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Q: Air Pressure ( No Answer,   18 Comments )
Subject: Air Pressure
Category: Science
Asked by: loophole-ga
List Price: $2.00
Posted: 21 Aug 2006 13:22 PDT
Expires: 20 Sep 2006 13:22 PDT
Question ID: 758154
a cup of coffee, a 10 gal aquarium, and bottle of ginger ale are on a
table.  Which one has the greatest amount of air pressure on it?
There is no answer at this time.

Subject: Re: Air Pressure
From: jack_of_few_trades-ga on 21 Aug 2006 13:40 PDT
"How much pressure are you under? Earth's atmosphere is pressing
against each square inch of you with a force of 1 kilogram per square
centimeter (14.7 pounds per square inch). The force on 1,000 square
centimeters (a little larger than a square foot) is about a ton!"

Air pressure is a function of the area on top of the object.  I will
assume that the aquarium has the greatest area on top (unless it is
very oddly shaped), so the aquarium is under the greatest amount of
air pressure.
Subject: Re: Air Pressure
From: polebear-ga on 21 Aug 2006 15:02 PDT
Air Pressure is a function of the height of the column of air on top
of it, as per low air pressure at the top of a mountain, so the cup of
coffe is most likely to have the greater airpressure on it, if it is
the lowest item
Subject: Re: Air Pressure
From: redhoss-ga on 21 Aug 2006 15:14 PDT
I say polebear-ga is correct.
Subject: Re: Air Pressure
From: pinkfreud-ga on 21 Aug 2006 15:18 PDT
I think we'll need to know whether or not the aquarium contains water.
Subject: Re: Air Pressure
From: myoarin-ga on 22 Aug 2006 05:02 PDT
Doesn't the air press against all surfaces exposed to it?  It does  -
not just vertically downwards.

Two flat pieces of glass that have been pressed together are just as
difficult to separate if they held vertically.  It is the air pressure
that makes them stick together.

That doesn't help me understand the question, however.
Subject: Re: Air Pressure
From: mikewa-ga on 22 Aug 2006 05:33 PDT
I agree with polebear: it is the altitude of the object that will
determine air pressure, not its size. The larger object will have a
greater *force* acting on it, but that is not what the question asks.
Subject: Re: Air Pressure
From: jack_of_few_trades-ga on 22 Aug 2006 05:38 PDT
Pole, I'll gladly admit that I'm no air pressure expert... but here's
my question about your comment:

Is there less air pressure affecting the Empire State Building than my
house?(yes, my house is much smaller in every dimension than the ESB,
and the base of my house is at the same distance from sea level as the

I don't doubt that air pressure changes with distance from sea level,
but I do doubt that the size of the object is not taken into
Subject: Re: Air Pressure
From: jack_of_few_trades-ga on 22 Aug 2006 05:56 PDT
Ah, I see where the disagreement is now that I read Mikewa's
comment...  Air pressure by itself is defined by the pressure on a
single point of an object.

However the questioner asked "Which one has the greatest amount of air
pressure on it?"  which I would take to mean the sum of all the air
pressure... since "on it" would include the entire object and not just
a single point.
Subject: Re: Air Pressure
From: bcattwood-ga on 22 Aug 2006 06:01 PDT
To figure it exactly you would have to integrate the air pressure as a
function of height over the differential surface area of the object
from bottom to top.  Given that all the objects in question are
relatively short though, you could just assume constant pressure and
say the one with the largest surface area is under the greatest
Subject: Re: Air Pressure
From: myoarin-ga on 22 Aug 2006 06:39 PDT
I agree that altitude probably has little to do with it.  The items
are all on one table.
Is the answer to the question related to whether or not the bottle of
ginger ale is opened or still closed  - relative to the cup's and
aquarium's being open?
Does the internal pressure of a closed bottle of pop, which is greater
than the ambient air pressure on its outside suffice to explain that
the bottle has the greatest air pressure on it, because surface area
and relative heighth are not factors?

