86 Common Science 



your hand, you are lifting up your body a little to do it ; 

 that is action and reaction. But after you take your 

 hand off the scale the pan springs back up : first it was 

 pushed down, then it springs back to its original posi- 

 tion ; it does this because of the elasticity of its spring. 



Application 23. Explain why basket balls are filled with 

 air; why springs are usually made of steel; why we use 

 rubber bands to hold papers together; why a toy balloon 

 becomes small again when you let the air out. 



Inference Exercise 



Explain the following, being especially 'careful not to confuse 



action and reaction with elasticity : 



in. When you want to push your chair back from a table, you 

 push forward against the table. 



112. The pans in which candy is cooled must be greased. 



113. Good springs make a bed comfortable. 



114. Paper clips are made of steel or spring brass. 



115. A spring door latch acts by itself if you close the door tightly. 



116. On a cold morning, you rub your hands together to warm 



them. 



117. If an electric fan is not fastened in place and has not a heavy 



base, it will move backward while it is going. 



118. Doors with springs on them will close after you. 



119. When you jump down on the end of a springboard, it throws 



you into the air. 



1 20. You move your hands backward to swim forward. 



NOTE. There are really two kinds of elasticity, which have nothing 

 to do with each other. Elasticity of form is the tendency of a thing to 

 go back to its original shape, as rubber does. If you make a dent in 

 rubber, it springs right back to the shape it had before. Elasticity of 

 volume is the tendency of a substance to go back to its original size, as 

 lead does. If you manage to squeeze lead into a smaller space, it will 

 spring right back to the same size as soon as you stop pressing it on all 

 sides. But a dent in lead will stay there ; it has little elasticity of form. 



Air and water all liquids, in fact have a great deal of elasticity 

 of volume, but practically no elasticity of form. They do not tend to 



