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May 23

Salts for hygrometer checking and calibration

I'm interested in using some salts to check and calibrate some humidity sensors for personal (rather than scientific or industrial use). I'm following the tutorial here: https://www.allaboutcircuits.com/projects/how-to-check-and-calibrate-a-humidity-sensor/ where it says that the salts should be wet to the consistency of wet sand. The article references some standard procedure but the standard procedure costs $50 to download. My question is, does it make sense to add water to the lithium chloride? It's meant to be bringing the RH down to 11.3% so it seems to me that adding water would impede its absorption of water from the air. Is the idea to have the salt partially, rather than fully, saturated with water, so it's kind of analagous to a buffer accomodating acids and alkalis? If that's the case, I'd still expect adding water to be unnecessary for that salt since the starting humidity is always bound to be higher than 11.3%, at least in my country (United Kingdom). If the lithium chloride was not stored in an airtight container, would it keep absorbing water from the air until it was no longer able to keep doing so? That would be another good reason to not add water before use; if it would prolongue its working life?

Also, how much salt is needed? Could 30 grams be enough to ready the air inside a 9 L food storage container? According to some online calculator, there are 40 grams of water in a cubic metre (1000 L) of air at 30 °C so... lets say your container was 100 L, you'd only need to absorb a maximum of 4 grams of water if you were going from 100% RH to 0% or give up 4 grams if you were doing the opposite. That doesn't help much because I don't know how much water 30 grams of lithium chloride can absorb (certainly less if I add some to it at the outset, though, right?).

Would sulphate of potash intended for gardening be likely to give the same result as reagent grade K2SO4? Apparently reagent grade only means ">95% purity" anyway.

The potassium sulphate gives an RH of 97-98% - this might be a stupid question but at that humidity, is there a higher risk of the water condensing on stuff inside the container? I was also thinking of using potassium carbonate (43%) and sodium bromide (58-60%) because 33% to 75% is kind of a big jump through the region that I expect most of the readings to be taken but then maybe I'm going a little overboard. --88.111.17.83 (talk) 04:16, 23 May 2020 (UTC)[reply]

The idea with these is that the standard humidity is created by a saturated solution, so you need some of the saturated solution of the salt. If your lithium chloride absorbed enough water to get to the consistency of wet sand, then you don't need to add extra water. Any way the lithium chloride solubility is high: 83.05 g/100 mL of water at 20 °C or 84.25 g/100 mL at 25° according to our article. So it is close to one for one in mass of water. And yes, i would keep absorbing water from the air if you did not keep it airtight. Fertilizer grade potassium sulfate is likely to have other soluble contaminants, perhaps potassium chloride, magnesium or sodium. There is no incentive to make it super pure, just safe enough. That will likely reduce the humidity from a saturated solution. Graeme Bartlett (talk) 22:44, 23 May 2020 (UTC)[reply]

Thanks very much, Graeme; this helps a lot. I think that water solubility value is exactly what I was looking for. I've found a fairly cheap supplier for all these salts so there's no sense in trying to skimp on the potassium sulphate. The lithium chloride is several times more expensive than the others. 88.111.17.83 (talk) 00:19, 24 May 2020 (UTC)[reply]

Plant

Hi, I would like to know what is the name of this plant:

--Red-back spider (talk) 11:24, 23 May 2020 (UTC)[reply]

Note that, according to the image’s uploader, the title and description state the wrong species. Brianjd (talk) 11:47, 23 May 2020 (UTC)[reply]

There is some difficulty in identifying immature foliage; my first thought was one of the birches, like this for example, but this hawthorn looks quite similar. Someone else might do better though. PS I took the liberty of resizing your image, feel free to reinstate it if you are offended. Alansplodge (talk) 12:09, 23 May 2020 (UTC)[reply]

It could also be elder, the first four or five leaves are not yet pinnate. Must wait one week more or so. 2003:F5:6F08:8200:705A:5DF6:6175:3EA0 (talk) 14:16, 23 May 2020 (UTC) Marco PB[reply]

