294 HOW MACHINES LIGHTEN LABOR 



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 ma- 

 chines, 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. 



Applications of the lever. 



Fio. 158. - A slightly different form of lever. ~ % means f a lever > 3 



i6oopound 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 151, 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 200 pounds, but with that 

 amount succeeds in prying up the i6oo-pound bowlder. If, 

 however, we know that the force arm is 8 times as long as 

 the weight arm, it is quite evident that the smaller force 



