Why doesn't the sky crush us all?

[doubleB]

Why doesn't the sky crush us all??

I'm sure I've thought this through before but after an afternoon wondering I need to ask:
Why is it that the billions of tons of air above us doesn't crush us?
Thanks for the help!

 

[masudr]

The pressure of fluids inside you matches the pressure of the atmosphere.

 

[doubleB]

What do you mean?

 

[russ_watters]

When you breathe, your lungs are pressurized to the same pressure as atmospheric. And most of the rest of the volume of your body is water - what is there to crush?

People scuba dive down to several hundred meters, and every ten meters you get another atmosphere of pressure. As long as the pressure is equalized, there aren't any problems.

 

[Ideologue]

The pressure of the air above us is not all that great. A square inch column of air, from the ground to the edge of the atmosphere, only weighs around 14 pounds, give or take. Therefore one atmoshere of pressure, 1 bar, is the same as 14psi. And of course pressure decreases with altitude as the further up one goes the less air, and therefore mass, there is above them.

Edit: I think that's right. Maybe someone else more knowledgeable can confirm.

 

[doubleB]

It's just that the way I see it is that if there are billions of tonnes of air on us then although most of our bodies wouldn't compress, because it's water, some bits would? I think I just don't understand what is going on...Is the air actually pounding down on us? And I mean if a million tonne iron weight was on top of us that would squash us right? Is that because it is more dense?

 

[fargoth]

the air is not like this weight you imagine, because it's all around us, you feel the same pressure from all the directions, even from the inside of your body - so the net force is zero...

when you change altitudes very rapidly (like when your plan takes off, or lands), you may feel the net force isn't zero, because the pressure inside you is less in case youre landing, or greater in case youre taking off - that's why your ears pop...

you only feel the pressure difference.

 

[russ_watters]

The pressure of the air above us is not all that great. A square inch column of air, from the ground to the edge of the atmosphere, only weighs around 14 pounds, give or take. Therefore one atmoshere of pressure, 1 bar, is the same as 14psi. And of course pressure decreases with altitude as the further up one goes the less air, and therefore mass, there is above them.

Well, 14psi is still a lot: Lie down on the floor and imagine 2500 lb of bricks stacked on your chest (assuming a 10"x12" chest/stomach).

It's just that the way I see it is that if there are billions of tonnes of air on us then although most of our bodies wouldn't compress, because it's water, some bits would?

Well everything in your body was formed in this environment, so everything that might have air in it would also be at 14.7 psi.

I think I just don't understand what is going on...Is the air actually pounding down on us?

Pounding. Tough word - you do feel the air because molecules are hitting you, but adding all the impacts up and you only get 14.7 lb of force for every square inch. That is far less of an impact force than you get clapping your hands.

And I mean if a million tonne iron weight was on top of us that would squash us right? Is that because it is more dense?

If a million ton weight were on your chest, the forces would be unequal and you would be crushed. What is crucial is that the force acts the same in all directions. That's what pressure means: the air isn't just pushing down on you, it is pushing left, right, forward, backwards, and up. Its why a little rubber balloon doesn't get crushed when you blow it up.

Though it is kinda a side issue, you don't have a million tons of atmosphere pushing down on you - only the atmosphere directly above you. Ie, if you stretch 1"x1" a cylinder into space, the total weight of the air inside it would be 14.7 lb. A diver at 100m (typical max depth of low skill recreational dive) has a column of water above him/her weighing about ten times that.

 

[Danger]

Although Russ singled out the lungs, I'd like to stress that every single cell in your body, as well as the intrathoracic, ear, and cranial sinus air spaces are pressurized to ambient (except in rapid changes as fargoth pointed out).
That having been said, air is a fluid and will therefore resist movement. A good demonstration is to set a yardstick or similar wood hanging over the edge of a table, just far enough back that it won't fall off. Cover the end that's on the table with a single 2-page sheet of newspaper, then whap the other end straight downward really fast with a bat. The stick will break rather than fling the paper off. That is a direct consequence of the roughly 4 1/2 tonnes of air not wanting to get off of the paper.

edit: Ooops, just spotted you new post, Russ. Quit sneaking in while I'm typing. :tongue:

 

[Andrew Mason]

I'm sure I've thought this through before but after an afternoon wondering I need to ask:
Why is it that the billions of tons of air above us doesn't crush us?
Thanks for the help!

A simple answer is: if the amosphere crushed us, we wouldn't be here to ask the question. Life adapts to its environment. Our environment is at the bottom of a gaseous sea.

AM

 

[arildno]

doubleB:
What un-nice effects do you think we would experience if we suddenly were thrust into the vacuum of outer space?

 

[russ_watters]

The one that kills you is probably asphyxiation, but while that's happening, you also get to watch your skin boil away...

 

[arildno]

Well, you would also get a severe case of divers' syndrome, I would think..

 

[doubleB]

Thanks to everyone for the help; I think I'm starting to understand it.
To Russ: I'm just slightly confused about what you said to Ideologue - you said that 14psi is actually a lot, like 2500lb of bricks stacked on your chest. But you then said that 14.7lb per square inch is less than clapping your hands. Is it because our bodies are pushing against the air pushing against us with the same force?