Thank you. I think it is not a tree. The species mentioned are non-existent (or extremely rare) in that forest, and this seedling is common. I will upload more photos soon.--Pere Orga (talk) 19:27, 23 May 2020 (UTC)[reply]

It looks like a tree seedling to me, and does bear resemblance to Crataegus monogyna, or possibly another Crataegus, though here in the UK C. monogyna isn't usually a forest tree. It's not Sambucus nigra ("elder") - there's a pic of an elder seedling here, and it's quite different. Has the uploader not looked at the trees that are growing around the seedlings? If seedlings are plentiful in an area, it usually isn't too hard to find their parents growing nearby. Another place to ask would be at WT:PLANTS. PaleCloudedWhite (talk) 01:04, 24 May 2020 (UTC)[reply]

The plant could be one of these, but there just my guess:

@Brianjd: Yes, I know that the species name in the title is wrong, in fact the picture is getting deleted because of that and I want to save it. After all it can be used in an article. @Pere Orga: It might be a tree, Google says so but who knows, Google is right or wrong. --Red-back spider (talk) 05:33, 24 May 2020 (UTC)[reply]

@Red-back spider: I have outdented your entire comment so it is aligned correctly. My comment about the species name being wrong was for the benefit of other users. Brianjd (talk) 07:39, 24 May 2020 (UTC)[reply]

Hoheria ("lacebark") and Plagianthus regius are endemic to New Zealand, so seems less likely that there are lots of their seedlings sprouting in a forest near Barcelona. Crataegus laevigata might be a fit, at least better than C. monogyna anyway - these images of C. monogyna seedlings have more serrated leaves than the unidentified image. PaleCloudedWhite (talk) 08:06, 24 May 2020 (UTC)[reply]

More photos in the same forest. This ones of the same species:

Even if these have a different shape, I think this could be the same, too:

My opinion is that this is a liana, and that when it grows it becomes this one:

@PaleCloudedWhite: I have identified all the trees that grow in that forest (sorbus, pines, quercus, savinas, acer... and Fraxinus angustifolia and Celtis australis, but rare) and I have concluded that this is very unlikely to be a tree.--Pere Orga (talk) 22:24, 25 May 2020 (UTC)[reply]

Your "liana" pictures are a Clematis, though I haven't tried to ascertain which species. You could be right regarding the original seedling image - the seedlings of Clematis urophylla shown here, especially the slightly more mature ones in Image C, look quite similar. The images you took of seedlings with a different leaf shape are not the same as the original - I think they are Acer seedlings. PaleCloudedWhite (talk) 20:58, 25 May 2020 (UTC)[reply]

The only Acer I've seen growing there is Acer monspessulanum, seedling: https://commons.wikimedia.org/wiki/File:Aur%C3%B3_negre_(Acer_monspessulanum)_acabat_de_n%C3%A9ixer_-_Santa_Perp%C3%A8tua_de_Gai%C3%A0.jpg. I'll see how this one evolves too :) --Pere Orga (talk) 22:24, 25 May 2020 (UTC)[reply]

May 24

Electroplating bare copper soldering iron tips with nickel and then maybe with iron?

There are some special soldering iron tips with a little groove in the end which helps improve contact area with the workpiece but they're only available for certain models of soldering iron. I've seen bare copper soldering iron tips (compatible with my own iron) for sale which I think I could cut my own groove into but they aren't good for soldering because copper dissolves in solder so the lifetime of bare copper tips is very limited (I'm not even sure why they're sold). However, I've seen online that you can electroplate nickel onto copper at home using vinegar to make a solution of nickel acetate. Someone uploaded a YouTube video doing this for soldering iron tips. I was just wondering, with regard to maximising life-time and functionality, would it be worth plating with iron after the nickel? Any idea what thickness the platings should be? Could sodium dodecyl sulfate be added as a wetting agent or is that only suitable in the conventional industrial nickel plating solutions? --88.111.17.83 (talk) 06:42, 24 May 2020 (UTC)[reply]

In my experience to work with bare copper is more agreable because copper is more easily wetted by tin than nickel or iron (Well it depends, for Tiffany glass work it is maybe even better so, but when soldering on electronic parts it is in my opinion better if the tip is wet with tin). I had some iron plated tips but even if they last forever they are not easily wetted by tin. Bare copper tips have a shorter lifetime but they don't really dissolve away in three minutes but rather in hours, secondly you can adjust them several times with hammer and file before you have to throw them away and lastly they are not so expensive that you have to renounce to the advantage of easy wetting.

