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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