MECHANICAL WORK in 



Mechanical Work. The work which this chapter dis- 

 cusses is mechanical work. It is the kind of work which 

 can be measured in foot-pounds or horse-power. It is the 

 kind of work done for us by animals and by machinery. 



An ancient legend illustrates the principle which man 

 has chiefly used in his simplest and oldest labor-saving 

 devices. A king had six sons. He decided to bestow his 

 kingdom upon the one having the most intelligence. To 

 test this he gave to each a bundle of sticks, and asked 

 them to see who could break his bundle in two. Five of 

 them tugged and strained and did their best to break the 

 bundles by main strength. But the sixth untied his 

 bundle and broke the sticks one by one. On him the father 

 bestowed his kingdom. 



This simple principle of doing things gradually explains 

 many of our simple machines. Let us see how it applies. 

 Why is it easier to climb a steep hill by a crooked path 

 than by a straight one? Why is it easier to roll a heavy 

 barrel up a tilted board than it is to lift it straight up to 

 the same height? Both of these are cases of doing things 

 gradually, cases in which the principle used is called the 

 principle of the inclined plane. If a barrel weighing three 

 hundred pounds is to be lifted to a height of one foot, 

 three hundred foot-pounds of energy must be used. If it 

 is lifted straight up all this energy must be applied at 

 once. But if it is pushed up an inclined plane three feet 

 in length, and the pushing power is applied parallel to the 

 plane, then only one hundred foot-pounds need be applied 

 at once. In other words, we have to move the barrel 

 three times as far, but, as compared with lifting it up 

 straight, we have to work only one-third as hard while 

 we are doing it. 



