EXERCISE 63 



HOW ARE PULLEYS USED AS MACHINES, AND WHAT ARE THE ADVANTAGES OF 

 DIFFERENT SYSTEMS OF PULLEY ARRANGEMENT? 



Materials. Some single and double pulleys ; a sensitive spring balance ; weights ; some small cord. 



Directions. Arrange a pulley, spring balance, and weight as shown 

 in the figure at the right of the diagram. The weight should be 100 

 grams or 200 grams. Attach the spring balance and raise it through a 

 known distance, noting and recording at the same time the distance 

 through which the weight rises. Now raise and lower the balance sev- 

 eral times, taking as many readings while raising as while lowering. 

 Average the readings. This is the reading that should be secured if 

 there were no friction. Why ? Keep a record of all your measurements 

 and readings. Tabulate the averages in the blanks given below. 



Arrange an apparatus as shown in the central figure and again make 

 record of your results. 



Do the same with the arrangement as shown at the left. 



Having secured your data as above, compare the force applied at 

 the end of the cord, the weight lifted, and the number of strands of cord 

 supporting the weight; and from this state (1) the mechanical advan- 

 tage of the pulley system you are using ; and (2) the rule whereby one 

 may always tell from inspection of any pulley system what advantage 

 it will give. This last statement should not be made until the results secured by all members of 

 the class have been tabulated on the board and compared. 



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Additional problems. What is the advantage of using pulleys in lifting hay when stacking it or put- 

 ting it into barns ? 



If a piano weighing 1000 pounds is to be moved from the street level to the fourth floor of a build- 

 ing 60 feet from the street level, and two men are to pull a total of 250 pounds in lifting the piano, what 

 arrangement of pulleys and ropes will enable them to lift the piano (friction being neglected) ? 



[89] 



