APPLICATIONS OF THE LEVER 159 



The distance from the force to the fulcrum is called the 

 force arm. The distance from the weight to the fulcrum is 

 called the weight arm ; and it is a law of levers, as well as of 

 all other machines, that the force multiplied by the length of 

 the force arm must equal the weight multiplied by the length 

 of the weight arm. 



Force x force arm = weight X weight arm. 



A force of I pound at a distance of 6, or with a force arm 6, 

 will balance a weight of 2 pounds with a weight arm 3 ; that is, 



1x6=2x3. 



Similarly a force of 10 pounds may be made to sustain a 

 weight of 100 pounds, providing the force arm is 10 times 

 longer than the weight arm ; and a force arm of 800 pounds, 

 at a distance of 10 feet from the fulcrum, may be made to 

 sustain a weight of 8000 pounds, providing the weight is I foot 

 from the fulcrum. 



153 Applications of the Lever. By means of a lever, a 600- 

 pound bowlder can be easily pried out of the ground. Let 

 the lever, any strong metal bar, be supported on a stone which 

 serves as fulcrum ; then if a man exerts his force at the end of 

 the rod somewhat as in Figure 91 (p. 1 54), the force arm will be 

 the distance from the stone or fulcrum to the end of the bar, 

 and the weight arm will be the distance from the fulcrum to the 

 bowlder itself. The man pushes down with a force of 100 

 pounds, but with that amount succeeds in prying up the 600- 

 pound bowlder. If, however, you look carefully, you will see 

 that the force arm is 6 times as long as the weight arm, so 

 that the smaller force is compensated for by the greater dis- 

 tance through which it acts. 



At first sight it seems as though the man's work were done 

 for him by the machine. But this is not so. The man must 



