# g, G

## 1

A unit of acceleration approximately equal to the acceleration due to gravity at the Earth's surface, internationally standardized at 9.80665 meters per second per second (about 32.1740 feet per second per second). Its symbol is a lowercase g.

The force of gravity varies from place to place, and with altitude, and is measured in g. Such measurements play a major role in exploring for oil, for example. Portable instruments can measure g to 1 part in 10⁸, and laboratory instruments can obtain even higher precision.

## 2

A unit of force, sometimes called the G-force. The symbol is often, erroneously, g. One G is the amount of force needed to produce an acceleration of 1 g, in the sense of definition 1 above. G is used to describe forces produced, for example, in car crashes and roller coasters, or those experienced by an airplane pilot making a sharp turn. In this use, it should always be capitalized to distinguish it from g, the symbol for gram.

The number of G’s a human can stand depends on the force’s duration and also on the direction from which it comes. A seated person exposed to 30 G’s acceleration downwards for a fiftieth of a second will suffer severe injury, while accelerating forward for a fiftieth of a second, 50 G’s is needed to cause severe injury. The greatest acceleration the average person can withstand for a sustained period is about 3 to 4 G’s, but higher levels are tolerated briefly (some amusement park rides involve forces of more than 6 G’s for a fraction of a second).

Contrary to the popular impression, the launch forces on astronauts are not very great: only about 3 G’s. Fighter pilots, however, are routinely exposed to forces of 9 G’s or more when they make high speed turns. In training, fighter pilots are subjected to high G’s in a centrifuge: 15 G’s chest-to-back for 5 seconds, 12 G’s chest-to-back for 15 seconds. Humans cannot endure such forces head-to-foot without losing consciousness, since blood is forced from the brain. To prevent this, fighter pilots wear G-suits, which squeeze the legs and abdomen to force blood to the head.

The greatest acceleration experienced by a human who survived occurred on 10 December 1954, during the deceleration of the rocket sled Sonic Wind I, with Dr. John Paul Stapp aboard. It went from 632 miles per hour to a full stop in five seconds, subjecting Dr. Stapp to 46.2 G's.

The National Transportation Safety Board requires aircraft flight recorders to withstand 3,400 G's for 6.5 milliseconds. (They are tested by being shot from a cannon into a “calibrated pillow.”²)

1. M. F. Fraser.
in J. F. Parker and V. R. West, editors.
Bioastronautics Data Book, 2nd edition.
Washington: National Aeronautics and Space Administration, 1973.

Pages 149-219.

2. American Heritage of Invention and Technology, Fall 1996, page 30.

## resources

A biography of Stapp.

www.nmspacemuseum.org/hallof fame/detail.php?id=46

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