THE LEVER. 



89 



FIG. 38. 



tween the power and weight (P. F. W. ), the lever is of 

 the first class (Fig. 37); e, g., crowbar, balance, steelyard, 

 scissors, pincers. 



(2.) If the weight is be- 

 tween the power and the 

 fulcrum (P. W. F.), the 

 lever is of the second class 

 (Fig. 38) ; e. #., cork-squeezer, 

 nut-cracker, wheel-barrow. 



(3.) If the power is be- 

 tween the weight and the ful- 

 crum (W. P. F.), the lever is ^ 

 of the third class (Fig. 39); 

 e. g., fire- tongs, sheep-shears, 

 human fore-arm. FlG - 39- 



17O. Static Laws of the Lever. It will be 

 clearly seen or may be geometrically shown that the ratio 

 between the arms of the lever will be the same as the ratio 

 between the velocities of the power and the weight, and 

 the same as the ratio between the distances moved by the 

 power and the weight. If the power-arm be twice as long 

 as the weight-arm, the power will move twice as fast and 

 twice as far as the weight does. The general laws of ma- 

 chines may therefore be adapted to the lever as follows : 



P x power-arm =W x weight-arm, or P x PF =W xWF. 



/. P : W :: 



(1.) In the case of the lever, the power and weight are 

 inversely proportional to the corresponding arms of the 

 lever; or, 



