Time rate at which a velocity is changing. Because velocity has both magnitude and direction, it is called a vector quantity; acceleration is also a vector quantity and must account for changes in both the magnitude and direction of a velocity.

If the velocity of a roller coaster moving on a straight path is increasing (i.e., if the speed, which is the magnitude of the velocity, is increasing), the acceleration vector will have the same direction as the velocity vector. If the velocity is decreasing (that is, the coaster is decelerating), the acceleration vector will point in the opposite direction. So, whether the coaster is increasing speed down a hill or decreasing speed up a hill, it is regarded as acceleration in the mathematical sense.

Changes in acceleration greatly contribute to the thrill of a roller coaster ride. A rider may feel greater sensations in a low-speed coaster with sharp acceleration changes than on a faster coaster with a smoother ride. Pure speed is often not as recognizable as the surge of acceleration during a coaster ride.

A common fallacy is that acceleration increases along with the weight of the riders and the car, when actually the acceleration of a coaster in free-fall is independent of its mass. Acceleration does increase, however, with a steeper angle of descent.

AT THIS POINT IN THE RIDE . . . at the bottom of the second hill, the coaster, in theory, reaches an instant of zero acceleration, at the point where its speed is greatest, relative to anywhere else on that hill. Zero acceleration also occurs for an instant at the top of a hill, where in turn the speed is at its lowest level. At the bottom of the dip, the riders experience compression, or acceleration stress, owing to forces greater than their usual weight.

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