## Centripetal Force

To discuss the motion of an object is traveling in a circle, we often use the terms centripetal acceleration and centripetal force. For something to travel in a circle, the direction of its velocity must be constantly changing. Any change in velocity, in magnitude or direction, means the object is accelerated. In the case of motion in a circle, we call this the centripetal acceleration. The acceleration points toward the center of the circle and, in fact, "centripetal" means "center-seeking". By Newton's Second Law, such acceleration implies that there must be a force acting on the object, directed at the center of the circle. Whatever its cause (for example, friction or ground reaction), we call this force the "centripetal force". This change in velocity, and the centripetal acceleration, are illustrated in the diagram of a skater skating in a circle.

An easy way to experience centripetal acceleration is in the game of crack the whip. Crack the whip is commonly played on an ice surface where a large group of skaters hold hands. One skater gets to be the inside person who starts to swing all the other skaters around in a circle. This game can also be played on land in any wide open space.

If the same group of skaters start to go faster, the harder the inside skater must pull to keep the group swinging in a circle. The skater on the outside, will feel an inward pull on his or her arm as he or she is swung in a circle. This pull is the centripetal force. The pull will get bigger as the whip goes faster or as more skaters join the whip.

© April, 1998, Montana State University-Bozeman