19.High School English Listening

Remember seesaws? Those things were great.
But there wasoften a problem.
Suppose there's a big kid on one side and a smaller kid on theother side.
The seesaw just stays in one position:
big kid on theground, smaller kid up in the air. No fun.
After a few differenttries, you probably realized that moving the big kid in towardthe center made it possible to rock a little.
Move the big kid evenmore 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 pointsomewhere along the plank that holds the two kids where their average weight falls.
If you can getthe 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 heavyas the little kid, then their mutual center of mass will be two times closer to the big kid than to thelittle 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 theirmutual center of mass is on the heavy kid's side of the pivot.
To make the seesaw work, theheavy kid must push that center of mass forward until it just touches the pivot.
Then theiraverage weight pushes right on the middle, letting the ends rock up and down.
译文：
还记得跷跷板吗？那些东西很不错。
但往往有一个问题。
假设有一个大孩子在一边，另一边是一个小孩子。
跷跷板只是停留在一个位置。
大孩子在地上，小孩子在空中。这可不好玩。
经过几次不同的尝试，你可能意识到，把大孩子移到中心，就可以摇晃一下。
进一步移动大孩子，你可以达到平衡。
这里发生了什么？
答案就在 "质心"。
质心是沿着木板的某一点，它承载着两个孩子的平均重量。
如果你能让质心在支点上，跷跷板就能保持平衡。
有一个简单的方法可以计算出质心的位置。
如果大孩子的重量是小孩子的两倍，那么他们的质心就会比小孩子的质心更接近大孩子的两倍。
三倍重，质心就会接近三倍。以此类推。
因此，重的孩子坐在地上而轻的孩子在空中的原因是，他们的共同质心在枢轴的重的孩子一边。
为了使跷跷板工作，重的孩子必须将质量中心向前推，直到它刚刚接触到枢轴。
然后，他们的平均体重就会推到中间，让两端上下摇晃。
重点词汇：
kid on 欺骗，哄骗
in one 集为一体
up in the air 悬而未决 ; 未决定
a few 少许 ; 几个 ; 〔反语〕很多 ; 一点点
a little 一点
going on 进行 ; 发生 ; “go on”的现在分词
center of mass 质心，质量中心
be over 结束 ; 完了