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PolyphemusEye
Post  Post subject: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 8:58 am

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Dear Science Forum,

I have a question about water pressure. The water pressure at the bottom of a pool is equal to the weight of the water divided by the area of the water.
If we now take out most of the water from the pool and replace it with peebles, is the water pressure at the bottom of the pool the same?
I say no because you will not have a continal column of water acting at the base of the pool. The pebbles will also react against the water weighing down on them.


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DrKrettin
Post  Post subject: Re: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 9:06 am
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I say yes - the pressure is still that of the column of water above it. The peebles, or pobbles, make no difference.

Nice username by the way - just keep out of Nobody's way.


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PhDemon
Post  Post subject: Re: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 9:09 am

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But if the pool now has less water in it and is half full of pebbles there is a smaller column of water so the water pressure will be lower as the column of water is not as high, or have I misread the OP?

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Olinguito
Post  Post subject: Re: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 9:20 am
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PhDemon wrote:
But if the pool now has less water in it and is half full of pebbles there is a smaller column of water so the water pressure will be lower as the column of water is not as high

Unless the addition of the pebbles raises the water to the same level as before.

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PhDemon
Post  Post subject: Re: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 9:36 am

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But then the water column is still shorter... The total pressure (pressure exerted by the water plus the pressure exerted by the pebbles) on the bottom of the pool will be the same but the water pressure will still be lower...

EDIT: I've just checked with one of our physics teachers - he agrees with me ;)

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Pong
Post  Post subject: Re: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 9:58 pm
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Yet BBs weigh more than cheese curds...

Does buoyancy help here? I mean, is it a completely different question about my boat in the pool?


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Chrispen Evan
Post  Post subject: Re: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 10:59 pm
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If the column of water is the same height then surely the pressure will be the same, pebbles or no pebbles?


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PhDemon
Post  Post subject: Re: Water pressure in a pool  |  Posted: Mon Feb 13, 2017 11:40 pm

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But it isn't, as the OP says most of the water is removed and replaced with pebbles, how can the water column then be the same height?

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g o r t
Post  Post subject: Re: Water pressure in a pool  |  Posted: Tue Feb 14, 2017 10:55 am

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PhDemon wrote:
But it isn't, as the OP says most of the water is removed and replaced with pebbles, how can the water column then be the same height?


My ice tea is less than half a glass without the ice.

The water column can be a mile hole or a bunch of soda straws, as long as the column height is the same the pressure is the same.

Same goes for wet sand, at least until the space between particles becomes small enough for other effects.

This is water pressure. the pressure of the pebbles is much higher.


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PhDemon
Post  Post subject: Re: Water pressure in a pool  |  Posted: Tue Feb 14, 2017 12:20 pm

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Maybe I misread the OP... When he said remove most of the water and replace it with pebbles I assumed he meant basically decreasing the depth of water by filling up the bottom of the pool with rocks changing the column height (he actually said there would be no continual column of water). I agree the pressure only depends on the column height and if any column of water reaches the bottom of the pool the pressure will be the same.

Maybe the OP can clarify what he meant.

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Chrispen Evan
Post  Post subject: Re: Water pressure in a pool  |  Posted: Fri Feb 17, 2017 7:54 am
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Quote:
(he actually said there would be no continual column of water)


not quite

Quote:
I say no because you will not have a continal column of water acting at the base of the pool.


that is their reasoning and may not in fact be the case.


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