G-force refers to the force of gravity, equal to a value of 1 for a body at rest at sea level, but which can be increased significantly through vertical or horizontal acceleration. An example of g-force increasing through acceleration is the effect felt by riders on a roller coaster during its downward plunge. The amount of accelerated g-force that a human body can endure is limited, and at levels beginning between 9 and 18 g, it can cause a loss of consciousness.
The symbol for g-force measurement is "g," and should not be confused with the "g" symbol representing grams. The term "g-force" is somewhat misleading because it is not a measure of force, but is instead a measurement of acceleration experienced as an increase in weight. This acceleration is the effect of a force acting upon a body and is not the effect of gravity itself.
Acceleration and speed are not the same, which accounts for the lack of a g-force effect on passengers in fast-moving trains, airplanes or vehicles that are traveling at a steady speed.