weightlessness, the absence of any observable effects of gravitation. This condition is experienced by an observer when he and his immediate surroundings are allowed to move freely in the local gravitational field. All bodies in the weightless environment experience the same acceleration. The more massive bodies (see mass) in the surroundings experience a stronger gravitational force, but they also have more inertia, or resistance to acceleration. As seen by a stationary outside observer, they appear to move together without any constraint. To the observer being accelerated, objects appear to float freely in space and to move with uniform speed in a straight line when given a push. Three examples of situations where weightlessness is encountered are: (1) an elevator falling freely in a vacuum; (2) a space capsule orbiting the earth; (3) a spacecraft drifting in outer space with its engines off. For the effects of weightlessness on the body, see space medicine.

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