WORK AND MACHINES 165 



(6) the effort put forth ("pull" exerted) to lift the body. 1 

 Enter on your drawings the values found by the teacher, 

 marking the resistance as W (weight), and the effort as P 

 (power). 



Be sure to verify the correctness of your drawings by 

 reference each time to the pulley arrangement as set up by 

 the teacher. 



3. Let the teacher take the readings of the spring balance 

 for the value of P (a) when pulling down over the last of the 

 fixed pulleys; (6) when lifting up, and not using this last 

 fixed pulley. Infer (a) the only purpose of this fixed pulley ; 

 (6) whether the section of cord for pulling downward shall 

 be counted in any case as a supporting cord for W. 



4. From inspection of your diagrams, and of the values 

 of P and W shown thereon, state what appears true (a) of the 

 number of cords supporting W relative to the number of 

 movable pulleys ; (6) of the fractional part of W supported 

 by each cord; (c) of the ratio (approximately) of W to P 

 relative to the number of such cords. 



5. Measure the distances through which P moves in all 

 of the cases above when W is raised through a distance of 

 one foot every time. State for each case how many times 

 greater the power-distance (Pd) is than the weight-distance 

 ( Wd) . How does this compare with the number of support- 

 ing cords in each case? 



6. Since P traverses the distance Pd in the same time 

 that W moves through Wd, what must be true of the rela- 



1 For this value of P take the average of the readings of the spring balance 

 as W is drawn slowly upward, and then again as W is allowed to descend 

 slowly. In this way the value of the resistance due to friction of the 

 pulleys is quite largely eliminated. 



