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Pressure is force divided by area, and force is mass times acceleration.

Now, the newton [N] is the force needed to accelerate 1 kg by 1 m/s, and the kilogram-force [kgf] is the force needed to accelerate 1 kg by g m/s, where g is the standard gravity. The standard atmospheric pressure is set at 101325 Pa, which would translate to ~10333 kgf/m^2.

So why don't we all implode? What is missing from the picture?

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  • $\begingroup$ There is no kgf. kg is mass, and Newtons is force. Now, to answer the question: atmospheric pressure exists inside of you as well as outside of you, due to small amounts of atmospheric oxygen and nitrogen (the equilibrium amount, based on gas solubility in your blood and tissues) that exist inside of you. This means that the net atmospheric force on you is zero. $\endgroup$ – David White Dec 9 '16 at 0:14
  • $\begingroup$ There was a kilogram-force. And thanks for mentioning solubility, that clears up a few things. $\endgroup$ – setun-90 Dec 9 '16 at 0:28
  • $\begingroup$ For the most part kilogram-force is an thing in engineering where it is a convenience, not in physics were we have the luxury of insisting on making a strong distinction. $\endgroup$ – dmckee Dec 9 '16 at 1:47
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Missing from the picture is that all macroscopic bodies in fluid or solid state do not like being compressed. These macroscopic bodies can be thought of as made up of molecules or atoms which below some distances will always start to repel each other. This causes them to exert a force against being compressed, i.e., brought more close to each other. So pressure is balanced by the resistance of bodies to be compressed beyond a certain point.

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  • $\begingroup$ Ah yes, the condensated states of matter (I guess I was thinking of something supercritical). My bad. $\endgroup$ – setun-90 Dec 9 '16 at 0:01
  • $\begingroup$ You should never feel sorry about having asked a question and tested your physical intuition for consistency - that's something which will in general make you learn a lot and understand physics better :) $\endgroup$ – Sanya Dec 9 '16 at 0:03
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Solids and liquids are what we call incompressable. They do not change their volume a great deal when a pressure is applied to them. A comparable metaphor for you is that you can park the wheel of a car on top of a phone book. That phone book is under thousands of pounds of force, but it doesn't implode because solid materials tend to not change volume when the force is applied.

Meanwhile, the gasses in our body are all at roughly atmospheric pressure, so they already press outwards with a pressure equal to the pressure going in. However, if you rapidly change pressures (such as a rapid decompression in the event of an airline cabin losing pressurization), you very quickly feel just how much force was being applied by all of those gasses for a few moments while they all equalize.

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  • $\begingroup$ I had also thought about the gasses in our body, but isn't there too little gas for this to be significant? I think the water in our bodies would boil first. $\endgroup$ – setun-90 Dec 9 '16 at 0:06
  • $\begingroup$ @setun-90 Have you ever felt your ears hurt and you resolved it by "popping" them? That pain was caused by a difference in pressure inside and outside the eardrum. Yawning or other approaches to cause "popping" opens up a tube known as the Eustachian Tube and permits the pressures to equalize. $\endgroup$ – Cort Ammon Dec 9 '16 at 1:57

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