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February 21

what is the U-shaped piece of wire in the front for?

example: https://en.wikipedia.org/wiki/File:Makita_DJV181.jpg All (newer) power jigsaws seem to have it. My old tool (90's) doesn't. What is this thing for? Thank you in advance 78.50.150.23 (talk) 12:40, 21 February 2018 (UTC)

As a general rule of thumb, modifications along the lines of "all the old ones don't have this, and all the new ones do, and I can't quite see what it does" are often safety-related, and result from either a regulatory change or an expensive lawsuit. (Rarely, changes to improve the safety of equipment result from manufacturer initiative—but that's not usually how capitalism works.)

In this particular instance, the bit of wire is a safety wire or guard wire (see the parts diagram and list, item 10). It's to help keep stuff – stray junk, power cords, the user – clear of the moving parts of the jigsaw. I expect a wire is used rather than a solid window because it's light, it's cheap, it's durable, and it won't get clogged up with sawdust. TenOfAllTrades(talk) 12:56, 21 February 2018 (UTC)

Thank you! I had downloaded like 3 manuals from different companies (Bosch, Einhell and Ryobi) but none had such a nice diagram. The purpose of the wire is not immediately obvious so I was wondering. Thanks again! 78.50.150.23 (talk) 13:28, 21 February 2018 (UTC)

It just kills me the old saw has become unusable because of a tiny part (blade pick-up clip) they for some reason chose to make from some brittle zinc or aluminium type stuff... and that I could probably machine myself -- if I had the tools 78.50.150.23 (talk) 13:40, 21 February 2018 (UTC)

What is a direct-view portable TV?

I don't have a lot of time when I'm at libraries to search for these things. But Wikipedia doesn't seem to have the information. Here is where I saw the term. At home, I try to stay away from unfamiliar sites.— Vchimpanzee • talk • contributions • 21:37, 21 February 2018 (UTC)

Philco Safari

• Literally, it's quite simple: it means it was a portable TV where you looked directly at the screen. Seems obvious now, but this wasn't how the previous ones had worked.

In particular, Sony's was the second portable TV (counting the well-known ones) and the first had been the Philco Safari [1]. This used a tiny tube, and to make it appear larger it was set inside the case, projecting onto a screen. To make the case more compact, that in turn had needed the tube to be pointed vertically and the image turned with a mirror. Sony achieved a couple of things: the screen was big enough width-wise to be visible when viewed directly, it was still short enough (the deflection angle needed to be big enough, despite the limits of tube technology) to not be a huge case and then it was still efficient enough to work as a portable set. Andy Dingley (talk) 21:59, 21 February 2018 (UTC)

Thanks for the intelligible reply Andy Dingley, could something be added to our History of television article about non-direct-view TV sets like these for example as there doesn't seem to be any mention at present? Alansplodge (talk) 13:02, 22 February 2018 (UTC)

That was my objective as well. Thank you. My first memory of Sony was of a very small TV in a restaurant.— Vchimpanzee • talk • contributions • 19:29, 22 February 2018 (UTC)

February 22

Missed approaches

How come almost all of the holding patterns charted at the end of missed approach procedures are set up in such a way as to require a parallel entry (or in a few cases a teardrop entry), but hardly ever a direct entry? Is there a reason for this? 2601:646:8E01:7E0B:0:0:0:64DA (talk) 07:56, 22 February 2018 (UTC)

IANAP, but naĩvely it would seem to me that the paths flown to achieve a parallel entry would minimise the likelihood of conflict or collision with other aircraft in the vicinity. However, doubtless a more knowledgeable contributor can shoot me down in flames! {The poster formerly known as 87.81.230.195} 90.220.212.253 (talk) 14:58, 22 February 2018 (UTC)

I know nothing about this, but this site seems to have useful graphics that are more detailed than the ones in holding pattern. If I understand #3 rightly, the pilot who misses an approach has to turn around and fly back to the holding area. The holding area appears to be set up with pilots already in a holding pattern flying toward the navigational fix in a straight line while they're flying toward the airport, hence one flying away from the airport is going the "wrong way" to do the direct approach and has to do something else. Does that make sense? Wnt (talk) 17:18, 22 February 2018 (UTC)

I'm not sure the claim is accurate - maybe I'll do a quick survey of the plates for approaches I commonly use, and see if "most" or "almost all" missed approach procedures do satisfy this claim; but here are a few observations:

• as a proficient instrument pilot, you should be equally comfortable with all entries to the hold - so it doesn't really matter;
• ATC doesn't care how you enter the hold as long as you stay in protected airspace. I just checked the current AIM, and it says: "other entry procedures may enable the aircraft to enter the holding pattern and remain within protected airspace, the parallel, teardrop and direct entries are the procedures for entry and holding recommended by the FAA, and were derived as part of the development of the size and shape of the obstacle protection areas for holding." (5-3-12 3.(d)). However, I'm certain that previous versions of the AIM actually had text to the effect that the PIC could select any entry they liked, so long as that procedure remained in the protected airspace. I can't find that quote in the October 2017 edition.
• Missed approach points, published entry procedures, holding points, and so on, are designed to guarantee protected airspace does not overlap (and obstacle clearance requirements are met). They aren't primarily designed to be easy to fly!
• On a practical note - if you're operating in a RADAR environment, you'll frequently receive RADAR vectors long before you reach the hold or have to fly a hold entry or course-reversal of any kind (unless you specifically ask to fly the published missed). Obviously, don't expect vectors unless you are instructed to expect, (§91.185(c)(1)(iii)).

Be sure to consult the AIM (5-3-8), and the Instrument Procedures Handbook (Chapter 4, in particular), and always check with a CFII if you're really unsure.

Nimur (talk) 21:38, 22 February 2018 (UTC)

As far as the accuracy of the claim: it's true for the missed approaches for the Sea-Tac VOR Runway 34R approach (parallel), Paine Field VOR Runway 16R (parallel), Yakima VOR/GPS-A (parallel), Pullman/Moscow VOR Runway 5 (parallel), Arlington LOC Runway 34 (parallel), Juneau LDA X Runway 8 (parallel), Paine Field ILS Runway 16R (parallel), Hoquiam VOR/GPS Runway 6 (parallel), Montgomery Regional RNAV/GPS Runway 28 (missed approach course is exactly opposite to the holding course inbound -- so either parallel or teardrop entry at the pilot's discretion), Boeing Field ILS Runway 13R (same as at Montgomery), Portsmouth (Pease) RNAV/GPS Runway 16 (ditto), Bremerton GPS Runway 1 (teardrop), Tacoma Narrows ILS Runway 17 (parallel), Tacoma Narrows NDB Runway 35 (parallel), Olympia VOR-DME/GPS Runway 35 (teardrop), Salem LOC BC Runway 13 (same as at Montgomery, Boeing Field and Portsmouth), and the infamous Yakima LOC/DME BC-B approach (parallel); the only exceptions I know of are Shelton GPS Runway 23 (direct), Oak Harbor RNAV/GPS Runway 7 (direct, although at a rather sharp angle) and Friday Harbor RNAV/GPS Runway 34 (direct). And that's all the approach plates I have with me (unless you count the really old ones for Berlin Tegel and Tempelhof from the days of the Berlin Airlift), but you see the pattern, don't you? 2601:646:8E01:7E0B:0:0:0:64DA (talk) 08:06, 23 February 2018 (UTC)

I have seen "some" published procedures that imply direct entry; other procedures do not. Have a look at all the plates for KSJC - for example, the approaches for the 12 runways have a missed approach hold at GILRO, but each hold is different. I'll reserve comment on statistics until I have reviewed enough procedures to be statistically significant! I've browsed maybe a dozen plates, and that's hardly enough to merit a bold claim - but with a booklet full of paper plates, one can quickly review hundreds of approach procedures!

If you don't have paper copies, you can search one-at-a-time via d-TPP (or Foreflight). But consider purchasing the paper copies! Electrical failures are real, not-hypothetical occurrences. Just nine dollars will buy you the better half of California.

I've got a paper copy of all the plates for Northern and Southern California, so I can collect some actual statistics when I've got time... it's Friday, and studying missed approach procedures is a great way to start the weekend!

Nimur (talk) 16:26, 23 February 2018 (UTC)

evolution and human Reproduction special case; specially subordination of lunar cycle

Reproduction of many many animals occurs as they have season for fertilize and baby birth ,as we know human fertilize time occurs in all over the year. the question of evolution specialists is that when in human evolution process it had begin? how man forget year duration periodic fertilize ,the women monthly period may be remained from that time. kindly guide me please. --Akbarmohammadzade (talk) 09:05, 22 February 2018 (UTC)

This book, page 144, does seem to answer your question, and contains citations to more detailed studies. To wit "Early humans also probably had seasonal birth peaks, as these persist in modern day humans" Which is to say, humans do have a vestigal mating season, as evidenced by the fact that births are not evenly distributed throughout the year, but vary with the seasons; This article for example suggests that there is a nadir of birthrates in the spring owing to lower sperm counts during the warmer summer months. --Jayron32 11:48, 22 February 2018 (UTC)

Our closest article is at concealed ovulation. The two things are not completely synonymous, but obviously related; a female could not be said to be "in heat", if there was no way of detecting that it happened. Matt Deres (talk) 13:40, 22 February 2018 (UTC)

• To describe the issue as one of "forgetting" to have a mating season is to miss the issue. It begs the question of why there are mating seasons in the first place. That is, why aren't animals always fertile? The human system is one of nominally constant frtility, limited by the fact that we are mammals, and hence tend to develop and shed a uterine lining conducive to implantation (menstrual period), and we release an egg (ovulation) in synchronization with the receptive part of that cycle. That the cycle is 'hidden' as mentioned above encourages male mate loyalty; at least the attentiveness of a 'steady date' who will be a potential father to one's children, rather than just a random hook-up, which could have disastrous consequences for an unwed mother both as a social stigma and due to lack of biparental support through much of human history.

