On a hypothetical lower-gravity world, like perhaps Europa, what would it be like to swim? Would a person (or an object) be any more buoyant? Would more ballast be required to say submerged?
Off the top of my head, I do not believe that bouyancy would be affected by gravity. A given volume of water would weigh less in a lower gravity field, but the immersed object would also weight less, in the exact same proportion.
Now, microgravity is a different kettle of fish.
If the density of an object is less than the density of the fluid medium in which it’s immersed, then it will always be buoyed upward by the fluid; the buoyant force is proportional to the difference in density of the object and the fluid, and it’s also proportional to the gravitational attraction of the planet on which the fluid and object are located.
However, reduced gravity also pulls downward less forcefully on a buoyant object. This is also a simple proportionality.
The net result is that if you float with your head above water on earth, you will also float with your head above water on any other planet. Icebergs are 90% submerged here on earth, and they would also be 90% submerged on any other planet.
If an object is of fixed geometry (e.g. a submarine), the same amount of ballast will be required to achieve neutral buoyancy on any other planet.
In reduced gravity, ambient pressure will increase with depth at a lower rate than it does here on earth. So if an object is compressible (e.g. a SCUBA diver using conventional diving gear), his lungs, wetsuit and buoyancy compensator (BCD) will shrink less at depth than they do here on earth; he wouldn’t need to adjust his BCD as frequently during ascent and descent to maintain neutral buoyancy.
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.
Could be tough in a place where the atmospheric density is less than a billionth of the Earth’s.
Well, you need something like the giant air storage tank on the moon Heinlein describes in his story. Flight is even more difficult in a virtual vacuum.
I can think of couple of interesting things that might happen :
1> Giant waves due to the low gravity.
2> Swimmer will have very less control in the vertical direction due to the low buoyancy. That is if a swimmer jumped into the water, he or she may just keep going longer straight down.
3> Maybe difficult to control your orientation in water - kinda like a guy on a space station. I believe different parts of the body have different buoyancy (due to air in our bodies) and we learn to use this to “balance” or “maintain our orientation” in water.
4> Taking your head out of the water to breathe while swimming may not be feasible - since surface tension will be dominating and you may get a huge blob of water instead of the breakup you see on earth.
5> If the air pressure is low, like someone mentioned then you will not have water since it will all evaporate
6> If the gravity is too low - the water surface will not be flat where it meets the shore / edges. It will be like a slope - somewhat like in a straw.
In really low gravity would surface tension have an effect on larger objects? That would have an effect on the buoyancy of the objects in question.
To avoid getting sidetracked, assume the hypothetical world somehow has an earth normal atmosphere despite the lowered gravity.
Maybe some sort of planet-wide semi-elastic shield that holds the gasses. Or something like Star Wars’s Kessel with huge atmosphere factories that produce atmosphere to replace the stuff that escapes into space.
Also, my first thought was how the reduced gravity would affect the relative power of surface tension. How would the effect affect the effect?
How could it not be affected? In zero gravity, there’s no buoyancy, in Earth gravity there’s normal buoyancy. What happens in between?
If gravity doesn’t affect buoyancy, how does a centrifuge work?
The buoyant force is equal to the weight of the water you displace. With any kind of gravity, no matter how low, the water you displace will weigh something. In low gravity, the buoyant force will be low, but your own weight will also be low, so the effect cancels out.
Statically: yes - the buoyant object will float at the same level.
Dynamically: no. For example, if you are submerged, it will take longer to rise to the surface.
Indeed. This goes for anything dynamical that happens due to gravity. For example, a pendulum will hang with the string taut no matter what the local gravity is, as long as it’s greater than 0. But once the pendulum starts swinging, in lower gravity it will be slower.
Would the water itself not be less dense due to the lower gravity?
Agreed.
My off-the-cuff answer is yes, because the relative densities will likely change. You can do the same experiment on earth, using two set-ups: one tank with water, another with a less dense fluid like gasoline. Put in similar objects (size) but with density ratios equal to their respective fluids. Who will float higher?
Yes, but only by a very tiny margin. The compressibility of water is very low:
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Why? Density is a function of mass and volume. Mass is not affected by where you are.
Doesn’t seem very relevant. Something that floats half out of the water on Earth will float half out of the water on the moon (inside a pressurised environment - let’s not complicate things). It will just take longer to come to rest in that configuration.
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.
So in the specific case of a person in water, I don’t think buoyancy will change much across different amounts of gravity, but with other materials it could change a lot.