Quantity, peculiar to a particle moving on a circular path, that has the same magnitude and dimensions as the force that keeps the particle on its circular path (the centripetal force) but points in the opposite direction.

Centrifugal force can be increased by increasing either (1) the speed of rotation, (2) the mass of the body, or (3) the radius, which is the distance of the body from the center of the curve. Increasing either the mass or the radius increases the centrifugal force proportionally, but increasing the speed of rotation increases it in proportion to the square of the speed; that is, an increase in speed of 10 times, say from 10 to 100 revolutions per minute, increases the centrifugal force by a factor of 100. Centrifugal force is expressed as a multiple of g, the symbol for normal gravitational force (strictly speaking, the acceleration due to gravity).

Although it is not a real force according to Newton's laws and is sometimes referred to as a pseudoforce, the centrifugal-force concept is a useful one, because it helps explains the sensations a rider experiences while on a roller coaster. For example, when analyzing the experience of a vertical loop, it is convenient to study the rider's sensations relative to the looping coaster rather than relative to the Earth. In order that Newton's laws be applicable in such a rotating frame of reference, an inertial force, or a fictitious force (the centrifugal force), equal and opposite to the centripetal force, must be included in the equations of motion. In a frame of reference attached to the looping coaster, the car is at rest; to obtain a balanced force system, the outward-acting centrifugal force must be included.

AT THIS POINT IN THE RIDE . . . the coaster enters a horizontal, helix-shaped corkscrew--which is a horizontal spiral, or helix--that carries the riders through two inversions. In the physics of motion, the corkscrew functions much the same way as a vertical loop. As the riders enter the first inversion, the actual centripetal acceleration is upward, because that is the direction the coaster is heading, acting upon the riders through the seat toward the center of the loop. This force is greater than the weight of the riders and necessary for circular motion. But the riders perceive the force differently. They experience the acceleration stress of being pushed back into their seats, and the apparent centrifugal force on the riders is downward. As the car reaches the top of the corkscrew loop, the apparent centrifugal force is now upward, contributing to the riders' sensation of feeling lighter than normal.

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