 

[russ_watters]

Yes, that's all it is.

 

[fargoth]

The one that kills you is probably asphyxiation, but while that's happening, you also get to watch your skin boil away...

The Answer

From the now extinct page http://medlib/jsc.nasa.gov/intro/vacuum.html:

How long can a human live unprotected in space?

If you don't try to hold your breath, exposure to space for half a minute or so is unlikely to produce permanent injury. Holding your breath is likely to damage your lungs, something scuba divers have to watch out for when ascending, and you'll have eardrum trouble if your Eustachian tubes are badly plugged up, but theory predicts -- and animal experiments confirm -- that otherwise, exposure to vacuum causes no immediate injury. You do not explode. Your blood does not boil. You do not freeze. You do not instantly lose consciousness.

Various minor problems (sunburn, possibly "the bends", certainly some [mild, reversible, painless] swelling of skin and underlying tissue) start after ten seconds or so. At some point you lose consciousness from lack of oxygen. Injuries accumulate. After perhaps one or two minutes, you're dying. The limits are not really known.

You do not explode and your blood does not boil because of the containing effect of your skin and circulatory system. You do not instantly freeze because, although the space environment is typically very cold, heat does not transfer away from a body quickly. Loss of consciousness occurs only after the body has depleted the supply of oxygen in the blood. If your skin is exposed to direct sunlight without any protection from its intense ultraviolet radiation, you can get a very bad sunburn.

At NASA's Manned Spacecraft Center (now renamed Johnson Space Center) we had a test subject accidentally exposed to a near vacuum (less than 1 psi) in an incident involving a leaking space suit in a vacuum chamber back in '65. He remained conscious for about 14 seconds, which is about the time it takes for O2 deprived blood to go from the lungs to the brain. The suit probably did not reach a hard vacuum, and we began repressurizing the chamber within 15 seconds. The subject regained consciousness at around 15,000 feet equivalent altitude. The subject later reported that he could feel and hear the air leaking out, and his last conscious memory was of the water on his tongue beginning to boil.

 

[jaredkipe]

A better question might be why isn't all the air compressed on the floor? Nothing is holding the air up, so why does it not accelerate to the surface of the earth?

This is my second post so I don't know if I'm supposed to backup a suggestion like this. But it is a statistical mechanics problem, specifically chemical potential balancing gravitational potential.

 

[russ_watters]

A better question might be why isn't all the air compressed on the floor? Nothing is holding the air up, so why does it not accelerate to the surface of the earth?

Air is moving like any other gas. That motion and the billiards-like elastic collisions are what gives the air its density.

 

[KingNothing]

A better question might be why isn't all the air compressed on the floor? Nothing is holding the air up, so why does it not accelerate to the surface of the earth?

Perhaps a better question might be, why do you think it isn't? Sure the outer edge of our atmosphere is about 200 miles above the earth, but the Earth's diameter is around 7100 miles.

Imagine for a second that all the atmosphere was compressed in a 1-inch layer around the earth. What would happen? The difference in pressure above this layer would cause a force (this is how wind works). If that force outweighs gravity, the air moves outward.

 

[jaredkipe]

Air is moving like any other gas. That motion and the billiards-like elastic collisions are what gives the air its density.

There are not too many collisions between air particles and other air particles, that is a requirement to use ideal gas law approximations. That still doesn't really explain it.
So air, unlike everything else has mass but when it is accelerated to the ground it bounces off and back up into the upper atmosphere? Can you detect this apparent movement of the air?

 

[masudr]

There are actually quite a lot of collisions.

The r.m.s. speed of nitrogen molecules at RTP is of the order of $10^2 ms^{-1}$ but we never feel that because they don't get very far til they hit another air molecule.

 

[Danger]

Can you detect this apparent movement of the air?

That's a bit of a loaded question, I think. We definitely detect air movement all the time what with wind, convection, etc.. In the final analysis, it's the gaseous nature of air that makes that possible, so I suppose that the answer is yes. Whether or not that fits the proper scientific breakdown of the question isn't within my knowledge.

 

[jaredkipe]

That's a bit of a loaded question, I think. We definitely detect air movement all the time what with wind, convection, etc.. In the final analysis, it's the gaseous nature of air that makes that possible, so I suppose that the answer is yes. Whether or not that fits the proper scientific breakdown of the question isn't within my knowledge.

It wasn't loaded it was somewhat sarcastic. My point is, that if you think of air as a billiard ball in classical mechanics, why doesn't it go to the lowest gravitational potential possible, meaning the ground. There is no good reason why it wouldn't, so why would a bunch of billiard balls suddenly stop behaving like one, and start bouncing off each other and never going into the ground state on the ground. (exclusion principal beside)


Why nobody has brought up the fact that at the usual temperatures, these tiny billiard balls have quite a lot of energy. Maybe its just me, but it seems people are reacting oddly to my original post.