But if you'd like and try, you can directly plate copper with iron without the nickel step. They say it is not so easy for hobbyists to do at home though, but here you will find some guidance: [[1]] 2003:F5:6F08:8200:B9A3:5B4F:C199:54F1 (talk) 11:38, 24 May 2020 (UTC) Marco PB[reply]

What does it mean when N,N- comes before a chemical's name, such as in N,N-Dimethyltryptamine?

And what page on Wikipedia should I add to the top of N,N (N,n) in an {{about}} template? Psiĥedelisto (talk • contribs) 15:08, 24 May 2020 (UTC)[reply]

It's dimethyl tryptamine, meaning that it's tryptamine wherein two of the hydrogen atoms are substituted by methyl groups. The N,N bit means that both substitutions are on a nitrogen atom. I don't believe we should have a page specifically on this notation. Probably we can live without a hatnote in the article you mention. --Trovatore (talk) 16:13, 24 May 2020 (UTC)[reply]

@Trovatore: Yes, but a hatnote to where?

Thanks for your help by the way, it makes a lot of sense that N means nitrogen and not n in the typical math sense, that's why I was confused. Psiĥedelisto (talk • contribs) 16:57, 24 May 2020 (UTC)[reply]

It's a locant for a substituent, but our locant article only mentions numbered positions of a chain, not heteroatoms. And the parent IUPAC nomenclature of organic chemistry article also does not have any examples of it. The other common (but not nearly as common s N) are O (for oxygen) and S (for sulfur), and those two are only usually as infixes not prefixes. DMacks (talk) 17:38, 24 May 2020 (UTC)[reply]

Prefixes for sulfur and oxygen substitution do exist: eg S-Methylmethionine, O-Ethyl methylphosphonothioic acid. If the o is lower case it will mean ortho, and if you see lower case n- (eg n-Butylamine) it will mean normal or straight. You will only get one of those n's though. Yet another variant is N,N'- eg N,N'-Dicyclohexylcarbodiimide meaning two different nitrogen atoms are substituted. And another: N,N,N- N,N,N-trimethyl-4-phosphoryloxytryptamine meaning three things substituted on a nitrogen. Graeme Bartlett (talk) 23:35, 24 May 2020 (UTC)[reply]

If these prefixes are chemical symbols, why are they presented in italics? --Lambiam 06:12, 25 May 2020 (UTC)[reply]

IUPAC/CAS says so and we all have to agree with some standardized system if we are to communicate clearly. I have no idea how to trace back the history of that:( DMacks (talk) 23:25, 25 May 2020 (UTC)[reply]

I would guess as a sort of reminder that they do not apply to primary alphabetization. S-Methylmethionine is sorted under "M", not "S".--Khajidha (talk) 18:26, 27 May 2020 (UTC)[reply]

That's a clever thought! DMacks (talk) 22:43, 27 May 2020 (UTC)[reply]

And it looks correct. IUPAC Blue Book 1993 edition says as a general convention "R-0.1.6 Italicization. Italicizing mainly serves to mark letters which are not involved in the primary stage of alphabetical ordering." DMacks (talk) 23:03, 27 May 2020 (UTC)[reply]

Thanks. Shouldn't capitalization then follow suit, as seen here (although not fully consistently): "S-M..." when sentence-initial, otherwise "S-m..."? --Lambiam 06:50, 28 May 2020 (UTC)[reply]

