Paired Forces
The figure at right (top panel) shows a ball in uniform circular motion held to its path by a massless string tied to an immovable post. In this system a centripetal force upon the ball provided by the string maintains the circular motion, and the reaction to it, usually called the reactive centrifugal force acts upon the string. In this model, the string is assumed massless and the rotational motion frictionless, so no propelling force is needed to keep the ball in circular motion.
Newton's first law requires that any body not moving in a straight line is subject to a force, and the free body diagram shows the force upon the ball (center panel) exerted by the string to maintain the ball in its circular motion.
Newton's third law of action and reaction states that if the string exerts an inward centripetal force on the ball, the ball will exert an equal but outward reaction upon the string, shown in the free body diagram of the string (lower panel) as the reactive centrifugal force.
The string transmits the reactive centrifugal force from the ball to the fixed post, pulling upon the post. Again according to Newton's third law, the post exerts a reaction upon the string, labeled the post reaction, pulling upon the string. The two forces upon the string are equal and opposite, exerting no net force upon the string, but placing the string under tension.
Read more about this topic: Reactive Centrifugal Force
Famous quotes containing the words paired and/or forces:
“The paired butterflies are already yellow with August
Over the grass in the West garden;
They hurt me. I grow older.”
—Li Po (701–762)
“Ah! how much a mother learns from her child! The constant protection of a helpless being forces us to so strict an alliance with virtue, that a woman never shows to full advantage except as a mother. Then alone can her character expand in the fulfillment of all life’s duties and the enjoyment of all its pleasures.”
—Honoré De Balzac (1799–1850)