Fig. 12. The upper tube is filled with water and the lower with oil. What will happen when she pulls the cardboard out?
Oil is lighter than water, as you know, because you have seen a film of oil floating on water. When you have the two test tubes in such a position that the oil and water can change, the water is pulled down under the kerosene because gravity is pulling harder on the water than it is pulling on the kerosene. The water, therefore, goes to the bottom and this forces the kerosene up.
Application 6. Three men were making a raft. For floats they meant to use some air-tight galvanized iron cylinders. One of them wanted to fill the cylinders with cork, "because," he said, "cork is what you put in life preservers and it floats better than anything I know of." "They'd be better with nothing in them at all," said a second. "Pump all the air out and leave vacuums. They're air-tight and they are strong enough to resist the air pressure." But the third man said, "Why, you've got to have some air in them to buoy them up. Cork would be all right, but it isn't as light as air; so air would be the best thing to fill them with."
Which way would the floats have worked best?
Application 7. A little girl was telling her class about icebergs. "They are very dangerous," she said, "and ships are often wrecked by running into them. You see, the sun melts the top off them so that all there is left is under water. The sailors can't see the ice under water, and so their ships run into it and are sunk." Another girl objected to this; she said, "That couldn't be; the ice would bob up as fast as the top melted." "No, it wouldn't," said a boy. "If that lower part wasn't heavier than water, it never would have stayed under at all. And if it was heavier at the beginning, it would still be heavier after the top melted off."
Who was right?
Inference Exercise
Explain the following:
11. When you wash dishes, a cup often floats on top of the water, while a plate made of the same sort of china sinks to the bottom of the pan.
12. If you put the cup in sidewise, it sinks.
13. The water in the cup, when lying on its side, is exactly as high as the water in the dish pan.
14. If you put a glass into the water, mouth first, the water cannot get up into the glass; if you tip it a little, there are bubbles in the water and some water enters the glass.
15. If you let a dish slip while you are wiping it, it crashes to the floor.
16. It is much harder to hold a large platter while you are wiping it than it is to hold a small butter plate.
17. If you set a hot glass upside down on the oilcloth table cover, the oilcloth bulges up into it when the hot air and steam shrink and leave a partial vacuum within the glass.
18. If you spill any of the dishwater on the floor, it flattens out.
19. You may use a kind of soap that is full of invisible little air bubbles; if you do, the soap will float on top of the water.
20. When you drop a dry dishcloth into water, it floats until all the pores are filled with water; then it sinks.
Section 5. How things are kept from toppling over: Stability.
Why is it harder to keep your balance on stilts than on your feet?
Why does a rowboat tip over more easily if you stand up in it?
