MECHANICAL WORK 



plied three times as applied to lifting the heavy stone 

 one inch. Do you see that the same principle is involved ? 



Take the case of a pulley (see Fig. 49). r __ i 



The object to be lifted is held up by two 

 lengths of rope, one of these being fastened 

 to the hook under the upper pulley, the 

 other passing over it and down to your 

 hands. Now do you see that to lift that 

 object one foot you must pull the rope two 

 feet? This means that your effort as ap- 

 plied to lifting the weight will be doubled, 

 though it will operate through only half the 

 distance that you pull the rope. By arrang- 

 ing a third pulley, the strength of the pull 

 on the weight 

 may be trebled, 

 the distance 

 through which 

 the pull must be 

 made being, of course, also 

 trebled. You may have seen 

 houses moved by a combination 

 of windlass and pulleys, and you 

 must have noted that the horses 

 turning the windlass move much 

 faster than the house which they 

 are moving. It is the same prin- 

 ciple at work. The more you 

 increase, by extra pulleys, the 

 power applied to the object to be moved, the longer must 

 be the pulling done at the other end (see Fig. 50). 

 This principle may be expressed mathematically. The 



FIG. 49. The sim- 

 ple pulley. 



FIG. 50. Compound pulleys. 



