Water seeks its own level. This means that when one pours a bunch of
water on a table it won't end up like a little mountain, but it will
very quickly spread out pretty widely (subject to surface tension,
When you pour a lot of sugar or sand, or dirt it won't spread out as
much, I presume because of friction between the sugar, sand, and dirt
particles. If you have a box of the stuff and open a door at the
bottom, some of the stuff will come out the door
What about air?
If you have a heated building and open an overhead door 12 to 16 feet
high, for trucks, how fast will the cold air outside pour into the
building, and how fast will the hot air nearer the ceiling be forced
I was recently at such a building, when it was below freezing out, and
I was close to the door and every time the door was opened, I'd feel a
bit of cold air, but not as much as I expected, and when the door was
shut, the building seemed back to normal very quickly. If it had been
full of water, in a twentieth of the time it took a car to drive in or
out, all the water would have run out. Does air not flow even at
1/20th the speed of water? What am I missing?
What you're missing is the density of the air just inside the door
opening is only a little less dense (because it's warm), than the
outside air that's on the other side. If the room were full of water,
there would be a huge density difference and gravity would cause it to
Or create a water example, where you had a pool with a vertical
divider, seperating warmer water from cooler water. If you pull up
the divider for a minute, the water will begin to mix, but the warm
water won't just rush over in a minute.
So does air, the pressure across the earth varies by 10% max.
It is spread out evenly (more or less), see above.
With just one opening, the cold air has to get in and the arm air get out.
A nice circulatory arrangement may occur, but it's better if there are two
openings, especially if they are arranged in the right places. Check out
Air can flow at 100 m/s or so on earth, in jet streams, can you find water
moving at anywhere near that speed?
Doesn't the tital waves move neat this speed ? I don't think in
meters/second but miles per hour. Thought the tital waves could hit several
Air will expand to fill all the space, but cold and hot air are almost the
same density. Take a clear class and heat some water so it is about 150 deg
F and drop in a couple of drops of food coloring and see how slow the cold
fluid moves if you don't sitr it.
Time to get educated and join the rest of the world.
The wave moves, not the water. In the middle of the ocean, the tsunami
does not consist of water moving fast like in river rapids. As it
comes on shore, that changes, of course, as all the video shots from
You need something to move the air. When you open the door, there is
already air on both sides so there is little exchange. If there is a
breeze, or temperature differences, there will be some air movement, but it
is not like spilling water onto a table. To prove this, put a divider in a
container of water. Color the water on one side,let it stand and be very
still, then pull up the divider. It will eventually mix, but not very fast.
Just as some cocktails are poured in layers.
On a windy day, open the door and you will feel more air, but if you have
openings at the other end of the building, no matter how large, you will get
a draft through the building. At one of our buildings on a breezy day, I
can tell when a big door is opened as far as 1000 feet away. On a calm day,
you don't feel anything even 50 feet away.
Air movement in side a closed or semiclosed area is extremely complex.
Although air densitytemperature gradients play an important role, air
mixing is highly dependent on the existing HVAC and placement of
furniture or equipment in the room. Although I could not find the
report in a short imte, I have seen a Department of Energy study from
quite a few years ago that discussed the result of smoke tests in a
variety of room configuratoins. What amazed me is that in some parts
of rooms with what might be consdered good circulation, there were
dead air spots that persisted for very long periods.
How fast would water have run out of the building if you had water
outside, at same level as inside? The building is not surrounded by
Mati Meron | "When you argue with a fool,
email@example.com | chances are he is doing just the same"
How about this: I know of a grocery store that had a somewhat larger
opening, as its main entrance, that was open maybe 14 hours a day (during
normal open hours). Or was it open 24 hours? I forget. The difference
between this entrance and a simple opening is there was a substantial
downward blast of air as you entered or left through it. The air blast
somehow kept warm inside air and cold outside air separated. (How?)
And since this was in a suburb of Buffalo NY, the outside air could get
rather cold (and windy!)
In sci.physics, Androcles
on Sun, 18 Feb 2007 04:22:46 GMT
Androcles does have a point here; if the water doesn't
adhere to the tabletop (e.g., wax), it will bead up.
These are definitely "little mountains", although more in
the US Eastern sense, not the Sierra Nevada. :-)
Even if the water does adhere to the tabletop, it will
still bead up, although not as much.
To be sure, surface tension can only go so far; if one
pours a cubic meter (1 metric tonne) of water on a 1 m^2
wax-coated tabletop, it won't all fit as little beads.
However, neither will it all fall off the edges, either.
Most of it, yes -- but not all. The rest will have to
be wiped off with a towel or left to evaporate; if one is
working with impure water, one gets salt -- a fact noticed
by many after a rainfall on their car rooftops, windshields,
Useless C++ Programming Idea #8830129:
Actually, air and water are pretty much the same thing when you study
the dynamics of fluids and gasses. The physical properties of both are
a bit different, water is for instance much less compressible than
air, water is more dense and the specific heat differs. Yet, there
isn't really a lot of difference when the dynamics of fluids and
gasses are modeled, it is all Navier Stokes maybe in a slightly
different form. Maybe a good book is "Ocean Atmosphere Dynamics" by
Gill which treats both topics next to each other.
(Amazon.com product link shortened)
Your building example is not really fair, a building filled with water
in an air environment is something different than a building filled
with air in an air environment. Why not a building filled with warm
water in a cold water environment?
Gas behaves similar to liquid--both move faster with higher
temperatures. But there is one important difference: gas (air) is
spongy meaning that it easily expands and contracts in volume,
depending on temperature and pressure. Liquids (and solids) stay
about the same volume. Molecules are close together in solids, far
apart in gases. Water will move faster than a gas in an air
environment due to the differences in specific gravity. Also, nature
tends to dissipate substances.
If you have enough gravity that the kinetic velocities of the gas
molecules are "small" then air will fall to lower levels like this. In
general the gas will expand to make its sound speed roughly equal to the
square root of the layer thickness times gravity. Of course that's
nothing but hydrostatic equilibrium.
The other consideration is the mean free path between inter-particle
scattering events be small compared to any dynamical scale.
On both counts, air is treatable as a fluid (and is treated with fluid
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