Centripetal Force, a center-seeking force that causes an object to move in a circular path. For example, suppose a ball is tied to a string and swung around in a circle at a constant velocity. The ball moves in a circular path because the string applies a centripetal force to the ball. According to Sir Isaac Newton's first law of motion, a moving object will travel in a straight path unless acted on by a force (see Mechanics). So, if the string were suddenly cut, the ball would no longer be subject to the centripetal force and would travel in a straight line in a direction tangent to the circular path of the ball (if not for the force of gravity). As another example, suppose a person is riding on a merry-go-round. As the merry-go-round rotates, the person must hang onto the ride to keep from falling off. Where the person grasps the ride, a centripetal force is applied to the individual that keeps the person moving in a circular path. If the person were to let go, he or she would travel in a straight line (if gravity were absent). In general, the centripetal force that needs to be applied to an object of mass m that is traveling in a circular path of radius r at a constant velocity v is the following:

Often, centripetal force is confused with centrifugal force. While centripetal force is a real force,—that is, the force is due to the influence of some object or field—centrifugal force is a fictitious force. A fictitious force is present only when a system is examined from an accelerating frame of reference. If the same system is examined from a non-accelerating frame of reference, all the fictitious forces disappear. For example, a person on a rotating merry-go-round would experience a centrifugal force that pulls away from the center of the ride. The person experiences this force only because he or she is on the rotating merry-go-round, which is an accelerating frame of reference. If the same system is analyzed from the sidewalk next to the merry-go-round, which is a non-accelerating frame of reference, there is no centrifugal force. The individual on the sidewalk would only note the centripetal force that keeps the individual moving in a circular path. In general, real forces are present regardless of whether the reference frame used is accelerating or not accelerating; fictitious forces are present only in an accelerating frame of reference.