The g-force experienced by an object is its acceleration relative to free-fall. The term g-force is considered a misnomer, as g-force is not a force but an acceleration. The use of "force" in the term has been popular because such proper accelerations cannot be produced by gravity, but instead must result from other forces which usually cause stresses and strains on objects. For example, the "1 g-force" acing on an object sitting on the Earth's surface is not caused by gravity, but by the stressful mechanical force of the materials (such as the ground) which keep the object from going into free fall.
The unit of measure used is the g- the acceleration due to gravity at the Earth's surface and it can be written g, g, or G. The unit g is not one of the SI International System of Units which uses "g" for gram, and "G" could be confused with the standard symbol for the gravitational constant, but they are both distinct. The SI unit of acceleration is m/s2. However, objects experiencing g-forces are not necessarily changing velocity or position, so standard units of acceleration, which do not necessarily require stress in free fall, are by convention not used to express g-force.
Measurement of g-force is typically achieved using an accelerometer (see discussion below in Measuring g-force using an accelerometer). In certain cases g-forces may be measured using suitably calibrated scales.
The term g-force is technically incorrect as it is a measure of acceleration, not force. The mechanical stresses which a g-force produces creates forces which are however felt. This is so even if there is no overall ("coordinate") acceleration, as in the example of an object sitting on the ground. As accelerations, g-forces are considered as a vector quantity; thus g-forces can be negative. A classic example of negative g is in a fully inverted roller coaster. In this case, the roller coaster riders are accelerated toward the ground faster than gravity when going over the top of the loop and are pinned upside down in their seats by a net pull in the same direction to gravity. In this case, the mechanical force exerted by the seat causes the g-force by altering the path of the passenger. Whether one is accelerating in a free‑fall, or acting against gravitational acceleration by standing in one spot on the surface of the earth, or accelerating or decelerating in any direction, all accelerations, or the lack thereof, are described by Newton's laws of motion as follows:
The Second Law of Motion, the law of acceleration states that: F = ma., 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 your hand must generate an equal and opposite force upwards if the rock is to remain stationary. If you drop the rock, there are no longer equal forces acting upon the rock and it will accelerate downwards.
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). If the pilot were to suddenly 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. 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).
Acceleration due to mechanical forces, and consequentially g-force, is experienced whenever anyone rides in a vehicle because it is the rate at which speed (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 = 9.80665 m/s2" means that for every second that elapses, velocity changes 9.80665 meters per second (≡35.30394 km/h). This rate of change in velocity can also be denoted as 9.80665 (meter per second) per second, or 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.
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.
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.
Resistance to positive or "downward" g, which drives blood to the feet, 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, which drives 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.
In aircraft, g-forces are often positive (towards the feet) which forces blood away from the head; this causes problems with the eyes and brain in particular. As g-force is progressively increased the pilot may experience:
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") 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. The record for peak experimental horizontal g-force tolerance is held by acceleration pioneer John Stapp, in a series of rocket sled deceleration experiments in which he survived forces up to 46.2 times the force of gravity for less than a second. Stapp suffered lifelong damage to his vision from this test.
Toleration of g-force also depends on its duration and the rate of change in acceleration, known as jerk. In SI units, jerk is expressed as m/s3. In non-SI units, jerk can be expressed simply as gees per second (g/s). Very short durations or high jerk forces of 100g have been claimed.
|The gyro rotors in Gravity Probe B and the
proof masses in the TRIAD I navigation satellite
|exactly 0 g|
|A ride in the Vomit Comet||approximately 0 g|
|Standing on the Moon at its equator||0.1654 g|
|Standing on the Earth at 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||3.5–6.3 g|
|Formula One car, maximum under heavy braking||5 g|
|Standard, full aerobatics certified Glider||+7/-5 g|
|Apollo 16 on reentry||7.19 g|
|Typical max. turn in an aerobatic plane or fighter jet||9-12 g|
|Maximum for human on a rocket sled||46.2 g|
|Death or serious injury likely||>50 g|
|Sprint missile||100 g|
|Brief human exposure survived in crash||>100 g|
|Shock capability of mechanical
|Rating of electronics built into military artillery shells||15,500 g|
|9 × 19 Parabellum handgun
(average along the length of the barrel)
|9 × 19 Parabellum handgun bullet, peak||190,000 g|
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. But if the car driver brakes sharply, the accelerometer will read about −0.9 g, corresponding to a deceleration.
However, if the accelerometer is rotated by 90°, so that its axis points upwards, it will read +1 g upwards even though still 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 (accelerometers measure only the mechanical components of accelerations, and thus directly read "g-force" acceleration only).
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 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. This is the "g-force" exerted by the ground. When flying at a stable altitude (or at a constant rate of climb or descent), the accelerometer will continue to indicate 1 g, as the g-force is provided by the aerodynamic lift. 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). If the pilot pulls back on the stick until the accelerometer indicates 2 g, the force acting downwards on him will double to 19.6 N/kg.
Faller, James E. (November-December 2005). "The Measurement of Little g: A Fertile Ground for Precision Measurement Science". Journal of Research of the National Institutes of Standards and Technology 110 (6): 559-581. http://nvl-i.nist.gov/pub/nistpubs/jres/110/6/j110-6fal.pdf.
Symbol G: Lyndon B. Johnson Space Center: ENVIRONMENTAL FACTORS: BIOMEDICAL RESULTS OF APOLLO, Section II, Chapter 5, Honywell: Model JTF, General Purpose Accelerometer
Symbol g: MEMSIC: ACCELEROMETER PRIMER