Yes. DMacks (talk) 09:20, 28 May 2020 (UTC)[reply]

@Lambiam: An uneducated guess: possibly an upright letter could be confused as a structural symbol, denoting a presence of the element at a specific position in the molecule. An italic letter doesn't say 'here is a nitrogen atom' but rather 'look for nitrogen to attach that group'. --CiaPan (talk) 21:10, 27 May 2020 (UTC)[reply]

Italicized element symbols are also used to indicate "here is this element". The name "2H-isoindole" means that specific hydrogen atom is attached to position 2 of the structure, not that there is something else to say about hydrogen #2 itself. DMacks (talk) 22:43, 27 May 2020 (UTC)[reply]

May 25

What is the "major splice donor" and how is it different from the Retroviral Psi packaging element?

According to this article on lentiviral packaging, safe lentiviral pseudoviruses can be produced by excluding gag/pol etc. from the viral genome proper and incorporating them instead with plasmids. These genes will not be packaged into the pseudoviruses because they lack "the HIV-1 elements that must be present in lentiviral vectors" which "include the RNA packaging signal (Ψ), the major splice donor, and the Rev-response element (RRE)." I get what the Ψ packaging signal is, as well as the RRE, but what is the major splice donor? I am surprised we do not have an article for it. Yanping Nora Soong (talk) 09:13, 25 May 2020 (UTC)[reply]

Here I read: "This region [viz. the non-coding part of the HIV genome between the LTR promoter and the first Gag open reading frame] encodes the 5ʹ-untranslated region (5ʹ-UTR) of the HIV RNA genome and contains many post-transcriptional replication signals, including the major splice donor (MSD) that is used in the generation of all spliced HIV transcripts and the packaging signal Ѱ that ensures the selective encapsidation of HIV RNA in assembling virion particles." From Figure 1 in the article I get the impression that MSD and Ψ are adjacent. --Lambiam 20:15, 25 May 2020 (UTC)[reply]

Okay, so this major splice donor will be present in all mRNA transcripts and serve as a packaging signal, whereas a generic 5'UTR or intron would not serve this purpose? Yanping Nora Soong (talk) 22:47, 25 May 2020 (UTC)[reply]

What (I think) I understand from the text in that article, the MSD and the Ψ packaging signal are distinct elements with distinct functions. This seems to be confirmed by the lengthy title of this article: "The packaging signal of simian immunodeficiency virus is upstream of the major splice donor at a distance from the RNA cap site similar to that of human immunodeficiency virus types 1 and 2". --Lambiam 08:15, 26 May 2020 (UTC)[reply]

I understand they are different sites, but I don't actually what the major splice donor does differently compared to the Ψ signal. Would the major splice donor be found in the 5' UTR of all retroviral mRNA transcripts, thus being a reliable packaging signal? Yanping Nora Soong (talk) 11:42, 26 May 2020 (UTC)[reply]

It is not my field, and the scholarly texts I find are research articles, apparently written for an audience of researchers in the field and scanty on explanation. --Lambiam 15:09, 26 May 2020 (UTC)[reply]

But reading e.g. this summary [[2]] I tend to imagine something like this: after the viral RNA has been replicated some of this whole viral genome (let us call it primary RNA) must be spliced to give the different messengers from which the viral proteins (including the capsid's components) are synthesized, and some must remain intact in order to be packaged into the finished capsids. So the MSD seems to control how much of the primary RNA is spliced vs. not spliced. Maybe acting like some kind of promoter for the splicing machinery. 2003:F5:6F08:8200:3179:F503:CF2E:D769 (talk) 18:18, 28 May 2020 (UTC) Marco PB[reply]

May 26

Why Large Synoptic Survey telescope launching for mapping milky way galaxy?

