Would you weigh less in an elevator? – Carol Hedden

Translator: tom carter
Reviewer: Bedirhan Cinar 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? 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 motion — net force equals mass times acceleration — tells 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 acceleration — down. 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.

100 thoughts on “Would you weigh less in an elevator? – Carol Hedden

  1. Your professors definitely sound like douchebags. Thankfully, I never had such jerks – I've literally loved physics before I even knew what the subject was called. YAY independent thinking before preschool 🙂

  2. I would expect the elevator to only accelerate for a short time until it reaches a constant velocity then the scale would be the same as if you weighing yourself outside the elevator.

  3. In 2:40 the acceleration is non-zero. If you were to jump, you'd hit the ceiling because the distance between two bodies with the same acceleration and different velocities will only grow if the direction of the acceleration favors the faster body and that body is already in the lead. This is corrected in 2:52 (They got the words right, but the picure was wrong for those 12 seconds)

  4. This video confuses me; I don't feel like the posed questions in the beginning of the video were properly answered in the end. I got the feeling that this video is telling that there basically is no such thing as being weightless, which is crazy.

  5. Yeah, a really crap video – worse than having no video as it is actually completely wrong and misleading on many occasions. Some examples they give are only true when the velocity is constant (i.e no acceleration), while other examples would only be true if the elevator was accelerating at a constant rate, yet they make no distinction between the two and fail to provide any clarification.

  6. No, we can't agree, as I'd rather people still ponder the question and seek the (right) answer, rather than settling for the plain wrong and misleading answer this video provides.

  7. If you just listen to the video, it's not wrong at any point. You may feel it is misleading because it doesn't ram home a distinction between velocity and acceleration, but it does differentiate between them if you actually listen to what is said. I encourage you to post your own education video on the topic and show us all "how it should be done", since you clearly think this video is wrong.

  8. It's insinuating some of the most prevalent misconceptions by early physics students, which teachers have to un-teach due to most educational systems replicating this system, keeping velocity and acceleration together, because they're really difficult for some children to differentiate.

  9. The video does mention about acceleration, which is the reason why your weight changes. However, in reality, the elevator come to a constant velocity after a certain time (acceleration=0) . At this point, your weight remains the same as if you were outside the elevator.

  10. then what should she have explained differently???…how should she have explained velocity and acceleration…(just curios:L)

  11. Science is built on intellectual honesty/scepticism and RIGOUR. An inability to LISTEN to what other people say is not part of science. Even if everyone thinks they heard A, if the recorded evidence says the speaker definitely said B, then they said B, and for people to attack a video because THEY didn't pay attention and think it through carefully is not a valid criticism. Sure, if the speaker was clearer, it would compensate for others' intellectually laziness, but that doesn't make them wrong

  12. It needs to be said simply, but not wrong. Science without accuracy is worthless and a fake knowledge is more dangerous than none.

  13. While the understand that unique animation is cool and all…traditional animation techniques could have been just as effective and much less disturbing

  14. If elevetor go down faster than you fall- you dont hit roof of elevator, it woupd hit you

  15. if the elevator fell fast enough, you will float around. this is how zero gravity simulator aircraft work and why we don't feel gravity when orbiting our planet; because the enclosure that we're in is free falling to the ground. The International Space Station is in a constant free-fall as it orbits fast enough to avoid hitting our planet thus zero gravity is simulated on board.

  16. hmmm sad. I hope Ted Ed does not start producing poor videos like this one. If anyone wanted to know how an elevator worked then this will certainly confuse them. I so was looking forward for a good video to show my class…

  17. Worst video ever. And that girls voice is annoying as fuck! And it feels like this video was made for a 2 year old.

    Its fuckin' simple:

    If the elevator acceleration = 0, then p=p1 where p is the weight on solid ground, and p1 is the weight in the elevator

    if the elevator acceleration > 0 (going up and accelerating) then p<p1

    if the elevator acceleration < 0 (going down adn accelerating) then p>p1

    Its not rocket science.

  18. If somebody cut the cable well then racks on the side would push into another rack on the elevator to stop it prom falling all the way to ground and it would stay suspended.

  19. This animation is so bad and the narrator is so childish it earned itself a hate comment and a thumbs down

  20. The part where she describes you weighing less from the elevator isn't true at all.  Gravity doesn't increase unless the mass of the earth dramatically increased in the microsecond that you started the elevator.  The normal force doesn't decrease, it stays the same. The fact that you have less push on the scale is from your inertia, you weren't moving and all the sudden your environment is. This video was just confusing to people and I hope Ted doesn't do this type of video again.

  21. Try using a barometer in an elevator; it will read wrong! Also wondering, what if you jump in an elevator falling at terminal velocity?

  22. if the rope was cut, then the lift would be free falling and although your weight stays the same, the weighing scale would read 0

  23. i think the scale would show 0.
    i dont know am i right:
    can i say that the acceleration is a constant?

  24. This wasn't informative. Thank goodness I already knew the answers. If the Cable was cut, when you jumped upwards, you will be floating weightless. But since the elevator is attached to a cable, and the acceleration is controlled, your weight will still be positive, but less than normal. Once it has reached cruising speed, your weight will return to your regular weight, since the velocity doesn't change.

  25. If someone burst the cables holding the elevator and it free fell, you would be completely weightless due to the acceleration and as it reaches terminal velocity you will be levitating in the elevator.

  26. I like the animation, it make this very boring video a bit more interesting. This is a poorly written script and I can't believe it is from Ted

  27. Why are there no answers for all these questions…. Wasting my time….. I had liked Ted-Ed but I didn't expect such a boring video from them…… Disappointingly disappointing….

  28. Get this: Say the acceleration of gravity in the elevator of 9.8m/s^2 and the elevator accelerates downwards at 4.9m/s^2 and that the elevator is in a large vacuum chamber (how coincedental, huh?). That means your acceleration *Relative to the elevator and the air in the elevator and the scale is 9.8-4.9 = 4.9m/s^2 which means you will weigh half as you normally do, and if you jumped, you could jump 4 times higher, not twice. That is because the formula of acceleration based speed or distance traveled in freefall in a vacuum would be 1/2 at^2, where a is acceleration, and t is time falling. If you jumped and your velocity was 4.9m/s at when your feet left the ground, in normal gravity, you would go up for half a second, plug the numbers, 1/2 * 9.8 * 0.5^2 and you get 1.225 meters, or about 4 feet. But in half of the gravity, you will be going up for about 1 second. Plug the numbers again, 1/2 * 4.9 * 1^2 and you get 4.9 meters, or 16 feet plus 0.8 inches. Now if the elevator accelerates down at 9.8m/s ^2, you would accelerate at 0m/s^2 relative to the elevator, meaning you would be floating. But If the elevator is accelrating down at 19.6m/s^2, since gravity is overcoming half of that acceleration, you would accelerate up towards the ceiling of the elevator at 9.8 m/s^2, meaning you will feel like the world is upside down. If the elevator was accelerating up at 9.8m/s^2, you would feel twice as heavier, because of the elevator's acceleration, and gravity added together. You would also jump 1/4 as high as you previously could as well and could possibly pass out now that your blood is twice as heavier and falls 41.41% faster.

  29. Probably one of the worst ted ed videos ever. You should take it down and not confuse people and especially kids with this

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