Lesson VII ] 



ilANICS. 



P. Th'-y are pulleys fixed to some place, ! 

 such as a beam. The object of the fixed 

 pulley is not to gain power, but to afford 

 a mure convenient mode of raisinga weight; I 

 and as it requires the same amount of 

 power to raise the weight as the amount of 

 the weight itself, it is evident that it does 

 not possess any mechanical advantage. 

 : ii-n in frontispiece raising a cask 

 with two fixed pulleys.] 



86. T. What are movable pulleys? 



/'. Th< y are of the same form as the 



fixed pulleys, but instead of being fixed or 



stationary, they sustain the weight, and 



are suspended by means of the cord that 



f .hangs under them. In 



23 we have a re- 

 A presentation of a fixed 

 (A) ard a movable 

 pulley (B). C repre- 

 sents a hook inserted 

 into a beam, and sup- 

 porting one end of! 

 a cord, which passes 

 under B, the movable 

 pulley, and proceeding 

 M. upwards, passes over 



the fixed pulley A -ends to P, 



is the power, acting against the 

 ce or weigi 



s the use of the fixed 

 pulley in the machine you have des> 



1 o change the direction of the power. 



88. T. How is equilibrium maintained 

 in a pi 



nay be explained by looking 

 which represents a pulley 

 (c) moving round 

 d axis or 

 pivot, and tlie line 

 forces 



acting in tli< 

 tions a b and d t. \ 

 If we could pro- 



forces meeting at f, which could only be 

 in equilibrium if their resultant were so. 

 If the two forces meeting in/, and acting 

 in the directions/ 6 and/e are equal, their 

 resultant will bisect, or cut into two, the 

 angle bfe, and will then pass through the 

 fixed central point c on the axis ; the effect 

 of which will be a state of equilibrium, but 

 if one of the forces (f b) is greater than the 

 other (ft), then the resultant will not pass 

 through the axis, and equilibrium cannot 

 be maintained. 



89. T. How can you estimate the pres- 

 sure which the axis of the pulley has to 



P. Easily. It is evident from what we 

 have learned before, that the pressure to 

 be sustained is equal to the resultant of the 



long these lines, 

 they would n 

 the point/ and u 

 it therefore evident 

 that if / were a 



1 * . y, we 



from a 



and it to /, without alter 

 any *.._-. we hould have two 



two forces. But if the directions of the 

 forces be parallel, as in f'/> 2*>. the pres- 

 sure upon the axis um of 

 forces (p and w), and the weight of 

 tlu- pulley. [See Catechism I. Fig. 20.] 



/'. How i* a movable pulley main- 

 tained in equilibrium ? 



P. By the lorers which si- 

 ends of the cord being equal to one 

 r, and causing their resultant to 



pUB through the central point 



The action of this resultant is not stopped 



owing to the axis i hut because 



I a third power in tlie axis, in the 

 he resultant, which is equal 

 and opposed to it. 



91. T. What is the third power you 

 have in' 



usually attached to a hook fastened 



T. What advantage H< 

 able po**?* ey T 



P. It halve* the weight. 



