Weightlessness (or zero-g) is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall. The physical path of an inertial trajectory depends only on the direction and strength of the sum of the gravitational attractions outside of the inertial reference frame.
Zero-g is subtly different from zero gravity which literally only refers to the complete absence of gravity, something which is impossible due to the presence of gravity everywhere in the universe. However, gravity causes gravitation gradients, which make themselves apparent to any object of finite size in a gravitational field even in free-fall. These gradients cause very small tidal effects which are impossible to remove by inertial motion, except at a single point in space. All other points near this point feel mechanical stresses from the gradient, as a result of being made to travel along with the inertial motion of the reference point, which is a motion not perfectly inertial for points near it. However, "zero-gravity" is usually used synonymously to mean effective weightlessness, neglecting tidal effects. Microgravity (or µg) is used to refer to situations that are substantially weightless but where g-force stresses within objects due to tidal effects, as discussed above, are around a millionth of that at the Earth's surface.
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