Wikipedia:Reference desk/Archives/Science/2022 February 23

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February 23

Are there any animals that have knees that bend the opposite way to ours?

I know it's a common misconception that birds have backwards knees. What looks like a knee is actually the ankle and what looks like the lower leg is actually the foot and what looks like the foot is just the toes.

Are there any animals that do have knees that bend backwards, compared to our knees? --Iloveparrots (talk) 01:43, 23 February 2022 (UTC)

Insects? --←Baseball Bugs ^{What's up, Doc?} carrots→ 06:01, 23 February 2022 (UTC)

Stricly speaking, only primates have knees. See Knee#Other_animals.--Shantavira|^{feed me} 09:20, 23 February 2022 (UTC)

So, bees don't have knees after all? --Bumptump (talk) 00:02, 25 February 2022 (UTC)

I do not believe the uncited claim on Wikipedia that "only primates have knees". It is trivially easy to find scientific papers about, eg elephant knees, avian knees, cow and horse knees. DuncanHill (talk) 00:18, 25 February 2022 (UTC)

They probably meant to say that only primates have -well- primate knees. Elephants, on the other hand, have -mmm- elephant knees. They differ, for example, that our knees don't have an extended resting position (like horses' and so on).

Historically, it was a common misconception to believe that animal had no knees, and could not stand-up if tipped over: see Cow_tipping#Historical_origins. Bumptump (talk) 14:15, 25 February 2022 (UTC)

I mean, this is strictly a matter of word-choice: who defines the word "knee," and on whose authority shall we accept this definition? Suppose a zoologist defines the word "knee" differently than an orthopedic surgeon (let's say, a surgeon trained for human anatomy, as opposed to a veterinary surgeon...)?

I'm "going out on a limb" here, but I think this is exactly the reason why anatomy textbooks use all those "absurd" Latin-esque words: we say "femur" instead of "the big leg bone" because the word femur is more precise, more specific; and if we study structures using the concepts of comparative anatomy, we speak of homologous structures and analogous structures. So, one does not say "the bee has a knee"; nor does one say "the bee has no knee"; rather, one says "the bee, as an arthropod, has a jointed limb structure with similarities to, and differences from, the jointed limb structure of a vertebrate." Or, we say, "the absence of a tibiofemoral/patellofemoral articulation in the arthropod is obvious and therefore unsuited to protection under American intellectual property law."

And then, the scientist is misunderstood by the general public, because that formal written style is unacceptable to many of today's readers, even when it is meant to be humerus...

Nimur (talk) 15:23, 2 March 2022 (UTC)

Gravity Question (I couldn’t come up with a better title)

Is there a name for the point between any two celestial bodies A and B, that any object C placed at that point will be equally affected by the gravity of both celestial bodies (feel the same amount of newtons of gravitational force from both celestial bodies)? Also, is there an equation to calculate how far said point is from the two celestial bodies, provided that they’re M meters apart, Body A has a mass of K kilograms, and Body B has a mass of H kilograms? Primal Groudon (talk) 04:22, 23 February 2022 (UTC)

I've come across "gravitational midpoint" used with this meaning -- either in reference to a point on the straight line between A and B, or to a point on the orbit that object C is following that brings it near both bodies (such as a spacecraft from the Earth to the Moon).

Newton's law of universal gravitation tells us that if object C is at the gravitational midpoint on the straight line between A and B, and is at distance X from body A, then K/X² = H/(M-X)². This simplifies to give a quadratic equation that can be solved for X. --184.144.97.125 (talk) 05:51, 23 February 2022 (UTC)

Lagrange point? --←Baseball Bugs ^{What's up, Doc?} carrots→ 05:59, 23 February 2022 (UTC)

Wrong, that's a different concept. --184.144.97.125 (talk) 21:01, 23 February 2022 (UTC)

Hence the question mark. --←Baseball Bugs ^{What's up, Doc?} carrots→ 05:15, 26 February 2022 (UTC)

I'm not aware of any name of this gravitational equilibrium point between two stationary masses. Yes, there an equilibrium point, and 184.144... has given the equation for it, but it's usually not a very interesting point. It's an unstable equilibrium and it only remains there when the the masses are kept stationary, despite the gravity they exert on each other. In space there's no way to keep these masses at fixed positions. They can be kept at a fixed separation if they orbit each other. In that case there's a new unstable equilibrium point, called the L1 Lagrange point, where the test mass can orbit with the other masses, but this is displaced from the point of no force towards the heavier mass. PiusImpavidus (talk) 09:23, 23 February 2022 (UTC)

Two stationary masses will not remain stationary for long. If there is no relative motion between them, they begin to will move towards each other along a straight line with a relative acceleration proportional to their masses, until they crash into each other. That's how gravity works. --Jayron32 13:43, 23 February 2022 (UTC)

I'd have thought that it gets a bit more complicated that that. Bodies A and B will be in orbit around their combined centre of mass. In the case of the Earth-Moon system the barycentre is just over 1,000 miles below the Earth's surface. In such a case as this an object at the gravitational neutral point would be in orbit around the Earth and the circular path would contribute its own forces to add to the simple linear equation above. I beg to differ slightly with PiusImpavidus; in the media it was a very interesting point when the Apollo spacecraft started to accelerate towards the Moon (or the Earth on the return journey). IIRC it was a headline over the lead story for Apollos 8 and 11. Martin of Sheffield (talk) 16:10, 23 February 2022 (UTC)

