COMMON TYPES OF WOKK 179 



190. Some facts about pulleys. Questions must certainly 

 arise regarding the advantage of using rope and pulleys 

 either in moving a boat or for other purposes. Anyone who 

 has observed the use of pulleys, as when a building is being 

 moved through the street or when a piano is being moved 

 to an upper floor of a house, has probably observed that the 

 load is moved more easily by their use. It is also true that 

 the load is not moved rapidly. Ability to move a greater load 

 is an advantage, but the slow motion is often a disadvantage. 

 It is important for the practical use of pulleys that one should 

 know precisely the relation between the force exerted and the 

 load moved, as well as understand the accompanying rate of 

 motion. It is only by the performance of experiments with 

 pulleys that these questions can be satisfactorily answered. 



We might arrange a set of small pulleys and cord similar to 

 the arrangement for moving the boat, but employ them for lift- 

 ing a weight. It will be noted that in this arrangement, as 

 snown in the figure, the hanging, or movable, pulley is sup- 

 ported by six strands of cord (fig. 91). It may be shown, first, 

 that the force exerted by tlie apparatus in lifting the weight is 

 much greater than the force applied to the free end of the cord, 

 and second, that the weight moves a much shorter distance 

 than does the hand which is pulling at the end of the cord. 



If the distances are measured, it will be found that tho 

 hand travels six times as far as the weight in the case of 

 the pulley system given in the figure. A measurement of 

 the force applied at the end of the cord will show that it is 

 a little more than one sixth of the amount which must be 

 applied to the weight in order to lift it. There is, of course, 

 some friction in the apparatus, and if care is taken to reduce 

 this as much as possible, it will be found that the force 

 applied is very little greater than one sixth of the weight 

 lifted. We may conclude that if friction could be wholly 

 eliminated, we should find that the ratio of the weight lifted 

 to the force applied is 6 to 1 , 



