英文科學(xué)讀本 第一冊(cè)·Lesson 40 What we mean by Elastic

Lesson 40 What we mean by Elastic

Come and sit down, Norah, said Willie. "I've got something new to show you. This is a piece of india-rubber band. Look, if I pull it I can stretch it nearly twice as long."

Well, what of that? said Norah. "Anybody could do that."

Don't be in such a hurry, said Will. He let go one end, and the india-rubber sprang back. "This is what I wanted to show you," said he. "The india-rubber will stretch when we pull it, and it will spring back to its proper size and shape when we let go."

Look, he added. "These pieces of cloth and flannel do the same. We say that india-rubber, cloth, and flannel are elastic. They stretch when we pull them, and spring back when we let go."

But teacher showed us, said Fred, "that there are other things, besides india-rubber, that spring back and are elastic. He showed us a cane, and a stick of whalebone, and bent them to make the two ends meet. They sprang back when he let go, because they are elastic."

He squeezed a sponge up in his hand. It sprang back when he opened his hand, because it is elastic.

He forced a cork into a bottle. The cork went in, because he made it smaller by squeezing it. When he pulled the cork out again, it sprang back to its right shape and size, because it is elastic.

So you see, Norah, all these things—the india-rubber, flannel, cloth, cane, whalebone, sponge, and cork—are elastic, but they are not elastic in quite the same way. One shows that it is elastic when it is pulled, another when it is bent, and another when it is squeezed out of shape.

SUMMARY

India-rubber stretches when we pull it, and springs back when we let go. It is elastic, and so are cloth and flannel. Whale-bone and cane bend easily, but spring back when we let go. They are elastic.

Sponge and cork can be squeezed, and made smaller, but they spring back when we let go. Sponge and cork are elastic.