If they have existing motion, then yes you are correct. I was making the point about stationariness (and the lack thereof ion the real world) that PiusImpavidus stated when he said "gravitational equilibrium point between two stationary masses". There is no equilibrium between two stationary masses that are not in contact; they will move towards each other. Such equilibrium (whether stable or meta-stable) will only exist if the bodies are in relative motion, at which point the most common stable set up is some kind of elliptical orbit. That orbit has lagrange points. If you were to place two bodies in space stationary to each other, they would start moving towards each other instantly (well, speed-of-light instantly, but you know). --Jayron32 17:04, 23 February 2022 (UTC)

The Apollos entered the Moon's Hill sphere at that point (in time!). This is delimited by a surface, not defined by a point (in space). --Wrongfilter (talk) 16:23, 23 February 2022 (UTC)

The intersection of a surface and a line (the trajectory) is a point though! Martin of Sheffield (talk) 17:47, 23 February 2022 (UTC)

In the lab it can be done. You can put two large lead spheres on a very strong table, put a test mass inbetween and measure the horizontal force on the test mass. The force will be small, nanonewtons, but it can be done and there will be an unstable equilibrium point where the force on the test mass is zero. Such force measurements can be used to determine the gravitational constant. The lead spheres won't move that easily. But with celestial bodies, no way to keep the masses stationary. PiusImpavidus (talk) 09:17, 24 February 2022 (UTC)

is the Huisheng (Kengan Ashura) have a basis on reality?

So in the Kengan Ashura manga, there's a technique called Huisheng. (more details in the wikia article here.

TLDR version of the technique is that a person tells a child all about himself (memories, likes, dislikes etc.) several times in his lifetime until he dies. In turn that child, now a grown up, will pass it to the younger generation. Of course you can also use audio recordings and earphones for more convenience. The point of the technique is that the personality is passed down orally through the generations, making it some sort of pseudo immortality for the original personality.

Does a similar technique exists IRL? --Lenticel ^(talk) 06:28, 23 February 2022 (UTC)

If audio recordings are used, one can skip a few generations. And why tie this to spoken text? One can also write it down. This technique exists IRL and is known as "autobiography".  --Lambiam 10:48, 23 February 2022 (UTC)

Oral tradition is the term, and is known in some form in just about every culture throughout history. --Jayron32 13:40, 23 February 2022 (UTC)

Thanks, just with a little brainwashing thrown in I guess. --Lenticel ^(talk) 01:17, 24 February 2022 (UTC)

Another fictional version is Fahrenheit 451 (1966 film), in a dystopian future where all books are banned. The Book People hide in a forest, each memorises an entire novel before destroying the physical book and then has to teach it word-for-word to a child before they die. Alansplodge (talk) 09:53, 24 February 2022 (UTC)

That's a nice movie article. Thanks for the recommendation. --Lenticel ^(talk) 09:50, 28 February 2022 (UTC)

Is there any equation for tidal heating in a subsurface ocean?

I've been looking for an approximative tidal heating equation akin to the one presented at Tidal heating

$${\dot {E}}_{\text{Tidal}}=-\operatorname {Im} (k_{2}){\frac {21}{2}}{\frac {GM_{h}^{2}R^{5}ne^{2}}{a^{6}}}$$

for bodies with a subsurface ocean, but I can't find one. Publications about Enceladus, Europa and the like don't provide one, either, only solutions specific for these bodies and these aren't general enough. Jo-Jo Eumerus (talk) 10:05, 23 February 2022 (UTC)

There are a bunch of results from Google Scholar. Have you gone through them and not found what you're looking for? GeorgiaDC (talk) 04:54, 24 February 2022 (UTC)

No, all what I found are calculations specific for one moon or the other. They aren't generalizable. JoJo Eumerus mobile (main talk) 13:06, 25 February 2022 (UTC)

Academic Union Oxford; What's it?

Is this organisation an academic body of any academic standing? https://oau.ebaoxford.co.uk/ Recently they seem to have conferred some honorary position on one Sabu Thomas who blows his own trumpet rather too loud, if his fancy website http://www.sabuthomas.com/ is any indication. (The article on the scientist could be autobiographical. Most of the contributions come from the IP of the institution the subject works at. Then there is a contributor named Iiucnn who seems to have made no contribution to any other article.) — Preceding unsigned comment added by Narrativist (talk • contribs) 15:23, 23 February 2022 (UTC)

See this press release. It does not read like a rejoinder by a respectable organization. Not only is the style that of a scam artist who is exposed, but the content is demonstrably dishonest. As reported, journalist Prega Govender of the (South African) Sunday Times was indeed rebuked, but not for an article about this Academic Union Oxford. The rebuke was for an article about a new sex-ed textbook rolled out in South Africa. Govender did write an article under the title "Oxford scam artists turn Bloemfontein principal into a fake prof";^[1] the "scam artists" are the AUO. It is behind a paywall, but the basic facts revealed in the article can be found here.  --Lambiam 16:57, 23 February 2022 (UTC)

Thanks a lot, Lambiam. Guess that unmasks the organisation enough.--Narrativist (talk) 17:25, 23 February 2022 (UTC)