# I Why atmospheric pressure does not crush us?

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1. Jun 11, 2016

### Ravi Singh choudhary

1atm is typically something 101.13 kPa that means 100kN force is acting on 1 square meter surface area of our body. How we are capable for taking such high pressure? Sometime I think on 1 square cm area what immense force they are creating. I feel something is wrong with my concept of atmospheric pressure, please help.

2. Jun 11, 2016

### cnh1995

I'm not an expert but I believe blood pressure doesn't let atmospheric pressure crush the body.

3. Jun 11, 2016

### Ravi Singh choudhary

I am thinking about the force. I know barometer is not crushed at top due to strength of the glass material. But under the same vacuum a big container collapsed. The container was for storing steam but somehow steam was not released and also it was sealed completely; steam cooled and that big container collapsed. So it is due to larger surface the same atmospheric pressure is able to collapse such huge container.

So here I am concerning about kilo newtons of force.

4. Jun 11, 2016

### Staff: Mentor

Take a plastic bottle. It doesn't get crushed by atmospheric pressure. Any idea why?

5. Jun 11, 2016

### Staff: Mentor

To a small amount it does. I once bought PET bottles at about 700m NN and when I arrived at home at 50m NN they were a little bit dented.

6. Jun 11, 2016

### mathman

Pressure inside your body balances atmospheric pressure.

7. Jun 11, 2016

### Grinkle

If your body were basically filled with gas and it were sealed to the outside world, it would behave like a sealed air-filled plastic bottle behaves.

Your lungs and gut are filled with gas but they are not sealed to the outside environment, so they behave like an opened plastic bottle that is filled with gas, not a sealed plastic bottle, at least for slowly changing external pressures. You need to think about the difference between a sealed bottle and an open bottle, and for the sealed bottle, the conditions under which the bottle was sealed.

Increase the external pressure enough in a liquid environment, and a human will be crushed - there are limits to how deep underwater humans can survive underwater descent without being shielded from the pressure. Your gut and lungs are open to the passage of gas, but not to liquid. Think about the difference this makes to pressure changes in a gas environment vs a liquid environment.

The answer to your question is what mathman said, and once you understand that mathematically, you need to think about why it is the case that internal pressure is able to balance external pressure in order to really get it.

8. Jun 11, 2016

### Tazerfish

The answer was pretty much given already by Grinkle, Mathman and Dr Claude
I get the feeling that you assume that we are not internally "pressurized".
But seriously, we are.
If you couldn't exhale and you were brought into a vacuum your lungs would burst.
And if you weren't magically kept from breathing the air out it would "explode" through your mouth.Any attempt to keep that from happening would be absolutely futile.
In contrast, your lungs (and all other gas-filled parts of your body) would collapse if you were put in a very high pressure enviroment.
So yes, if there are big pressure differences between the gasses in your body and the atmosphere outside that is terrible news.

(That is a nice question to ask people who get confronted with atmospheric pressure for the first time.)
You see there is gas inside the bottle.NOT A VACUUM! In fact, the gas inside pushes on the outside just as much as the outside gasses push inwards.

Now something that hasn't been said: yes the gas molecules colliding with your skin push on it with a considerable force, yet nothing happens.
Solids and liquids are not very "impressed" by forces that seem large to us.(no pun intended)
Look up a "bulk modulus" table if you want some numbers on it.
Even the unimaginably strong pressure at the core of our planet (roughly 300 GPa or 300.000.000 atmospheres(source: Wikipedia))
cannot compress the atoms in the earths core very much:
According to this website: http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry57.html the density of the inner(iron) core is about $12*10^3 \frac{kg}{m^3}$ to$13*10^3 \frac{kg}{m^3}$ That is just approximately 1.6 times more dense than iron at the surface of the planet... not a very high factor for 300GPa
[I was very tempted to use strong language considering the magnitude of these pressures, but I will not since I have gotten into trouble for that previously ]
I don't know a lot about the physics which you have to consider in this problem, but I assume the electrostatic repulsion between electrons of "neighbouring" atoms together with the fact that it takes a lot of energy to "compress" electrons into a smaller space make it very hard to change the volume or properties of materials(as long as they are solid or liquid).
A less extreme example would be deep sea fish which live kilometers below the surface.They have to withstand hundreds of atmospheres of pressure and they still have no problem surviving. And some animals, namely deep diving whales, can even go through intense pressure changes without beeing harmed.
It is just that chemical reactions and the structure of materials are usually not affected by pressure diffrence as "low" as the ones you encounter on earth.
I hope the confusion has been resolved.

