MACHINES 



55 



if a one-pound weight is lifted two feet, we say that the 

 work done is equal to two foot pounds. 



Experiment 1 1 . Pass a cord over a pulley as shown in Figure 

 46. Attach a spring balance F to the end of the cord and a weight 

 W to the other end. What force is required to 

 support the weight ? Weigh the weight W. The 

 force and weight in this case will be found to be 

 the same. If F moves a certain distance, W will 

 be moved up an equal distance. The force multi- 

 plied by the distance through which it moves will 

 just equal the weight multiplied by the distance 

 through which it moves. If d is the distance F 

 moves and d' the distance W moves, then 



Fd = Wd' 



This is the general law of all machines. 

 The power or force multiplied by the dis- 

 tance through which it acts equals the 

 resistance or weight multiplied by the 

 distance through which it acts. 



FIG. 46. The 

 advantage of a fixed 

 pulley is simply one 

 of direction. 



The Lever. This is the most common of all the sim- 

 ple machines. It is a bar of any kind arranged to turn 

 on a rest or pivot, called its fulcrum. The power is the 



FIG. 47. 



force applied to any part of the lever and the weight is 

 the body to be moved or balanced by the application 

 of power. 



