Jan. 2st. 2006
MECHANICS 1.0: CENTRIPETAL FORCE
About 1680, Issac Newton correctly described motion along a curve and dispelled the erroneous idea, held since Aristotle, that revolving objects are thrown radially outward by a "center-fleeing" centrifugal force.
The velocity of a ball tied to a string and whirled in a horizontal circle continuously changes. Its velocity changes continuously even at constant speed because the direction of velocity keeps changing. That means there must be an acceleration and, by Newton's Second Law of Motion, a force causing that acceleration. Velocity , acceleration, and force are vector quantities that is, having magnitude and direction. Speed is simply the magnitude of velocity; "how fast" without regard to orientation. Back to the whirling ball my hand pulls the string inward toward the center of motion, and the string continuously pulls the ball off its straight-line inertial course. A centripetal, or "center-seeking" force must be exerted if any object is to move in a curved path. If the inward centripetal force is removed, the ball's motion instantly becomes straight-line inertial and tangent to the circle the way sparks fly from the edge of a rotating grinding wheel not outward along the former radius.
Centripetal force is not a unique force but rather a name given to any force directed toward the center of motion. For the Moon revolving around the Earth, centripetal force is gravitational. For a motorcycle or car rounding a flat curve, centripetal force is frictional. No force is needed to keep an object moving along a straight line at constant speed. In fact, such motion as that can be thought of as circular with an infinite radius. If the object is made to follow an arc with a defined radius, a centripetal force must be applied. The smaller the radius, the tighter the curve and the greater the force necessary to pull the object away from its straight-line path.