Animals have mating seasons mostly based on the availability of food and the advantage of producing offspring all at once, for which see masting. Humans who breastfed their children also do not tend to ovulate during this period, to avoid the burden of having children back to back, rather than, say, every two years, when the newborn's elder sibling will at least be of the age to eat and walk without having to be carried and manually fed non-adult food. Putting the question in terms such as "forgetting" a breeding cycle sounds like old-wives tales or folk religion, not evolutionary biology. μηδείς (talk) 20:14, 22 February 2018 (UTC)

thank you -and about lunar cycle ; and how human period obey moon rotation cycle , why accurately lunar based duration ?--Akbarmohammadzade (talk) 05:25, 25 February 2018 (UTC)--Akbarmohammadzade (talk) 05:08, 25 February 2018 (UTC)

It's not entrained to the "moon rotation cycle" at all. The moon rotates in the same time that it revolves (relative to the stars). The strength of the tides (which have some influence on the human body, which is 97% water) depends on the distance. The intervals between closest approach (perigee) are slightly longer. The major factor affecting the strength of the tides is the angular separation between the moon and the sun - the time between succcessive alignments is longer still. 86.131.187.242 (talk) 12:41, 25 February 2018 (UTC)

You might like to have a look at Culture and menstruation. "Menstruation" and "moon" have an etymological link (not only in English) and people around the world have asked the same question as you, about the similar length of the lunar and (human) menstrual cycle. As for times of the year, women generally only ovulate when they have enough fat to sustain the pregnancy. Modern athletes often cease to menstruate, and in times of famine, so too would our ancestors. Carbon Caryatid (talk) 16:16, 26 February 2018 (UTC)

The association seems reasonable. Remember, the rod photoreceptors in the human eye are basically designed to see by moonlight. They don't seem much good under the stars alone, at least. In an era before artificial lighting, the moon might have meant an all-night party. And what do people do to pass the time? Wnt (talk) 13:05, 27 February 2018 (UTC)

February 23

Olive oil

How much alpha-linolenic acid does 1 tablespoon of olive oil have in it? --User777123 (talk) 02:57, 23 February 2018 (UTC)

The answer is there in olive oil, under constituents. Matt Deres (talk) 04:22, 23 February 2018 (UTC)

WP is not a reliable source!

Try http://nutritiondata.self.com/facts/fats-and-oils/509/2 which has great resources. It is not loading for me pending a flash update, so I have no info, but this was my goto site when I went from a 39 to a 23 BMI. μηδείς (talk) 04:35, 23 February 2018 (UTC)

No one said it is. But it is supposed to cite them. And the article does cite a source for the info. If you feel it is not a RS per wikipedias definition then tag it as {{unreliablesource}}. If you have your own weird definition of reliable source, you should at least provide it before asking. Nil Einne (talk) 08:25, 23 February 2018 (UTC)

Medeis's link, which opened OK for me, does not actually list the different types of fatty acid. The source quoted in the article looks reasonable: it does seem to give a very thorough description of the chemistry, and quotes other sources for that data. In any case, the answer seems to come down to a range of 0% to 1.5% - which clearly suggests that the actual level will vary according to variety and location. The bottle in my kitchen doesn't include that information - it just lists saturated and unsaturated fatty acids. The implication may be that the identical bottle, bought somemonths apart, could have a slightly different chemical composition. Wymspen (talk) 12:02, 23 February 2018 (UTC)

An educated guess would be that it varies by commercial grade (virgin, extra-virgin, etc.) and probably also by cultivar used. Matt Deres (talk) 16:13, 23 February 2018 (UTC)

Olive oil seems to contain between very little and nothing at all of it. Maybe you are interested in this table of oils from seeds, with alpha-linolenic oil (ALA) content (average percent, first table from above): http://www.my-personaltrainer.it/nutrizione/alimenti-grassi-essenziali.html Language is not English but you have the Latin name of all plants. Olive is missing by the way. It is funny that many articles and ads about olive oil cite with much ballyhoo this ALA as an important constituent, but not the quantity contained. The only one somewhow doing so says "so much ALA as in mother milk", but again without figures. 194.174.76.21 (talk) 20:04, 23 February 2018 (UTC) Marco Pagliero Berlin

I just noticed there's some explanation in the source.

Regarding the polyunsaturated fatty acids (PUFAs), there is a wide range acceptable for extra virgin olive oil, however the linolenic acid has to be less than 0.9% per the International Olive Oil Council (IOOC) guidelines. Higher levels, e.g. 1.5%, do not present a nutritional problem, but the IOOC uses the linolenic acid level to establish the authenticity of the olive oil. Seed oils like canola oil have higher levels of linolenic acid.

So if you're getting quality olive oil, it should indeed be low. Nil Einne (talk) 00:02, 24 February 2018 (UTC)

Actually it does list different types of fatty acids. It just doesn't show them by default. (I noticed this while evaluating it.) If you click more details under 'Fats & Fatty Acids', it should show a list. That said, it only shows 18:03 which I assume to mean Linolenic acid. They don't list anything for 18:3 n-3, c,c,c or 18:3 n-6, c,c,c, so it may seem we don't know if it's alpha-Linolenic acid or gamma-Linolenic acid. Except that 'Total Omega-3 fatty acids', is the exact same as 18:03, strongly suggesting it's all alpha/n-3. I don't know why 18:03 rather than 18:3 as they do for all the other ones as well as the subtypes for 18:3. It seems we're not the first to wonder this [2] (who also come to a similar conclusion about alpha vs gamma). I did initially question that source, however their data is supposedly from USDA SR-21 so it may not be too bad even at a minimum their interpretation and presentation seems confusing. Anyway so their number is 0.761% which is within the range the source our article uses provides. (I initially thought it was around 1.6% but the default values are for 1 cup, not for 100g.) Nil Einne (talk) 00:00, 24 February 2018 (UTC)

See also here. Count Iblis (talk) 00:33, 24 February 2018 (UTC)

Current Scientific opinion on Piaget and Kohlberg

I’ve been doing research on ethical theory from the standpoint of evolutionary theory, but I want to take in ideas from any discipline that have something scientifically rigorous to add. I’m very familiar with the philosophical literature on ethics but not with the psychological research. I recently came across Kohlberg’s work. I’ve just started to look at it but so far I’m very impressed and he seems to have some good empirical evidence. I’m wondering is there any modern scientific consensus on his work? I’m mostly interested in Kohlberg but I include Piaget because from what I understand their two approaches are closely related. Are there any major Issues with Kohlberg’s work? Are there competing theories I should focus on? I’m interested only in scientific approaches, and theories specifically about ethical development not general theories of personality or development. I’ve looked at Moral Foundation’s Theory. I looked at the Wikipedia page on Kohlberg’s ethical theory but was wondering if people had anything more to add that isn’t in the article, even non-encyclopedic POVs ;) Any feedback would be appreciated --MadScientistX11 (talk) 06:05, 23 February 2018 (UTC)

Are you talking about Jean Piaget and Lawrence Kohlberg? ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:00, 23 February 2018 (UTC)

@Baseball Bugs: Sorry, I thought it would be obvious from the context but yes Jean Piaget and Lawrence Kohlberg. --MadScientistX11 (talk) 15:56, 23 February 2018 (UTC)

Since ethics is part of the field Philosophy you should move your question to Wikipedia:Reference desk/Humanities. --Kharon (talk) 18:33, 23 February 2018 (UTC)

Since Kohlberg's work was in the field of developmental psychology,which is" the scientific study of how and why human beings change over the course of their life," the question is located fine right here. That assumes that Kohlberg did experiments or some sort of science-based observation rather than just sitting in his office philosophizing. Edison (talk) 22:06, 23 February 2018 (UTC)

@Edison: Thanks, yes I agree, Kohlberg was a psychologist not a philosopher and I think my question belongs here. One of the things that surprised me was that he did indeed do experimental work where he tried to test his theory of moral development. For example, he had subjects who he followed at various points in their lives and had them respond to stereotypical scenarios (was what this person did wrong? if so why?) and then he had other researchers independently rate the answers based on his model and he achieved pretty strong consistency across evaluators and some good evidence that supports his model of how moral values evolve as people age. Similar to Piaget's theories about child development and my understanding is that Piaget was a strong influence on Kohlberg. I found it interesting because Kohlberg's work is pretty old and in the more modern work I'm reading now in evolutionary psychology he's not mentioned that much even though he seems to be as or more rigorous as much of the work I'm reading now. But I was also wondering, if there was some later work that pointed out major flaws in his methodology or otherwise. Sorry, I'm rambling on here, since no one has spoken up I'm going to follow up on my own and read some more of his work, but I'm still interested if people have any opinions either for or against his approach. --MadScientistX11 (talk) 18:37, 26 February 2018 (UTC)

Is Iodised salt broken in water?