 

[Danger]

In the case of the others, it's because they're scientists. In my case, it's because I'm not. Get used to it; everybody around here is kind of odd.
Anyhow, if gravity were stronger, then the lower layer of the atmosphere would liquify.

 

[russ_watters]

My point is, that if you think of air as a billiard ball in classical mechanics, why doesn't it go to the lowest gravitational potential possible, meaning the ground. There is no good reason why it wouldn't, so why would a bunch of billiard balls suddenly stop behaving like one, and start bouncing off each other and never going into the ground state on the ground. (exclusion principal beside)

How do you figure? Newtonian mechanics suggests that a particle hitting the ground in an elastic collision will bounce right back up to where it started. That's conservation of momentum/energy.

Why nobody has brought up the fact that at the usual temperatures, these tiny billiard balls have quite a lot of energy.

"Quite a lot" is qualatative, but we most certainly have been discussing how much energy is in these molecules.

Maybe its just me, but it seems people are reacting oddly to my original post.

I, for one, am confused as to your reactions here. These concepts are pretty simple and you don't seem to be getting them.

 

[russ_watters]

So air, unlike everything else has mass but when it is accelerated to the ground it bounces off and back up into the upper atmosphere? Can you detect this apparent movement of the air?

It doesn't bounce back up to the upper atmosphere - there are more collisions than you imply. Molecular movement due to kinetic energy is called Brownian motion and you can most certainly detect it. Watch smoke disperse.

 

[Farsight]

I have to say I'm not quite happy with the Kinetic Theory of Gases.

 

[Farsight]

If I can elaborate, the Kinetic Theory of Gases says that hydrogen molecules at room temperature and pressure are traveling at circa 4000mph. So if I were to suddenly decap a canister of hydrogen in a vacuum chamber, I should see molecules coming out at 4000mph. This seems like an explosive velocity that ought to blast the opposite wall of the chamber in the twinkling of an eye. It doesn't seem to square with my layman's experience. Can any more knowledgeable posters tell me about any experiments that prove the high velocity of the kinetic gas molecules?

 

[jaredkipe]

If I can elaborate, the Kinetic Theory of Gases says that hydrogen molecules at room temperature and pressure are traveling at circa 4000mph. So if I were to suddenly decap a canister of hydrogen in a vacuum chamber, I should see molecules coming out at 4000mph. This seems like an explosive velocity that ought to blast the opposite wall of the chamber in the twinkling of an eye. It doesn't seem to square with my layman's experience. Can any more knowledgeable posters tell me about any experiments that prove the high velocity of the kinetic gas molecules?

First off pressure doesn't have anything to do with how fast a molecule is moving. Secondly... circa...doesn't really mean that...

By equipartition, <E>=kT, where k is the Boltzmann factor and T is its temperature in degree's Kelvin.
<E>=.5m<v>^2=kT

so the average velocity of a molecule is related to its mass and temperature <v>=(2kT/m)^(1/2), I haven't plugged in numbers to see if you get your 4000mph, so I won't comment on that. But just because something is fast doesn't mean there is a lot of energy behind it.

As for an experiment, you could do what I just did in reverse. Specifically, suspend something that is very sensitive to pressure or impact. Like maybe an extreamly light film that you can measure its angle by bouncing a laser or something off of it. (obviously accounting for the momentum of the laser light) and pump on the chamber till there are few molecules of a specific mass. This should make it so that when one of the few molequles hits the film it is deflected, and you can calculate the energy of the collision. Thus deduce the velocity of the molecule.

Since it can happen at a variety of angles you should either reduce those angles, by making an aperture the molecule must go through to strike the film, or simply average over a lot of collisions.

 

[jasc15]

If I can elaborate, the Kinetic Theory of Gases says that hydrogen molecules at room temperature and pressure are traveling at circa 4000mph. So if I were to suddenly decap a canister of hydrogen in a vacuum chamber, I should see molecules coming out at 4000mph. This seems like an explosive velocity that ought to blast the opposite wall of the chamber in the twinkling of an eye. It doesn't seem to square with my layman's experience. Can any more knowledgeable posters tell me about any experiments that prove the high velocity of the kinetic gas molecules?

I once took a class on vacuum technology and we learned that hydrogen has an rms velocity (I think it's rms velocity) around 3973mph, helium 2808mph, and O2 960mph. The hydrogen molecules do come out of the container at about 3973mph, but since the mean free path is so small (about 3.7x10^-10m for N2 @ 22C), it keeps bouncing around the other molecules and takes quite a bit of time to evenly distribute the room.
Im not sure if this topic is kinetic theory, or statistical mechanics (im not even too sure what the difference is), but when i start studying for physics in the fall, I am excited to learn about this stuff.

 

[Farsight]

I guess I ought to start a new thread to ask my question.

It's a little off the Why Doesn't The Sky Crush Us All topic.

 

[jaredkipe]

Just because there are "tons" of matter somewhere above you, doesn't mean it is exerting any force on you. Why does your roof not crush you? Because something is holding it up. The reason air doesn't crush you is because more air is holding the higher air up.

EDIT: Oh no, I thought you were asking it again, but its different.