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Topic: Floating Hourglass (Read 5819 times) |
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ThudnBlunder
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A glass cylinder is filled with water and has an hourglass floating at the top. If the cylinder is inverted a curious thing happens; the hourglass remains at the bottom of the cylinder until a certain quantity of sand has flowed into its lower compartment. Then it rises slowly to the top. It seems impossible that a transfer of sand from the top to the bottom of the hourglass would have any effect on its overall buoyancy. What is its modus operandi?
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THE MEEK SHALL INHERIT THE EARTH.....................................................................er, if that's all right with the rest of you.
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redPEPPER
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Re: Floating Hourglass
« Reply #1 on: Oct 31st, 2003, 3:10pm » |
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A guess:
The hourglass is barely light enough to float to the surface. However, with the sand on top, the hourglass tries to flip, so it presses against the glass cylinder. Maybe friction is enough to prevent it from going up, until the center of gravity is low enough that it doesn't try to flip as much.
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harpanet
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Re: Floating Hourglass
« Reply #2 on: Nov 1st, 2003, 8:59am » |
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Buoyancy has always confused me so I spent an interesting time reading up on it and I think I've got a better understanding of it. However, my new knowledge does not really help me with this problem!
This is my guess of what is happening:
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I think the effect is due to the kinetic energy of the sand as it falls from the upper globe to the lower. Initially the sand has further to fall than when the lower globe has started to fill up. The initial kinetic energy in the falling sand is enough to impart sufficent downward pressure to prevent the hourglass from floating upwards. As the sand continues to fall it has a increasingly shorter distance to drop as the sand pile grows, thus it has less kinetic energy and therefore exerts less downwards pressure. At some point the kinetic energy will be insufficient to prevent the hourglass from floating upwards.
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Whether I'm right or wrong it is certainly an interesting and thought-provoking puzzle.
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rmsgrey
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Re: Floating Hourglass
« Reply #3 on: Nov 1st, 2003, 6:58pm » |
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I'm with redPEPPER. I've heard of devices for sale with two glass tubes, in one of which floats an hourglass, in the other an hourglass sits at the bottom. When the apparatus is inverted, the floating glass remains at the bottom and the sinking glass remains at the top until, after a certain amount of time (and sand) has passed, they rise and sink respectively. This counters harpanet's guess.
I came across a description of the above device in a book somewhere - I think it was Martin Gardner, though it could possibly have been Ian Stewart or even someone else...
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BNC
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Re: Floating Hourglass
« Reply #4 on: Nov 1st, 2003, 9:37pm » |
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There's a very through discussion of this problem here.
This (excelent) document was posted in this forum some time ago -- I think (no sure) by James -- but I can't find the original post now.
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How about supercalifragilisticexpialidociouspuzzler [Towr, 2007]
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TimMann
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Re: Floating Hourglass
« Reply #5 on: Nov 1st, 2003, 11:15pm » |
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I posted a link to the book rather recently. I don't know if James posted one earlier when I wasn't coming around. Anyway, the legitimate site to get the book from is http://www.g4g4.com/contentsmmpp.html, and the correct title is The Mathmagician and the Pied Puzzler, edited by Elwyn Berlekamp and Tom Rodgers. The other sites that have copies seem to be violating the downloading agreement.
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| « Last Edit: Nov 1st, 2003, 11:17pm by TimMann » |
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Speaker
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Re: Floating Hourglass
« Reply #6 on: Nov 10th, 2003, 9:25pm » |
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I agree with RedPepper also. That is because it seems like a principle that someone could predict. Which would lead them to build the device, in order to test the idea.
Maybe you could predict friction also, but it doesn't seem to merit the effort of building the device to discover that friction occurs between two glass surfaces under water.
I intend to read the book to get more information.
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rmsgrey
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Re: Floating Hourglass
« Reply #7 on: Nov 14th, 2003, 7:27am » |
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I was under the impression that redPEPPER's answer was friction...
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Speaker
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Re: Floating Hourglass
« Reply #8 on: Nov 16th, 2003, 6:01pm » |
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Doh! My mistake.
But, now I know that friction is the right answer, so, so, so there!
And, my theory that a person would need to be motivated by something more unusual than finding out about friction on a glass surface under water is also shot down. If the glass blower made it I can imagine him playing around and trying to find another product and coming up with this idea.
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They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety. <Ben Franklin>
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