SIMPLE MACHINES 



191 



FIXED PULLEY 



class lever. When the pulley is a second-class lever, 

 the weight is at the center, the force is at one end of the 

 diameter, and the fulcrum is at the other end. 



(a) It is often more convenient to have a force act in one 

 direction than in another, and the pulley is a very effec- 

 tive device with which to do this. It is 

 much easier to have a pulley at the top of 

 a flag pole, with a rope around it, and pull 

 the flag up by pulling downward on the 

 rope, than to climb the mast and pull up 

 the flag. When a pulley is fastened in 

 one position, as the one at the top of a 

 flag pole, it is called a single fixed pulley, 

 such as is also shown in the illustration. 

 When a single fixed pulley is used we can 

 easily find by experiment that the force- 

 applied is always equal to the weight lifted; 

 hence the mechanical advantage is one, that is, no ad- 

 vantage at all except in changing the direction of the force. 

 (6) The single movable pulley is one 

 which is fastened to the weight and moves 

 f as the weight moves, hence it is called 

 movable. In the figure, one end of the 

 rope is fastened on the hook and the 

 other end is attached to the force /. The 

 pulley with the weight W attached is rest- 

 ing on the rope. Each strand of rope, c 

 or 6, holds one-half the weight, just the 

 same as if the weight were hanging on two 

 separate ropes. The pulley is now acting 



as a second-class lever whose fulcrum 

 MOVABLE PULLEY , i i , mi < i 



moves as the weight moves. The fulcrum 



is at the end of the diameter which the strand of rope, 



