G-force is not really a force, and gravity alone never produces it
Astronauts orbiting Earth are still firmly in its gravity, yet they feel no g-force at all. That is because g-force measures the push of floors, seats and engines against free fall, not gravity itself. Sitting in your chair right now, you are feeling roughly 1 g, supplied by the chair.
The name misleads. A g-force is an acceleration expressed per unit of mass, measured in multiples of standard gravity, which is fixed at 9.80665 metres per second squared. Physicists use the unit to separate acceleration relative to free fall from a plain change of speed. The lowercase g is not an SI unit and should not be confused with the gram, nor with capital G, the gravitational constant.
The crucial point is that g-force only arises when one surface pushes on another. An object resting on the ground experiences 1 g upward because the ground stops it from falling toward the Earth's centre. Remove that contact and let something fall freely in a vacuum, and it registers zero, the state loosely called zero-g. The same weightlessness would occur inside an elevator falling freely, and to a good approximation it is what happens inside a spacecraft in orbit, which is constantly falling around the planet.
Because the push arrives through a surface and spreads inward, it creates internal stress: a standing person's body is compressed, while hanging arms are stretched. The body cannot tell the source apart. Accelerating upward in a lift feels identical to standing on a world with stronger gravity. The unit does not change with location either, so standing on the Moon registers almost exactly one sixth of the Earth value.
Everyday and extreme examples show the range. A dragster pulling away can generate a horizontal 5.3 g. An inverted roller coaster diving toward the ground pins riders into their seats with negative g, since the seat drives them downward faster than gravity would. Collisions produce huge g-forces for tiny fractions of a second, and military and aerobatic pilots must resist high loads to avoid g-induced loss of consciousness. Accelerometers, and sometimes carefully calibrated scales, are used to measure it.
Source: G-force
