 0 g |
| Standing on Mimas, the smallest and least massive known body rounded by its own gravity | 0.006 g |
| Standing on Ceres, the smallest and least massive known body currently in hydrostatic equilibrium | 0.029 g |
| Standing on Pluto at average ground level | 0.063 g |
| Standing on Eris at average ground level | 0.084 g |
| Standing on Titan at average ground level | 0.138 g |
| Standing on Ganymede at average surface level | 0.146 g |
| Standing on the Moon at surface level | 0.1657 g |
| 2000 Toyota Sienna from 0 to 100 km/h in 9.2 s | 0.3075–0.314 g |
| Standing on Mercury | 0.377 g |
| Standing on Mars at its equator at mean ground level | 0.378 g |
| Standing on Venus at average ground level | 0.905 g |
| Standing on Earth at sea level–standard | 1 g |
| Saturn V Moon rocket just after launch and the gravity of Neptune where atmospheric pressure is about Earth's | 1.14 g |
| Bugatti Veyron from 0 to 100 km/h in 2.4 s | 1.55 g |
| Gravitron amusement ride | 2.5–3 g |
| Gravity of Jupiter at its mid-latitudes and where atmospheric pressure is about Earth's | 2.528 g |
| Uninhibited sneeze after sniffing ground pepper | 2.9 g |
| Space Shuttle, maximum during launch and reentry | 3 g |
| High-g | 3.5–12 g |
| Hearty greeting slap on upper back | 4.1 g |
| Top Fuel drag racing world record of 4.4 s over 1/4 mile | 4.2 g |
| First world war aircraft (ex:Sopwith Camel, Fokker Dr.1, SPAD S.XIII, Nieuport 17, Albatros D.III) in dogfight maneuvering. | 4.5–7 g |
| Luge, maximum expected at the Whistler Sliding Centre | 5.2 g |
| Shock Wave, current highest g-force rollercoaster in operation | 5.9 g |
| Formula One car, maximum under heavy braking | 6.3 g |
| Tower Of Terror, highest g-force steel rollercoaster | 6.3 g |
| Formula One car, peak lateral in turns6 g has been recorded in the 130R turn at Suzuka circuit, Japan. Many turns have 5 g peak values, like turn 8 at Istanbul or Eau Rouge at Spa | 6–6.5 g |
| Standard, full aerobatics certified Glider aircraft | +7/−5 g |
| Apollo 16 on reentryNASA: Table 2: Apollo Manned Space Flight Reentry G Levels Lsda.jsc.nasa.gov | 7.19 g |
| Maximum permitted g-force in Sukhoi Su-27 plane | 9 g |
| Maximum permitted g-force in Mikoyan MiG-35 plane and maximum permitted g-force turn in Red Bull Air Race planes | 10 g |
| Flip Flap Railway, highest g-force wooden rollercoaster | 12 g |
| Jet Fighter pilot during ejection seat activation | 15–25 g |
| Gravitational acceleration at the surface of the Sun | 28 g |
| Maximum g-force in Tor missile system"Russia trains Greek Tor-M1 crews". RIA Novosti. 27 December 2007. Retrieved 2008-09-04. | 30 g |
| Maximum for human on a rocket sled | 46.2 g |
| Formula One 2021 British Grand Prix Max Verstappen Crash with Lewis Hamilton | 51 g |
| Formula One 2020 Bahrain Grand Prix Romain Grosjean Crash | 67 g |
| Sprint missile | 100 g |
| Brief human exposure survived in crash"Several Indy car drivers have withstood impacts in excess of 100 G without serious injuries." Dennis F. Shanahan, M.D., M.P.H.: Human Tolerance and Crash Survivability , citing Society of Automotive Engineers. Indy racecar crash analysis. Automotive Engineering International, June 1999, 87–90. And National Highway Traffic Safety Administration: Recording Automotive Crash Event Data | > 100 g |
| IndyCar 2003 Texas Kenny Bräck Crash | 214 g |
| Formula One 2014 Japanese Grand Prix Jules Bianchi Crash | 254 g |
| Formula One 1994 Monaco Grand Prix Karl WendlingerMellor, Andrew. "Formula One Accident Investigations." SAE Technical Paper 2000-01-3552 (2000). https://doi.org/10.4271/2000-01-3552. Crash | ≈360 g |
| Coronal mass ejection (Sun) | 480 g |
| Formula One 1994 San Marino Grand Prix Roland Ratzenberger Qualifying Crash | 500 g |
| Space gun with a barrel length of 1 km and a muzzle velocity of 6 km/s, as proposed by Quicklaunch (assuming constant acceleration) | 1,800 g |
| Shock capability of mechanical wrist watches | > 5,000 g |
| V8 Formula One engine, maximum piston acceleration | 8,600 g |
| Mantis Shrimp, acceleration of claw during predatory strike | 10,400 g |
| Rating of electronics built into military artillery shells | 15,500 g |
| Analytical ultracentrifuge spinning at 60,000 rpm, at the bottom of the analysis cell (7.2 cm)(rpm·π/30)2·0.072/g | 300,000 g |
| Calculated acceleration of the mandibles of the ant species Mystrium | 607,805 g |
| Acceleration of a Cnidocyte: the fastest recorded acceleration from any biological entity. | 5,410,000 g |
| Mean acceleration of a proton in the Large Hadron Collider(7 TeV/(20 minutes·c))/proton mass | 190,000,000 g |
| Gravitational acceleration at the surface of a typical neutron star Extract of page 322 note: = | |
| Acceleration from a wakefield plasma accelerator(42 geV/85 cm)/electron mass |
Accelerometers are often to measure g-force along one or more axes. If a stationary, single-axis accelerometer is oriented so that its measuring axis is horizontal, its output will be 0 g, and it will continue to be 0 g if mounted in an automobile traveling at a constant velocity on a level road. When the driver presses on the brake or gas pedal, the accelerometer will register positive or negative acceleration.
If the accelerometer is rotated by 90° so that it is vertical, it will read +1 g upwards even though stationary. In that situation, the accelerometer is subject to two forces: the gravitational force and the ground reaction force of the surface it is resting on. Only the latter force can be measured by the accelerometer, due to mechanical interaction between the accelerometer and the ground. The reading is the acceleration the instrument would have if it were exclusively subject to that force.
A three-axis accelerometer will output zero‑g on all three axes if it is dropped or otherwise put into a Ballistics trajectory (also known as an inertial trajectory), so that it experiences "free fall", as do astronauts in orbit (astronauts experience small tidal accelerations called microgravity, which are neglected for the sake of discussion here). Some amusement park rides can provide several seconds at near-zero g. Riding NASA's "Vomit Comet" provides near-zero g-force for about 25 seconds at a time.
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