PULLEYS, INCLINED PLANES, AND WEDGES 37 



FIG. 20. A Dumb-Waiter Pulley. 



sheaves, assembled in one block on the same pin. Some- 

 times three, four, or more sheaves are placed thus side by 

 side, a strong pin 

 serving as an axis. 

 In this way a force 

 can move two, three, 

 or four times its own 

 resistance. Thus in a 

 three-sheaved mov- 

 able block, 100 Ibs. 

 would balance 300 Ibs. Since the entire movement of the 

 pulley is made up of a series of stops and starts, the movable 



pulley acts during its 

 motion on the principle 

 of a lever of the second 

 class. As a result, the 

 force applied times the 

 diameter of the pulley 

 will always equal the 

 weight lifted times the 

 radius of the pulley. 

 Figures 20, 21, and 22 show common forms of pulleys. 



FIG. 21. Tackle or Awning Pulley. 



Problems on Pulleys 



1. How much pull at P would be required to lift 150 Ibs. at W? 

 (Fig. 18.) 



2. What force at W would just balance 200 Ibs. at Pf 



3. With what force or how many Ibs. is the rope C pulling on 

 its fixed end when 300 Ibs. is being lifted at Wf (This force or pull 

 is called the tension at C.) 



4. If a rope is carried around six pulleys as shown in Fig. 19 

 and a pull of 100 Ibs. is exerted at P, what weights would be lifted 

 at A, B, and C? 