If I put Iodised salt in water it's broken into salt (NaCl) and Iodine? 185.191.178.183 (talk) 16:40, 23 February 2018 (UTC)

There's a lot of chemistry to correct there. Let's see how much I can get through in the space of this forum. Iodised salt does not contain a mixture of sodium chloride and elemental iodine, it contains a mixture of sodium chloride and some other salt of iodine (i.e. an iodide salt or iodate salt such as potassium iodide or potassium iodate). Secondly, the word broken is imprecise here. All salts, when they dissolve in water, dissolve through a process called solvation, which includes a sort of "breaking"; ion-ion interactions in the solid salt are replaced with ion-solvent interactions in the solution. I suppose that involves "breaking" of a sort. Both the sodium chloride and potassium iodide undergo this process. --Jayron32 16:47, 23 February 2018 (UTC)

Thank you for the info. I'd like to know please, if this solvation has to do something with temperature? (the higher temperature the dissolving faster - for example) I'm asking because I want to understand if I'm adding iodised salt to food while cooking, it's dissolved in the water to be potassium and iodain separately and goes to the blood via the digestive system. 185.191.178.183 (talk) 03:24, 24 February 2018 (UTC)

Yes, all ions are dissociated from each other when in solution. The ion channels responsible for transport of ions across membranes are highly selective, and your body will absorb and process potassium ions and iodide ions in different ways; your body does of course use iodine ions so it has ways of dealing with them, but yes, when they enter your blood stream, the potassium ions and the iodide ions are dissociated from each other in solution, the same way they would be in any water-based solution (blood being a water-based solution. This article describes specifically how iodide is absorbed. --Jayron32 03:58, 24 February 2018 (UTC)

Can someone reconcile the caldera and volcanic crater articles?

The former claims the two terms are distinct, while the latter claims the former is a subtype. I'm sure there are disagreements. 161.185.160.21 (talk) 17:23, 23 February 2018 (UTC)

I do not think that volcanic crater article claims that caldera is a subtype of volcanic crater. Ruslik_Zero 20:36, 23 February 2018 (UTC)

I've changed "crater" to "depression" in the volcanic crater article to help make the distinction. Does this help? Dbfirs 20:50, 23 February 2018 (UTC)

Clinical trial paradox

Not seeking medical advice (this originated as a math-related question rather than a medical one) but science desk seems better because I'm wondering how actual drug trials and the medical profession deal with the issue.

There's a syndrome S for which there are a bunch of prescription drugs D₁, D₂, etc. If you treat S with a placebo, about 25% of patients get better. Similarly if you treat S with D₁, about 25% get better, and the same with D₂, with D₃, and so on. The obvious reaction is "well those drugs are no better than placebos, they're being overmarketed by greedy pharma companies bla bla, the only cure is clean living so quit those filthy habits like editing Wikipedia!". In fact I think the FDA generally won't approve drugs that don't outperform placebos in clinical trials, which seems logical.

The above seems paradoxical for the following reason. Experiment might show that placebos are the same as a class: if placebo P₁ doesn't work then P₂ won't either, while the drug actions are much less correlated. So the medical strategy doctors observably use for treating S is: try drug D₁. If it doesn't work, try D₂, etc. If the drugs are independent, then after k of them, $1-\left({3/4}\right)^{k}$ of the patients should get better, and in the $k\rightarrow \infty$ limit, all of them should get better. So the drugs are (collectively) useful after all. In fact the same could be said even if the clinical trials showed each drug to be *less* likely to help than a placebo.

Drug trials afaict usually test one drug at a time, or there might be a multi-armed test where different sub-populations get different drugs. Do they ever test correlations between drugs? Is there a way to do that without testing multiple drugs on the same patients? Is this a known issue in the clinical trials field and something that gets reasonable study? It's obvious that doctors know about it informally which is why they try multiple drugs (this question is inspired by something I saw on a medical blog) but maybe there's a disconnect. Thanks. 173.228.123.121 (talk) 20:38, 23 February 2018 (UTC)

If a drug works even for some patients, it will still work better than placebo on average. The situation that you describe can only be possible if the drug actually makes the syndrome S worse in some patients. Then it will be up to researches to devise criteria when the drug should be used and when not. Ruslik_Zero 20:56, 23 February 2018 (UTC)

I don't see why a sometimes-working drug would be better than placebo on average (maybe it helps 20% of patients and leaves 80% with their condition unchanged or makes them worse, while placebo improves 25% and does nothing for the rest). But anyway that's a corner case. The controversy I see is that drugs get approved after (maybe only slightly) beating placebo, and then there's reproducability concerns or claims that the effect wasn't statistically significant enough, etc., yet doctors still consider the drug to be a useful part of their toolbox.

I do think it's normal to dispense drugs that have a potential for making the illness worse (or even for causing illness, like vaccines given to healthy people). It's just a trade-off like anything else. 173.228.123.121 (talk) 00:12, 24 February 2018 (UTC)

You still seem to be conflating different things. A drug which doesn't help 80% of patients is a very big difference from a drug which makes the condition worse in 80% of patients. If the drug helps enough patients without making things worse in the rest, it's easily possible this improvement will be picked up even in a simple statistical test of all patients. Also I think you're overestimating the number of drugs which are approved which are known to make the condition worse in a subset of patients, especially if we're talking about something which is easily measured e.g. blood pressure, cholesterol levels, uric acid levels. More likely the drug has known side effects which sometimes could be worse than whatever you are trying to resolve. Also some drugs have a narrow therapeutic range and require careful monitoring because you may overcorrect what you're trying to adjust. E.g. warfarin. BTW, drugs are not always tested against placebos even in the initial approval stages. They may be tested against some existing drug for the same condition. Nil Einne (talk) 01:34, 24 February 2018 (UTC)

Patients don't know whether they're being given the placebo or not. So the same 25% that heal through the power of placebo will heal anyway, leaving us to assess the drug's usefulness on the basis of the other 75% (in the long run of course). 93.142.92.135 (talk) 01:33, 24 February 2018 (UTC)

Nil, I think you're focusing on about the least relevant part of the question, but obviously just as a math tautology, a drug can cause expected worsening of a condition and still be worth trying, depending on how bad the potential worsening is and whether it's reversable. E.g. if I'm nearsighted and I'm offered a drug that has 20% chance of making my nearsightedness better and 80% chance of making it worse (but it must taken every day to continue having any effect at all), then of course I'll want to try it. If it works, great. If not, I stop taking it and I'm back where I was. If there are 10 such drugs each with an independent 20% chance of helping, I try one after another and have a 90% chance of eventually finding one that helps, seems like a good deal. (That's obviously an artificial conceptual example rather than a common practical situation).

The question is simply about a line of reasoning that I see a lot, that goes: "Drug X was compared against a placebo (or against drug Y) in a clinical trial for treating disease D and it didn't do significantly better according to standard statistical tests. Therefore, drug X's claimed usefulness against disease D is unsupported by sound evidence, the actual experiences of patients getting better after taking it should be treated as meaningless anecdotes, and doctors should quit prescribing it". That reasoning is tempting (I believed it myself without thinking about it much) but on closer inspection it's clearly fallacious. So I'm wondering how recognized the fallacy is, whether it's taken into account in trial processes, etc. Of course I recognize that this may be far down in the weeds compared with other sources of bias in those experiments, usually in the other direction. 173.228.123.121 (talk) 03:17, 24 February 2018 (UTC)

I never said it's impossible. I'm simply said it's anywhere as common as your response seemed to suggest. (Not simply because of how many drugs work, but also because if a drug is like that, for a lot of conditions it's probably going to be abandoned maybe fairly early on. And that you need to differentiate between the far more common example where are drug does not appear to improve the condition in some patients, and the case where the drug appears to make the condition worse for some patients since you seem to conflate the two in your first 2 responses. You need to properly understand the basics before you try and understand more complicated things. To give an obvious example you said "I don't see why a sometimes-working drug would be better than placebo on average". But as Ruslik_Zero had already said, and I emphasised, it should be fairly obvious why a sometimes working drug is often statistically better than a placebo on average if you understand the basics i.e. well statistics and that most drugs do not make the condition worse, and the placebo effect still applies to the drug. (The other IP's point on the placebo effect was another thing I noticed your response seemed to be confused about, but I didn't comment on.) Or to put it a different way, as long as you are confused by the basics, you are liable to be even more confused if you try and analyse more complicated things, you can't simply dismiss understanding the basics as the "leave relevant part". Nil Einne (talk) 03:45, 24 February 2018 (UTC)

P.S. No part of my responses are intended to suggest a drug will always be statistically significantly better if it works in some people even if it doesn't generally work in others despite not making things worse. Isosorbide dinitrate/hydralazine is an obvious example here [3]. A number of cancer drugs are also approved dependent on some genetic indicator either in the cancerous cells or the patient. (Although they may not have actually bothered to test it for effectiveness on others initially, depending on their understanding of the mechanism of action etc. One of the points here of course is that with better understanding of what we're trying to achieve and why, we are able to better guess whether it's actually going to work. So in some ways you're still actually targeting a specific problem aiming to achieve a specific outcome. It's just more complicated than 'lowering high blood pressure'. And something like 'malignant breast cancer' is actually very non specific.) Nil Einne (talk) 04:05, 24 February 2018 (UTC)

(edit conflict) Ruslik0 wrote "If a drug works even for some patients, it will still work better than placebo on average" which is a non-sequitur for reasons I explained. It might often be true in practice, but it's not a valid logical inference. I interpreted an unqualified "if X then Y" as "if X then necessarily Y as a matter of pure deduction", rather than as "if X then often Y in the context of how people tend to test drugs". If Ruslik0 meant it the second way then oops, I misinterpreted their words, but that's different from being confused by the basics. If they meant it the first way, then it's a math error as far as I can tell.

