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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，如果你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?
还是会在电梯下降时悬在空中呢？
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，那种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.
其一是地心引力。
Two, the scale pushes you up.
其二是体重计的推力。
What? You didn't know a scale could push?
什么？你不知道体重计有推力？
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.
这种向上的推力叫做正向力。
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, tells us that if the acceleration equals zero, the net force must equal zero.
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。
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，向下拉你的地心引力必等于体重计向上支撑你的正向力。
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上。
