Zero Gravity

Zero Gravity

Weightlessness, or an absence of 'weight', is in fact an absence of stress and strain resulting from externally applied forces, typically contact forces from floors, seats, beds, scales, and the like. Counterintuitively, a uniform gravitational field does not by itself cause stress or strain and a body in free fall in such an environment feels weightless.

When bodies are acted upon by non-gravitational forces, as in a centrifuge, a rotating space station, or within a space ship whose rockets are firing, a sensation of weight is produced as the forces overcome the body's inertia. In such cases, a sensation of weight, in the sense of a state of stress can occur, even if the gravitational field were zero. Far from any source of gravitation, and in the absence of any stress producing forces, a body would be 'doubly weightless'. It would be stress free and mg free.

When the gravitational field is non-uniform, a body in free fall suffers tidal effects and is not stress free. Near a black hole, such effects can be very strong. In the case of the Earth, the effects are minor, especially on objects of relatively small dimension (such as the human body or a spacecraft) and the overall sensation of weightlessness in these cases is preserved.