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• You step into an elevator.

你進了電梯

• It starts going down, fast.

電梯開始快速下降

• What would happen if you jumped right when it started going down?

此時，如果突然往上跳 會發生什麼事？

會撞到天花板嗎？唉唷！

• Do you stay suspended in the air while the elevator plummets down?

還是會懸在空中呢？

• 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.

首先，想像一個體重計

• You know, the kind of scale you weigh yourself on.

一般用來量體重的體重計

• When you step on a scale to weigh yourself,

你站上體重計量體重時

• there are two forces involved.

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 是體重計的垂直抗力

• What? You didn't know a scale could push?

什麼？ 你不知道體重計有垂直抗力？

• 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.

這種垂直抗力叫做正向力

• 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 根據牛頓第二運動定律

• Newton's second law of motion -- net force equals mass times acceleration --

施加於物體的淨外力等於 此物體的質量與加速度的乘積

• tells us that if the acceleration equals zero, the net force must equal zero.

說明，如果加速度等於零 淨外力也必等於零

• 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，地心引力必等於正向力

• Now let's suppose you're standing on that scale in an elevator.

現在， 試想你站在電梯裡的體重計

• At first, the elevator is standing still,

一開始，電梯靜止不動

• so you and the scale are standing still.

正如你和體重計一樣 靜止不動

• 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.

看體重計就知道正向力有多大

• We call that your weight.

也就是所謂的 體重

• Then, the elevator starts falling down.

於是，電梯開始下降

• You and the scale are in the elevator, so you are falling down too,

你和體重計也跟著一起下降

• faster and faster.

速度越來越快

• That means you are accelerating downward.

代表加速度向下

• Now there is a net force in the same direction as the acceleration -- down.

此時， 有一淨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 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？

還是會撞到天花板呢？

• Well, what's pulling the elevator down? Gravity.

想想為何電梯會下降？ 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.

也不會撞到天花板 除非你本來就能跳到天花板

• Now consider: what would happen if the elevator accelerated upward

最後，想像一下 如果電梯上升時

• and, even more frightening, if someone snuck in and cut the cable holding the elevator,

突然有人發瘋， 闖進電梯井道剪斷鋼索

• what would happen then?

會發生什麼事？

想想看 -繁體中文翻譯：盧曉天

You step into an elevator.

B1 中級 中文 TED-Ed 電梯 體重 加速度 地心引力 天花板

【TED-Ed】我在電梯裡會更輕嗎？ Would you weigh less in an elevator? - Carol Hedden

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VoiceTube 發佈於 2013 年 05 月 02 日