WORK AND MACHINES 



61 



1. They help man to apply force in a more convenient 

 direction. The pulley (Fig. 45) and lever (Fig. 46) are 

 common examples of this. A bit of 



thought will show how handy it 

 may be, at times, to thus change 

 the direction of motion. 



2. We may use other forces 

 than our own to run them. Steam 

 engines, windmills, electric mo- 

 tors, water wheels, and tread- 

 mills all serve to call such forces 

 to mind. 



3. They help us to store energy 

 to be used at another time. For 

 example, the spring of a watch, 

 in unwinding, does only the work 

 which was done upon it in 

 winding it up. We could not 



FIG. 45 



easily exert force directly upon the wheels all day long. 



4. By their use we may exchange strength of force for 



speed, and speed for strength of force. That is, we may 



