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First of all, how can atmospheric pressure be same in a closed room when it clearly depend on the height of the air column. Compared to atmosphere, the room few metres high so how can air pressure be same.

Secondly, how can human body sustain such high pressure. If a body gets hits by a truck on all side with the same pressure, he will be crushed. Even if internal pressure is equal why would not the skin get crushed. Why aren't everyday crushed.

Thirdly, if book is held on a table with few mm of separation, will the air pressure underneath the book be same and how. Will the there be air flow.

What happens to air pressure between small gaps between objects

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    $\begingroup$ Would you expect a vacuum under a carport? $\endgroup$ May 20, 2021 at 16:59

4 Answers 4

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In the case of air pressure difference over a height change of order ~2 meters, that difference does indeed exist but is too small for humans to notice. Note that the best mechanical altimeters as used in airplanes can read out height differences based on pressure differences to a resolution of 20 feet, and electronic altimeters can sense height differences via pressure measurements that are of order ~one foot.

The human body can sustain such pressures because the same pressure exists inside your body as outside your body. And if you climb a mountain up into less dense air where the ambient pressure is less, there's plenty of time available for the inside and outside pressures in your body to come into equilibrium.

The pressures on two sides of a book near a table top will be almost exactly the same and there will be no air flow, for the same reasons cited above.

Air pressure gets evenly distributed throughout our environment because gases cannot support shear forces; the only forces remaining (in the absence of winds) are hydrostatic.

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  • $\begingroup$ But if air is present in small gaps between the book and the surface how such less air can generate such enormous pressure and i am still confuse that how such small volume of air in room can be 760mm of Hg $\endgroup$ May 20, 2021 at 17:10
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The air pressure in a closed room is the same as the atmospheric pressure outside because even a closed room has small gaps through which air can enter or leave. If the pressure in the room were different from the external pressure then air would enter or leave the room until the pressures were equal. The only way to achieve an inside pressure that is different from the outside pressure is to completely hermetically seal the room, in which case you have created a pressure vessel or a hypobaric chamber.

Your skin is not crushed because all of the cells in your body have an internal pressure which is also the same as atmospheric pressure. There are some serious physiological effects of high or low pressure (such as HPNS or altitude sickness) but these are not due to mechanical crushing of cells.

Even with a flat book on a flat table, there are small gaps between the book and the table which molecules of air can get into (molecules are really small). So the pressure underneath the book is again the same as atmospheric pressure, because if it were different air would move in one direction or the other until it was equal. To achieve a lower pressure underneath the book you would first have to make an airtight seal between the book and the table. This is how a suction cup works.

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  • $\begingroup$ But if air is present in small gaps between the book and the surface how such less air can generate such enormous pressure and i am still confuse that how such small volume of air in room can be 760mm of Hg $\endgroup$ May 20, 2021 at 17:10
  • $\begingroup$ @ParthDeodhar If we take two different volumes of air (or any other gas) at the same temperature and pressure and join them together, the pressure does not change because it does not depend on volume. On the other hand, if we take a fixed amount of gas and double its volume then its pressure halves - it does not increase. $\endgroup$
    – gandalf61
    May 20, 2021 at 17:57
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how can atmospheric pressure be same in a closed room when it clearly depend on the height of the air column.

It isn't. The pressure will depend on height according to the normal equation $P = P_0e^{-gh/\alpha}$. The difference between air at the ceiling and the floor is probably 0.0001% or something.

If a body gets hits by a truck on all side with the same pressure, he will be crushed.

This analogy isn't the same because the human body is not air tight. Each individual skin molecule gets hit by air molecules from all directions. Not each individual skin molecule gets hit by molecule-sized trucks from all directions.

In your analogy there are no trucks hitting you from inside your body. And that can't happen cause trucks are huge. But air molecules are tiny.

will the air pressure underneath the book be same and how

Air molecules are always moving in random directions trying to go everywhere. If there's no air molecules in the small gap under the book there will be soon enough! It's like if you had a billion ants in a tank and then put a crumpled piece of paper on the floor of it. As long as the ants can fit they'll be crawling all over it - not because they're excited about the fresh piece of paper, but just by random movement alone.

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  • $\begingroup$ @ParthDeodhar Where did you get the idea that a large volume increases pressure? It's quite the opposite. PV = NRT so P = NRT/V $\endgroup$
    – Señor O
    May 20, 2021 at 16:59
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You have some good answers here but you still seem to be concerned about the air pressure under a book on a table so I will attempt to address this. While air pressure is depth dependent and increases with lower altitudes, the small amount of air between the book and the table is not constrained from the air pressure beside the book. If it is not completely sealed off from the outer pressure you can consider it at the same distance from the topmost atmosphere. Air pressure also pushes sideways and upwards, not just downwards so the pressure beside the book will push air molecules sideways under the book until the pressures are equal. Hope this helps.

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