168 



MAN^S WAY OF HELPING HIMSELF 



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. When 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. 



162. Combination of Pulleys. A combi- 

 nation of pulleys called block and tackle is 

 used where very heavy loads are to be 

 moved. In Figure 1 1 1 

 the upper block of pul- 

 leys is fixed, the lower 

 block is movable, and 

 one continuous rope 

 passes around the vari- 

 ous 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. If 8 pulleys were used, a force 



equivalent to one eighth of the load would 



suffice to move W, but this force would have 



to be exerted over a distance 8 times as 



great as that through which W was raised. 



163. Practical Application. In our childhood many of us 



saw with wonder the appearance and disappearance of flags 



FIG. no. A movable 

 pulley lightens labor. 



FIG. in. An effec- 

 tive arrangement of 

 pulleys known as 

 block and tackle. 



