What is necessary to give motion to a body? Force of some kind is necessary to give motion to a body. How long will the motion given to a body continue? The motion will continue until it is checked. What is necessary to check the motion of a body? Force of some kind is necessary to check its motion. What force is always acting so as to bring bodies to a state of rest? The force of gravity, which is constantly drawing all bodies to the earth. What else may check the motion of a body? The air, the water, and solids of all kinds may check the motion of a body. In what direction will a body move when its course is not obstructed? It will move in a straight line. Why does a ball struck by a bat make a curve to the earth? Because the force of gravity gradually overcomes the force given by the bat, so that the ball moves in a curve. Why does a bullet make a curve to the earth? Because the force of gravity gradually overcomes the force given by the powder, so that the bullet moves in a curve to the earth. When a person aims directly at a mark some distance from him, where will the bullet strike? The bullet will strike below the mark. Why will the bullet strike below the mark? Because the force of gravity draws it towards the earth, while it is going from the gun to the mark. When a ball attached to a string is whirled around, as in Fig. 38, what is the tendency of the ball? The tendency of the ball is to fly off in a straight line. [Illustration: Fig. 38.] What is the force tending to make the ball fly off called? It is called _centrifugal_ force. What keeps the ball from flying off? The string keeps the ball from flying off. What is the force exerted by the string called? It is called _centripetal_ force. What then is centrifugal force? It is the force tending to fly off from the centre. What is centripetal force? It is the force tending to draw towards the centre. Why does mud fly off from the wheel of a wagon when in motion? Because the motion of the wheel gives centrifugal force to the mud, and causes it to fly off. What does the rim of a wheel when in motion, tend to do? The rim tends to fly off from the centre of the wheel. How is it prevented from flying off? It is prevented by making the wheel very strong, so that all parts of it are held firmly together. What does this centrifugal force sometimes cause? The centrifugal force is so great that it sometimes causes grind-stones and mill-stones, when revolving rapidly, to burst into pieces. What becomes of these stones when they burst? When they burst, their pieces fly off in different directions, doing more or less injury to whatever obstructs their course. When a horse turns a corner, why does the rider lean in the direction in which the horse is turning? He leans so as to overcome the centrifugal force, and is thus prevented from falling off the horse.