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Gravity Debate: Need help with argument

  1. Dec 30, 2008 #1
    Hello. I am having an argument with two of my friends. They went to Harvard and MIT and are making fun of me for saying you would experience weightlessness in a free falling elevator. They say you would be stuck to the ground and unable to lift your legs since the faster you fall the harder you are pulled to the floor of the elevator.

    But Einstein's Equivalence Principle says that two objects of different mass would fall and accelerate at the same rate so you would basically be floating in the elevator as it free-fell. No? Is that correct?
    Thanks for any help.
     
  2. jcsd
  3. Dec 30, 2008 #2
    Your friends are silly. You would only be pulled to the floor when the elevator accelerates towards the top, not free falls. Shame to MIT and Harvard!:!!)
     
  4. Dec 30, 2008 #3
    Pity that they went to Harvard and MIT but never really grasped high school physics, because that's what this is. Ask them which brand of washing detergent they got their PHD's with.
     
  5. Dec 30, 2008 #4
    I call B.S., however it depends how you fall =).

    Is this elevator in an ideal world where there's no curvature in space, and the world isn't spinning. Or is it an elevator where the world isn't ideal and there's a curvature in space and the world is spinning.
     
  6. Dec 30, 2008 #5

    stewartcs

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    Your friends obviously did not go to Harvard or MIT if they are indeed asking you a serious question. I have a feeling they are just messing with you.

    If you are in an elevator and are in free-fall, then you will feel weightless (i.e. an absence of gravity). That is, until the elevator hits the ground! Then you will be stuck to the ground...err...floor of the elevator (and probably be really flat too)!

    CS
     
  7. Dec 30, 2008 #6

    G01

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    I find it very hard to believe that your friends went to Harvard and MIT if they are serious about their argument. I find it impossible to believe that, if they did go to those schools, that they majored in science or engineering, which I think everyone has been assuming.

    If the elevator was in free fall, you would indeed feel weightless, since the floor of the elevator does not push up on you with a normal force. (You and the floor are both accelerating downward at the same rate.) It is the normal force from the floor of the elevator that causes the feeling of having weight.

    So, while you still have gravity pulling on you in this situation, you will feel weightless since there is no normal force.
     
  8. Dec 30, 2008 #7
    Elevators have braking systems to prevent frictionless free-fall. Additionally, even if the braking system would fail, the column of air they displace as the elevator descends limits its terminal velocity so you won’t actually experience the equivalence of weightlessness.

    Case in point, I was in a Center City Philadelphia elevator a number of years ago that dropped from the 35th floor to a basement floor (and yes, I was reasonably certain “judgment day” had come). Upon stopping, the doors opened only about 6 inches then closed just as quick as they had opened. The elevator then shot straight up to the 32nd or 33rd floor and its doors again opened, but only about ¾ of the way and again began to close immediately, but I had sprung out of that elevator as soon as it gave me 1 foot of open doors!

    I told maintenance which elevator was giving the “6 Flags - Life Flash Before Your Eyes” ride, and they responded with, “Yeah, it’s been acting up all morning.” Doh!

    During the quick descent, I could feel that I was noticeably lighter on my feet, but not weightless and the elevator slowed so fast at the end that it caused my knees to buckle (which sure beats splattering!). My main point here is, elevators don’t make very good examples for free-fall because they aren’t typically “free-falling” devices.
     
  9. Dec 30, 2008 #8

    stewartcs

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    I'm relatively certain they are speaking purely theoretically and not practically. Hence, one may ignore friction and drag.

    CS
     
  10. Dec 30, 2008 #9
    Based upon their initial argument, it is just as likely that they are not aware of that which I've stated. It's best not to "assume".
     
  11. Dec 30, 2008 #10

    stewartcs

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    It is not just as likely actually, and I'm not assuming. It is explicitly stated in their argument that the elevator is in free-fall. By definition, free-falling objects do not encounter air resistance (although it may certainly be accounted for if one choses to).

