# Would buoyancy in water change in lower gravity? **URL:** \ **Category:** Factual Questions\ **Created:** [February 13, 2013, 2:03pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175 "2013-02-13T14:03:52Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 14, 2013, 5:33pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/21 "2013-02-14T17:33:04Z")
> [@DrCube](#): > > Yes, but volume is a function of pressure which is a function of gravity. In the specific case of water (and thus also people), the volume doesn’t change that much with gravity, but gases and some compressible liquids and solids could vary widely across different planets and in comparison to each other. True, but: a)aren’t we working around/ignoring the pressure factor? (otherwise, the answer to the question “What’s it like to swim on Europa” is simply “lethal”) and b)I don’t think that’s what **the\_diego** was talking about (I’m not sure what it was, but it doesn’t look like that). ---
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [February 14, 2013, 5:37pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/22 "2013-02-14T17:37:30Z")
> [@CalMeacham](#): > > Buoyancy wouldn’t be affected, but the low gravity would have HUGE implications. you weigh 2/15 on Europa of what you do on earth. You could probably push yourself out of the water, leap like a porpoise. The water you displace in swimming will tend to stay in the air longer and fall weirdly. Think of it like Heinlein’s _The Menace from Earth_, but with water. > > Bonus, as in that story, with proper wings, you ought to ge able to fly. Europa, no. [But Titan yes.](http://what-if.xkcd.com/30/) “In fact, humans on Titan could fly by muscle power. A human in a hang glider could comfortably take off and cruise around powered by oversized swim-flipper boots—or even take off by flapping artificial wings. The power requirements are minimal—it would probably take no more effort than walking.” ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [February 14, 2013, 8:39pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/23 "2013-02-14T20:39:06Z")
> [@The\_Controvert](#): > > Europa, no. [But Titan yes.](http://what-if.xkcd.com/30/) > > “In fact, humans on Titan could fly by muscle power. A human in a hang glider could **comfortably** take off and cruise around powered by oversized swim-flipper boots—or even take off by flapping artificial wings. The power requirements are minimal—it would probably take no more effort than walking.” the weather is a bit nipply, though. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [February 14, 2013, 10:29pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/24 "2013-02-14T22:29:35Z")
The relative densities would not change. Say, on Earth, you float up to your ears and a 1kg lead sinker makes you exactly neutrally buoyant. Then on any planet you would float up to your ears and the same sinker would make you neutrally buoyant. But if you were to swim (without the sinker) on a high-gravity planet you would find it would take more energy to submerge yourself. When you push yourself under you are essentially lifting up a volume of water. The higher the gravity the more energy you need to lift that same volume of water. ---
**Author:** ![mikelepage](https://avatars.discourse-cdn.com/v4/letter/m/ce7236/32.png) [@mikelepage](https://boards.straightdope.com/u/mikelepage)\ **Post date:** [October 8, 2013, 4:25am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/25 "2013-10-08T04:25:17Z")
Okay, I’m coming at this from a slightly different angle and you guys seem physics inclined. Imagine a Elysium style space station ring, in which there is a lake, on which there is a boat. Question 1) When the ring spins at slow, medium and fast angular velocities (ie different centripetal accelerations), how does the boat sit in the water? a) the same height for all three, b) higher when spinning faster c) lower when spinning faster. Question 2) As the ring angular velocity increases (centripetal acceleration increasing), the boat a) stays at the same height, b) rises to a new height, c) rises during acceleration, but falls back to the original height once the angular velocity becomes constant, d) falls to a new low, e) falls during acceleration, but rises back to the original height once the angular velocity becomes constant. Thanks for your help. FYI This is actually relevant to a proposal I’m going to make. ---
**Author:** ![Fish\_Cheer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fish_cheer/32/17179_2.png) [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)\ **Post date:** [October 8, 2013, 5:36am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/26 "2013-10-08T05:36:11Z")
> [@Mangetout](#): > > If gravity doesn’t affect buoyancy, how does a centrifuge work? A centrifuge separates by relative density, doesn’t it? It only speeds up what would happen anyway, that is, denser stuff sinks to the bottom and vice versa. ---
