Physiological changes that occur in the human body in motion as a result of rapid increase of speed. Rapid acceleration and surges in acceleration are felt more critically than are gradual shifts.

Riders of roller coasters are especially subject to the effects of acceleration because of the high-speed drops, turns, and inversions of the coaster. Acceleration forces are measured in units of gravitational acceleration, or g. One g is a force equivalent to the weight of the coaster on the surface of the earth. A force of 3 g, for example, is equivalent to an acceleration three times that of a body falling near the Earth.

Three kinds of acceleration stress are distinguished--positive, negative, and transverse--according to the position of the body with relation to the direction of acceleration.

Positive acceleration stress occurs when the direction of acceleration is along the long axis of the body from head to foot, experienced, at 1 to 2 g, by a general feeling of heaviness in the seat, hands, and feet.

Negative acceleration stress occurs when the direction of acceleration is from feet to head, causing a slight displacement of the internal organs in the abdomen and chest and a rush of blood to the face accompanied by the feeling of congestion. The average endurable times for negative stress are a few seconds at 5 g, 15 seconds for 4.5 g, and around 30 seconds for 3 g.

Transverse acceleration stress occurs when the direction of acceleration is sideways with relation to the long axis of the body. Accelerations up to 6 g directed across the body produce sensations of increased pressure on the part of the body that supports the weight.

AT THIS POINT IN THE RIDE . . . the coaster approaches the vertical loop, entering at a high rate of speed. Entry may create a heavy force on the riders as high as 5 g, but only for a few seconds--any longer and riders may blackout from acceleration stress. Modern vertical loops are tear-shaped, clothoid loops that have a tighter curve at the top, where the velocity is smaller, than at the exit and entry points at the bottom. The larger radius at the bottom of the loop reduces centripetal acceleration, thereby lowering the overall g-forces, which naturally increase by 1 g owing to gravitation. Often the designer will adjust the radius at the top of the loop to create a centripetal force of 2 g, allowing the riders, after subtracting the value of gravitation (1 g), to experience normal weight levels while upside down. The queasiness associated with looping rides is owing to some internal organs floating upward, counter to their natural hanging position.

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