3 ms·

This is pretty interesting, but perhaps the new question would be at what point does the thickness of a liquid slow down a swimmer. For example, there's no way

by hkuo 16y ago

This is pretty interesting, but perhaps the new question would be at what point does the thickness of a liquid slow down a swimmer. For example, there's no way a person could swim equally fast through water vs mud. Perhaps the thickness of the syrup was not a significant enough difference to prove this.

obsaysditto 16y ago
you can read the study here http://www3.interscience.wiley.com/cgi-bin/fulltext/109667863/PDFSTART http://www3.interscience.wiley.com/cgi-bin/fulltext/10966786... [pdf]
ImFatYoureFat 16y ago
I only skimmed the article so they might have covered this, but I would think that this comparison would, at least in theory, hold true for any two substances. Pressure is pressure. No liquid is going to react differently to pressure applied by your head and shoulders than it does to pressure applied by your arms and legs. In other words: swimming is essentially pushing against something with your head while you push against something else with your arms and legs. The pressure and force received from these two actions should stay comparably constant in any PURE substance. So maybe you would swim at a different speed in mud, if there were rocks in it.
- CWuestefeld 16y ago
  I don't think so. I think that at the extreme (say, the consistency of peanut butter), the amount of force you expend simply in inserting and extracting your arms (that is, not in the direction that propels you forward) becomes excessive.
  - sliverstorm 16y ago
    Or, to put it in terms of a simple thought experiment- can you 'coast' or 'glide' in water? can you do the same in molasses? How about caramel?
- SamReidHughes 16y ago
  Change the density and the swimmer's buoyancy will change. Change the viscosity and the optimal swimming algorithm will change. As you increase a liquid's viscosity, the swimmer loses its ability to coast on the inertia provided by previous effort. At a high enough viscosity (given a swimmer of constant size and strength), it's as if the swimmer has no inertia. The movement of the swimmer and fluid will be effectively a function of what forces the swimmer is putting on the fluid at that point in time, because any inertia will immediately get "stolen" by the fluid. For example, scallops can swim in water, but with a high enough viscosity (or a microscopically sized scallop), their swimming style would get them nowhere. See http://en.wikipedia.org/wiki/Scallop_theorem http://en.wikipedia.org/wiki/Scallop_theorem . At high enough viscosities, the best way for a human to swim underwater would be to grow new skin on the scalp and reabsorb excess skin at the feet. Viscous fluids are better at stealing the effort you've put in, so you're going to have a harder time swimming in them, unless you can find a good way to push yourself along without donating your velocity back to the fluid.
- lmkg 16y ago
  You'd like it to be the case that all you have to worry about is pressure and force, but that's not always true. A chemistry major in my dorm once made a non-newtonian fluid just with water and the appropriate concentration of corn starch. The viscosity is not a constant in such a fluid, but rather scales with the force applied. If you struck it hard with a pencil, the pencil would bounce off like the fluid was solid--but if you pushed slowly, it would act like a thick liquid. It was like... a liquid chinese finger trap. Since your hands move faster than your head, I think that means you would swim faster in such a liquid, up to a point, although after a certain point swimming faster gets harder to. It hurts my head to think about it though.
  - masterj 16y ago
    I believe this is the seminal work on the subject: http://www.youtube.com/watch?v=f2XQ97XHjVw http://www.youtube.com/watch?v=f2XQ97XHjVw
- ImFatYoureFat 16y ago
  thanks for the explanations
- jlangenauer 16y ago
  No, pressure is not pressure. There are two ways a fluid or gas can exert a force on another body: through normal pressure ("normal" here meaning the local normal perpendicular to the surface of the body at that point), and through shear forces, which are dependent (mainly) on the viscosity of the liquid, but also local fluid effects such as boundary layers etc can come into play.
baddox 16y ago
I'm also curious about the other end of the spectrum. How thin could the fluid be? Obviously, it would have to be more dense than the human body, but beyond that how thin could we make it?
- electromagnetic 16y ago
  Density doesn't necessarily correspond with viscosity. Water's viscosity varies greatly simply by temperature. Water at 90C has under 1/4 the viscosity of 10C water, but we're fully buoyant in it.
  - baddox 16y ago
    I know, I was just mentioning that density would be an important factor when searching for less viscous fluids.

