26 FORCE AND MOTION. 



any cause that tends to produce, change, or destroy 

 motion. 



It follows from inertia that bodies are incapable of 

 changing their condition of rest or motion. Any cause 

 capable of producing a tendency to change either of these 

 conditions, is called a force. 



(a.) We say that the tendency of a force acting on a body at rest 

 is to move it. Motion will be produced if the body is free to move. 

 This motion may be prevented by the simultaneous action of another 

 force or of other forces. Or the body may be fixed so that a given 

 pull or pressure, *. e., the application of force, will produce no 

 motion. In this case, opposing forces are called into action as soon 

 as the given force begins to act, and thus the new force is neutralized. 

 For instance, a small boy may exert all of his muscular power upon 

 a large stone and not lift it at all. The force employed produces no 

 motion. The attraction between the earth and the stone ( 33) is a 

 force acting in a downward vertical direction. This force is exactly 

 balanced by the upward pressure of the supporting earth or floor 

 ( 93). If the stone weighs two hundred pounds and the boy lifts 

 fifty pounds, the supporting body exerts an upward pressure of only 

 one hundred and fifty pounds. One quarter of the weight of the 

 stone or a downward force of fifty pounds is thus liberated or called 

 into play by the very act of lifting with a force of fifty pounds. 

 Hence no motion is produced, because an opposing force is called 

 into action as soon as the given force begins to act, and thus the 

 new force is neutralized. 



(&.) In this case, the greatest opposing force that can be set free 

 or called into play is a force of two hundred pounds, the full weight 

 of the stone. If, therefore, the stone be lifted with a force of more 

 than two hundred pounds, the new force can not be wholly neutral- 

 ized and motion will take place. If there be no opposing force to be 

 thus called into action, or, in other words, if the body be free to 

 move, the smallest conceivable force will overcome the inertia and 

 produce motion. 



65. Elements of a Force. In treating of forces, 

 we have to consider three things : 



(1.) The point of application, or the point at which 

 the force acts. 



