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  • You step into an elevator.


  • It starts going down, fast.


  • 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?

  • Would the ceiling hit your head? Ouch!


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


  • Let's examine the elevator problem one step at a time.


  • First, consider a scale.


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


  • When you step on a scale to weigh yourself,


  • there are two forces involved.


  • 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.


  • Since you're just standing there on the scale, you're not moving.


  • 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.


  • 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,


  • 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.


  • You can read how hard the scale is pushing by looking at it.


  • We call that your weight.

    也就是所謂的 體重

  • Then, the elevator starts falling down.


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


  • faster and faster.


  • That means you are accelerating downward.


  • 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?

  • Would the elevator hit your head?


  • 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.


  • 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?


  • Think about it.

    想想看 -繁體中文翻譯:盧曉天

You step into an elevator.


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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