WORK AND MACHINES 



63 



A lever is any device having force applied at one 

 point and resistance at another, the whole turning on a 

 point called a fulcrum. A crowbar may be used as 

 a lever (Fig. 48). a , 



As the force acts ^^\ 



from a 1 to 6', the \ 



weight moves only 



]&' 



FIG. 48 



from a to b ; hence 

 a small force at 

 a 1 will, in acting 

 through a greater distance, a'b', move a greater weight 

 at a the lesser distance, ah. Also a great force at b could 

 move a lesser weight at b' with greater speed. Fig. 49 



shows three classes of 

 levers. In Class I the 

 fulcrum/ is between the 

 points where the force p 

 and the resistance w are 

 applied. In Class II / is 

 at the end, w being ap- 

 plied between it and p ; 

 this lever gives us a 

 gain in force at the 

 expense of speed. In 

 Class III the force p is 

 applied beween the ful- 

 crum and the resistance, so that we gain in speed at 

 the expense of force. 



Experiment 55. Pulleys and levers are common, and many 

 experiments may be made, according to the time and mate- 

 rial available. A movable pulley is not hard to find ; but for a 



fl 



P 



w 



III 

 FIG. 49 



