Читать книгу Common Science - Carleton Washburne - Страница 32

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Which do you think is the stronger, the pull of gravity which makes some of the water drip off, or the pull of adhesion which makes some of the water cling to your finger?

If the pull of gravity is stronger, would not all the water drop off, leaving your finger dry? If the pull of adhesion is the stronger, would not all the water stay on your finger, none dropping off?

The truth of the matter is that gravity is stronger than adhesion unless things are very close together; then adhesion is stronger. The part of the water that is very close to your finger clings to it in spite of gravity; the part that is farther away forms drops and falls down because of the pull of gravity.

Adhesion, then, is the force that makes things cling to each other when they are very close together.

Why it is easier to turn a page if you wet your finger. Water spreads out on things so that it gets very close to them. The thin film of water on your finger is close enough to your finger and to the page which you are turning to cling to both; so when you move your finger, the page moves along with it.

Why dust clings to the ceiling and walls. The fine particles of dust are wafted up against the ceiling and walls by the moving air in the room. They are so small that they can fit into the small dents that are in plaster and paper and can get very close to the wall. Once they get close enough, the force of adhesion holds them with a pull stronger than that of gravity.

Oily and wet surfaces catch dust much more readily than clean, dry ones, simply because the dust can get so much closer to the oil or water film and because this film flows partly around each dust particle and holds it by the force of adhesion. This is why your face gets much dirtier when it is perspiring than when it is dry.

Application 12. Explain why cobwebs do not fall from the ceiling; why dust clings to a wet broom; why a postage stamp does not fall off an envelope.