by hkuo 16y ago

obsaysditto 16y ago

you can read the study here http://www3.interscience.wiley.com/cgi-bin/fulltext/109667863/PDFSTART http://www3.interscience.wiley.com/cgi-bin/fulltext/10966786... [pdf]

ImFatYoureFat 16y ago

I only skimmed the article so they might have covered this, but I would think that this comparison would, at least in theory, hold true for any two substances. Pressure is pressure. No liquid is going to react differently to pressure applied by your head and shoulders than it does to pressure applied by your arms and legs. In other words: swimming is essentially pushing against something with your head while you push against something else with your arms and legs. The pressure and force received from these two actions should stay comparably constant in any PURE substance. So maybe you would swim at a different speed in mud, if there were rocks in it.

CWuestefeld 16y ago
I don't think so. I think that at the extreme (say, the consistency of peanut butter), the amount of force you expend simply in inserting and extracting your arms (that is, not in the direction that propels you forward) becomes excessive.
- sliverstorm 16y ago
  Or, to put it in terms of a simple thought experiment- can you 'coast' or 'glide' in water? can you do the same in molasses? How about caramel?
SamReidHughes 16y ago
Change the density and the swimmer's buoyancy will change. Change the viscosity and the optimal swimming algorithm will change. As you increase a liquid's viscosity, the swimmer loses its ability to coast on the inertia provided by previous effort. At a high enough viscosity (given a swimmer of constant size and strength), it's as if the swimmer has no inertia. The movement of the swimmer and fluid will be effectively a function of what forces the swimmer is putting on the fluid at that point in time, because any inertia will immediately get "stolen" by the fluid. For example, scallops can swim in water, but with a high enough viscosity (or a microscopically sized scallop), their swimming style would get them nowhere. See http://en.wikipedia.org/wiki/Scallop_theorem http://en.wikipedia.org/wiki/Scallop_theorem . At high enough viscosities, the best way for a human to swim underwater would be to grow new skin on the scalp and reabsorb excess skin at the feet. Viscous fluids are better at stealing the effort you've put in, so you're going to have a harder time swimming in them, unless you can find a good way to push yourself along without donating your velocity back to the fluid.
lmkg 16y ago
You'd like it to be the case that all you have to worry about is pressure and force, but that's not always true. A chemistry major in my dorm once made a non-newtonian fluid just with water and the appropriate concentration of corn starch. The viscosity is not a constant in such a fluid, but rather scales with the force applied. If you struck it hard with a pencil, the pencil would bounce off like the fluid was solid--but if you pushed slowly, it would act like a thick liquid. It was like... a liquid chinese finger trap. Since your hands move faster than your head, I think that means you would swim faster in such a liquid, up to a point, although after a certain point swimming faster gets harder to. It hurts my head to think about it though.
- masterj 16y ago
  I believe this is the seminal work on the subject: http://www.youtube.com/watch?v=f2XQ97XHjVw http://www.youtube.com/watch?v=f2XQ97XHjVw
ImFatYoureFat 16y ago
thanks for the explanations
jlangenauer 16y ago
No, pressure is not pressure. There are two ways a fluid or gas can exert a force on another body: through normal pressure ("normal" here meaning the local normal perpendicular to the surface of the body at that point), and through shear forces, which are dependent (mainly) on the viscosity of the liquid, but also local fluid effects such as boundary layers etc can come into play.

baddox 16y ago

I'm also curious about the other end of the spectrum. How thin could the fluid be? Obviously, it would have to be more dense than the human body, but beyond that how thin could we make it?

electromagnetic 16y ago
Density doesn't necessarily correspond with viscosity. Water's viscosity varies greatly simply by temperature. Water at 90C has under 1/4 the viscosity of 10C water, but we're fully buoyant in it.
- baddox 16y ago
  I know, I was just mentioning that density would be an important factor when searching for less viscous fluids.