**Author:** ![Glazer](https://avatars.discourse-cdn.com/v4/letter/g/a88e57/32.png) [@Glazer](https://boards.straightdope.com/u/Glazer)\ **Post date:** [October 8, 2013, 5:44am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/27 "2013-10-08T05:44:10Z")
> [@Machine\_Elf](#): > > Icebergs are 90% submerged here on earth, and they would also be 90% submerged on any other planet. But would I crystallize in the same way under high G? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 8, 2013, 7:42am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/28 "2013-10-08T07:42:15Z")
> [@mikelepage](#): > > Okay, I’m coming at this from a slightly different angle and you guys seem physics inclined. Imagine a Elysium style space station ring, in which there is a lake, on which there is a boat. > > Question 1) When the ring spins at slow, medium and fast angular velocities (ie different centripetal accelerations), how does the boat sit in the water? a) the same height for all three, b) higher when spinning faster c) lower when spinning faster. > > Question 2) As the ring angular velocity increases (centripetal acceleration increasing), the boat a) stays at the same height, b) rises to a new height, c) rises during acceleration, but falls back to the original height once the angular velocity becomes constant, d) falls to a new low, e) falls during acceleration, but rises back to the original height once the angular velocity becomes constant. > > Thanks for your help. FYI This is actually relevant to a proposal I’m going to make. In theory, it should sit at the same level all the time - because the weight of the boat, and the weight of the water it displaces both increase or decrease in proportion to the force of (real or fake) gravity. If you’re just launching the boat, it will settle to its floating position quicker under higher gravity (and sinking objects will sink faster, and floating objects will rise faster). ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 8, 2013, 7:43am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/29 "2013-10-08T07:43:27Z")
> [@Fish\_Cheer](#): > > A centrifuge separates by relative density, doesn’t it? It only speeds up what would happen anyway, that is, denser stuff sinks to the bottom and vice versa. Yes, as agreed in post #15. Dynamic systems will speed up. Static systems will not be affected. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [October 8, 2013, 7:46am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/30 "2013-10-08T07:46:32Z")
> [@Fish\_Cheer](#): > > A centrifuge separates by relative density, doesn’t it? It only speeds up what would happen anyway, that is, denser stuff sinks to the bottom and vice versa. The centrifuge does increase the rate of separation, but it can also cause it to occur where it would not occur without. eg small particles may stay in suspension, eg due to their stickiness. The artificial gravity of the centrifuge can overcome the stickiness to make the particles drop. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 8, 2013, 7:55am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/31 "2013-10-08T07:55:27Z")
> [@Isilder](#): > > The centrifuge does increase the rate of separation, but it can also cause it to occur where it would not occur without. > eg small particles may stay in suspension, eg due to their stickiness. The artificial gravity of the centrifuge can overcome the stickiness to make the particles drop. I imagine there are a few boundaries that change - gels become fluids in a higher gravity environment - or in a lower gravity environment, things we know as fluids start to act more like gels (I’m not actually sure if I want to include the surface tension behaviour of water in microgravity in this category or not) ---
**Author:** ![Glazer](https://avatars.discourse-cdn.com/v4/letter/g/a88e57/32.png) [@Glazer](https://boards.straightdope.com/u/Glazer)\ **Post date:** [October 8, 2013, 8:00am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/32 "2013-10-08T08:00:34Z")
I thought Brownian motion kept particles in suspension. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [October 8, 2013, 8:23am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/33 "2013-10-08T08:23:54Z")
> [@Mangetout](#): > > Yes, as agreed in post #15. Dynamic systems will speed up. Static systems will not be affected. Exactly so; if your boat is pushed deeper than the equilibrium point, then the restoring force is dependent on the excess weight of the displaced water, and acts on the mass of the boat. One of these is affected by gravity and the other is not. At equilibrium, the weights of the boat and displaced water balance out, as long as they are non-zero (which is why you require _some_ gravity). ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [October 8, 2013, 8:26am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/34 "2013-10-08T08:26:22Z")
> [@For You](#): > > the weather is a bit nipply, though. I think you have invented a new and useful word: Nipply - adjective (nipplyer, nipplyest) informal of the weather - very chilly: it’s a bit nipply this morning (as in it makes ones nipples stand out) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 8, 2013, 9:07am UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/35 "2013-10-08T09:07:06Z")
