COMBINATION OF PULLEYS 



33 



12 inches. You must exercise a force of 100 pounds over 12 

 feet of space in order to raise a weight of 200 pounds a distance of 

 6 feet. Whether we raise 100 pounds through 

 12 feet or 200 pounds through 6 feet the total 

 work done is the same ; but the pulley enables 

 those who cannot furnish a force of 200 

 pounds for the space of 6 feet to accomplish 

 the task by furnishing 100 pounds for the 



space of 12 feet. 



Combination of pulleys. 



A combination of pulleys 



called block and tackle is 



used where very heavy loads 



are to be moved. In Fig- 

 ure 172 the upper block of 



pulleys is fixed, the lower 



block is movable, and one 



continuous rope passes 



around the various pulleys. 



The load is supported by 6 



strands, and each strand 



bears one sixth of the load. 



If the hand pulls with a force of I pound at 



P, it can raise a load of 6 pounds at W, but 



the hand will have to pull downward 6 feet at 



P in order to raise the load at W I foot. 



Practical application. In our childhood 

 many of us saw with wonder the appearance and disappear- 

 ance of flags flying at the tops of high masts, but observation 

 soon taught us that the flags were raised by pulleys. In tene- 

 ments, where there is no yard for the family washing, clothes 

 often appear flapping in mid-air. This seems most marvelous 

 until we learn that the lines are pulled back and forth by pulleys 



P 



FIG. 171. A movable 

 pulley lightens labor. 



FIG. 172. An effec- 

 tive arrangement 

 of pulleys known as 

 block and tackle. 



