MOVABLE PULLEYS 



I6 7 



a mast, it is not necessary to climb the mast, and so pull 



up the flag; the same result is accomplished much more easily 



by attaching the flag to a movable string, 



somewhat as in Figure 109, and pulling 



from below. As the string is pulled down, 



the flag rises and ultimately reaches the 



desired position. 



If we employ a stationary pulley, as in 

 Figure 109, we do not change the force, be- 

 cause the force required to balance the load 

 is as large as the load itself. The only ad- 

 vantage is that a force in one direction may 

 be used to produce motion in the opposite 

 direction. Such a pulley is known as a 

 fixed pulley. 



161. Movable Pulleys. By the use of 

 a movable pulley, we are able to support 

 a weight by a force equal to only one half 

 the load. In Figure 109, the downward 

 pull of the weight and the downward pull 

 of the hand are equal ; in Figure 1 10, the FIG. 109. By means of 

 spring balance supports only one half the S^^p^dSm^ 

 entire load, the remaining half being borne tion . in the opposite di- 



i i T i 1-11 11 rection. 



by the hook to which the string is attached. 

 The weight is divided equally between the two parts of the 

 string which passes around the pulley, so that each strand 

 bears only one half of the burden. 



We have seen in our study of the lever and the inclined 

 plane that an increase in force is always accompanied by a i 

 decrease in distance, and in the case of the pulley we naturally 

 look for a similar result. If you raise the balance (Fig. 1 10) 12 

 feet, you will find that the weight rises only 6 feet ; if you 

 raise the balance 24 inches, you will find that the weight rises 



