Wikipedia:Reference desk/Archives/Science/2021 August 22

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August 22

Why do swimmers go hairless?

This line from here doesn't explain clearly: The goal is to reduce drag (a small factor) and enhance the mental gain from a more streamlined sensation in the water (a big factor).

How can a body hair drag someone in fluids? Rizosome (talk) 06:03, 22 August 2021 (UTC)

What makes you think it wouldn't? The article seems to be totally clear on the subject. Sure, it's a small factor. But every millisecond counts. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:21, 22 August 2021 (UTC)

In theory, smooth skin reduces the boundary layer improving laminar flow which in turn reduces turbulence. While this might be important for sharks (the skin on fast-moving sharks even have have "riblet" shaped scales that act as laminar flow tabs), the benefit for swimmers is probably not measurable. However, performing the ritual of shaving might help in mental preparation. --2603:6081:1C00:1187:D1BC:575A:563:109E (talk) 07:14, 22 August 2021 (UTC) ... See also: Skin friction drag

I expect the flow for active human swimmers to be turbulent anyway. The drag on a golf ball is less with turbulent flow, and I guess the same holds for Olympic swimmers. Shark riblets also reduce the skin friction drag of turbulent flow.^[1][2]  --Lambiam 08:14, 22 August 2021 (UTC)

The golf ball is a very poor example here; it has dimples precisely to make its friction coefficient drop due to the drag crisis (more precisely, the dimples make the drag crisis occur at a lower Reynolds number i.e. lower speed than it would for a smooth sphere). I have not done the calculation but a swimmer (about 20 times bigger and slower than a golf ball) in water (much more viscous than air) is certainly well below the Reynolds of the drag crisis, so hair is probably a net negative. Tigraan^{Click here for my talk page ("private" contact)} 12:07, 23 August 2021 (UTC)

The point, however, was which of the two dominates, the turbulent flow, or the laminar flow. If there is no laminar flow, improving it will not have much effect.  --Lambiam 15:50, 24 August 2021 (UTC)

Whether or not it's an unintended consequence, shaving the head also eliminates the minor inconvenience of getting wet hair and having to dry it afterwards.

Anecdotally, the mildly eccentric long-distance runner David Bedford used to wear his hair long by contemporary standards, but appeared with it trimmed very short just before one important race. When asked why by a TV commentator, he replied: "Streamlinin', mate."

I think we need hard numbers for the assertion "the benefit for swimmers is probably not measurable." In a sport where the results are measured to a hundredth of a second, or from .03% to .001% of the overall times for current olympic-level performances at various distances, I suspect the effect of head-hair drag might be quite significant. Most accessible studies seem to focus on the effect of shaving swimmers' body hair, like [this one the one linked by Alansplodge below whose linking I managed to cock up without noticing], and show it to be very significant. The effect of shaving specifically the head can largely be mimicked by wearing a swimming cap, but a scientific comparison of swimming with head hair, hair under a cap, and a shaven head may exist somewhere.

The OP appears to be confusing (deliberately?) the phenomenon of drag, that would slow a swimmer down, with that for being dragged (presumably forward). {The poster formerly known as 87.81.230.195} 2.121.162.207 (talk) 13:55, 22 August 2021 (UTC)

The Effect of Shaving Body Hair on the Physiological Cost of Freestyle Swimming Alansplodge (talk) 14:54, 22 August 2021 (UTC)

Another anecdote is Mark Spitz, who won gold medals in swimming, while famously and controversially wearing a mustache, at the 1972 Olympic games.[3] ~Anachronist (talk) 15:00, 22 August 2021 (UTC)

Mark Spitz is suspected of having increased his speed by emitting supersonic bubbles of flatulence. A sort of Alcubierre distortion in the Olympic time / space continuum.

Some experiments on superluminal flatulence were conducted which resulted in athletes finishing the race before it was even started. --Cookatoo.ergo.ZooM (talk) 16:45, 22 August 2021 (UTC)

I'm thankful that Alan brought us the source above to keep this discussion grounded and not so based in speculation: I was surprised by the findings of a significant effect from shaving torso and limbs, as found in that research. To be fair, it's just one study and with a relatively tiny sample size, and no real controls for the psychological factors that it kind of just hand-waves away without much empirical reasoning other than begging the question. But it's not nothing, and the effect observed was not just measurable but statistically significant.

All of that said, it's worth noting that the shaved body look is hardly confined to swimming: indeed, I would suspect (admittedly as an impressionistic matter) that is is the dominant look for most athletes of both sexes competing at the national and international level in sports which are represented at the Summer Games. While swimming seems intuitively to be the sport which has the best claim to a justified advantage in performance from this practice, it would seem that there is a more significant answer to the question the OP presents in the title of this thread, in the form of cultural and aesthetic trends (both in cosmopolitan society generally and in elite athletics in particular). With the important caveat that your average athlete in many of these sports probably does believe in some performance advantage, however slight. Nevertheless, this rationalization (and the apparently actual, if slight, advantage in certain sports) is clearly only a part of the explanation for the prevalence of the practice. SnowRise ^{let's rap} 00:21, 24 August 2021 (UTC)

Lithium extraction

Is there any data out there regarding the partition coefficients for liquid-liquid extraction of lithium salts from aqueous solution? And if so, is there also data about how (if at all) this is affected by the presence of potassium chloride and/or potassium sulfate? 2601:646:8A81:6070:7484:4D7C:C258:55B2 (talk) 10:12, 22 August 2021 (UTC)

You would probably want to use some derivative of 12-Crown-4 to do lithium extraction in the presence of potassium ions. I did a substructure search on ChemSpider and there are several candidate compounds that could be used but I didn't find any detailed references. Mike Turnbull (talk) 15:16, 22 August 2021 (UTC)