62 



MOTION AND FORCE 



use great force slowly and cause a small body to move 

 rapidly, or use small force rapidly and move a great 

 I 1 weight slowly. A few ex- 



S amples of this will be given 



Weight 



Force 



FIG. 46 



in 72 and 73. 



71. Law of Machines. The fourth of these uses is 

 one of much importance. It will be more easily under- 

 stood when we have clearly in mind the following Law 

 of Machines: The force and the resistance vary inversely 

 as the distances through which each acts. This means 

 that if a certain force causes motion against a resist- 

 ance that is greater or smaller than itself, the distance 

 through which the resistance acts must be just as many 

 times smaller or greater than 



the distance through which the 

 force is applied. 



72. Pulleys and Levers. - 

 A few simple machines will 

 serve as examples of this law. 

 Fig. 47 shows one movable pul- 

 ley B attached to a weight W\ 

 F is the point at which the 

 force is applied. Notice that 

 as F moves, W will move only 

 half as far. From the law, this 

 shows that the force need be 

 only half as great as the weight. 



With two movable pulleys the force would act through 

 four times the distance and lift a weight four times 

 as heavy. 



FIG. 47 



