WHEN DO WE WORK? 



155 



devices which would relieve muscular strain and lighten 



physical 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. 94), and transport 



huge weights on their 



shoulders and heads. 



Any mechanical device 



(Figs. 95 and 96), whereby 



man's work can be more 



conveniently done, is called 



a machine ; the machine it- 

 self never does any work 



it merely enables man to use his own efforts to better advantage. 

 150. 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 be- 

 cause you use force 

 and move the knap- 

 sack through a distance 

 equal to the height of 



the table. If the knapsack were twice as heavy, you would 



exert twice as much force to raise it to the same height, and 



hence you would do double thje work. If you raised the 



FIG. 92. The wheelbarrow lightens labor. 



FIG. 93- Rolling barrels up a plank. 



