Weightlessness, or Zero Gravity, a condition in which an object appears to have no weight. Weightlessness occurs when an object is in free fall—that is, when there is no resistance to a gravitational force acting on the object.

A spacecraft coasting in orbit around the earth is in a state of free fall. (The craft's forward speed keeps it from falling to the earth's surface.) Any astronauts in the craft are also in a state of free fall and thus experience weightlessness. For a discussion of the physiological effects of weightlessness,

Short periods of weightlessness can be experienced in an aircraft that flies upward and then downward along a curved path. Astronauts in training take such flights to become familiar with the sensations produced by weightlessness.