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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,如果你突然往上跳的話會發生什麼事?

  • 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?

    還是會懸在空中呢?

  • Uh, 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.

    就是一般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.

    當然有!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。

  • Newton's second law of motionnet force equals mass times accelerationtells us that if the acceleration equals zero, the net force must equal zero.

    根據牛頓第二運動定律,施加於物體的淨外力等於此物體的質量與加速度的乘積。意思就是,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,地心引力必然等於體重計施加在你腳上的正向力。

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

    現在, 試想你站在電梯裡的體重計上。

  • At first, the elevator is standing still, so you and the scale are standing still.

    一開始,電梯靜止不動,所以你和體重計也一樣靜止不動 。

  • The two forces on you are equal and opposite.

    在你身上的兩股作用力相等且相反。

  • You can read how hard the scale is pushing by looking at it. We call that your weight.

    看體重計就知道體重計對你施加的正向力有多大,而這就是所謂的體重。

  • Then, the elevator starts falling down.

    接著電梯開始下降。

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

    你和體重計也跟著電梯一起下降,速度越來越快

  • That means you are accelerating downward.

    這代表你正在向下加速。

  • Now there is a net force in the same direction as the accelerationdown.

    此時, 淨外力和加速度的方向相同:向下。

  • 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,便代表體重計的垂直抗力不夠大。

  • So the scale is reading a smaller number.

    因此讓體重計上顯示的數值較小。

  • 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.

    所以你在電梯的相對位置是不會改變的。

  • The ceiling will only hit you if you can jump up to the ceiling.

    除非你本來就能跳到天花板的高度,否則你並不會撞到天花板。

  • 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?

    最後,想像一下電梯正在上升,而恐怖的是這時有人突然闖進電梯井道並剪斷鋼索。這樣的話會發生什麼事?

  • Think about it.

    試著想想看吧。

You step into an elevator.

你進了電梯。

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B1 中級 中文 TED-Ed 電梯 體重 地心引力 加速度 天花板

【TED-Ed】如果在電梯裡體重會更輕嗎? Would you weigh less in an elevator? - Carol Hedden

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    VoiceTube 發佈於 2021 年 12 月 05 日
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