That placebos sometimes have a real effect is widely reported (Wikipedia's placebo article describes various mechanisms for this, though some studies claim the opposite;[4] I should update the article) and I've heard this is sometimes true even when the person knows they're getting a placebo.

The disconnect between reported experience and clinical trial results is substantial (the whole field of alternative medicine could be described as people taking stuff that failed clinical trials and saying that it works anyway). Some of it is obviously bogus but some could possibly be explained by the effect under discussion. I don't know how important the issue is, but I do get the impression that it's overlooked. Anyway I appreciate the responses though at this point we've probably run out of meaningful ideas. Thanks! 173.228.123.121 (talk) 05:46, 24 February 2018 (UTC) (Added: I see now that by "on average" Ruslik0 may have meant averaged over drugs, or averaged over trials, or whatever. I had read it as averaged over patients participating in a specific trial). 173.228.123.121 (talk) 05:53, 24 February 2018 (UTC)

No they were simply wrong since it might affect some people badly. Many drugs can have some bad side effects and it is a question of balance of the benefits against the possible side effects. Unfortunately people react a bit differently to things. You'll find a list of possible side effects on a leaflet with any packet of medicine. The assesment has been that the risk of these is acceptable compared to the benefits. The balance is normally tipped to the side of avoiding bad side effects and that will cause some drugs to not be approved unless a good criterion can be found for spotting when the bad effects might occur and avoiding them. It is a bad idea to use such drugs and risk being unlucky. Dmcq (talk) 10:23, 24 February 2018 (UTC)

I think that you misunderstand the statistics involved. If the probability of placebo effect in a given patient is $p_{1}=1/4$ (as you said) and the probability that a drug will help this patient is $p_{2}$ then the probability that the drug will make a difference is $P=1-(1-p_{1})(1-p_{2})=p_{1}+p_{2}(1-p_{1})\geq p_{1}$. The equality will mean that $p_{2}=0$, which implies that the drug is ineffective. So, in your hypothetical example when all drugs have the same efficiency as placebo, the efficiency of all those drugs is zero. So, regardless of how you combine them the efficiency that you will get is the efficiency of the placebo. Ruslik_Zero 18:54, 24 February 2018 (UTC)

Agreed. In a properly controlled trial, where the control patients are influenced to the same level of conviction that they're taking the real drug as the actual users of the drug, the placebo effect works equally on both sides. So, even for a terribly ineffective drug with 5% success rate, 25% of the patients (in your example) will heal from the placebo effect, and 5% of the other 75% will heal from the drug, giving a total of 28.75%. Technically, it's possible to have a drug's success rate lower than the placebo effect, for example if 15% of the patients heal and the other 85% experience horrible side effects negating the placebo effect. In that case though, this might create and ethical dilemma since giving the patients the drug appears to be worse than doing nothing. 93.142.90.67 (talk) 19:26, 24 February 2018 (UTC)

Ruslik0, sure, if you model it that way then that formula is valid. But IMHO it's an unwarranted assumption about what the drug does. I'd start with the experimental results. If you see 25% of the placebo patients and 20% of the drug patients get better (and it's a big enough sample that that's unlikely to be a chance result) you have to take the numbers as they come, rather than saying there's something wrong with them because they don't fit your model. You similarly can't rigorously infer that the drug didn't help any patients. There could be a confounder that shuts off the placebo effect for the drug patients, so the 20% that got better that really did get helped by the drug. I agree that this doesn't sound likely with real drugs, but it's ok mathematically as far as I can tell. 93.142.90.67 yes I think we're saying similar things, more or less. 173.228.123.121 (talk) 01:51, 25 February 2018 (UTC)

February 24

Weather control to prevent cold snaps and snowstorm s

Has anyone ever proposed weather control methods for preventing cold snaps and snowstorm s? — Preceding unsigned comment added by 73.118.181.38 (talk) 00:00, 24 February 2018 (UTC)

If by "cold snap" you're meaning the sort of unexpected late frost which damages seedlings and grape vines, then yes, they have. "Smudge pots" were a popular attempt, where orchards and vineyards would light smokey stoves between the trees. The smoke reduces radiative cooling, thus the temperature drop overnight. Andy Dingley (talk) 00:11, 24 February 2018 (UTC)

Andy Dingley's example and cloud seeding are about as far as this goes with our technology. Weather is pretty much impossible to control directly, as it is a system with many times the energy output that humanity can produce. Instead of spending energy in a futile effort to change the weather we typically tend to adapt our tools to let it do the work for us: e.g. windmills, wind turbines, hydroelectric power plants... Besides, cold snaps, snowstorms and other apparently extreme events can often be useful to the environment in unexpected ways. For example, sequoia relies on forest fires to procreate, while cold weather in the Arctic winter encases Alaskan and Siberian north coasts in ice, preventing coastal erosion by winter storms. 93.142.92.135 (talk) 01:27, 24 February 2018 (UTC)

I was about to reply with a supporting statement that cold snaps kill off insect pests, but a quick Googling has an amusing fifty-fifty split between articles supporting that it does and articles debunking that as a myth. Whether it kills them or not, my personal observation is that it does induce them to hibernate or otherwise quite being such pains. I quite look forward to it for that reason - and because a sharp frost typically stops my ragweed-induced hay fever. Matt Deres (talk) 15:08, 24 February 2018 (UTC)

Loss of cabin pressure

I was on a plane and they gave me a bread roll in a sealed plastic bag. I didn't eat it, but kept it.

During the trip, the bag got puffed up and under pressure, like a little football. Then it went back to normal.

I understand that the pressure in the cabin must have dropped. But is that normal and okay? Anna Frodesiak (talk) 00:58, 24 February 2018 (UTC)

Was the bag given to you on the ground? After you'd been in the air for a while? Nil Einne (talk) 01:03, 24 February 2018 (UTC)

The bag was given to me an hour into the flight, and it was baggy then. Then an hour or so later it was blown up. Then, hour(s) later, still in the middle of the flight, it went back to being baggy. Anna Frodesiak (talk) 01:08, 24 February 2018 (UTC)

AFAIK, barring stuff like damage, For a given commercial airline plane, cabin pressure will depend mostly on altitude, the higher the plane, the lower the cabin pressure. Nimur or someone would probably explain more, but I believe it's common for cruising altitude to increase over time as the plane gets lighter (burns up fuel), see [5] [6]. So it's not unexpected the cabin pressure will reduce, particularly if it's only been 1 hour. Altitude may also need to be adjusted for other reasons, particularly weather conditions. If you happen to have access (sorry I suspect you don't), you can see how the altitude and cabin pressure varied over one flight from London to Bangkok here [7]. Nil Einne (talk) 02:01, 24 February 2018 (UTC) 03:33, 24 February 2018 (UTC)

I don't think the pressure really depends much on altitude. As I understand it they pressurize the cabin to about 8000 ft for cruising, regardless of cruising altitude (which is obviously always higher than that). Used to be 7000, but 8000 saves on fuel, and makes the passengers slightly groggier and slightly less likely to cause trouble. --Trovatore (talk) 06:17, 24 February 2018 (UTC)

That mostly doesn't seem to be what the Respirology article found or says, unless I misunderstood it. Actually you don't need access to the whole source for this part although the graph may be useful, the abstract itself mentions "There was a linear fall in cabin pressure as the aircraft cruising altitude increased." That said, I did miss until now that this relationship only held during normal cruising, as the cruising altitude increased into the flight. On the 3 occasions the aircraft descended to a lower cruising altitude mid-flight, the cabin pressurisation was higher than predicted. (I'm pretty sure they mean in hPa, i.e. the pressurisation was at a lower altitude equivalent than expected.) It sounds like possibly they didn't have any occasions where the plane went to an unexpectedly high cruising altitude, so we don't know what would have happened then. And finally this was only during cruising, it doesn't apply during the descent and ascent phrase.

So my statement was at least a little wrong as the variation only seems to apply during normal cruising and probably won't apply during unexpected changes such as the weather example I mentioned. Also they only looked at 747-400 so it's possible other planes vary.