Most famous Hubble space telescope failed to map milky way galaxy? So why Large Synoptic Survey telescope launching for mapping milky way galaxy? Ram nareshji (talk) 09:11, 26 May 2020 (UTC)[reply]

The Hubble Space Telescope was not designed to map large portions of the sky – it has a very small field of view and it would take ages for it to map the Milky Way. You need different types of telescope for performing different tasks. --Wrongfilter (talk) 09:38, 26 May 2020 (UTC)[reply]

If it is not designed to map large portions of the sky, then what it is designed for? Is it just a test telescope before launching Large Synoptic Survey telescope? Ram nareshji (talk) 11:35, 26 May 2020 (UTC)[reply]

It was designed to observe individual objects at high angular resolution. --Wrongfilter (talk) 11:39, 26 May 2020 (UTC)[reply]

Courtesy links: the Hubble Space Telescope, launched into orbit in 1990, and the Vera C. Rubin Observatory (previously called the Large Synoptic Survey Telescope), a ground-based instrument due to commence observations this year. Cutting-edge telescope design advances considerably over several decades, and as the previous responders have said, the two instruments operate in totally different circumstances and perform very different kinds of observations. {The poster formerly known as 87.81.230.195} 90.202.168.206 (talk) 13:19, 26 May 2020 (UTC)[reply]

Mybe this comparison can help: you need a wide angle lens for your camera if you want to take a picture of the whole Symphonic Orchestra (this is LSST), but you need a telephoto lens if you want a good portrait of the third violinist from left, and this is Hubble. Two different instruments for two different tasks. 2003:F5:6F08:8200:3179:F503:CF2E:D769 (talk) 18:40, 28 May 2020 (UTC) Marco PB[reply]

How far can a spider descend hanging from its silk on a windless day?

Likely variety-dependent of course. Sagittarian Milky Way (talk) 16:06, 26 May 2020 (UTC)[reply]

No farther than the ground. Googling the subject "length of spider silk", here's an item about an African species whose thread can extend 25 yards or more.[3] ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:07, 26 May 2020 (UTC)[reply]

Don't forget holes! InedibleHulk (talk) 19:35, 26 May 2020 (UTC)[reply]

@Sagittarian Milky Way: The article linked above by Baseball Bugs refers further to: Agnarsson I, Kuntner M, Blackledge TA (2010) Bioprospecting Finds the Toughest Biological Material: Extraordinary Silk from a Giant Riverine Orb Spider. PLoS ONE 5(9): e11234. https://doi.org/10.1371/journal.pone.0011234 --CiaPan (talk) 09:35, 27 May 2020 (UTC)[reply]

You may be asking about the strength to weight ratio of spider silk, i.e. how many feet of silk does the silk have sufficient tensile strength to hold up (plus the spider's weight). If you look in that article, there is a table of specific strengths for different materials, that includes spider silk. You should be able to do the math from there. 2601:648:8202:96B0:9BC:E0D4:15B8:4B0A (talk) 05:56, 29 May 2020 (UTC)[reply]

What would a camera show at this specific situation related to speed of light?

Imagine there is a train moving in a circle motion near the speed of light with a camera inside it. Inside that small circle, there is a small building with a computer and a screen that will receive the data and show at the screen what the camera is recording. The circle has a small radius, so the distance between the camera the computer is always small despise the train move at near light speed.

Anyway at this very specific situation, what the computer monitor would show?2804:7F2:686:36B7:2910:A06E:9111:2C9D (talk) 23:07, 26 May 2020 (UTC)[reply]

How many frames per second is your camera recording? ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:58, 27 May 2020 (UTC)[reply]

It is easier to imagine a recording camera, where you watch the recording afterwards. We need to suspend disbelief; a real camera would get crushed by the centripetal force needed to keep it on track. (See also Wikipedia:Reference desk/Miscellaneous#Calculating "g" forces.) Here is a video by Carl Sagan showing a simulation of what a voyager would see near lightspeed. The circularity of the track should not make an essential difference. --Lambiam 13:39, 27 May 2020 (UTC)[reply]

