You step into an elevator. It starts going down, fast.
你进了电梯，电梯开始快速下降。
What would happen if you jumped right when it started going down?
此时，如果突然往上跳，会发生什么事？
Would the ceiling hit your head? Ouch!
会撞到天花板吗？唉唷！
Do you stay suspended in the air while the elevator plummets down?
当电梯垂直下降时，你是会悬在空中吗？
Let's examine the elevator problem one step at a time.
我们一步一步来剖析这个问题。
First, consider a scale. You know, the kind of scale you weigh yourself on.
首先，想象一个体重计。一般用来量体重的体重计。
When you step on a scale to weigh yourself, there are two forces involved.
你站上体重计量体重时，牵涉到两种力。
One, gravity pulls you down. Two, the scale pushes you up.
其一，是地心引力。其二，是体重计的垂直抗力。
What? You didn't know a scale could push?
什么？你不知道体重计有垂直抗力？
Of course it can! If it didn't push up on you, you would go crashing through the floor.
当然！如果没有它，体重计早被踩坏了。
This upward push is called the normal force and yes, it is normally there.
这种垂直抗力叫做正向力，是的，它是正向的。
Since you're just standing there on the scale, you're not moving.
因为你站在体重计上，静止不动。
Therefore, you're not accelerating.
所以，你没有加速度。
Newton's second law of motion -- net force equals mass times acceleration --
根据牛顿第二运动定律，施加于物体的净外力等于此物体的质量与加速度的乘积
tells us that if the acceleration equals zero, the net force must equal zero.
说明，如果加速度等于零，净外力也必等于零。
Which means that the force of gravity pulling you down must be equal to the force of the scale pushing you up.
也就是说，地心引力必等于正向力。
Now let's suppose you're standing on that scale in an elevator.
现在，试想你站在电梯里的体重计。

At first, the elevator is standing still, so you and the scale are standing still.
一开始，电梯静止不动，正如你和体重计一样静止不动。
The two forces on you are equal and opposite.
在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. We call that your weight.
看体重计就知道正向力有多大，也就是所谓的体重。
Then, the elevator starts falling down.
于是，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.
这表明你在向下加速。
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，都是向下的。
Since gravity hasn't changed, that must mean the scale isn't pushing up as hard.
由于地心引力从未改变，这就意味着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要小。
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? Would the elevator hit your head?
如果你向上跳呢？你会停止下降吗？还是会撞到天花板呢？
Well, what's pulling the elevator down? Gravity.
想想为何电梯会下降？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.
所以你在电梯的相对位置不会改变。
The ceiling will only hit you if you can jump up to the ceiling.
天花板不会碰到你，除非你本来就能跳到天花板的高度。
Now consider: what would happen if the elevator accelerated upward
最后，想象一下：如果电梯上升时会发生什么
and, even more frightening, if someone snuck in and cut the cable holding the elevator,
甚至更可怕的是，突然有人闯进电梯井道并剪断钢索，
what would happen then? Think about it.
那么会发生什么事？想想看吧。