Molecular Attraction 49 



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 un- 



starched 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 neces- 

 sary 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 



