The g-force (with g from gravitational) is a measurement of the type
of acceleration that causes a perception of weight. Despite the name,
it is incorrect to consider g-force a fundamental force, as "g-force"
(lower-case character) is a type of acceleration that can be measured
with an accelerometer. Since g-force accelerations indirectly produce
weight, any g-force can be described as a "weight per unit mass" (see
the synonym specific weight). When the g-force acceleration 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 weight for every unit of an object's mass. The types of
forces involved are transmitted through objects by interior mechanical
stresses. The g-force acceleration (except certain electromagnetic
force influences) is the cause of an object's acceleration in relation
to free fall.
The g-force acceleration experienced by an object is due to the vector
sum of all non-gravitational and non-electromagnetic 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.
Gravitation acting alone does not produce a g-force, even though
g-forces are expressed in multiples of the acceleration of a standard
gravity. Thus, the standard gravitational acceleration at the Earth's
surface produces g-force only indirectly, as a result of resistance to
it by mechanical forces. These mechanical forces actually produce the
g-force acceleration on a mass. For example, the 1 g force on an
object sitting on the Earth's surface is caused by 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. (
1 Unit and measurement
3.1 Vertical 3.2 Horizontal
4 Short duration shock, impact, and jerk 5 Other biological responses 6 Typical examples 7 Measurement using an accelerometer 8 See also 9 References 10 Further reading 11 External links
Unit and measurement
The unit of measure of acceleration in the International System of
Units (SI) is m/s2. However, to distinguish acceleration relative to
free fall from simple acceleration (rate of change of velocity), the
unit g (or g) is often used. One g is the acceleration due to gravity
at the Earth's surface and is the standard gravity (symbol: gn),
defined as 7000980665000000000♠9.80665 metres per second
squared, or equivalently 7000980665000000000♠9.80665 newtons
of force per kilogram of mass. Note that the unit definition does not
vary with location — the g-force when standing on the moon is about
The unit g is not one of the SI units, which uses "g" for gram. Also,
"g" should not be confused with "G", which is the standard symbol for
the gravitational constant. This notation is commonly used in
aviation, especially in aerobatic or combat military aviation, to
describe the increased forces that must be overcome by pilots in order
to remain conscious and not
G-LOC (G-induced loss of
Measurement of g-force is typically achieved using an accelerometer
(see discussion below in Measurement using an accelerometer). In
certain cases, g-forces may be measured using suitably calibrated
Specific force is another name that has been used for g-force.
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:
The g-force acting on a stationary object resting on the Earth's surface is 1 g (upwards) and results from the resisting reaction of the Earth's surface bearing upwards equal to an acceleration of 1 g, and is equal and opposite to gravity. The number 1 is approximate, depending on location. The g-force acting on an object in any weightless environment such as free-fall in a vacuum is 0 g. The g-force acting on an object under acceleration can be much greater than 1 g, for example, the dragster pictured at top right can exert a horizontal g-force of 5.3 when accelerating. The g-force acting on an object under acceleration may be downwards, for example when cresting a sharp hill on a roller coaster. If there are no other external forces than gravity, the g-force in a rocket is the thrust per unit mass. Its magnitude is equal to the thrust-to-weight ratio times g, and to the consumption of delta-v per unit time. In the case of a shock, e.g., a collision, the g-force can be very large during a short time.
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
All "coordinate accelerations" (or lack of them), are described by
Newton's laws of motion
This acrobatic airplane is pulling up in a +g maneuver; the pilot is experiencing several g's of inertial acceleration in addition to the force of gravity. The cumulative vertical axis forces acting upon his body make him momentarily 'weigh' many times more than normal.
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 725 newtons (163 lbf). 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 (163 lbf). This mechanical
force provides the 1.0 g-force 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 1,450 N (330 lbf) downwards into his
seat and the seat is simultaneously pushing upwards with an equal
force of 1,450 N (330 lbf).
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 "1 g = 7000980665000000000♠9.80665 m/s2" means
that for every second that elapses, velocity changes
7000980665000000000♠9.80665 meters per second
(≡7000980665000000000♠35.30394 km/h). This rate of change in
velocity can also be denoted as 7000980665000000000♠9.80665 (meter
per second) per second, or 7000980665000000000♠9.80665 m/s2.
For example: An acceleration of 1 g equates to a rate of change
in velocity of approximately 35 kilometres per hour (22 mph) for
each second that elapses. Therefore, if an automobile is capable of
braking at 1 g and is traveling at 35 kilometres per hour
(22 mph) 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, 105 km/h (65 mph), 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/(2s).
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.
