g-force

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.

Objects allowed to free-fall in an inertial trajectory, under the influence of gravitation only, feel no g-force – a condition known as weightlessness.

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).

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.

For example: An acceleration of 1 g equates to a rate of change in velocity of approximately 35 km/h (22 mph) for each second that elapses.

Human tolerances depend on the magnitude of the gravitational force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body.

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 resonant frequency of organs or connective tissues.

[citation needed] To some degree, g-tolerance can be trainable, and there is also considerable variation in innate ability between individuals.

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.

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:[citation needed] Resistance to "negative" or "downward" g, which drives blood to the head, is much lower.

[citation needed] Early experiments showed that untrained humans were able to tolerate a range of accelerations depending on the time of exposure.

[15] These forces were endured with cognitive facilities intact, as subjects were able to perform simple physical and communication tasks.

[16] 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.

For example, a stiff and compact object dropped from 1 m that impacts over a distance of 1 mm is subjected to a 1000 ɡ0 deceleration.

[citation needed] Recent research carried out on extremophiles in Japan involved a variety of bacteria (including E. coli as a non-extremophile control) being subject to conditions of extreme gravity.

Analysis showed that the small size of prokaryotic cells is essential for successful growth under hypergravity.

Notably, two multicellular species, the nematodes Panagrolaimus superbus[21] and Caenorhabditis elegans were shown to be able to tolerate 400,000 × g for 1 hour.

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.

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).