Centrifugal << sehn TRIHF yuh guhl or sehn TRIHF uh guhl >> force is often incorrectly defined as the force that pulls an object outward when it moves in a circle. Actually, an object moving in a circle is being pulled inward. If no force pulled it inward, it would continue to move in a straight line with constant speed. Physicists call the force that pulls the object inward centripetal force.
If you tie a string to a stone and whirl the stone around, you must exert a centripetal force to keep the stone from moving in a straight line. In the same way, the earth’s gravity exerts a centripetal force on a speeding satellite and keeps it from flying into space.
Physicists find the idea of centrifugal force useful in certain situations. For example, when you ride on a merry-go-round, you can feel yourself being thrown away from the center of rotation. If you observe your motion with respect to the merry-go-round, you could say that centrifugal force pulls you away from the center. Physicists would call the merry-go-round a rotating reference frame. You do not need the idea of centrifugal force if you observe your motion with respect to the ground instead of to the merry-go-round. You would then say the merry-go-round exerts centripetal force, the result, let us say, of friction between your shoes and the platform. This keeps you from moving in a straight line with a constant speed.