    CS
     
  12. Dec 30, 2008 #11

    russ_watters

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    My guess would be B.A. Not everyone who goes to Harvard studies physics, guys.
     
  13. Dec 30, 2008 #12

    arildno

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    Well, they might have studied feminist physics or ethno-mathematics..
     
  14. Dec 30, 2008 #13
    I think there was a misunderstanding when you asked them.

    No-one is stupid enough to think you would be pulled to the floor in a falling elevator.

    Maybe the last time they went in an elevator they thought they were going downwards when they were actually going up? That would explain it :p
     
  15. Dec 30, 2008 #14
    Sorry but you don't need a physics degree to come to such insights. It's high school physics, or even just a little education + intuition. You don't go to Harvard and not understand the concept of free fall. "Feet stuck to the elevator"? :rofl:


    Amazing story, Gnosis, i can't imagine what was going through you after it went up again. What was the reason for the relatively "gently" deceleration after the semi-free fall?
     
  16. Dec 30, 2008 #15

    Dale

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    :rofl:

    I agree here. If true, it would be embarassing that they let such people in, let alone graduate. I don't care what degree they are seeking, this is basic stuff that anyone who has jumped off a fence or wall should be able to understand viscerally.

    I'm guessing your friends are either teasing you or are the unholy spawn of some elite politician :devil:
     
  17. Dec 30, 2008 #16

    D H

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    No reason to derogate groups that don't deserve it. Reserve it for groups that do (at least with regard to understanding simple scientific concepts). As in

    Well, they might have taken "Physics for Poets".
     
  18. Dec 31, 2008 #17

    arildno

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    I do not derogate those groups of people at all.

    I DO derogate the wished-for post-modernist pseudo-disciplines like feminist physics and ethno-mathematics that have been espoused by crackpots like Lucie Irigaray, S. Harding and many others beside them who regard the sciences as merely a "social discourse" that has attained a malignant "hegemony" over other "discourses".

    You might do well to read "Higher Superstition" by Levitt (and some other guy), or "A House built on Sand", edited by Noretta Koertge to get an inkling of what I referred to.
    (Those books are written by scientists tackling the crackpottery of post-modernism when it ventures into the field of science)
     
  19. Dec 31, 2008 #18

    D H

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    You mean stuff of this ilk? "Scientists have to gal to demand large sums of public money, forcibly taken from common people to fund schemes such as military research. Scientists are a privileged bunch living in an ivory tower, supposedly searching for this illusive truth. Well, the search has ended here, in failure, the house of cards starts to collapse and the word is out that science is a fraud!" (http://www.optionality.net/mag/apr98a.html)

    or this, "The problem with maths is that it expresses the philosophy of uniformity, of universal truth, of singularity, etc. Some people may agree to use a symbol or abbreviation to refer to a specific process. This may be useful in whatever they are doing. But what maths does is to elevate such an abbreviation or symbol to a uniform system that everyone has to use, as if it is the one and only true reflection of universal reality. Of course, this is nothing but dictatorship, which may seem an easy way for the dictator to deal with society, but which clearly is detrimental for all involved, including that very dictator." (http://www.optionality.net/mag/nov98a.html)

    or this, "In that case, why is it not as illuminating and honest to refer to Newton's laws as "Newton's rape manual" as it is to call them 'Newton's mechanics'?", Sandra Harding, "The Science Question in Feminism"?

    Yowsers. I stand completely corrected.
     
  20. Jan 12, 2009 #19
  21. Mar 1, 2009 #20
    If the elevator were descending, but slowing down, you would be accelerating upwards so you would be pressed to the floor.

    I can only pray that they meant something like this. If you go to Harvard or MIT-

    Scratch that, if you go to college-

    In fact, if you've been in an elevator, you'd never in a million years say that you would be pressed to the floor while the system was in free fall.
     
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