評論
打印
At first, the elevator is standing still, so you and the scale are standing still.
一開始，電梯靜止不動，你和體重計一樣靜止不動。
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.
看體重計就知道正向力有多大了。
We call that your weight.
這就是所謂的體重。
Then, the elevator starts falling down.
然后，電梯開始下降。
You and the scale are in the elevator, so you are falling down too, faster and faster.
你和體重計也跟著一起下降，速度越來越快。
That means you are accelerating downward.
這代表你加速向下。
Now there is a net force in the same direction as the acceleration down.
此時，有一凈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.
既然地心引力從未改變，就代表體重計的垂直抗力不夠大。
So the scale is reading a smaller number.
因此體重計上顯示的數值較實際小。
The faster the elevator accelerates, the less the scale pushes up.
由此可知，電梯下降速度越快，體重計的正向力越小。
What if you jumped? Would you stop falling?
如果你向上跳呢？你會停止下降嗎？
Would the elevator hit your head?
還是會撞到天花板呢？
Well, what's pulling the elevator down? Gravity.
想想電梯為何會下降？是因為地心引力。
Is gravity pulling on you? Of course.
那地心引力是否也對你作用？當然。
So your relative position in the elevator will stay the same.
所以你在電梯的位置不變。
The ceiling will only hit you if you can jump up to the ceiling.
你也不會撞到天花板，除非你本來就能跳到天花板上。
Now consider: what would happen if the elevator accelerated upward and, even more frightening, if someone snuck in and cut the cable holding the elevator, what would happen then?
最后，想象一下，如果電梯上升時更恐怖的事情發生了，如果有人闖進電梯井道、剪斷鋼索會發生什么事？
Think about it.
想想看。
