22 FORCE AND ENERGY 



out of the way and hence made the ball move more and 

 more slowly; (2) the earth, which by its downward pull 

 made the ball move in a path curving downward instead 

 of in a horizontal one. 



This illustration shows us the ways in which one body 

 may exert force upon another: (1) it may set the other 

 body in motion; (2) it may change the direction of the 

 motion; (3) it may stop the motion. In addition, (4) one 

 body may change the velocity, or rate of motion, of another 

 and so exert force upon it. 



Air exerts force, not simply by resisting other bodies 

 as they pass through it, but also as wind air in motion. 

 We use the force of the wind to sail a kite, to run a wind- 

 mill, or to drive a ship through the sea. 



24. Force of Expanding Gases. What exerts the force 

 that sets a bullet in motion? The cartridge used in 

 modern firearms is partly filled with powder (Fig. 18), 

 and the open end of the cartridge is closed by the bullet. 



Besides powder, the cartridge 

 contains a small amount of a 

 white solid called mercury ful- 

 minate, which, when given a 

 FIG. is. blow (percussion), sets the pow- 



Kinds of Powder: (1) Ordinary -, " n rrn IT / , i 



Form; (2) Giant Powder. der on fire. The pulling of the 

 trigger releases the hammer of 



the gun; the blow of the hammer causes the mercury 

 fulminate to ignite the powder, so that it "explodes." 

 By the explosion of the powder a gas is formed that will 

 take up, when released, perhaps 300 times as much space 

 as the original powder. In the cartridge the gas is under 



