英語(yǔ)聽(tīng)力文摘506 蹺蹺板的樂(lè)趣

Remember seesaws? Those things were great. But there was often a problem.

Suppose there’s a big kid on one side and a smaller kid on the other side. The seesaw just stays in one position: big kid on the ground, smaller kid up in the air. No fun. After a few different tries, you probably realized that moving the big kid in toward the center made it possible to rock a little. Move the big kid even more and you can achieve balance. Now the seesaw works.

What’s going on here? The answer is in the “center of mass.” The center of mass is a point somewhere along the plank that holds the two kids where their average weight falls. If you can get the center of mass to be over the pivot point, the seesaw will be balanced.

There’s a simple way to figure out where the center of mass is. If the big kid is two times as heavy as the little kid, then their mutual center of mass will be two times closer to the big kid than to the little kid. Three times heavier and the center of mass will be three times closer. And so on.

So the reason the heavy kid sits on the ground while the light kid is up in the air is that their mutual center of mass is on the heavy kid’s side of the pivot. To make the seesaw work, the heavy kid must push that center of mass forward until it just touches the pivot. Then their average weight pushes right on the middle, letting the ends rock up and down.

還記得蹺蹺板嗎?它可是偉大的發(fā)明，但仍然存在一個(gè)問(wèn)題。

假設(shè)蹺蹺板的一邊坐一個(gè)大點(diǎn)的孩子，而另一邊坐一個(gè) 小點(diǎn)的孩子。蹺蹺板就只停留在一邊：大孩子那邊，小孩子則懸在空中。這樣就失去了蹺蹺板的樂(lè)趣。幾次嘗試后，你也許會(huì)發(fā)現(xiàn)讓大孩子往中間坐久可以搖動(dòng)一點(diǎn)。再讓大孩子讓中間挪動(dòng)直至平衡后蹺蹺板就可以搖動(dòng)了。

這到底是怎么回事?答案就是“質(zhì)心”。質(zhì)心是蹺蹺板上支撐兩個(gè)孩子平均重量的一個(gè)點(diǎn)。如果找到了是新的支點(diǎn)，蹺蹺板就平衡了。

有一個(gè)簡(jiǎn)單的方法找出質(zhì)心。如果大孩子的重量是小孩子的2倍，那么質(zhì)心的位置離大孩子的距離就比離小孩子距離近2倍。重三倍，質(zhì)心的位置就進(jìn)三倍，以此類推。

因此，重的小孩坐在地上而輕的小孩懸在空中的原因是，他們共同的質(zhì)心在重的孩子這邊。要想讓蹺蹺板搖動(dòng)，重的孩子就得往中間挪直到質(zhì)心移到中心。然后當(dāng)他們的平均質(zhì)量正好在中間，這樣兩頭就可以上下?lián)u動(dòng)了。