Note also in this study from 2004 to 2006, they don't seem to have generally reached the maximum allowed cabin pressure of 8,000 feet altitude equivalent. (Maximum seems to be what the sources use since they generally refer to the cabin pressure altitude equivalent.) The time frame suggests that this was probably mostly before high fuel prices really began to hit, and also before budget airlines really began to take off so these may have affected modern practices, even in full service airlines like those studied (Qantas, British Airways and a single Malaysian Airlines). And on that point, maybe only those airlines have/had these practices or even they only do/did it on the London to Bangkok route. Also it's possible that the airlines were aware that the study was being carried out and varied their practices. But I'm not sure as the study seemed to use a wristwatch altimeter and in-flight displays (which I take to mean any data shown on the entertainment systems).

That said, there also seems to be a push to higher Cabin pressurisation (i.e. lower altitude equivalents), e.g. the widely touted 6,000 feet for the 787. Our article also mentions something similar "A design goal for many, but not all, newer aircraft is to provide a lower cabin altitude than older designs. This can be beneficial for passenger comfort."

Our article also says "One study of 8 flights in Airbus A380 aircraft found a median cabin pressure altitude of 6,128 feet (1,868 m), and 65 flights in Boeing 747-400 aircraft found a median cabin pressure altitude of 5,159 feet (1,572 m)." referencing a 2010 study (albeit the link doesn't seem to work for me). So it doesn't seem that the findings of the earlier study on typical cabin pressurisations changed much in those 4-6 years. (I believe I looked into the issue of typical cabin pressurisations a bit more before, it's probably somewhere in the RD archives.) My memory is I found other stuff which supports the view the cabin pressurisation often doesn't reach the maximum allowed.

Anyway ultimately I don't know how well supported outside this study the finding on the variation of cabin pressurisation with cruising altitude during normal flight is either in the ~2005 period or nowadays. (My impression from vague memories of what I read before when looking into average or median cabin pressurisations is this is an area where there tends to be a lot of theory. What airliners actually do isn't well outlined or studied.)

Nil Einne (talk) 09:25, 24 February 2018 (UTC)

Hmm. Well, my phone can do both barometric pressure and GPS altitude, so maybe I'll start taking some data points and see what I can see. I suppose it's slightly less expensive to pressurize to 7000 ft if you're at 25000 than if you're at 35000, so if the airlines actually see any upside to pressurizing more than they have to, then I suppose they could. But I'm a little skeptical that they do — it's not something your average passenger tracks, so it probably doesn't give much competitive advantage. --Trovatore (talk) 09:58, 24 February 2018 (UTC)

Passengers rarely know it explicitly, but they feel more comfortable when cabins are pressurized to lower equivalent-altitudes. This is a major selling point for new airplanes like 787 and 777X: here's a blurb from Boeing.

Most of the time, the pilot configures the cabin pressure for "the lowest cabin altitude the plane can sustain," and that is basically limited by a few factors: how perfectly airtight is the cabin? How much energy can the engine bleed (e.g., the "air pumps") sustain? How much structural strength does the fuselage provide for the gauge-pressure difference between inside and outside?

As far as measuring this with your phone - I encourage the experiment, but I doubt it will work... I would not trust either the GPS altimeter and the baro-altimeter to accuracy of even +/- 1000 feet. Those kinds of consumer devices usually use software tricks and sensor-fusion to "improve" the data in a way that makes for very poor scientific instruments - especially when compared to a true altimeter. But if you collect data, please let us know! Nimur (talk) 17:46, 24 February 2018 (UTC)

I use the "GPS Status" app by MobiWIA, which reports the pressure in hPa, not as altitude. So it seems unlikely that it's doing any Kalman filtering or anything like that at the app level. I don't know enough about the internals to know whether the OS could be doing any such thing, but there's no obvious evidence of it as you watch the numbers change. --Trovatore (talk) 20:26, 24 February 2018 (UTC)

Don't pretend to know all the answers to this but here are some more likely possibilities. One doesn't want ones bread rolls to go pop. So when it was a football shape is was possibly out gassing (ie bags are not air tight). Second (you haven't mentioned which type of hairycraft you where in, nor the route) Most jet propelled aluminum cans in service today still use engine bleed air to pressurize the cabin. Bleed air is hot! So to increase the cabin temp, more air is vented out of the cabin (ie pressure is lowered) to allow more warm air to enter cabin from engine. To cool, venting is restricted and cabin pressure increases, slowing down the flow of warm air from engine. The battery in my slide-rule has gone flat so I can't use Log _e but fortunately there is this handy widget air-pressure-at-altitude-calculator. So by regulating the temperature, the cabin altitude pressure can fluctuate between 6000 to 8000 feet. Which equates to 11.78 to 10.92 PSI. Nearly one pound per square inch on the raper of said roll which has degassed whilst at lower pressure of 8000 ft. More than enough to deflate it. --Aspro (talk) 02:45, 24 February 2018 (UTC)

I once bought a soda at 700 feet and broke the seal at only 2 or 3 thousand feet and it burped fog. Air pressure's interesting stuff. (this was natural ground altitude) Sagittarian Milky Way (talk) 03:54, 24 February 2018 (UTC)

Very interesting indeed. Thank you all. I understand now that pressure changes within the cabin are nothing to worry about, and that strange things can happen. Many thanks to all. :) Anna Frodesiak (talk) 05:59, 24 February 2018 (UTC)

The flight attendants have to go through a canned routine set of announcements at the start of every flight, going through the "safety features of our Boeing 737 aircraft". One of the things they always say is ...although we never anticipate a change in cabin pressure.... Liars. Oh well. I know what they mean, and I guess that's what matters. --Trovatore (talk) 06:13, 24 February 2018 (UTC)

You will find they say something like "in the unlikely event of a cabin depressurization ..." The cabin pressure changes almost continuously throughout a flight, but uncommanded depressurization followed by deployment of the emergency oxygen system is indeed a rare event. Dolphin (t) 12:24, 24 February 2018 (UTC)

No, they don't. They say precisely "although we never anticipate a change in cabin pressure". I've heard it enough times that I can recite it in my sleep. This may vary by airline, but I guarantee you that that is word-for-word exactly what they say. --Trovatore (talk) 20:58, 24 February 2018 (UTC)

This seems to depend a lot on the airline and probably even the precise video. This American Airlines video for example says [8] "now, if the airplane loses pressure". This United Airlines video [9] "if necessary, an oxygen mask". (I came across another more standard safety video from United Airlines which said the same thing, but not linking to it because I'm not sure that Global Airline Safety Videos Youtube channel ensured they had the appropriate permissions for redistribution although I doubt UA will actually care. I had higher hopes for another video again with very similar wording albeit also fancy on Canal Plus Producciones's channel but looking a bit more they seem to be a relative small Venezuelan production company so I'm less certain. And I strongly doubt FrienldySKy made sure they had the appropriate permissions although the wording is slightly different on their old video.) This Virgin America one says [10] "if the cabin pressure's changin'". This KLM one says [11] "if there is a sudden decrease in cabin pressure". This Cathay Pacific one says [12] "oyxgen masks will drop automatically if they're needed". (An older one which coming from the production company which made it hopefully they know what they are doing [13] says "should oxygen be required". The smoking version says the same thing [14] although does remind you to extinguish your cigarette later.) Not sure what airline you are thinking of but I wonder if they still say that. With the modern trend of trying to produce fancy inflight safety videos, the would likely have a new one and perhaps the wording was changed. Nil Einne (talk) 02:14, 25 February 2018 (UTC)

Most of my flights are on Southwest Airlines. Yes, they definitely still say it. --Trovatore (talk) 03:17, 25 February 2018 (UTC)

Can't find any safety videos. But it seems unlike many other airlines who are competing to have some viral hit safety video, Southwest Airlines instead known for their flight attendants spicing up the safety presentations. So I can find videos of the flight attendants saying this [15] [16] [17] or similar [18]. Although not all seem to say it [19] [20] [21] [22] (while some of these look to be semi spontaneous or invented by the flight attendant, this one at least looks scripted probably by the airline since other than the fact she seems to be reading it, it sounds very similar to this [23] [24]). Nil Einne (talk) 11:51, 25 February 2018 (UTC)

Southwest doesn't use "videos", at least not on their bread-and-butter short-haul flights. The 737s have no video screens. --Trovatore (talk) 20:21, 25 February 2018 (UTC)

Loss of pressure can carry effects, ranging from "nothing" to "passenger discomfort" to "potentially serious physiological response," to "serious medical emergency"... Commercial airplanes never (intentionally) expose passengers to hazardous cabin pressure. In aviation, the two types that are of interest are rapid cabin pressure loss, and slow pressure loss. In the event of a sudden depressurization, most passengers will lose "useful consciousness" within a moment or two. (During my High Altitude Hypoxia training, I lasted almost 300 seconds at equivalent of 29000 feet altitude, with a little fuzziness on the "useful" bit - in this case "useful consciousness" is a technical term meaning that I had enough energy to put on my oxygen mask - barely - I had some blackout periods in there, and could not do basic aviation arithmetic correctly). If you're interested in developing a sense for what altitude hypoxia does, have a look at the famous Four of Spades video. While watching that training video, bear in mind that you are watching a high-IQ, high-adrenaline, college-educated Air Force officer describing his symptoms. Absent oxygen, your brain does things differently. If you want to survive a depressurization event, the only thing your brain must do is to remember is to put the oxygen mask on. If you can not remember to don the mask, it will not matter if you are awake. If the depressurization occurs slowly, you might not notice the onset of hypoxia, and you may lose useful consciousness without noticing it. If the crew informs you to do so, put the mask on immediately. Nimur (talk) 17:59, 24 February 2018 (UTC)

