In mechanics, state of a body that moves freely in any manner in the presence of gravity. Newton's laws of motion show that a body in free-fall follows an orbit such that the sum of the gravitational and inertial forces equals zero. This explains why a rider in a roller coaster following a parabolic path may experience the condition of weightlessness. Because of the motion of the coaster, the effect of gravity is canceled by the equal and opposite force of inertia or, in this case, centrifugal force.

The term zero gravity, or 0 g, is often used to describe this condition, but excluding spaceflight, true weightlessness can be experienced only briefly. Gravitational forces are never uniform, and therefore, in theory, only the center of mass--or the center car in a train of coasters--is ever in free-fall. All other cars are subject to tidal forces because they move in a slightly different gravitational field.

AT THIS POINT IN THE RIDE . . . the car speeds down the inclined track of the first hill under the force of gravitation and the riders experience downward centripetal acceleration and the sensation of being lifted out of their seats. Free-fall, or airtime, is felt by the riders in the front cars of the coaster train while cresting the hill and by the riders in the back cars while on the descent. During free-fall the car exerts no pressure on the riders or the track; the riders feel only the force of gravity. The seat restraint, often a lapbar, holds them in the coaster, just as the car's upstop wheels hold it to the track, otherwise both riders and cars would be thrown into projectile motion above the track.

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