Tazer

Last edited: Jun 11, 2016
9. Jun 12, 2016

### davenn

yes it will if you are changing altitude, but that wasn't the question posed

10. Jun 13, 2016

### A.T.

We have evolved under this conditions.

11. Jun 14, 2016

### Complex Root

The force of atmospheric pressure, like so many other concepts in physics, greatly challenges our common sense notions. One of the early investigations into this force was the Magdeburg Hemispheres experiment in the 1600's (see Wikipedia) where teams of horses could not pull apart two copper hemispheres held together only by atmospheric pressure, a high vacuum having been created between them.

1. Why do our senses not register any sensation' of the force of the atmosphere? After all, on placing a 1kg weight on 1cm$^{2}$ on our skin we would definitely feel a very distinct sensation of pressure, yet the atmosphere already exerts such a force at all times, and on every 1cm$^{2}$ sq of our whole body.
2. If atmospheric pressure is acting all around the very complicated shape of our body, and it amounts to actual tons of weight (approx 1 tonne on every 30 x 30cm square), then could this not affect our mechanical stability? In classical mechanics if we drew a force diagram, we would be interested in the net external force and the net external torque acting on the object - is it obvious that these are both zero for the contribution of atmospheric pressure? We might surmise it might be, and in fact we can prove this for any uniform pressure field. The equalization of pressure from inside our bodies involves internal forces, and these do not enter into the two conditions of equilibrium in mechanics.
Aspect (1) :-
This brings into question how we actually perceive forces in general. It is clear we are highly sensitive to contact' forces, which are ultimately forces between molecules. What our nerves detect is pressure acting on the tissues of our body, ie. force $\div$ area, and so for example a moderate force over a very small area (eg a knife edge or pinprick) will register as a very strong' signal to our brain. From the moment our life began as a single cell we have been inside atmospheric pressure, and though our nervous system could detect this pressure, it simply has no reason to send a signal to our brain prompting us to action - this is our normal environment for which we are designed. Likewise a deep sea fish does not register the high pressures it is accustomed to, though it would be sensitive to higher pressures, eg. if it came into contact with a sharp object. Throughout the millions of years of evolution all lifeforms have been accustomed to their natural environment, which in some cases such as tardigrades is very extreme - their nervous systems are designed to register only variations from their normal environment. It is an interesting fact to note in the passing that these contact forces of pressure appear to be the only forces we can directly sense, in particular we cannot directly sense the force of gravity (for example astronauts in the ISS experience exactly the same sensation of weightlessness' as they would do if travelling in a spacecraft at constant velocity in interstellar space away from any sources of gravity). (In fact this was one of the key insights of Einstein that led him to his general theory of relativity - eg see Wikipedia articles on Weightlessness, and Ficticious Forces). In keeping with our perception of atmospheric pressure as a kind of base or zero level of pressure, the notion of gauge pressure' means pressure above ambient atmospheric pressure, eg tyre pressure (when its completely flat the air inside it is at atmospheric pressure, so there is no net force on the tyre walls), or blood pressure (eg a systolic blood pressure of 120 mmHg means 120 mmHg above atmospheric pressure (the latter is of the order of around 750 mmHg) - the heart's pumping action is largely responsible for the additional 120 mmHg of pressure).

Aspect (2) :-
In solving mechanics problems, unless we are dealing with fluids, we never usually stop to think about the atmospheric pressure forces acting on the body, and they don't appear in our force diagram, even though these are very large forces. This is because in a uniform pressure field, such as for everyday objects on the surface of the earth, net external force of pressure is zero, and wrt any reference point the net external torque of the pressure forces is zero, so we can completely disregard the effect of pressure. To prove the former, we can use the fact that the net force of a uniform pressure on a surface in a particular direction is the pressure multiplied by the area of the projection of the surface onto a plane perpendicular to the direction, eg as calculated for a spherical soap bubble when considering the notion of surface tension. The pressure forces on the closed surface $S$ of a body in a particular direction must then sum to zero. (For more complicated shapes we can piece them together from simpler shapes, in a similar way as we do with Gauss's and Stokes' Theorems). Thus the net vector force is zero, as the above applies to any direction. To prove the net torque of the pressure forces is zero, choosing an arbitrary reference point $O$, we can write the total torque as :- $$\tau = -p \iint_{S} \mathbf{r} \times \mathbf{n} \, dS$$ where $p$ is the uniform pressure, $\mathbf{n}$ is unit outward normal to $S$, and $\mathbf{r}$ is the position vector of a surface element wrt $O$. We can write this out one component at a time, and express it as a volume integral using Gauss's Theorem (ie Divergence Theorem), which is then readily shown to be zero.

