Application 13. Explain why mercury cannot wet your fingers; why rain falls in drops; why it is harder to drive a nail into wood than into soap; why steel is hard.
Inference Exercise
Explain the following:
51. Ink spilled on a plain board soaks in, but on a varnished desk it can be easily wiped off.
52. When a window is soiled you can write on it with your finger; then your finger becomes soiled.
53. A starched apron or shirt stays clean longer than an unstarched one.
54. When you hold a lump of sugar with one edge just touching the surface of a cup of coffee, the coffee runs up the lump.
55. A drop of water on a dry plate is not flat but rounded.
56. It is hard to write on cloth because the ink spreads out and blurs.
57. If you roughen your finger nails by cleaning them with a knife, they will get soiled much more quickly than if you keep them smooth by using an orange stick.
58. When you dip your pen in the ink and then move it across the paper, it makes ink marks on the paper.
59. If you suck the air out of a bottle, the bottle will stick to your tongue.
60. You cannot break a thick piece of iron with your hands.
Section 9. Friction.
What makes ice slippery?
How does a brake stop a car?
Why do things wear out?
It would not be such a calamity if we were to turn off friction from the world. Still, I doubt whether we should want to leave it off much longer than was necessary for us to see what would happen. Suppose we imagine the world with all friction removed:
A man on a bicycle can coast forever along level ground. Ships at sea can shut off steam and coast clear across the ocean. No machinery needs oiling. The clothes on your body feel smoother and softer than the finest silk. Perpetual motion is an established fact instead of an absolute impossibility; everything that is not going against gravity will keep right on moving forever or until it bumps into something else.
But, if there is no friction and you want to stop, you cannot. Suppose you are in an automobile when all friction stops. You speed along helplessly in the direction you are going. You cannot steer the machine—your hands would slip right around on the steering wheel, and even if you turn it by grasping the spoke, your machine still skids straight forward. If you start to go up a hill, you slow down, stop, and then before you can get out of the machine you start backward down the hill again and keep on going backward until you smash into something.
A person on foot does not fare much better. If he is walking at the time friction ceases, the ground is suddenly so slippery that he falls down and slides along on his back or stomach in the same direction he was walking, until he bumps into something big or starts to slip up a slope. If he reaches a slope, he, like the automobile, stops an instant a little way up, then starts sliding helplessly backward.