Weightlessness

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.

The definition and use of 'Weightlessness' is difficult. Weight means the force exerted by gravity, weightless means the absence of such forces and weightlessness formally means the condition of zero gravitational force. In common use, however, 'weightlessness' (often with quotation marks) has the meaning given in the preceding paragraph. Astronauts and cosmonauts in the International Space Station are said to experience 'weightlessness', even though, at an altitude of a few hundred km, their weight (the gravitational force acting on them) is only about 10% less than on earth. Their orbit has a large centripetal acceleration towards the earth and their weight is the centripetal force producing it. However, the space station has (almost) exactly the same acceleration towards the earth. Consequently, in the frame of the space station, an unsupported person appears to have no acceleration and so his relative motion in this frame is the same as that of a person without weight in a spacecraft that is not accelerating. Hence the name 'weightlessness'.

If objects are far from a star, planet, moon, or other such massive body, so that they experience very little gravitational interaction with them, they would approach the condition of zero gravity. If they are close to a massive object, but are freely accelerating towards the mass by gravitational acceleration only, they are in free fall and are weightless. Physically, they both follow Newton's first law of motion which describes linear motion. Such a situation, except for microgravity effects and the inhomogeneity of the gravitational field, cannot be distinguished from weightlessness due to the absence of gravity from a body nearby.

As an example, an accelerated free fall trajectory results in the weightlessness of objects in a falling elevator. The same type of accelerated free fall trajectory causes weightlessness of objects in orbit about the Earth. Such objects are in free fall toward the Earth, as in the falling elevator, but they do not strike the Earth because their forward speed is such that the curved surface of the Earth drops downward and away from the object as fast as the object falls toward the Earth. An astronaut inside an orbiting vehicle has the experience of weightlessness because the action and acceleration due to gravity by itself does not cause a sensation of weight, and all of the other types of forces that do cause such sensations (such as mechanical pushes from the floor or other surfaces that cause g-force acceleration) are absent.