=78. Curvilinear Motion.=--Curvilinear motion occurs when a moving body is pulled or pushed away from a straight path. The pull or push is called _centripetal_ (center-seeking) force. A moving stone on the end of a string when pulled toward the hand moves in a curve. If the string is released the stone moves in a tangent to the curve. The string pulls the hand. This phase of the pull is called _centrifugal_ force. The _centripetal_ force is the pull on the stone. Centripetal and centrifugal force together cause a tension in the string. Examples of curvilinear motion are very common. The rider and horse in a circus ring lean inward in order to move in a curve. The curve on a running track in a gymnasium is "banked" for the same reason. Mud flying from the wheel of a carriage, the skidding of an automobile when passing rapidly around a corner, and sparks flying from an emery wheel, are illustrations of the First Law of Motion. Cream is separated from milk by placing the whole milk in a rapidly revolving bowl, the cream being lighter collects in the center and is thrown off at the top. (See Fig. 60.) Clothes in steam laundries are dried by a centrifugal drier. In amusement parks many devices use this principle. (See centrifugal pumps, Art. 70.) [Illustration: FIG. 61.--The two balls reach the floor at the same time.] =79. The Second Law of Motion,= sometimes called the _law of momentum_, leads to the _measurement of force_, by the momentum or the quantity of motion, produced by it. The law is stated as follows: _Change of motion, or momentum, is proportional to the acting force and takes place in the direction in which the force acts._ In other words, if two or more forces act at the same instant upon a body each produces the same effect that it would if acting alone. If a card be supported on two nails driven horizontally close together into an upright board (see Fig. 61), and two marbles be so placed on the ends as to balance each other, when one marble is snapped horizontally by a blow, the other will fall. Both reach the floor at the same time. The two balls are equally pulled down by the earth's attraction and strike the ground at the same time, though one is shot sidewise, and the other is dropped vertically. As gravity is a constant force, while the blow was only a momentary force, the actual path or resultant motion will be a curved line.