INERTIA OF MATTER 



25 



obstruction, it does work: it breaks the rock, or splits the tree, or flat- 

 tens itself. 



When a body does work by lifting another body against gravity, the 

 amount of work done is obtained by multiplying the weight lifted 

 by the vertical distance it is lifted: 



Work = weight X distance. 



Thus, a workman carrying 50 Ibs. up a ladder 20 ft. high does just 

 as much work as one raising 40 Ibs. 25 ft., or 10 Ibs. 100 ft. 



26. Power. In calculating the amount of work done 

 by a force we have not taken account of the time required. 

 Yet if we were choosing a 

 horse or an engine to raise 

 a weight (Fig. 20), we would 

 take the horse or engine that 

 could do the work most rap- 

 idly. The rate of doing work 

 is power. 



The common unit of power 

 is the horse-power (written 

 H.P.) . James Watt, who de- 

 vised the unit, thought that 



an average horse could raise 33,000 Ibs. 1 ft. in 1 minute, 

 or 550 Ibs. 1 ft. in 1 second. As a matter of fact, the 

 average American horse can exert only about three- 

 fourths of a horse-power; that is, it can raise about 25,000 

 Ibs. 1 ft. in 1 minute. 



Fio. 20. 

 Horse Raising Weight of a Pile-Driver. 



27. Inertia of Matter. In Chapter I we learned that 

 matter occupies space and has weight. What we have 

 now learned about force shows us that matter has another 



