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HOW MACHINES LIGHTEN LABOR 



devices which would relieve muscular strain and lighten physi- 

 cal labor. 



If all mankind had depended upon physical strength only, 



the world to-day would be 

 in the condition prevalent 

 in parts of Africa, Asia, and 

 South America, where the 

 natives loosen the soil with 

 their hands or with crude 

 implements (Fig. 154), and 

 transport huge weights on 

 their shoulders and heads. 



Any mechanical device 

 (Figs. 152, 153, 155, 156), 

 whereby man's work can 

 be more conveniently done, 

 is called a machine. The 

 machine itself never does 



FIG. 152. -The wheelbarrow lightens labor. aRy wor k _ j t me rely en- 

 ables man to use his own efforts to better advantage. 



When do we work? Whenever, as a result of effort or force, 

 an object is moved, work is done. If you lift a knapsack from 

 the floor to the table, 

 you do work because 

 you use force and move 

 the knapsack through a 

 distance equal to the ta 

 height of the table. If 

 the knapsack were twice 



as heavy, you would FIG. 153. Rolling barrels up a plank. 



exert twice as much 



force to raise it to the same height, and hence you would do 



double the work. If you raised the knapsack twice the distance, 



