weightless

Weightless describes the condition in which an object experiences no contact force supporting it against gravity, making its weight effectively zero. Technically, weight is the gravitational force on a mass, W = m g. Weightlessness does not imply zero mass or the absence of gravity; it occurs when an object is in free fall or in a free-falling reference frame, such as orbit around a planet, where there is no surface to exert a normal force. In orbital motion, gravity is still present, but the object is continually falling toward the planet, matching its orbital speed.

Weightless conditions are most commonly encountered in spaceflight and during experiments that remove contact support: orbital flight, parabolic flights that produce short periods of free fall, and drop towers that briefly simulate microgravity.

Effects can be observed in humans, who may experience fluid shifts toward the head, changes in balance and coordination, and, with prolonged exposure, bone density loss and muscle atrophy. Fluids and objects also behave differently in microgravity: droplets become nearly spherical, convection is reduced, and liquids can form floating films or droplets.

Microgravity is the term often used for near-weightlessness, with residual accelerations due to vehicle motion and gravity. Scientists measure it with accelerometers and by observing residual forces in the environment.

Common misconceptions include confusing weightlessness with zero gravity; gravity is not eliminated, but the absence of a supporting surface creates the sensation of weightlessness.