According to the law of inertia, in the absence of forces, an object moves in a straight line at a constant speed. An outside force must act on an object to make it move in a curved path. When you whirl a stone around on a string, you must pull on the string to keep the stone from flying off in a straight line. The force the string applies to the object is the centripetal force. The word centripetal is from two Latin words meaning to seek the center .
Centripetal force acts in other ways as well.
For example, a speeding automobile tends to move in a straight line. Centripetal force must act on the car to make it travel around a curve. This force comes from the friction between the tires and the pavement. If the pavement is wet or icy, this frictional force is reduced. The car may then skid off the road because there is not enough centripetal force to keep it moving in a curved path.