06 May 2024

# Physics Test

### Niels Bohr, Physics, Problem Solving

The following concerns a question in a physics degree exam at the University of Copenhagen: “Describe how to determine the height of a skyscraper using 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 instructor 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 answer was indeed correct, but did not display knowledge of physics. To resolve 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 the 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 restoring force T = 2 pi sq. root (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 meters 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.

5 Feedbacks on "Physics Test"

### Bilderback

Whereupon the arbiter said: I see, Mr. Bohr, that rather than a wise young man, you are a wise ass.

### OneGuy

I only took one year of college physics. I am no expert. But my observation was that almost everything in physics was at one time believed by physics experts to be false. And later to be true. About half of everything in physics went through this sequence multiple times, i.e. true, untrue, true, untrue, hell we have no idea. So I just guessing here but I suspect about half of what we fervently believe in physics today is not true and worse we don’t even know which half to suspect.

### rocdoctom

There is always someone smarter than you.

### Craig Furlong

Funny story but not the great Bohr.

### Reading Lounge | Evocatively Ambiguous

[…] Physics Test (with a certain Niels […]

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