By now we've established that the ambient cabin pressure does change - and this is safe and normal on a commercial airliner. You'll typically lose about 5 or 8 inches of mercury worth of pressure (25 kPa) by the time you hit cruise altitude. At that point, the snack bag (which has sea-level air inside it) will tend to puff up. For me, the most interesting detail is that the snack-bag eventually re-equilibrates to ambient pressure, but only after some delay! Those little bags of cookies or peanuts are frequently bags made from plastic or metallized plastic - sort of like mylar - and sealed using hot-bar pressed seams. Mylar is amazing aerospace material - compared to our best engineered materials, it's darned near airtight - but not actually perfectly airtight! We don't know whether air leaks through the seams or through the material itself. But you can sort of develop an intuition about how fast air flows into and out of the bag based on how effectively it withstands its internal pressure against the ambient cabin pressure! Nimur (talk) 17:46, 24 February 2018 (UTC)

As I understood the OP's comments, the bag was not puffed up when they received it 1 hour into the flight, so the plane was likely already at some level of cruising altitude. This puffing up happened due to changes in cabin pressure (probably due to changes in cruising altitude) after the ascent phase. Nil Einne (talk) 02:14, 25 February 2018 (UTC)

February 25

A low viscosity liquid which will harden after 20+ minute working time?

I have a small hobby project in which I'm printing some keyboard keycaps with an SLA 3D printer with the intention of filling in an inset legend with some kind of paint or resin which will harden in a contrasting colour (white since the SLA resin is black) but I don't know exactly what to use. The spaces I fill will be really small so I need to use something with low viscosity which I can inject with a (blunt, narrow needle) syringe and it will self-level and harden very slowly so I have time to inject about 70 characters in total. I know two-part resins are available but I don't know their viscocity or if there are more suitable alternatives. Can anyone advise a suitable material available in small quantities for this purpose? 185.222.217.213 (talk) 14:34, 25 February 2018 (UTC)

There'll be lots of low-viscosity potting epoxy on the web. But I'd have thought some pot of enamel paint for hobbyists would do the job just as well. I think it would probably be worthwhile painting the flat area of the keycap with some masking fluid too. Dmcq (talk) 14:58, 25 February 2018 (UTC)

• Stupid-sounding idea, but what about solder, if you are OK with a metal-silvery finish? The idea is that you might already have some training and equipment for soldering. (I am assuming that since you hobby-print 3D pieces there's a good chance you have standard electronics material already. If not, follow Dmcq's advice instead.)

I would try the standard tin-lead at first on a test cap (to check that you can reliably apply the solder without melting the plastic too much; you may need to thicken the plastic cap though). If you can 3D-print freely, that is easy enough to test. If this does not work, you could switch to using a low-temperature solder (Solder#Solder_alloys lists many; I could find some Cerrolow 117 for about $40 on ebay), which should be easier to use (you can overheat the fusion temp by more, so you have more time to wipe mistakes, and viscosity is lower, while being at lower absolute temperatures for the plastic cap). Tigraan^{Click here to contact me} 13:40, 26 February 2018 (UTC)

Feynman Lectures. Exercises. Exercise 19-17 JPG

. .

...

A yo-yo like spool consists of two uniform discs, each of mass M and radius R, and an axle of radius r and negligible mass. A thread wound around the axle is attached to the ceiling, and the spool is released from rest a distance D below the ceiling.

a) If there is to be no pendulum -- like swinging motion, what angle should the thread make with the vertical as the spool is released?

b) What is the downward acceleration of the center of the spool?

—  R. B. Leighton , Feynman Lectures on Physics. Exercises

I have found the acceleration with assumption the motion is vertical (with zero angle ).On time 6:16 of a video youtube.com/watch?v=kdTLZ6-hVq8 it seems there is no sidewise motion. But I don't understand why the angle is zero. The acceleration of the yo-yo is $g<a<0$. So it has apparent weight = $2M(g-a)$ like in the going down elevator. This net force must create a torque and shift the yo-yo so that the center of mass comes to be under the attachment point on the ceiling . Username160611000000 (talk) 16:05, 25 February 2018 (UTC)

The tension in the thread and the true weight of the spool (acting vertically) cannot be collinear otherwise there would be no torque to provide the angular acceleration. From the moment of inertia and the angular acceleration, you can calculate the perpendicular distance of the line of action of the tension in the thread from the centre of the spool, and hence the initial angle of the thread. From the order of the questions, I assume that you were supposed to set up simultaneous equations for linear and angular acceleration. If you have used an energy method for the acceleration, then you have probably calculated the vertical component of the tension. (I can see now that you didn't.) Dbfirs 17:13, 25 February 2018 (UTC)

"the perpendicular distance of the line of action of the tension in the thread from the centre of the spool" = r. How from this can I find the angle? Username160611000000 (talk) 18:08, 25 February 2018 (UTC)

I have calculated the acceleration form next formulas.

$\alpha ={\tfrac {\tau }{I}}={\tfrac {2Mgr}{{\tfrac {2MR^{2}}{2}}+2Mr^{2}}}$

$a=r\alpha ={\tfrac {gr^{2}}{{\tfrac {R^{2}}{2}}+r^{2}}}$

The moment of inertia $I$ and the torque $\tau$ were calculated around a point A (see image PNG) where the thread touched the spool. From answers it is correct jpg. The tension is then $T=2M(g-a)$, but again using the assumption that the spool is moving vertically.

In The Solutions is said:

...

The spool is acted upon by gravity, directed vertically downward, and the tension force T along the thread. The spool will not swing if there are no horizontal forces, i.e. if the thread is vertical.

—  MEPhI , Solutions (Google Translate)

But it does not explain the case when the thread is fixed , then the spool will go so that the center of mass is under the point on the ceiling PNG . I.e. we hold the spool with fixed thread by hand in position, showed in the exercise PNG; then the spool is let fall. The spool then 100% will go to the right, but in the absence of horizontal forces at starting moment. I can understand it like next: the spool starts rotation and pulls the thread away from the vertical. The inclined thread generate a horizontal force. It proves that absence of horizontal forces CAN produce horizontal motion and it demolishes the arguments from The Solutions. Username160611000000 (talk) 18:20, 25 February 2018 (UTC)

• • From assumption that the center of mass must be under the ceiling fixing point I can calculate the angle: $\sin(\theta )={\tfrac {r}{OC}}$ (see image). It is not clear from the statement of the exercise is $D=OC$ or $D=AC$, but it is not a problem. The problem is how to prove that the center of mass must be under the point on the ceiling, moreover the experiment shows that it's not the case. Username160611000000 (talk) 08:18, 26 February 2018 (UTC)

Now I'm confused. It's too long since I did this type of problem. Perhaps someone else can help? I had assumed that the spool was released with the thread vertical, but there will be horizontal motion in this case as the spool rotates about the lower end of the thread. Perhaps you were meant to assume that the centre of mass is moved to be under the point of suspension before release, but then the tension in the string will cause a swing the other way. Dbfirs 10:11, 26 February 2018 (UTC)

@Dbfirs:but then the tension in the string will cause a swing the other way. The tension will create a torque about point O only in O - reference frame (ref. frame in which point O is at rest). First, the tension will simply increase angular velocity about point O, there is no guarantee that the point O will move horizontally. Second, O - ref. frame is an accelerating frame, so there may be some complications. On the other hand point A is at rest in ceiling ref. frame (at least during small time at start). Username160611000000 (talk) 17:09, 26 February 2018 (UTC)

Hmmm. I think the thread, wherever unspooled, does not actually move; thus all downward force on it is opposed by upward force. And at the moment of release, the spool has no downward velocity. Therefore, it applies a torque according to the newtons (M * g) multiplied by the lever arm = radius r. To oppose the torque, the string can be non-vertical, applying a contrary lever arm. That puts the center of mass of the spool directly under the string, AFAICT - in other words, the center of mass is stable when directly below the suspension point, like with anything else. That makes the angle sin-1(r/D).

