Читать книгу Common Science - Carleton Washburne - Страница 41

Table of Contents

Section 10. Levers.

How a big weight can be lifted with a little force; how one thing moving slowly a short distance can make another move swiftly a long distance.

Why can you go so much faster on a bicycle than on foot?

How can a man lift up a heavy automobile by using a jack?

Why can you crack a hard nut with a nutcracker when you cannot crack it by squeezing it between two pieces of iron?

"Give me a lever, long enough and strong enough, and something to rest it on, and I can lift the whole world," said an old Greek philosopher. And as a philosopher he was right; theoretically it would be possible. But since he needed a lever that would have been as long as from here to the farthest star whose distance has ever been measured, and since he would have had to push his end of the lever something like a quintillion (1,000,000,000,000,000,000) miles to lift the earth one inch, his proposition was hardly a practical one.

But levers are practical. Without them there would be none of our modern machines. No locomotives could speed across the continents; no derricks could lift great weights; no automobiles or bicycles would quicken our travel; our very bodies would be completely paralyzed. Yet the law back of all these things is really simple.

You have often noticed on the see-saw that a small child at one end can be balanced by a larger child at the other end, provided that the larger child sits nearer the middle. Why should it matter where the larger child sits? He is always heavier—why doesn't he overbalance the small child? It is because when the small child moves up and down he goes a longer distance than the large child does. In Figure 26 the large boy moves up and down only half as far as the little girl does. She weighs only half as much as he, yet she balances him.