Subject: Re: Air Pressure
From: efn-ga on 23 Aug 2006 18:47 PDT
The air pressure is the same for all of the objects.  Air pressure is
force per unit area, so the area doesn't matter.  Any difference due
to altitude is insignificant.
Subject: Re: Air Pressure
From: mohit_jain-ga on 29 Aug 2006 09:46 PDT
first , we want to know that what ar the atmospheric conditions and
what is in the coffee cup with its temperature ?

second thing we tend to know that what is the height at which they are placed ?

pressure on a surface is depend on the temperature  and vapour
pressure . atmospheric pressure depends on the height as well ;

so if all the given liquids are at the same temperature and at the
same height then pressure on that surface will be more whose vapour
pressure is high .
Subject: Re: Air Pressure
From: redkev-ga on 30 Aug 2006 03:09 PDT
There is only air pressure over the aquarium. The gas above the pop
will be too high in carbon dioxide and the water vapour pressure above
a hot cup of coffee will be too high. Neither qualify as "air". If you
wait long enough and the coffe gets cold and the pop goes flat then
you can measure the air pressure (in kg per square meter) over each.
Due to thermal effects and drafts an instrument capable of measuring
the difference in height of a few inches would be confused by the
noise these effects would have. In effect the pressure is the same
over each object.
Subject: Re: Air Pressure
From: knickers-ga on 02 Sep 2006 07:20 PDT
They all have the same force per unit area but have different total
pressures as the total pressure is related to the total projected
surface area of the object.

p.s. the two glass sheet comment is nothing to do with air pressure.
Thats a cohesive force issue.
Subject: Re: Air Pressure
From: witboy-ga on 04 Sep 2006 01:39 PDT
I think air pressure is formed by the weight of air molecules,which is
full of any space, and it is same in any location in same height. As
you say, every thing on table almost endure the same air pressure, but
the tiny difference of air pressure on liquid surface is depend on the
height of liquid in container.

Air pressure on sea level in general will be 1 bar(pressure unit),and
more less air pressure more height of your location. So if you stand
on the higher mountain,you will feel the lack of air, it means you
endure low air pressure.
Subject: Re: Air Pressure
From: sgaz-ga on 04 Sep 2006 06:12 PDT
The Ginger Ale haz the most air pressure acting upon it. But only
because of ith internal pressure created by the carbonated drink, all
others will not have a significant pressure difference
Subject: Re: Air Pressure
From: finale-ga on 05 Sep 2006 18:30 PDT
well, to be exactly, it depends on the heiht of the container,
however, the height is so mere to the atmosphere that it can be
ignored, which is they have the same pressure.
Subject: Re: Air Pressure
From: austinmoore-ga on 28 Sep 2006 18:42 PDT
Ok, mind my arrogance, however the VAGUELY closest answer is from
"efn-ga" and "sgaz-ga", but lets elaborate more.

In the most discrete way possible, the best way to think of any sort
of gas pressure (air pressure is a example of gas pressure) is the
kinetic force of all the gas molecules against an object.

The only two ways to increase gas pressure (in a closed system) is
either to pump more gas in, or increase the temperature.

Now in a generalization, we can say that for macro-atom objects that
gas pressure is a the force from the atoms in all directions of a
closed object. Therefore the force from the pressure of gas is not a
vector (it doesn't matter about the "column of air above the object").
The pressure of gas is a modular quantity perpendicular to all surface

With the application of air pressure (as a gas pressure) it gets
harded to explain. We have to think of the "local" atmosphere as a
closed container. It's complicated to explain fully, however just
think of the Earth as having an always-changing atmospheric air
pressure, with differences in air pressure in different regions due to
the weather/climate. It is so vast that a region of air pressure can
be considered a closed container

Now the problem with this question is it is a trick question! To
rewrite the question in other words, "loophole-ga" is saying

"[out of the objects] which one has the greatest POSITIVE net force
due to the force of constant atmospheric air pressure, whereas
positive net force is pressure from the atmosphere that is greater
than the pressure inside the container."

The simple answer is: --NONE--, no object is exibiting a positive net
force from the atmospheric pressure.

With an open container (coffee cup, aquarium), the air/liquid inside
the container and the air outside the conainer (atmosphere) create a
pressure equilibrium, so the gas pressure force on the ouside is the
same on the inside. So the forces cancel each other out, and the net
force is zero.

For the Ginger Ale, it actually has a negative net force. Since it is
a closed container within the atmosphere, and since it is highly
carbonated, it does have some gas pressure force on it from the
liquids inside. However this is not air pressure, and the question

The only positive net force from air pressure you will find is in any
closed partial vacuum.

ref- (covers
only some discrete basics)

         Austin Moore

p.s.- You can keep your two dollars.

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