> [@Glazer](#): > > I thought Brownian motion kept particles in suspension. For a certain range of conditions, it probably does, but Brownian Motion is just lots of little random, but finite forces - if the particles are too big/heavy, the forces will be insufficient to keep them suspended - but ‘heavy’ here is dependent on the force of gravity - in lower gravity, larger particles will be able to stay suspended. ---
**Author:** ![Fish\_Cheer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fish_cheer/32/17179_2.png) [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)\ **Post date:** [October 8, 2013, 3:36pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/36 "2013-10-08T15:36:08Z")
> [@Mangetout](#): > > For a certain range of conditions, it probably does, but Brownian Motion is just lots of little random, but finite forces - if the particles are too big/heavy, the forces will be insufficient to keep them suspended - but ‘heavy’ here is dependent on the force of gravity - in lower gravity, larger particles will be able to stay suspended. That makes sense. Thank you. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 8, 2013, 5:26pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/37 "2013-10-08T17:26:41Z")
If you’re getting your gravity from centrifugal force, and you’re changing the rate of rotation, then you’re going to get complications from the tangential acceleration, as well. In layman’s terms, it’ll slosh. Do it gradually enough that you minimize the sloshing, though, and the boat will just sit there at a constant height. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [October 8, 2013, 6:07pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/38 "2013-10-08T18:07:16Z")
Centrifugal gravity raises an interesting question, though. If in zero-G you have a cylinder containing some amount of air and water (say, half and half, at STP), when you spin the cylinder, what happens? I mean, water near the wall will cling to the wall, but water near the center will not be inclined to move outward, at least initially, and air near the sides will not tend to be forced inward. Has anyone experimented with this? Assuming a single axis of ritation, would such a cylinder eventually reach some sort of equilibrium with a layer of water at the wall (forming by surface-tension capture of water near the wall) with air toward the inside (like a bubble) and some globs of water in the middle? Would air moving inward eventually force water in the middle outward or to the end (where it would be flung outward)? And how would the smoothness of the inner surface affect the distribution of contents? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 8, 2013, 6:50pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/39 "2013-10-08T18:50:41Z")
Even though there’s an equilibrium for heavy material at the center, it’s an unstable equilibrium. Eventually (which in practice wouldn’t be a very long time), you’d have all the water on the outside, and all the air in the middle. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 8, 2013, 7:38pm UTC](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175/40 "2013-10-08T19:38:22Z")
> [@For You](#): > > Centrifugal gravity raises an interesting question, though. If in zero-G you have a cylinder containing some amount of air and water (say, half and half, at STP), when you spin the cylinder, what happens? I mean, water near the wall will cling to the wall, but water near the center will not be inclined to move outward, at least initially, and air near the sides will not tend to be forced inward. Has anyone experimented with this? Assuming a single axis of ritation, would such a cylinder eventually reach some sort of equilibrium with a layer of water at the wall (forming by surface-tension capture of water near the wall) with air toward the inside (like a bubble) and some globs of water in the middle? Would air moving inward eventually force water in the middle outward or to the end (where it would be flung outward)? And how would the smoothness of the inner surface affect the distribution of contents? Eventually, everything will rotate - because each of the layers is in contact with the next - even though they’re fluids, they are touching one another and will transfer forces. Think of it this way: Put a large bowl of water on a record player turntable and turn it on. At first, only the bowl rotates - the fluid stays as it is, but gradually (and beginning from the layers most closely in contact with the bowl, but eventually extending throughout the entire body of the fluid), it all starts to rotate, until eventually, everything is rotating at the same rate - and the fluid is piled up toward the edges of the bowl. [Previous page](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175.md?page=1) [Next page](https://boards.straightdope.com/t/would-buoyancy-in-water-change-in-lower-gravity/650175.md?page=3)