The term g-force is used to convey values of force and acceleration
- Force - expressed as a proportion of the nominal gravitational force experienced when standing on the earth's surface, or
- Acceleration - expressed as a proportion of the nominal gravitational acceleration experienced when in free-fall just above the earth's surface.
Astronauts, fighter pilots and Formula One drivers might, for example, be said to experience a force or acceleration of so many ‘g’ when performing a particular manoeuvre.
Unfortunately, use of the letter ‘g’ sometimes leads to the belief that the force involved is produced gravitationally, which is rarely the case. For example, when an astronaut alters trajectory, a pilot changes speed or direction or a racing driver goes around a corner, it is the rocket, jet or internal combustion engine that provides the force needed to accelerate them, not gravity.
In these cases the magnitude of the forces involved can be several times higher than the value of the Earth's gravitational force and hence, for example, they are said to experience 2 g, 3 g, 6 g, etc.
The effect of high acceleration rates on humans can be very significant and most humans start to pass out when subjected to a sustained acceleration of a few g. But it is only when the acceleration is sustained that such an effect is noticeable, let alone significant. Jumping from the lowest step of a stairway onto a hard floor, for example, can produce a deceleration of many g on landing but only for an instant (depending on what type of shoes you have on and how straight your knees are!).
On 13 July 1977, British racing driver David Purley survived a deceleration from 173 km/h to zero in a distance of about 0.66 m, enduring 180 g*.
* Source: Guinness Book of Records