ELEMENTARY SCIENCE 



TTPT 



power expended multiplied by the distance through which 

 it is expended is equal to the weight moved multiplied by 

 the distance through which it is moved. In other words, 

 the work done is equal to the force expended multiplied 



by the distance through 

 which that force acts. 



A lever, of which you 

 have seen the crowbar to 

 be an example, is evi- 



FIG. 51. Principle of the scales. r ' 



dently a device which 



increases power at the expense of speed, which is another 

 way of saying that the job is done gradually. Evidently, 

 also, three things are essential to a lever; namely, the 

 point at which the power is applied, the point at which 

 it is exerted, and the point of purchase (the fulcrum). 

 These points may be variously 

 arranged with reference to each 

 other, and on this basis levers are 

 divided into three classes. 



A lever of the first class is one 

 in which the fukrum is between the 

 applied power and the resistance. 

 The crowbar is a lever of this type. 

 So is a pair of scales on which your 

 weight is counterbalanced by a 

 much smaller weight at the end of 

 a bar. Another weight slides to 



and fro on the bar. The heavier you are, the more weight 

 it takes to bring down the bar (see Figs. 51 and 52). 



In a lever of the second class the resistance is between the 

 applied power and the fulcrum (see Fig. 53). This is well 

 illustrated by the nutcracker, which also well illustrates 



FIG. 52. A lever of the first 

 class. 



