Gravitation 23 



9. If you drop the seed, it falls to the floor. 

 10. If you tip the glass to drink the lemonade, the surface of the 

 lemonade does not tip with the glass, but remains horizontal. 



SECTION 4. Sinking and floating: Displacement. 



What keeps a balloon up ? 

 What makes an iceberg float ? 



Why does cork float on the water and why do heavier sub- 

 stances sink? 

 If iron sinks, why do iron ships not sink? 



Again let us imagine ourselves up in the place where 

 gravitation has no effect. Suppose we lay a nail on the 

 surface of a bowl of water. It stays there and does 

 not sink. This does not seem at all surprising, of course, 

 since the nail no longer has weight. But when we put 

 a cork in the midst of the water, it stays there instead 

 of floating to the surface. This seems peculiar, because 

 the less a thing weighs the more easily it floats. So 

 when the cork weighs nothing at all, it seems that it 

 should float better than ever. Of course there is some 

 difficulty in deciding whether it ought to float toward 

 the part of the water nearest the floor or toward the 

 part nearest the ceiling, since there is no up or down ; 

 but one would think that it ought somehow to get to 

 the outside of the water and not stay exactly in the 

 middle. If put on the outside, however, it stays there 

 as well. 



A toy balloon, in the same way, will not go toward 

 either the ceiling or the floor, but just stays where it 

 is put, no matter how light a gas it is filled with. 



The explanation is as follows : For an object to float 

 on the water or in the air, the water or air must be heavier 



