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Elevator physics. [very graphic]

  1. Nov 21, 2012 #1
    I've read the forum rules where it says that graphical/violent content cannot be posted on the forum and I completely understand that. That being said, I will post this in hopes that someone out there can help me resolve an argument that my friends and I have been having ever since we came across this. Basically it is a short clip of video we saw where there might be some physics incoherence with it. We are all university students studying engineering and we've all taken physics before. Anyhoo, without any further delays, here is the situation:


    In the video, a woman enters an elevator and begins to descend to ground floor with constant speed. However, not long after the elevator had started moving, the elevator wire snapped and it sends the elevator plummeting towards the ground. This is all fine and dandy except for the weird incoherence that bugs me and some of my other engineering friends. Although the elevator experiences a very abrupt acceleration from not being tethered to the wire anymore, the woman in the elevator appears as if she is falling at the same rate as the elevator. Is this correct? For further clarification, her feet were on the floor of the elevator and she is basically "standing" throughout the entire duration of the scene, until the elevator stops and she face plants onto the floor of the elevator. That part is the graphic part of this whole video and I implore you to pay no mind to it since I am merely here for the physics, not to be a troll. My interpretation of that scene is that it is false. The woman was initially at the same constant downward velocity as the elevator prior to the steel tether snapping. After the snap, the elevator experiences an acceleration greater than her, hence she should displace upward in reference to the elevator. Her feet should not be touching the ground and instead she should be in a somewhat free-fall state (I.E. "floating"). That is my argument for this case.

    My friend's argument is that everything fall at the same 9.81m/s^2 and that the scene is a legitimate representation of what should happen in real life. He argues that the woman falls as the exact same acceleration as the elevator and so she would not have any upward displacement in reference to the elevator, hence her feet touching the ground throughout the entire duration.


    That is the best I can do to describe the video. If anyone is interested in seeing it for themselves to better assess the situation, click on this https://www.youtube.com/watch?v=ef1o9VzlbA8.

    Mind you, it is very GRAPHICAL towards the end and I am giving you this warning ahead of time because I don't want to be breaking the forum rules and not have my question answered. I would really like some feedback from the Physics community on this matter because both of our arguments seems valid and we would like to reach a definitive conclusion. Your feedback would be greatly appreciated.


    Thank you!
     
  2. jcsd
  3. Nov 21, 2012 #2

    Doc Al

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    Your friend is correct. If the elevator were really in free fall, then both elevator and passenger would accelerate at the same rate. They both would be in free fall. She would be "weightless": the elevator would not be exerting the usual upward support force on her.

    (Of course, real elevators have emergency brakes that will kick in if the cable snaps.)
     
  4. Nov 21, 2012 #3
    Since they are both in freefall, the woman would probably move away from the floor; virtually any movement, such as a slight stretch in the legs, or pointing toes, will cause a movement away from the floor. You are correct, but not in the sense that they experience different accelerations.

    EDIT: Having just watched the video, all the panicking would have caused her to move away from the floor
     
  5. Nov 21, 2012 #4

    Doc Al

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    True, if the woman pushes against the floor by stretching her legs she'll start separating from the elevator floor. But that wasn't the OP's argument. If she just stands there frozen in fear, she and the elevator will move together. (The cartoon was a bit too chaotic to draw any conclusions or tell what was going on. At least for me!)
     
  6. Nov 21, 2012 #5

    mfb

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    That is very common for standing humans, otherwise they don't keep standing on the floor.

    The elevator in the video is "shaking" somehow - it has to have some contact to the walls or other objects to do that. This indicates an acceleration below g, which allows the woman to stand on the floor.
     
  7. Nov 21, 2012 #6

    K^2

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    In a shaft designed for single elevator, a falling elevator will very quickly reach terminal velocity due to air flow being very restricted. The actual weightlesness can be very brief. And if cable wasn't completely free, there could be none at all. So the animation isn't necessarily wrong.

    But yes, if you actually had the free-fall situation, the person would separate from floor. If nothing else, the tension in muscle used to support the stance before the fall would be enough to push you away.
     
  8. Nov 21, 2012 #7
    You won't get a free fall situation with an elevator because of air flow restriction on the outside and practicaly non in the inside on the passengers.
     
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