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Thread: Great Moments in Physics

  1. #1
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    Great Moments in Physics

    The following concerns a question in a physics degree exam at the University of Copenhagen.

    "Describe how to determine the height of a skyscraper with a barometer."

    One student replied:

    "You tie a long piece of string to the neck of the barometer, then lower the barometer from the roof of the skyscraper to the ground. The length of the string plus the length of the barometer will equal the height of the building."

    This highly original answer so incensed the examiner that the student was failed. The student appealed on the grounds that his answer was indisputably correct, and the university appointed an independent arbiter to decide the case. The arbiter judged that the answer was indeed correct, but did the problem it was decided to call the student in and allow him six minutes in which to provide a verbal answer which showed at least a minimal familiarity with the basic principles of physics.

    For five minutes the student sat in silence, forehead creased in thought. The arbiter reminded him that time was running out, to which the student replied that he had several extremely relevant answers, but couldn't make up his mind which to use.

    On being advised to hurry up the student replied as follows:

    "Firstly, you could take the barometer up to the roof of the skyscraper, drop it over the edge, and measure the time it takes to reach the ground. The height of the building can then be worked out from the formula H = 0.5g x t squared. But bad luck on the barometer."

    "Or if the sun is shining you could measure the height of the barometer, then set it on end and measure the length of its shadow. Then you measure the length of the skyscraper's shadow, and thereafter it is a simple matter of proportional arithmetic to work out the height of the skyscraper."

    "But if you wanted to be highly scientific about it, you could tie a short piece of string to the barometer and swing it like a pendulum, first at ground level and then on the roof of the skyscraper. The height is worked out by the difference in the gravitational restoring force T = 2 pi sqroot (l / g)."

    "Or if the skyscraper has an outside emergency staircase, it would be easier to walk up it and mark off the height of the skyscraper in barometer lengths, then add them up."

    "If you merely wanted to be boring and orthodox about it, of course, you could use the barometer to measure the air pressure on the roof of the skyscraper and on the ground, and convert the difference in millibars into feet to give the height of the building."

    "But since we are constantly being exhorted to exercise independence of mind and apply scientific methods, undoubtedly the best way would be to knock on the janitor's door and say to him 'If you would like a nice new barometer, I will give you this one if you tell me the height of this skyscraper'."

    The student was Niels Bohr, the only person from Denmark to win the Nobel prize for Physics.

  2. #2
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    But why would the janitor know the height of the building? As I heard the joke from my father, it was the "building superintendent" who was supposedly in possession of this secret knowledge.

    I doubt it was really Niels Bohr, but a great story nonetheless.

  3. #3
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    Re: Great Moments in Physics

    Quote Originally Posted by Silent Knight
    "But if you wanted to be highly scientific about it, you could tie a short piece of string to the barometer and swing it like a pendulum, first at ground level and then on the roof of the skyscraper. The height is worked out by the difference in the gravitational restoring force T = 2 pi sqroot (l / g)."
    If your string was that long in the first place, you could lower it to the ground, and swing it like a pendulum--and the period T will give you the length l directly.
    The student was Niels Bohr, the only person from Denmark to win the Nobel prize for Physics.
    What about the 1975 Nobel Prize for physics? It was won by Niels Bohr's son Aage, and Ben Mottelson, both from Denmark.

  4. #4
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    I can´t explain why, but when I saw the topic I somehow knew Bohr was involved.

  5. #5
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    This is discussed at length here and elsewhere. I agree that this is not a true story, at least not with regard to dropping the barometer. If I were the professor and a student decided to play games instead of answering the question, I would happily mark the student wrong because dropping the barometer would give the wrong answer unless you took wind resistence into account.

  6. #6
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    It is amusing how this one morphs over time. I first heard it about 22 years ago in a HS physics class. Back then, there were fewer solutions, and the student was nameless, as was his university. I did not see Niels pop up in this one until the internet hit full steam.

  7. #7
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    Another solution is to throw the barometer through the window of the city records office, so as to break in and find documentation on the building.

  8. #8
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    What always bugged me about this particular urban legend was, what physics teacher, especially university-level physics, wouldn't have followed that up with a question like, "given barometer reading A at the base of the building, B at the top, and given (if s/he's generous) atmospheric density C and acceleration of gravity g, calculate the height of the building"? And where would this Wunderkind have been on that question?

  9. #9
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    Quote Originally Posted by JohnOwens
    What always bugged me about this particular urban legend was, what physics teacher, especially university-level physics, wouldn't have followed that up with a question like, "given barometer reading A at the base of the building, B at the top, and given (if s/he's generous) atmospheric density C and acceleration of gravity g, calculate the height of the building"? And where would this Wunderkind have been on that question?
    Well, I suppose he would have had to be even more creative.

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