180 ELEMENTS OF GENERAL SCIENCE 



We find, therefore, that in the particular pulley system 

 which we have been studying, the applied force must move 

 six times as far as the weight, but that if friction is 

 neglected, it need be but one sixth as great as the resist- 

 ance of the weight. With different combinations of pulleys 

 the ratio always equals the number of strands supporting 

 the weight. 



191. Mechanical advantage of pulleys. The advantage in 

 the use of pulleys lies in the fact that by their means a small 

 force may be made to overcome a great resistance. This 

 advantage is measured by the number of times which the 

 force exerted against the resistance is greater than the force 

 applied. In the case considered in the preceding section the 

 mechanical advantage was 6. The mechanical advantage of a 

 pulley system may be determined very readily, for it is always 

 equal to the number of strands of rope which are supporting 

 the weight or other resistance. 



192. Work and pulley systems. If in section 188 the boy 

 uses rope and pulleys to pull the boat out of the water, he 

 does not escape the necessity of doing work in order to move 

 the boat, for he must pull on the rope. The pulley system 

 also does work upon the boat in moving it up the beach. 

 We may call the work done by the boy hi pulling on the rope 

 the work in, and the work accomplished in overcoming the 

 resistance of the boat and the friction on rope and pulleys 

 the work out. 



The part of the work performed in moving the boat is 

 the useful work, while that wasted in overcoming friction and 

 in the pulley system will be known as the wasted work. 



We have learned that the force which the boy applies is 

 less than that which is exerted upon the boat, but is the 

 same true of the work that he puts in and the work that 

 he gets out? Before we can solve this problem we must 

 agree upon a definition of work and upon some means of 

 measuring it. 



