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The standard scene follows: The good guys have finally captured the enemies and have them in an airlock chamber on a spaceship. A button is pressed and out they go, violently propelled by the blast of air moving from a pressurized zone to a zone of much less pressure (space).

Here's my question: If people don't immediately decompress, then why do airlocks do this in movies? If the air moves due to expansion to reduce pressure, then why wouldn't the human body do the same?

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    $\begingroup$ Recall that the human body is not gaseous. ;-) $\endgroup$ – Wouter Jul 3 '14 at 20:59
  • $\begingroup$ For the airlock, it depends how suddenly the air is released. People consist of lots of muscle, fat, bones, etc. plus liquid plasma in fairly strong tubes, plus air in lungs. If they don't try to hold their breath, there's not much to decompress. $\endgroup$ – Mike Dunlavey Jul 3 '14 at 21:03
  • $\begingroup$ @Wouter Gas expands in space because molecules bounce against each other, right? How would that be different than a solid? Is it just because gas isn't tightly held together (so it can move more freely)? $\endgroup$ – person27 Jul 3 '14 at 21:12
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    $\begingroup$ The atoms in a solid are bound together, such that they need energy to be pulled apart. Atoms in a gas are free to bounce around, so they do. $\endgroup$ – George G Jul 3 '14 at 21:25
  • $\begingroup$ Atoms in solids are bound and since your body is a solid shell to some extent, it can withstand pressure gradients for a while. In a gas atoms are free to move around (and quite quickly so at high enough temperatures). Emilio does make a valid objection to the "violently blasting people away" part: the air molecules might expand at a high velocity, but one might question if they carry enough momentum to blast people away like that. Of course, if you were to first add more air to the airlock chamber and could then get it to suddenly open, you probably would be able replicate the results. $\endgroup$ – Wouter Jul 4 '14 at 1:24
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People don't immediately compress because the body is more or less a pressure vessel. It's not a very good pressure vessel for dealing with vacuum, but it's something. It's your body's resistance to pressure that lets you do things like spray bodily fluids (from your mouth, or from your bladder, or from your arteries). When my wife was in labor with my son, one of the sensors monitoring her reported the internal pressure during her contractions in pascals (though you'll have to find an obstetrician to tell you what a typical uterine contraction pressure is, because I was mostly paying attention to other things at the time).

Gases don't have this constriction. Nitrogen and oxygen at room temperature have mean molecular velocities following \begin{align} \frac12 mv^2 &= \frac32 kT \\ \frac vc &= \sqrt\frac{3kT}{mc^2} \approx \sqrt\frac{3\cdot 25\,\mathrm{meV}}{30\,\mathrm{GeV}} \\ v &\approx 1.5\times 10^{-6}c \approx 450\,\mathrm{m/s} \approx 1000\,\mathrm{mph} \end{align} When you depressurize a gas volume at room temperature, this is the average speed of the gas that moves into the vacuum. If you depressurize an airlock by letting the air flow through a 2 m$^2$ doorway, you have a lot of momentum to carry things along with you.

Emilio Pisanty gives a nice example in a comment: if your airlock is the size of a bathroom (~ 20 m$^3$) and it's depressurized rapidly, so that all the air is "suddenly" moving away from the door at a thermal speed, the momentum of the air is comparable to the momentum of a car. You have a good intuition for what it's like to be hit by a car (hint: it sucks, even at low speed); getting thrown out the door of the airlock is a totally plausible outcome, but not a certainty. Keep in mind, if you start to calculate things, that the kinetic energy available, $p^2/2m$, goes way up if you take the momentum of a car collision and put it into a few kilograms of air.

Note that in a real airlock, you'd change the pressure slowly. If you were on a spacecraft where air was a precious resource, you would probably even pump the air from the airlock into the spacecraft rather that throwing it away.

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    $\begingroup$ Hmmm. It's a hard comparison to make, but air at STP has about a kilogram per cubic meter, so you would only have a few kilograms of the stuff, and only a small fraction of it will actually impart momentum to stuff and/or people caught in the flow. It is going rather fast, but if there's, say, 20kg of air, then the total momentum will be something like a 1-ton car going at 20mph=30kph, which is a large but not huge amount of momentum. Sending someone flying off at that speed takes about 10% of it, which is more of a transfer than I'd expect from rushing air. $\endgroup$ – Emilio Pisanty Jul 3 '14 at 22:09
  • $\begingroup$ So, if I get it right, in a nutshell you mean they get blown away by the immense force coming from the air diffusing out, before the pressure 'leaks' in human body take any effect? $\endgroup$ – 299792458 Jul 4 '14 at 6:09
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    $\begingroup$ @New_new_newbie Yes. The time it takes for all the air to leave the airlock is going to be comparable to the time it takes for an air molecule to zip across the lock, much less than a second; apparently it takes tens of seconds for pressure changes to really affect the body. $\endgroup$ – rob Jul 4 '14 at 17:28
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Then why do airlocks do this in movies?

Good question. Why do space vehicles have wings in movies? Why do producers have all kinds of non-physical nonsense going on in scifi movies?

The answer to these questions is simple: Because they make for such cool special effects. Don't take anything you see in a bad scifi movie as real.

Real airlocks currently operate very slowly. As much air as is possible is pumped to a reservoir before the outer lock is opened. Air will presumably still be a precious resource when/if space travel becomes commonplace. Why waste that resource?

The pressure difference between inside and outside may will bind the pod bay door shut, precluding the door from opening until at least some of the gas is removed from the airlock.

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  • $\begingroup$ This sort of sidesteps the question of what would happen if you did suddenly unlock a spacecraft airlock, which is I think the key part of the OP. $\endgroup$ – Emilio Pisanty Jul 4 '14 at 1:41
  • $\begingroup$ Except you can't. The OP's question was the aerospace engineering equivalent of a question along the lines of "What do the laws of physics say will happen if the laws of physics are wrong?" $\endgroup$ – David Hammen Jul 4 '14 at 1:45
  • $\begingroup$ Not at all what I said. I asked why air rushes out in airlocks in movies. I never asked about a scenario regarding how you could make air rush out of an airlock at crazy velocities or any such nonsense to suggest I was asking about a hypocritical situation. $\endgroup$ – person27 Jul 4 '14 at 16:20
  • $\begingroup$ As for "then why do airlocks do this in movies?" I wasn't asking what the purpose was, I was looking for confirmation that it is unrealistic. $\endgroup$ – person27 Jul 4 '14 at 16:22

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