LEVERS 



53 



will be found by careful measurement that, if the two 

 children are of exactly equal weight, they must sit at 

 equal distances from the fulcrum. In other words, the 

 two arms of the lever must be of equal length. If, how- 

 ever, one child is twice the weight of the other, the lighter 

 must sit twice as far from the fulcrum to balance the 

 heavier. 



It is thus seen that the long arm of a lever and a 

 light weight are equal to a short arm and a heavy weight. 

 The fundamental law of leverage is that the forces are 

 inversely proportional to the lengths of the arms on 

 which they act, that is, to their distance from the 

 fulcrum. 



Classes of levers. Levers are ordinarily divided 

 into three classes, as determined by the relative posi- 



W 100 pi 200 



FIG. 44. Examples of levers of the first class, with analysis of the forces involved. 

 The direction of the forces is indicated by the direction of the arrows ; their 

 relative amounts by both the length of the arrows and the figures. F = fulcrum ; 

 P = power ; W = weight. 



tions of the fulcrum, the power and the weight. To 

 the first class belongs the lever which has the fulcrum in 

 the middle; to the second, that which has the weight 

 in the middle ; to the third, that which has the power in 

 the middle. It will be readily seen that, given the same 

 \engths of arms and the same forces, the balance remains 



