82 Common Science 



SECTION 14. Elasticity. 



What makes a ball bounce ? 



How does a springboard help you dive? 



Why are automobile and bicycle tires rilled with air ?- 



Suppose there were a man who was perfectly elastic, 

 and who made everything he touched perfectly elastic. 

 Fortunately there is no such person, but suppose an 

 elastic man did exist : 



He walks with a spring and a bound ; his feet bounce 

 up like rubber balls each time they strike the earth ; 

 his legs snap back into place after each step as if pulled 

 by a spring. If he stumbles and falls to the ground, 

 he bounces back up into the air without a scar. (You 

 see, his skin springs back into shape even if it is scratched, 

 so that a scratch instantly heals.) And he bounces on 

 and on forever without stopping. 



Suppose you, seeing his plight, try to stop him. Since 

 we are pretending that he makes everything he touches 

 elastic, the instant you touch him you bounce helplessly 

 away in the opposite direction. 



You may think your clothes will be wrinkled by all 

 this bouncing about, but since we are imagining that 

 you have caught the elastic touch from the elastic man, 

 your clothes which touch you likewise become perfectly 

 elastic. So no matter how mussed they get, they 

 promptly straighten out again to the condition they 

 were in when you touched the elastic man. 



If you notice that your shoe lace was untied just 

 before you became elastic, and you now try to tie it and 

 tuck it in, you find it most unmanageable. It insists 

 upon flying out of your shoe and springing untied again. 



