So you’re motionless in a very large zero-g space (the middle of a very large spaceship, perhaps). There is breathable atmosphere at room temperature, and you are naked (or clothed but unable/unwilling to remove anything). If you do nothing, you will stay in the center of this space forever. Which is a faster method to cross to the one side or the other of this large space: Swimming, or inhaling deeply, then turning your head in the appropriate direction, and exhaling forcefully, over and over again.
If you had fins like a scuba-diver, would swimming be more effective? Is there any other way (let’s leave out any particularly disgusting NSFW ideas) that could work? Am I missing or forgetting some vital factor of physics in such a scenario?
No, you pretty much nailed it - just breathing in, then blowing out through pursed lips will give you a tiny push (because you breathe in air from a hemisphere of directions around your mouth, but you expel it in one direction).
Throwing something solid would be better than throwing air, so there is a more effective solution probably available, but you don’t want to hear it.
Swimming is almost certainly going to be better. Compare, for instance, the situation where you’re in water: You can move by swimming, or you can move by gulping down and spitting out mouthfuls of water. Nobody ever does the latter, because it’s horribly inefficient compared to swimming.
Both will be less effective in air, because of the lower density, but the decrease in effectiveness due to density should be the same for both methods, and hence the relative effectiveness of the two methods should also be the same.
Are you sure about this? This doesn’t match my limited recollection of fluid dynamics – I thought that, for the inhale-exhale method, the type of fluid wouldn’t really matter as for the force generated (i.e. you generate about the same force spitting water as spitting air), while the fluid is incredibly important for motions like swimming. But it’s been a long time since I studied it.
I have a funny feeling that there is an “it depends” here. It depends upon the Reynolds Number. Swimming requires you to move air by causing a flow relative to your limbs that eventually provides a net force where you want to go. As the viscosity of the fluid you are in drops, your ability to cause this drops. Blowing air from your mouth is only about conservation of momentum. The harder you blow the greater the momentum change. It is independent of the viscosity of the fluid you are in. Greater density helps. In water both fish and squid seem to zip about effectively.
But we can only blow (say) about a gram of air out per breath. And you would be lucky to get more than a few metres per second of velocity. So a thousand hard breaths would only change your velocity maybe a couple of mm per second. Still, a metre or so per hour. It isn’t infeasible. But you are not built like a squid.
Reynolds number changes with scale. Insects are so small that they are already effectively swimming more like in water than in air. As creatures get bigger the Reynolds number gets bigger, and swimming drops in efficiency. A blue whale might be at a crossover where jetting from its blowhole becomes more useful than trying to swim. (Probably not yet, but it would be more affected than a human.)
Hmm… iiiiandyiiiiiiii (making sure I’ve got all of them in there), you might actually be right, now that I think about it. And blowing on a cup of hot tea does seem like it moves about as much air as fanning it with one’s hand. I suspect that when you get your whole body involved, swimming is still more efficient, but it might be closer than my first guess suggested.
The astronauts on the ISS do describe it as swimming, but that might not be a fair comparison, since the ISS doesn’t have any large enclosed spaces, and so most motion is going to result from just pushing off of surfaces.
You actually exactly doubled the amount of i’s on each side, Chronos! Was it on purpose, or by accident? I’ve never seen that exact variation before; well done!
We have NOT clawed our way up the food chain only to become poo-flinging monkeys once again! Sir! How dare you! Laser pistols on the port solar array when the terminator crosses the Prime Meridian! Choose your second!
Using the exhalation method, you’d need to be sure you blow along your body’s long axis, or you’ll start spinning.
I was going to wonder whether it would be necessary to turn your head 180 degrees when you inhale, but then I recalled Feynman’s water sprinkler, so I think that’s not necessary.
Forces may be the comparable, but propelling yourself by spitting is action-reaction - air has a lot less reaction mass than water, same force on less mass equals less thrust
Ugh, almost but not quite. Thrust is force. It it were the same force in both cases the thrust would be the same. The trouble is that you can’t apply the force. You have an impedance mismatch problem. The issue is that you are velocity limited - same velocity change on less mass means less thrust. So a gram of air versus a kilogram of water means 1:1000 the thrust - for the same velocity. You can only exhale so quickly. If you could exhale at arbitrary speed you could generate any thrust.
I believe the Skylab astronauts did exactly this experiment.
Can’t find it yet, but I know I’ve seen a video of one of them flapping around with some sort of… flappers… trying to move around the large area inside Skylab. I’m not talking about the tests of the MMU, which they also did. I’ll keep looking.
Without having thought about this carefully, doesn’t the existence of blowguns suggest that you can do better than what Francis Vaughan has suggested, at least if you’re allowed a tube of some sort? Here, for example, is a claim that one kind of blowgun dart weighs about 0.8 g, and I presume we agree that we would expect your typical blowgun to propel a projectile at significantly more than 1 meter/second.
Added for clarity: Francis Vaughan’s number strikes me as a little pessimistic, but look: if a 100 kg person can exhale 1 gram of air at 1 meter/second, then his speed should increase by about 0.01 mm/sec by simple conservation of momentum. Do that 60 times in a minute and you’ve reached the lofty speed of 0.6 mm/sec. Even if this is pessimistic by a factor of ten, you’re still not going to get to the far wall anytime soon.
I don’t think our exhalation is velocity-limited; I think it’s pressure limited (at least until you get to very large volumetric flow rates, i.e. mouth wide open, at which point getting the air out of your lungs/trachea quickly is a challenge). If your lips are pursed to make a nozzle 1/4" across, your lungs/diaphragm can generate the same pressure (and therefore the same thrust) whether your lips are expelling air or water. Because of water’s much higher density, the velocity will be lower (by a factor of ~29), but pressure=thrust, so no difference in thrust, whether you’re squirting air or water from your mouth.
1 m/s for an exhalation is far too low. Haven’t you all heard that sneezes can exceed 100 MPH? Granted, most exhalations are going to be much slower than a sneeze, but not that much slower.
