Water pressure question
 

I have a question that has me questioning the laws of fluid physics. I will
be posting the question in a day or so, but first I want to get some others
input regarding water pressure at different depths. First, do we all agree
that pressure is only dependant on depth of water? Lets say you had an
aquarium that was 3 feet high and 3 feet long and 2 feet front to back.
Fluid laws say it would not matter what the front to back measurement is.
The pressure on the front glass would be the same if the back of the
aquarium was two feet or 200 feet or 200 miles away, right?
Dixon
--

On Mon, 29 Nov 2004 23:39:21 GMT, "dixon"
wrote:


I have a question that has me questioning the laws of fluid physics. I will
be posting the question in a day or so, but first I want to get some others
input regarding water pressure at different depths. First, do we all agree
that pressure is only dependant on depth of water? Lets say you had an
aquarium that was 3 feet high and 3 feet long and 2 feet front to back.
Fluid laws say it would not matter what the front to back measurement is.
The pressure on the front glass would be the same if the back of the
aquarium was two feet or 200 feet or 200 miles away, right?
Dixon
That's correct. Otherwise dams on rivers like the Colorado would
burst.

I would have agreed with the width/length of the tank/dam doesn't matter ...
but ... then why are dams so thick at the top? Dams can be 20 / 30 / ...
feet thick at the top where the water is only a few inches deep. There is
more to this than me thinks!

dixon wrote:
I have a question that has me questioning the laws of fluid physics. I will
be posting the question in a day or so, but first I want to get some others
input regarding water pressure at different depths.

Paging Jax!

.... First, do we all agree
that pressure is only dependant on depth of water?

Those of us who are not bull****ting pinheads, yes.

.... Lets say you had an
aquarium that was 3 feet high and 3 feet long and 2 feet front to back.
Fluid laws say it would not matter what the front to back measurement is.
The pressure on the front glass would be the same if the back of the
aquarium was two feet or 200 feet or 200 miles away, right?

Correct. The vertical distance between the given point and the surface
of the water is the determining factor... when the system is static...

Regards
Doug King

I would have agreed with the width/length of the tank/dam doesn't matter ...
but ... then why are dams so thick at the top? Dams can be 20 / 30 / ...
feet thick at the top where the water is only a few inches deep. There is
more to this than me thinks!


Many dams have a service road across the top, a place to stage heavy equipment
if need be for repairs, etc. That will require a certain practical width. You
also need some structural mass to frame spillway gates, etc- often located near
the top of the dam. Hoover Dam has a highway across the top, and so is probably
75 feet "thick" many feet above the surface. :-)

On Mon, 29 Nov 2004 16:58:51 -0700, James Gemmill wrote:

On Mon, 29 Nov 2004 23:39:21 GMT, "dixon"
wrote:


I have a question that has me questioning the laws of fluid physics. I will
be posting the question in a day or so, but first I want to get some others
input regarding water pressure at different depths. First, do we all agree
that pressure is only dependant on depth of water?

Notwithstanding:
- density of the water (salt, fresh...)
- Pressure at the surface of the water (ie we're talking differential
pressure, open tank)
- Changes to gravity, either from location (earth vs moon), acceleration,
or maybe Kurt Vonnigut is right and the gravitational "constant" isn't...

:)
Lloyd

--

"DSK" wrote in message
.. .
dixon wrote:
I have a question that has me questioning the laws of fluid physics. I
will
be posting the question in a day or so, but first I want to get some
others
input regarding water pressure at different depths.

Paging Jax!

.... First, do we all agree
that pressure is only dependant on depth of water?

Those of us who are not bull****ting pinheads, yes.

.... Lets say you had an
aquarium that was 3 feet high and 3 feet long and 2 feet front to back.
Fluid laws say it would not matter what the front to back measurement
is.
The pressure on the front glass would be the same if the back of the
aquarium was two feet or 200 feet or 200 miles away, right?

Correct. The vertical distance between the given point and the surface
of the water is the determining factor... when the system is static...

Regards
Doug King

Okay, looks like we are in agreement. I'll post my "mental dilemma" tomorrow
(11-30-04). It has troubled me for years. Get your thinking caps ready!
Dixon

"dixon" wrote in message
news:JqOqd.110213$V41.105580@attbi_s52...

I have a question that has me questioning the laws of fluid physics. I
will
be posting the question in a day or so, but first I want to get some
others
input regarding water pressure at different depths. First, do we all agree
that pressure is only dependant on depth of water? Lets say you had an
aquarium that was 3 feet high and 3 feet long and 2 feet front to back.
Fluid laws say it would not matter what the front to back measurement is.
The pressure on the front glass would be the same if the back of the
aquarium was two feet or 200 feet or 200 miles away, right?
Dixon
--



Yes. You are really looking at the PSI at the bottom of a column.

"Gould 0738" wrote in message
...
I would have agreed with the width/length of the tank/dam doesn't matter
....
but ... then why are dams so thick at the top? Dams can be 20 / 30 / ...
feet thick at the top where the water is only a few inches deep. There is
more to this than me thinks!


Many dams have a service road across the top, a place to stage heavy
equipment
if need be for repairs, etc. That will require a certain practical width.
You
also need some structural mass to frame spillway gates, etc- often located
near
the top of the dam. Hoover Dam has a highway across the top, and so is
probably
75 feet "thick" many feet above the surface. :-)


And parts of the dam are hollow. Inspection galleys, equipment areas, and
there are also waves hitting the top above normal water level. These have a
side force.

"WaIIy" wrote in message
...
On Mon, 29 Nov 2004 17:13:54 -0800, "Mark" wrote:

I would have agreed with the width/length of the tank/dam doesn't matter
....
but ... then why are dams so thick at the top? Dams can be 20 / 30 / ...
feet thick at the top where the water is only a few inches deep. There is
more to this than me thinks!


Dams are thicker on the bottom, not the top.

True, but they can be thick at the top also.