26 FORCE AND ENERGY 



striking property helplessness. This property is com- 

 monly called inertia. We define inertia when we say that 

 matter cannot move itself, cannot stop itself if moving, 

 and cannot change the direction or rate of its motion. 



There are many illustrations of inertia : 



When you played "tag," and your playmate came rushing toward 

 you at full speed, you took advantage of the inertia of his body when 

 you "dodged." You knew he could not stop himself at once. 



When you run around a corner, you go in a wide curve, because the 

 inertia of your body will not let you turn the corner sharply. 



When you shake a rug, you are able to jerk the rug away from the 

 dirt because of the inertia of the dirt. 



If you hit a suspended newspaper, you burst a hole in it, because 

 only the part struck moves forward while the rest of the newspaper 

 remains behind. 



The air also has inertia. If we try to push it away suddenly, as by 

 thrusting forward a paper fastened in a large hoop, the air remains 

 where it was and tears the paper, just as a tree or a rock would. We 



work against the inertia of air in mo- 



x --'" tion when we try to shut a door 



against a strong wind, or to "haul in" 

 \ a sail in a gale. 



28. " Flying from the Cen- 

 ter." If a stone attached to 

 / a string is whirled about the 

 hand, it moves in a circle (Fig. 

 ^' 21). The circular path is the 



FIG. 21. result of two forces: (1) the 



ine^a of the moving stone, 

 which, acting alone, would cause 



the stone to move off in a straight line ; and (2) the re- 

 sistance of the string, which compels the stone to remain 



