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Megan
Skeptic Friend
USA
163 Posts |
 Posted - 06/11/2002 : 20:03:31
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When you have a cup and you put it into the water with the open end in the water, how come the water doesn't go into the cup?
~Megan~
Questions: never leave home without them.
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James
SFN Regular
USA
754 Posts |
Posted - 06/11/2002 : 20:55:30 [Permalink]
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It's because the air pressure in the cup is equal to the air pressure pushing down on the surface of the water.
Or something like that.
________________________
Monday is an awful way to spend 1/7 of your life.
Two more years...Two more years...Two more years...Two more years...Two more years...
*whine* |
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Slater
SFN Regular
USA
1668 Posts |
Posted - 06/11/2002 : 21:25:16 [Permalink]
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We used to use this trick back in the 70's when we were doing reef work off the FL Keys. A plexiglas dome about 3' across anchored to the bottom by 8' cables in about 35' of water enabled us to talk to one another. Air has mass and it takes a fair amount of pressure to compress it.
It works the other way around too. Stick a soda straw into a glass of water and then cover the top end with your finger. So long as you keep the straw straight up and down the water will stay in when you raise it into the air.
Water also has mass but you cannot compress it.
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My business is to teach my aspirations to conform themselves to fact, not to try and make facts harmonize with my aspirations. ---Thomas Henry Huxley, 1860 |
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Chagur
New Member
USA
21 Posts |
Posted - 06/12/2002 : 16:04:57 [Permalink]
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Actually, Megan, the water does 'go' into the cup only you don't see it. If you were to submerge the cup deeper, the water would 'go' in farther and you would be able to see it.
It is the 'trapped' air which is being compressed that keeps the water out of the cup and since the amount of trapped air doesn't change, the deeper the cup is pushed, the more water enters the cup and the higher the air's pressure becomes because the air occupies less volume in the cup.
The amount of compression is proportional to the depth the cup is pushed down to since the trapped air has to balance the weight of the column of water above the cup. For each 33 feet deeper, the water pressure increases 14.7 lb (normal atmospheric pressure at the surface of the water) and as a result the air trapped in the cup looses half its volume and its pressure doubles.
Hope that answers your question.
Take care.
"Fie, fie how franticly I square my talk!"
- Edwin A. Abbott |
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Megan
Skeptic Friend
USA
163 Posts |
Posted - 06/12/2002 : 19:50:49 [Permalink]
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Ohh hehe, god I feel stupid...hehehe
~Megan~
Questions: never leave home without them. |
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James
SFN Regular
USA
754 Posts |
Posted - 06/12/2002 : 19:51:39 [Permalink]
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quote:
Actually, Megan, the water does 'go' into the cup only you don't see it. If you were to submerge the cup deeper, the water would 'go' in farther and you would be able to see it.
It is the 'trapped' air which is being compressed that keeps the water out of the cup and since the amount of trapped air doesn't change, the deeper the cup is pushed, the more water enters the cup and the higher the air's pressure becomes because the air occupies less volume in the cup.
The amount of compression is proportional to the depth the cup is pushed down to since the trapped air has to balance the weight of the column of water above the cup. For each 33 feet deeper, the water pressure increases 14.7 lb (normal atmospheric pressure at the surface of the water) and as a result the air trapped in the cup looses half its volume and its pressure doubles.
Oh, sure, give her the technical version.  
________________________
Monday is an awful way to spend 1/7 of your life.
Two more years...Two more years...Two more years...Two more years...Two more years...
*whine* |
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Chagur
New Member
USA
21 Posts |
Posted - 06/12/2002 : 20:16:47 [Permalink]
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Megan
That's there to feel stupid about? It was a decent question.
James
What can I say? I'm still a skeptical pedant
Take care
"Fie, fie how franticly I square my talk!" -Edwin A. Abbott -
Edited by - Chagur on 06/12/2002 20:19:07 |
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