g-force (also gee-force, gee-loading) is a measurement of an object's acceleration expressed in gees. The "gee" (pronounced ; symbol g) is a non-SI unit equal to the nominal acceleration due to gravity on Earth at sea level, defined as 9.80665 m/s2, or approximately 32.174 ft/s2. More precisely, g-force measures the difference between the acceleration that an object actually experiences and the acceleration that gravity is trying to impart to it, as explained further below. The symbol g is properly written in lowercase and italic, to distinguish it from the symbol G, the gravitational constant, which is always written in uppercase; and from g, the symbol for gram, which is not italicised.
Although strictly a measurement of acceleration, the term g-force, as its name implies, is popularly understood to refer to the force that an accelerating object "feels", expressed as a multiple of the force that it "feels" when resting stationary on the Earth's surface. The relationship between force and acceleration stems from Newton's second law, F = ma, where F is force, m is mass and a is acceleration. Thus, objects of different mass experience different forces under the same acceleration.
These so-called "g-forces" are experienced, for example, by fighter jet pilots or riders on a roller coaster, and are caused by changes in speed and direction. For example, on a roller coaster high positive g-forces are experienced on the car's path up the hills, where riders feel as if they weigh more than usual. This is reversed on the car's descent where lower g-forces occur, causing the riders to feel lighter or even weightless.
Because of the potential for confusion about whether g-force measures acceleration or force, the term is considered by some to be a misnomer. Scientific usage prefers explicit reference to either acceleration or force, and use of the appropriate units (in the SI system, metres per second squared for acceleration, and newtons for force).