Now as for the downward acceleration, we know it can move only r * however many radians it turns by. From the list of moments of inertia, I = 1/2 mr^2 for a disk. We use the quation from moment of inertia that tau = I alpha, where tau (the torque) is that M*g*r thing. So alpha = M*g*r / (1/2 MR^2) given that R is the disk radius and M is I think the same M, or alpha = 2*g*r/R^2. The distance, velocity, and acceleration should all be r* the angular versions I think, so I get 2*g*r^2/R^2 ... hmmm, units should be acceleration, it tends to zero for a very thin axle -- but, I get double speed falling if you wrap a string around a soup can, which is wrong. Sigh. This time I'll post my detritus in hope someone finds a fix, with apologies. Wnt (talk) 02:11, 27 February 2018 (UTC)

@Wnt: With the non-vertical thread, I'm pretty sure the spool would swing in the direction of the horizontal component of the string tension. Compare this with picture frame wires, which typically have two attachments with opposing tensions that can be adjusted (they tend to get misaligned anyway). Take away one of the attachments and the frame swings. Assuming an infinite thread and fall of the spool (with gravity not changing), the spool's mass should oscillate about the thread's vertical position as the horizontal tension is a restoring force towards vertical with each swing. -Modocc (talk) 19:14, 27 February 2018 (UTC)

Where the problem statement states "no pendulum -- like swinging motion" is a bit unclear, however the solution that the string (or thread) needs to be vertical such that the net horizontal force acting on the disks' axle is zero is a reasonable interpretation of that... any angle from vertical gives a measurable horizontal force component from the string's total tension and without any other horizontal force also acting on the mass center, that lateral string tension results in its horizontal acceleration. Now if the string is "fixed at point A" as in your diagram, the spool thread is essentially stuck as the spool rotates without the thread being released, creating a pendulum with lateral string tension acting on the spool and which you didn't account for and the motion of which is to be avoided. --Modocc (talk) 17:30, 27 February 2018 (UTC)

How well do Tanks and other tracked vehicles coast?

I was talking to a buddy of mine about tanks the other week, and we were wondering how well tanks coast and if it is possible to put regen braking on a tracked vehicle. Does anyone have any experience with tanks here? Our Tank steering article wasn't much help, and neither was Google. Thanks, L3X1 ◊distænt write◊ 16:36, 25 February 2018 (UTC)

I can't find any in service, but this paper discusses the concept and notes that there are some hybrid-electric tracked vehicle development programs in the works. I don't know how well tanks coast, but regenerative braking definitely seems to be seen as a non-laughable concept. TenOfAllTrades(talk) 17:49, 25 February 2018 (UTC)

Considering that the M1 Abrams gets only .6 miles per gallon, I doubt "coasting" is even the right word; it barely budges without eating a lot of fuel. Matt Deres (talk) 21:05, 25 February 2018 (UTC)

Well, the thing weighs 60-70 tons, so unless there is some jake or automatic engine braking going on, if you let off the throttle at 30 or 45mph inertia says it will keep on going for a little bit. I've driven some hydraulic transmission construction vehicles, but they always would come to a halt pretty quick if you let off the gas. Thanks, L3X1 ◊distænt write◊ 22:21, 25 February 2018 (UTC)

Common Tanks are very ineffective vehicles because of their weight and Caterpillar tracks. They also lose their speed fast, even on streets, because the traks are very sturdy and stiff. A recuperation system would not be worth the extra weight and besides it would have to be absurdly huge to take up and feed back the braking energy of 60+ tons at 60-70 km/h. Because of their ineffectiveness most modern Armoured personnel carrier constructions went back to wheels. --Kharon (talk) 05:39, 27 February 2018 (UTC)

Why do humans have body hair?

Why?86.8.202.234 (talk) 22:10, 25 February 2018 (UTC)

Because mammal! "It [ hair ] is a definitive characteristic of the class." {The poster formerly known as 87.81.230.195} 90.220.212.253 (talk) 03:16, 26 February 2018 (UTC)

For the lack of body hair in humans, see What is the latest theory of why humans lost their body hair? Why are we the only hairless primate? from Scientific American. Alansplodge (talk) 09:10, 26 February 2018 (UTC)

We're not hairless. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:34, 26 February 2018 (UTC)

Indeed, but a lot less hairy than all other primates (most people anyway). Alansplodge (talk) 11:20, 26 February 2018 (UTC)

"Hairless" does not simply mean "less hair". I'm reminded of Richard Armour's comment about the wireless, i.e. the radio: "Although it had a great many wires, it had less than it might have." ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:21, 26 February 2018 (UTC)

Still, the concept of the human as a hairless ape is a common enough trope, see, for example, the sentiment in the title of the famous book by Desmond Morris, The Naked Ape. --Jayron32 17:41, 26 February 2018 (UTC)

This is the bloke who wrote the offending article. Alansplodge (talk) 18:05, 26 February 2018 (UTC)

FWIW, the Aquatic ape hypothesis mentioned in that article is not really supported by any paleontological or archaeological evidence and is generally dismissed by most anthropologists (outside of a few proponents) as an interesting thought experiment. In terms of body hair, of course, we have about the same number of hairs per square inch as a chimp, it's just that the hairs are shorter and finer. Matt Deres (talk) 14:36, 26 February 2018 (UTC)

Thank you. Yes, some of us have particularly fine hair. Drmies (talk) 17:42, 26 February 2018 (UTC)

The problem about the lack of paleontological or archaeological evidence for the Aquatic ape hypothesis is that it would be extremely difficult to find any even if it were true. The hypothesis supposes the aquatic behaviour (leading to various physical adaptations) was practiced on marine shores, but since the postulated period in question sea levels have risen by hundreds of feet, so any such evidence will have been almost if not entirely obliterated, and any that remains would be almost impossible to find and excavate, even if we were looking in the right places (which we're not). In this case, the maxim "absence of evidence is not evidence of absence" is particularly apt. {The poster formerly known as 87.81.230.195} 90.220.212.253 (talk) 23:28, 26 February 2018 (UTC)

I think a "mud ape" hypothesis is more interesting. After all, polar bears can swim with thick body hair, but we are constantly reading about well-meaning humans pulling things out of mud pits. In earlier times they would also have meant well... meant to eat well. The Okavango delta in particular is known for periods of flooding and periods of mud and periods of wildfire, and seems like an interesting possible ancestral situation. The feet of humans, like lechwe, seem elongated -- could it have been for crossing that mud? Wnt (talk) 01:46, 27 February 2018 (UTC)

So you're saying it's unfalsifiable? Then I'm afraid it's not science. We have found and studied numerous human settlements that are now underwater, having been submerged by rising sea levels, so the implication that "evidence may be underwater so we just have to throw up our hands" is nonsense. All conclusions in science are provisional, so it's possible in the future we could unearth some stunning evidence for the hypothesis, and if we do, it will presumably be revisited. Until then, it fails in explanatory power relative to other hypotheses. RationalWiki has a good summary of its problems. Although as noted, a problem is that people often seem to mean different things when talking about the "aquatic ape hypothesis". The notion that marine environments may have played some role in human evolution is, I don't think, very crazy, though we need stronger evidence to state that more definitely. The idea that human ancestors at some point lived entirely in the water like whales or dolphins is ludicrous. --47.146.60.177 (talk) 08:23, 27 February 2018 (UTC)

Yes, that site is a good overview of the problems with the AAH, but the problem it goes deeper than that. We all have hooded noses, we all have sebaceous glands, we all have all those items that proponents of the theory say were likely derived from a semi-aquatic existence. In order to be true, aquaticism would have to have been a massively important evolutionary feature where divergence from that adaptation meant complete lack of procreation. It would require a bottleneck where all non-aquatic people were wiped out to the point where even recessive non-aquatic genes were virtually eliminated. Matt Deres (talk) 13:33, 27 February 2018 (UTC)

That's not really a valid argument at the end. Many racial features and even more substantial cosmetic alterations in the lineage leading to humans (such as hooded noses) can be seen as cosmetic features that potentially might have been basically under no selection at all. If on average a person had to dive into a creek once a generation to avoid a barrage of rocks, spears, or arrows, that would be enough selection to make big changes in even a thousand years. Wnt (talk) 15:14, 27 February 2018 (UTC)

February 26

Static discharge and lightning igniting fuel

According to the article Pan Am Flight 214, the plane was damaged due to lightning igniting fuel vapors. The response was to install Static discharger on aircraft. I read the article on that, and it seems to be focused on preventing static electricity build up from interfering with cabling and electronic equipment. What I don't get is how discharging static electricity would prevent a lightning strike from igniting fuel vapor. Can someone explain? RudolfRed (talk) 00:29, 26 February 2018 (UTC)

From reading our articles, Static discharger and that on the flight, I don't think it would have helped in the situation with Flight 214. If you look at the report, this was only one of a number of the recommendations made to FAA Wikisource:Aircraft Accident Report: Pan Am Flight 214/Attachment II in response to the accident which the FAA then implemented Wikisource:Aircraft Accident Report: Pan Am Flight 214/Attachment III. I'm fairly sure it's common the CAB/NTSB and FAA will feel that what was discovered from an accident suggests the need for something, even if that probably wouldn't have helped in the specific accident. This may have been even more common in the past, when there were still a lot of unknowns. See also the report itself, especially the end Wikisource:Aircraft Accident Report: Pan Am Flight 214. The modern FAA's lessons learnt also doesn't mention static dischargers but does suggest the accident was one of the key catalysts for further research etc that lead to improved protection against lightning strikes [25]. This, while maybe not the best source (NY Daily News) is also perhaps of interest [26]. As may be obvious from Inerting system, something which arose, at least in part from flight 214, is still an ongoing issue. Nil Einne (talk) 02:29, 26 February 2018 (UTC)

@Nil Einne: Thanks for the explanation and detailed answer. RudolfRed (talk) 02:43, 26 February 2018 (UTC)

I suspect an indirect lightning strike could create a static charge (flying through where a bolt passed, or by electromagnetic induction as the bolt increased and decreased in intensity). Wnt (talk) 02:15, 27 February 2018 (UTC)

February 27

Isn't the anthropic principle a tautology?

