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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?

  • Would the ceiling hit your head? Ouch!

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

  • Uh, 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.

  • Two, the scale pushes you up.

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

  • Of course it can! If it didn't push up on you, you would go crashing through the floor.

  • 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.

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

  • 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.

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

  • 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.

  • 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 accelerationdown.

  • Since gravity hasn't changed, that must mean the scale isn't pushing up as hard.

  • So the scale is reading a smaller number.

  • The faster the elevator accelerates, the less the scale pushes up.

  • What if you jumped? Would you stop falling?

  • Would the elevator hit your head?

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

  • Is gravity pulling on you? Of course.

  • 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.

  • 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?

  • Think about it.

You step into an elevator.

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B1 TED-Ed elevator scale gravity net force acceleration

【TED-Ed】Would you weigh less in an elevator? - Carol Hedden

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    VoiceTube posted on 2021/12/05
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