Property of the motion of a body traversing a circular path. The acceleration is directed radially toward the center of the circle and has a magnitude equal to the square of the body's speed along the curve divided by the distance from the center of the circle to the moving body.

The force causing this acceleration, which is supplied by gravity in most roller coasters, is directed also toward the center of the circle and is named centripetal force. Whenever there is a change in direction, centripetal acceleration will occur.

As a coaster enters a sideways curve, it undergoes inward centripetal acceleration, which bears upon the riders toward the center of the curve radius. Under pressure of lateral g-forces, though, the rider feels thrown to the outside of the curve in what he perceives as outward centrifugal force, which is actually a fictitious force.

AT THIS POINT IN THE RIDE . . . the coaster enters a banked curve, and as the track is curved and as the car is in a state of acceleration, it becomes subject to centripetal force, which may give the riders the sensation of being pushed sideways. But if the track is banked at the correct angle for the speed, the force the riders feel push them into their seats. Banking the turn converts the lateral g-forces the riders feel into positive g-forces, as pressure is felt from the floor as well as from the side of the car. If the speed of the coaster doubles, centripetal forces quadruple, unless the radius of the curve is also quadrupled. As a result, shallower curves are found on larger, faster coasters than on smaller ones.

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