Isn't the anthropic principle a tautology? Doesn't it basically say that we contemplate our existence, well, because we can? Or does it relate to the old cogito ergo sum (with which I incidentally disagree)? I feel I'm missing something important here. 93.142.74.255 (talk) 09:11, 27 February 2018 (UTC)

I don't see how that would make it a tautology? Have you read Anthropic principle?--Shantavira|^{feed me} 09:20, 27 February 2018 (UTC)

• The observation behind the "anthropic principle" is by itself a fairly mundane observation (though not strictly a tautology). Our article mentions that Others[ref] have criticised the word "principle" as being too grandiose to describe straightforward applications of selection effects. Its point is to explain apparent paradoxes by selection bias.

For instance, a classic creationist argument is to say that humans have unique characteristics among animals, Earth has unique climatic conditions and is the only place to host life, physical constants of the universe are tuned extremely precisely to allow matter to exist as we know it, etc.; all this would have had a ridiculous chance of happening by random chance, and a single deviation would have caused sentient life not to exist; hence, there must be some deity who fine-tuned all of this. The AP is in effect saying that we wouldn't be here to observe it if it was not the case, which invalidates the "hence" (because in both the deity-created and the random-dice universes, only the outcomes with a sentient species to observe it are sampled).

Of course, only the precise formulation is new, not the idea itself - the very similar "shipwrecked crew make no offerings" existed in the Antiquity. Tigraan^{Click here to contact me} 09:53, 27 February 2018 (UTC)

The anthropic principle is notable mostly in the use, and mostly in the use to dispel "it won't happen because it hasn't happened before" type arguments. For example, the Earth didn't collide with another planet a million years ago because then we wouldn't be here -- which means that we don't know (at least from Earth alone) whether such collisions happen all the time. Or, more practically, we know the Earth didn't overheat and form a Venus-like greenhouse because we're alive to look, but that means we don't know whether the odds of that happening are low in the future. In the extreme case, I recall a Nature paper from some decades ago arguing that because the position of a human today is random within the order of all humans ever to exist, humans probably will not produce more offspring than that in the future; hence an end of the world can be predicted within a few generations. But I didn't track it down just now. Wnt (talk) 13:01, 27 February 2018 (UTC)

@Wnt: You are describing the Doomsday argument. Tigraan^{Click here to contact me} 19:05, 27 February 2018 (UTC)

• The anthropic principle is tautological in the week sense (of course we have to live in a universe that supports our sort of life in order to support our sort of life) and is entirely arbitrary and unfalsifiable in the strong sense. In the strong sense, the claim is that the universe had to be the ways it is (or very close) because otherwise there would be no intelligent life to observe it.

We have no way of knowing that there being intelligent life to observe it is a necessary property of existence, and we have no way of knowing that in other universes without matter as we know it there might not be some sort of non-material life as we don't know it. We cannot enumerate these possible universes, we have no idea of the unknown unknowns or any way of actually modelling these existences other than in very crude parochial terms.

"week sense": is that when you sense how weeks pass? --Hofhof (talk) 18:37, 27 February 2018 (UTC)

Seems like they pass more quickly every year. --47.146.60.177 (talk) 19:00, 27 February 2018 (UTC)

Linking relevant article... Hofhof (talk) 19:25, 27 February 2018 (UTC)

To New Yorkers or isolated Pacific islanders, life is only possible in Manhattan or on a coral atoll. See the famous parochial New Yorker Magazine cover, and translate that into a cosmological model if you will. μηδείς (talk) 16:52, 27 February 2018 (UTC)

Said cover being titled View of the World from 9th Avenue. --47.146.60.177 (talk) 18:59, 27 February 2018 (UTC)

• Thnaks to IP 47's image link and key nigh. I flame WP for having an antic spell-checker with know glamour funk shin. As for the numerology Wnt alluded to, there was also an argument before 2001 that, since statistically, any entity is statistically 5% likely to be in its first %5 of lifespan (this assumes a very unlikely linear relation) and 5% likely to be in its last 5% of it's existence, we can be sure that the Catholic church, if assumed to be a round 2,000 years old, has existed that long, there is a 90% chance that it won't live fewer than another hundred and five years or longer than another 38,000 years. This is, of course, yet more meaningless gibberish, funded at taxpayer dollars. μηδείς (talk) 03:31, 28 February 2018 (UTC)

Deflating tyres to drive over snow or sand

When you deflate the tyres to drive in the desert or over snow, how much bigger does the surface in contact with the ground get? How meaningful is that (let's suppose that you could inflate the tyres as soon as you drive over a different terrain)? How much more friction can be expected? --Hofhof (talk) 18:20, 27 February 2018 (UTC)

From the basic definition of pressure, (pressure equals force times area): if the entire weight of a vehicle is supported by its pneumatic tires, then the area in contact with the ground is inversely related to the tire-pressure. Of course, the simple model assumes a perfect, nonrigid wheel. Automobile tire dynamics constitutes an entire field of advanced science and engineering.

The extent to which contact patch size affects friction and traction and rolling resistance is nontrivial, because the wheels are nonrigid and the surface contact dynamics are complicated. (For starters, read Slip (vehicle dynamics)). So, you cannot accurately use a simple model derived from first principles; there are so many confounding variables. Instead, empirical laws are derived for automobiles; for specific modes of operation, on specific road surfaces, and for specific vehicle models, you can obtain a table of tire performance.

Our article on SAE J2452 - an automotive industry standard for tire performance - has links to more detailed technical resources.

Nimur (talk) 19:24, 27 February 2018 (UTC)

• This is complicated, and in general don't do it.

It used to work, back in the days of crossply tyres. Tyres were also of a taller cross section. As practically all modern tyres are radials, they're a different matter: there aren't many modern tyres where you can deflate them sufficiently to make a difference to their shape where this isn't also going to cause so much bending in the sidewall as to damage the tyre.

In principle, the idea is to let the tyre flex more, thus increasing the size of the contact patch. Ideally this should increase lengthwise, but not sideways - if it increases much sideways, you're killing those sidewalls. Another problem is that reducing the pressure reduces the load carried pneumatically as a balloon tyre and increases the proportion being carried by the sidewalls - which are also getting bent out of their ideal shape.

There's no increase in friction as such - the "grip" of tyres on soft ground is far more complicated than such a simple term. In this case, it's more about the structural integrity of the surface layer of the ground and nothing to do with the friction to the tyre. A hard tyre with a small patch will have a greater loading on the surface (force/area) and this tends to break up the integrity of the mud or snow - the limit for the unbroken surface is higher than the force available once it starts to crumble.

Where deflation is used today it's mostly with CTIS and tyres designed specifically to cope. They might also need a beadlock system to stop the tyres pulling off the rims when there's not enough pressure to hold them otherwise. Tyres can be re-inflated as soon as you hit tarmac, which is important for allowing higher road speeds and avoiding damage. Andy Dingley (talk) 20:49, 27 February 2018 (UTC)

Tires are much more abused in high speed curves. Todays sidewalls or tires in general can take an awesome lot of "abuse", so its a minor problem to run them partly deflated for some hours to give them more contact on sand, mud and alike where there is danger of sinking in to a degree you get stuck. This does not work on snow and ice tho because you actually will get more grip by having your given weight on a very small contact area. --Kharon (talk) 21:42, 27 February 2018 (UTC)

Tyres "in high speed curves" aren't abused, they're run at proper pressures, so that the sidewalls aren't excessively flexed as they are when the same tyre is run under-inflated. If they were run like this, then they'd fail very rapidly. Tyres in high speed curves are heavily loaded, but that's not the same as abuse - it's what they're designed for -and their sidewalls are in the correct shape. Andy Dingley (talk) 22:24, 27 February 2018 (UTC)

Bison fur pattern

Why American bison has only that partial fur covering his upper parts, neck and head instead of the entire body? Perhaps some other animals have the same. 212.180.235.46 (talk) 18:34, 27 February 2018 (UTC)

Compared with the European bison, for example? And note that they both have plenty of fur. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:13, 27 February 2018 (UTC)

I don't know about the southern states, but in the northern states and in Canada, the fur over the shoulder hump grows long and thick as it gets colder. In the winter, the fur is very thick and helps them handle the cold (as cold as -40°). When it warms, the fur molts in large clumps and the difference is mainly in texture during the summer rather than length. 209.149.113.5 (talk) 20:46, 27 February 2018 (UTC)

Compared with more or less uniform fur distribution in other animals. Also, it it the same mechanism involved in uneven hair distribution in humans compared to apes who have more uniform fur? Thanks. 212.180.235.46 (talk) 21:23, 27 February 2018 (UTC)

It is hard to say if their hair has changed from behavior or if their behavior is based on their hair. Bison are known for facing into cold winds. Growing up on a reservation in Montana, we had a saying that you should stand and face harsh conditions the way the bison stand and face the cold bitter winter winds. They don't need thick hair on their rears because they take the wind to their face. 156.143.240.137 (talk) 23:44, 27 February 2018 (UTC)

• Horses, lions, and humans all have manes, see also sexual selection. μηδείς (talk) 03:17, 28 February 2018 (UTC)

February 28