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我会不定期在主题帖里记录我的学习内容
（主要为网易公开课和TED，有兴趣的坛友可以跟帖和我一起学习）
你进了电梯。电梯开始快速下降。
You step into an elevator. It starts going down fast.
此时，如果你突然往上跳会发生什么事？
What would happen if you jumped right when it started going down?
会撞到天花板吗？唉唷！
Would the ceiling hit your head? Ouch!
还是会在电梯下降时悬在空中呢？
Do you stay suspended in the air while the elevator plummets down?
我们一步一步来剖析这个问题。
Let's examine the elevator problem one step at a time.
首先，想象一个体重计，那种用来量体重的体重计。
First, consider a scale. You know, the kind of scale you weigh yourself on.
你站上体重计量体重时牵涉到两种力。
When you step on a scale to weigh yourself, there are two forces involved.
其一是地心引力。
One, gravity pulls you down.
其二是体重计的推力。
Two, the scale pushes you up.
什么？你不知道体重计有推力？
What? You didn't know a scale could push?
当然！如果没有它，体重计早被踩坏了。
Of course it can! If it didn't push up on you, you would go crashing through the floor.
这种向上的推力叫做正向力。
This upward push is called the normal force and yes, it is normally there.
因为你站在体重计上，静止不动。
Since you're just standing there on the scale, you're not moving.
所以，你没有加速度。
Therefore, you're not accelerating.
根据牛顿第二运动定律，净外力等于物体的质量与加速度的乘积，因此如果加速度等于零，净外力也必等于零。
Newton's second law of motion, net force equals mass times acceleration, tells us that if the acceleration equals zero, the net force must equal zero.
也就是说，向下拉你的地心引力必等于体重计向上支撑你的正向力。
Which means that the force of gravity pulling you down must be equal to the force of the scale pushing you up.
现在试想你站在电梯里的体重计上。
Now let's suppose you're standing on that scale in an elevator.
At first, the elevator is standing still, so you and the scale are standing still.
一开始，电梯静止不动，你和体重计一样静止不动。
The two forces on you are equal and opposite.
你身上的作用力相等且相反。
You can read how hard the scale is pushing by looking at it.
看体重计就知道正向力有多大了。
We call that your weight.
这就是所谓的体重。
Then, the elevator starts falling down.
然后，电梯开始下降。
You and the scale are in the elevator, so you are falling down too, faster and faster.
你和体重计也跟着一起下降，速度越来越快。
That means you are accelerating downward.
这代表你加速向下。
Now there is a net force in the same direction as the acceleration down.
此时，有一净外力和加速度同方向。
Since gravity hasn't changed, that must mean the scale isn't pushing up as hard.
既然地心引力从未改变，就代表体重计的垂直抗力不够大。
So the scale is reading a smaller number.
因此体重计上显示的数值较实际小。
The faster the elevator accelerates, the less the scale pushes up.
由此可知，电梯下降速度越快，体重计的正向力越小。
What if you jumped? Would you stop falling?
如果你向上跳呢？你会停止下降吗？
Would the elevator hit your head?
还是会撞到天花板呢？
Well, what's pulling the elevator down? Gravity.
想想电梯为何会下降？是因为地心引力。
Is gravity pulling on you? Of course.
那地心引力是否也对你作用？当然。
So your relative position in the elevator will stay the same.
所以你在电梯的位置不变。
The ceiling will only hit you if you can jump up to the ceiling.
你也不会撞到天花板，除非你本来就能跳到天花板上。
Now consider: what would happen if the elevator accelerated upward and, even morefrightening, if someone snuck in and cut the cable holding the elevator, what would happenthen?
最后，想象一下，如果电梯上升时更恐怖的事情发生了，如果有人闯进电梯井道、剪断钢索会发生什么事？
Think about it.
想想看。
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