Weightlessness

Weightlessness, or apparent weightlessness, is the condition in which a person or object experiences no contact force supporting it, so it feels weightless. In physics, weight is the force of gravity on a body. Weightlessness occurs when the only acceleration acting is gravity and there are no supporting forces, such as normal contact with a surface. In practice, this state arises in free fall or when an object shares the same acceleration as its supporting vehicle.

Common contexts include orbital flight, parabolic aircraft trajectories, and drop towers. In low Earth orbit, spacecraft and occupants are in continual free fall toward Earth while moving forward fast enough to keep circling the planet. Although gravity is still present, there is no net contact force inside the cabin, producing a microgravity environment. It is important to note that true zero gravity does not exist near Earth; gravity remains, but the observed weight is effectively zero.

Physiological and perceptual effects accompany weightlessness. Prolonged exposure can cause muscle and bone density loss, fluid redistribution, balance disturbances, and space adaptation syndrome. Short-term exposure via parabolic flights can produce motion sickness and disorientation.

Measurement and terminology: the term weightlessness describes an absence of contact force, not an absence of gravity. The environment is often described as microgravity, with residual accelerations ranging from 10^-3 to 10^-6 g, depending on conditions and duration.

Applications include astronaut training, biomedical experiments, and materials research conducted in space or on Earth analogs.