Fig. 209. The Single 

 Pulley. 



PHYSIOLOGICAL PHYSICS. [Chap. XLI. 



if the two forces are to be in equilibrium, so that the 

 pulley is not turned, the two forces must be equal. A 

 small power will not, therefore, raise a larger weight. 



The advantage of the single 

 fixed pulley is that it alters the 

 direction of the force. Thus, a 

 man wishing to raise a load from 

 the ground may do so by placing 

 himself above the load and pul- 

 ling it upwards ; by using a 

 pulley, however, he pulls down- 

 wards, and thus is able to add 

 the weight of his body to the 

 power. Similarly in Fig. 209 

 the load P (of moment PX BC) 

 requires an equal weight R on 

 the other side of the large pulley to counterpoise it. 



The single fixed pulley is used in the body for 

 altering the direction of a force. Thus the digastric 

 muscle and the oblique muscles of the eye have the 

 direction of their action changed by bands of fibrous 

 tissue, etc.., acting the part of pulleys. 



It is, however, otherwise with the movable pulley 

 represented in Fig. 210. Here 

 we have a rope, fixed by a 

 hook to a beam, passing down- 

 wards round a movable pulley, 

 and then upwards. It next 

 passes over a fixed pulley, and 

 its free end has a weight at- 

 tached. The fixed pulley is 

 placed merely for changing the 

 direction of pull, it being incon- 

 venient to pull on the free end of 

 the rope after it has passed 

 round the movable pulley. But 



the fixed pulley does not affect 



Fig. 210. The Movable 

 Pulley. 



