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You step into an elevator. It starts going down fast.
你進了電梯。電梯開始快速下降。
What would happen if you jumped right when it started going down?
此時，如果你突然往上跳會發生什么事？
Would the ceiling hit your head? Ouch!
會撞到天花板嗎？唉唷！
Do you stay suspended in the air while the elevator plummets down?
還是會在電梯下降時懸在空中呢？
Let's examine the elevator problem one step at a time.
我們一步一步來剖析這個問題。
First, consider a scale. You know, the kind of scale you weigh yourself on.
首先，想象一個體重計，那種用來量體重的體重計。
When you step on a scale to weigh yourself, there are two forces involved.
你站上體重計量體重時牽涉到兩種力。
One, gravity pulls you down.
其一是地心引力。
Two, the scale pushes you up.
其二是體重計的推力。
What? You didn't know a scale could push?
什么？你不知道體重計有推力？
Of course it can! If it didn't push up on you, you would go crashing through the floor.
當然！如果沒有它，體重計早被踩壞了。
This upward push is called the normal force and yes, it is normally there.
這種向上的推力叫做正向力。
Since you're just standing there on the scale, you're not moving.
因為你站在體重計上，靜止不動。
Therefore, you're not accelerating.
所以，你沒有加速度。
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.
根據牛頓第二運動定律，凈外力等于物體的質量與加速度的乘積，因此如果加速度等于零，凈外力也必等于零。
Which means that the force of gravity pulling you down must be equal to the force of the scale pushing you up.
也就是說，向下拉你的地心引力必等于體重計向上支撐你的正向力。
Now let's suppose you're standing on that scale in an elevator.
現在試想你站在電梯里的體重計上。