A couple of interesting home experiments that illustrate atmospheric pressure are (i) drill a hole (say 5-8mm) near bottom of a plastic bottle, fill it up and screw cap on - water will not flow out the hole! Unscrew cap slightly and it will flow immediately. Screw cap back on again and it will stop flowing! (ii) upturned glass of water and card experiment - search online, a remarkable phenomena, though requires a steady hand.

Both (i) and (ii) work by the same principle - the air cavity at the top expands slightly (only by about 1% or so, so it is not visible), causing its pressure to drop (by Boyle's Law) which then causes equalization of the pressure at the bottom of the water column to the external atmospheric pressure.
Yes the area is crucial, for example a small and a large boat may both sink into water the same depth, so that the pressure experienced is the same, but the bouyancy force on the large boat is much greater due to the larger surface area, so its greater weight can be supported. With a gas spring' it is area difference at either side of the piston head that produces the force (the pressure is the same at either side).

12. Jun 15, 2016

### Tazerfish

Interesting comment complex root.
One thing was particularly strange and i can't really wrap my head around it:
The thought that i have been experiencing pressures way beyond atmospheric pressure on a dayly basis is mind-bending.
Just pushing on a wall with one hand and a lot of stength will put an average pressure of somewhere between a quarter and say two atmospheres on my hand.(+1atm)
(yes, I know that isn't very precise.I dont really know my hand area or maximum pushing strength very well.)
And that is just the average! the spots of "meat" between my bones and the wall will experience significantly more pressure than that.
I don't even want to think of the pressures involved in getting your finger stuck in a closing door .
During a handstand some parts of your hands are actually experiencing higher pressures than during a dive to the bottom of a shallow lake.
That is so strange!

Finally i wanted to add that the hole in the bottom of the water bottle has to be small.
Otherwise the surface tension will not be able to keep a "smooth" interface between the water and air and water will flow out while air simultaneously bubbles up through it.

Last edited: Jun 15, 2016
13. Jun 15, 2016

### jbriggs444

What you feel is not "pressure". What you feel is a "stress". More properly, what you sense is the result of a deformation cause by a stress.

If you apply an even pressure to a blob of putty from all sides, it will not deform. It will retain its same shape. But if you apply a pressure from top and bottom but do not apply a pressure from the sides, it will deform. It will squeeze out the sides. If there were nerve cells embedded in that blob of putty, then an even pressure would cause no deformation and the nerves would sense nothing. But under an uneven pressure, some would be stretched and some would be compressed depending on their orientation. This deformation is what the nerves detect.

When you press on the wall, the wall is pushing your meat one way and your bones are pushing your meat another way. Your pressure-sensing nerves are stretched or compressed as a result -- because your meat behaves somewhat like putty.

14. Jun 15, 2016

### Tazerfish

Well that was stupid of me ...
I should have known that the same units don't imply the same physical quantity. :[

But I thought there were multiple sensors in human skin, some that measure stress and others that really measure pressure.
(I am not quite sure how the pressure sensors (would) work. They might have to contain gas)
For example when you suspend your arm in water you can feel something.(aside from the cold)
Wouldn't that only be possible if there were pressure sensors in human skin ?

Also, Thanks.I've been wondering how the pressure sensing "nerves" worked.
P.S.: Should i delete my older comment? It might confuse others.

15. Jun 15, 2016

### jbriggs444

Can you? They fill sensory deprivation chambers with [warm] water.

16. Jun 15, 2016

### Tazerfish

Now i feel delusional
There ARE multiple sensors in our skin but apparently none that could sense pressure.
So I guess I have been imagining things.
The most plausible non-mental cause would be that the veins in my arm were in an enviroment with higher pressure than the enviroment of my heart and i could feel that.
Just like you can feel underpressure because blood gets sucked into these parts of the body.
But then i would still be unable to percieve absolute pressure.

So this statement seems to be fundamentally wrong ...
We are most likely not able to sense absolute pressure at all.
Which would explain why diving a couple of meters down does not feel (much) different than beeing submersed under the surface.
Fascinating