58 



GENERAL SCIENCE 



Pulleys. Figure 51 represents the use of a single 

 fixed pulley. In this pulley the only advantage gained 



a is direction. By pulling down we may 



lift a body vertically, but we cannot 

 lift more than the equivalent of the 

 force applied at F. 



Arrange the pulley and spring bal- 

 ance as shown in Figure 52, so that a 

 single movable pulley is attached to the 

 weight W. It is quite evident that 

 the weight is supported equally by 

 each strand of the rope. 

 This may be verified * 

 by the spring balance. 

 FIG. 51. single Fixed it should be understood, 



Pulley. 



however, that the weight 

 at W includes the weight of the pulley. 



Make a test to see how far the force 

 must be moved to move the weight a 

 distance of 1 foot. Does the law of 

 machines hold in this case of the pulley? 



Other arrangements of pulleys are 

 shown in Figure 53. In each case there 

 are four strands of rope supporting the 

 weight. Then the force necessary to 

 support W is one fourth of the total 

 weight, or the weight equals the force FIG. 52. Measur- 

 multiplied by the number of supporting ft^USSS 



Strands Of rope. Movable Pulley. 



W - Fn 



With most arrangements of pulleys there is so much 

 friction that it may seem that the law does not hold. 



