g (also gee, g-force or g-load) is a unit of acceleration defined as exactly 9.806 65 m/s², approximately equal to the acceleration due to gravity on the Earth's surface. Gravity due to the earth is experienced the same as being accelerated upward with an acceleration of 1 g. The total g-force is found by vector addition of the opposite of the actual acceleration (in the sense of rate of change of velocity) and a vector of 1 g downward for the ordinary gravity (or in space, the gravity there). Weightlessness means a zero g-force, which is the result when acceleration due to movement is equal to that due to gravity.
The symbol g is always written in lowercase, to distinguish it from the symbol G, the gravitational constant, which is always written in uppercase.
The value of g defined above is an average over the whole of the Earth's surface. It is sometimes written as gN or g0 to distinguish it from the local value of g that varies with position. The actual acceleration of a body at the Earth's surface depends on the location at which it is measured, for two reasons. The first is that the rotation of the Earth imposes an additional acceleration on the body that opposes that due to gravity. The net downward force on the body is therefore offset by a centrifugal force that acts upwards, reducing its weight. This effect on its own would result in a range of values of g from 9.789 m/s² at the equator to 9.823 m/s² at the poles. The second reason is the Earth's equatorial bulge, which causes objects at the equator to be further from the planet's centre than objects at the poles. Because the force due to gravitational attraction between two bodies (the Earth and the object being weighed) varies inversely with the square of the distance between them, objects at the equator experience a weaker gravitational pull than objects at the poles. Measurements show that the combined result of these two effects is a variation of 0.052 m/s² in the value of g. Practically, this means that the weight of an object can vary by 0.5% depending on where on Earth it is weighed.
The UK's National Physical Laboratory gives the following formula for estimating g:
One often hears the term being applied to the limits that the human body can withstand without "blacking out", sometimes referred to as g-loc. A typical person can handle about 5 g before this occurs, but through the combination of special g-suits and efforts to strain muscles, modern pilots can typically handle 9 g.