Semilog graph of the limits of tolerance of humans to linear acceleration
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.:350 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 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. Vertical Aircraft pilots (in particular) sustain g-forces 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. Positive, or "upward" g, 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 varies. A typical person can handle about 5 g0 (49 m/s2) (meaning some people might pass out when riding a higher-g roller coaster, which in some cases exceeds this point) before losing consciousness, 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 a sustained 9 g0 (88 m/s2) (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:
Grey-out, where the vision loses hue, easily reversible on levelling out. Tunnel vision, where peripheral vision is progressively lost. Blackout, a loss of vision while consciousness is maintained, caused by a lack of blood to the head. G-LOC, a g-force induced loss of consciousness. Death, if g-forces are not quickly reduced, death can occur.
Resistance to "negative" or "downward" g, which drives blood to the
head, is much lower. This limit is typically in the −2 to
−3 g0 (−20 to −29 m/s2) range. This condition is
sometimes referred to as red out where vision is figuratively
reddened due to the blood laden lower eyelid being pulled into the
field of vision Negative g 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
The human body is better at surviving g-forces that are perpendicular
to the spine. In general when the acceleration is forwards (subject
essentially lying on their back, colloquially known as "eyeballs
in") a much higher tolerance is shown than when the acceleration
is backwards (lying on their front, "eyeballs out") since blood
vessels in the retina appear more sensitive in the latter
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 20 g for less than 10 seconds, to 10 g for 1
minute, and 6 g for 10 minutes for both eyeballs in and out.
These forces were endured with cognitive facilities intact, as
subjects were able to perform simple physical and communication tasks.
The tests were determined to not cause long or short term harm
although tolerance was quite subjective, with only the most motivated
non-pilots capable of completing tests. 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 25 g for 1.1 seconds, proving that the
human body is capable of this. Stapp lived another 45 years to age
89 without any ill effects.
The highest recorded
the shock on an object during impact is
is the distance covered during the impact. For example, a stiff and
compact object dropped from 1 m that impacts over a distance of
1 mm is subjected to a 1000 g 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 (g/s; 1 g/s ≈ 9.81 m/s3).
Other biological responses
Recent research carried out on extremophiles in
The gyro rotors in
Gravity Probe B
A ride in the
Standing on the Moon at its equator 0.1654 g
Standing on the Earth at sea level–standard 1 g
Space Shuttle, maximum during launch and reentry 3 g
High-g roller coasters:340 3.5–6.3 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
Formula One car, maximum under heavy braking 6.3 g
Formula One car, peak lateral in turns 6–6.5 g
Luge, maximum expected at the Whistler Sliding Centre 5.2 g
Standard, full aerobatics certified glider +7/−5 g
Maximum permitted g-force in
Maximum permitted g-force in
Maximum permitted g-force turn in
Red Bull Air Race
Maximum for human on a rocket sled 46.2 g
Sprint missile 100 g
Brief human exposure survived in crash > 100 g
Shock capability of mechanical wrist watches > 5,000 g
V8 Formula One engine, maximum piston acceleration 8,600 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) 300,000 g
Mean acceleration of a proton in the Large Hadron Collider 190,000,000 g
Gravitational acceleration at the surface of a typical neutron star 7008200000000000000♠2.0×1011 g
* Including contribution from resistance to gravity. † Directed 40 degrees from horizontal. Measurement using an accelerometer
The Superman: Escape from Krypton roller coaster at Six Flags Magic Mountain provides 6.5 seconds of ballistic weightlessness.
An accelerometer, in its simplest form, is a damped mass on the end of a spring, with some way of measuring how far the mass has moved on the spring in a particular direction, called an 'axis'. Accelerometers are often calibrated 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 ballistic 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 for about 25 seconds at a time. See also
Artificial gravity Earth's gravity Euthanasia Coaster Gravitational acceleration Gravitational interaction Load factor (aeronautics) Peak ground acceleration – g-force of earthquakes Relation between g-force and apparent weight Shock and vibration data logger Shock detector
^ G Force. Newton.dep.anl.gov. Retrieved on 2011-10-14.
^ Sircar, Sabyasachi (2007-12-12). "Principles of Medical Physiology".
^ BIPM: Declaration on the unit of mass and on the definition of
weight; conventional value of gn.
^ Symbol g: ESA: GOCE, Basic Measurement Units, NASA: Multiple G,
Astronautix: Stapp Archived 2009-03-21 at the Wayback Machine.,
Honeywell: Accelerometers Archived 2009-02-17 at the Wayback Machine.,
Sensr LLC: GP1 Programmable
Faller, James E. (November–December 2005). "The Measurement of Little g: A Fertile Ground for Precision Measurement Science" (PDF). Journal of Research of the National Institutes of Standards and Technology. 110 (6): 559–581. doi:10.6028/jres.110.082.
"How Many Gs Can a Flyer Take?", October 1944, Popular Science one of the first detailed public articles explaining this subject Enduring a human centrifuge at the NASA Ames Research Cen