28 APPLIED SCIENCE 



W X 24 = 100 X 12 

 24W = 1200 

 W = 50 Ibs. 



That is to say, it will take 50 Ibs. at the long end of the lever to 

 balance the 100 Ibs. at the short end.* 



30. Levers of the First Class. 



In levers of the first class, the fulcrum 



FIG. 10. A Lever of the is placed between the acting and re- 

 First Class. sisting forces as shown in Fig. 10. 



This figure illustrates the lifting of u heavy block by means of 

 a crowbar and a support. 



E - Effort 

 F = Fulcrum 

 W --= Weight 



By pressing down the end of the bar E the other end of the lever 

 raises the weight W and the center of motion is at the fulcrum F. 

 In other words, the applied force E acting on the lever supported 

 by the fulcrum F overcomes the resistance, called weight W. 



The force of the lifting power of the lever increases in 

 proportion as the distance of the effort E from the fulcrum 

 increases, and diminishes in proportion as the distance of 

 the weight W from the fulcrum increases. 



* It should be noted that when leverage problems are figured 

 by arithmetic no account is taken of the weight of the lever itself. 

 The results obtained by using simply weights and distances are 

 exact enough for all practical purposes. If the designer had to 

 allow for the weight of the lever itself, he would have to make a 

 long and difficult calculation. Such allowance, however, is not 

 necessary because, for safety, all parts of machinery are made at 

 least five times as strong as they need to be. 



