142 ILLINOIS STATE ACADEMY OF SCIENCE 



which acts in turn on the gear turning the screw on the 

 jack. The "weight" force which is overcome consists of 

 a number of large iron weights hung on the end of a lever. 

 The fulcrum of this lever is at the opposite end of the bar 

 and the "power" force on the lever is the force exerted 

 by the jack. The lever as constructed by us consists of 

 an oak beam 1.5 inches thick, about 4 inches wide and 

 about 70 inches long. The upper end of the jack screw 

 was fitted with a wedge-shaped piece and this rests on an 

 iron plate screwed on the lower part of the oak beam. 

 The point of application of this "power" force can thus 

 be shifted along the lever bar. This point of application 

 will not in general be under the center of gravity of the 

 lever bar so that the work done in raising the center of 

 gravity must be taken into account in the final calcula- 

 tions of the efficiency of the machine. 



The following is a sample set of readings made with 

 this apparatus. 



weight of lever 6490 grams 



pitch of screw .847 cm 



distance of fulcrum to jack 46.0 cm 



distance of fulcrum to center of gravity of lever 66.0 cm 



distance of fulcrum to weight 166.0 cm 



distance "weight" is shifted for two revolutions of disk. . . 6.35 cm 

 distance "power" force moves for two revolutions of disk. 177.8 cm 



weights at end of lever 12410.0 cm 



"power" weights in backet required to just lift above 



"weight" 2040.0 gm 



From the above data, the student calculates directly 

 the following: 



ideal mechanical advantage of jack 105. 



actual mechanical advantage of Jack 26.8 



ideal mechanical advantage of lever .266 



actual mechanical advantage of lever .230 



ideal mechanical advantagS of combined machine 28.0 



actual mechanical advantage of combined machine 6.08 



efficiency of jack 25% 



efficiency of lever 86% 



efficiency of combined machine 22% 



Laboratory of Physics 

 University of Illinois 

 April, 1924 