Yes, it'll show what the camera sees. The challenge for our quasi-magical recording and transmittal setup is it has to compensate for the blueshift/redshift of the transmission from the relativistic Doppler effect. But if we've already built a train system that can withstand relativistic speeds this is just another simple matter of engineering. To the questioner: is there some effect you were expecting? If you can help us understand your thinking, we can probably point you at where to look to learn more. --47.146.63.87 (talk) 02:11, 30 May 2020 (UTC)[reply]

May 27

Effect of very high redshift on neutrino velocity

If a galaxy were sufficiently far away, and thus with big redshift and big speed away from us, could that cause neutrinos emitted from that galaxy to travel, relative to us, at considerably below lightspeed, say 0.5c, 0.01c, or even less? — Preceding unsigned comment added by Richard L. Peterson (talk • contribs) 07:30, 27 May 2020 (UTC)[reply]

Redshift means that we measure a lower energy and momentum than the correponding values measured in the frame of the galaxy that the neutrino was emitted from, so yes, we would measure a lower velocity. However, neutrinos emitted from stars (like Solar neutrinos) have energies in the MeV range, much larger than their mass (at most eV), so they move effectively at the speed of light. As galaxies have redshifts less than about 10, this remains true for any neutrinos emitted from galactic sources in our Universe. If you want slow neutrinos, you should look at the cosmic neutrino background. This has a temperature of 1.95 K now, which would correspond to typical velocities of 0.02c for a neutrino mass of 1 eV (higher if the neutrino mass is lower). --Wrongfilter (talk) 08:20, 27 May 2020 (UTC)[reply]

Why do flies make so much noise during sex?

I have often been alerted to the presence of copulating flies (of various species) by the loud sustained buzzing noise they make while doing so. Why do they make such a lot of noise in this way? It surely makes them vulnerable to predation, indeed as reported here. PaleCloudedWhite (talk) 12:27, 27 May 2020 (UTC)[reply]

Reminds me of a true story I saw a while ago (unfortunately I can’t remember the details). A female human was stranded in the wilderness for a while, and had her period during that time; she was worried about the blood attracting predators. I’ve been wondering about that since then. Menstruation (for humans) and Menstruation (mammal) don’t seem to address this directly, although they do suggest that menstruation does not confer an evolutionary advantage. Brianjd (talk) 13:41, 28 May 2020 (UTC)[reply]

Humans and other animals are rather noisy in that situation too. Maybe the buzzing noise excites the flies just as the respective noises excite humans. If it helps to a succesful copulation it is a selective advantage which compensates for the disadvantage of drawing predators' attention. In the long run this 16 percent of flies eaten by bats can be of no relevance if the other 84 percent are not eaten while laiyng more eggs or copulating twice as often or what makes flies more successful in this domain if they are more noisy. Same for menstruation: maybe menstruating women are more vulnerable to predators, but first this is less true since humans live in large communities able to defend them, and importantly the advantage for a freshly fertilized egg to find a soft and fresh mucous membrane to implantate itself in the uterus instead of some dry and fibrous age old tissues must compensate for the disadvantages. In general it seems that 20 or 30 percent losses doesn't matter at all if another 20 or 30 percent ist successful enough. I think the problem comes from the idea that selection has to act only on individuals: this is only true for individuals living alone for themselves. As soon as several individuals live together and act as a group, the group becomes also an object for selection. I know that this view is deprecated today, but the paradigma 'selection can only act onto individuals' seems to me to be simply untenable under several aspects. 2003:F5:6F08:8200:25BC:4DFB:3A9F:92FF (talk) 18:33, 29 May 2020 (UTC) Marco PB[reply]

Considering their short lifespan, they've got to grab all the gusto they can. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:08, 29 May 2020 (UTC)[reply]

Wheelchairs and steps

Can a wheelchair go up a 5cm high step without much effort? 90.192.124.163 (talk) 20:06, 27 May 2020 (UTC)[reply]

What type of wheelchair? What sized wheels? What constitutes "much effort"? --OuroborosCobra (talk) 20:09, 27 May 2020 (UTC)[reply]

Assuming a wheel diameter of 24 inch (almost 61 cm), the initial effort to make the wheel go over the step is about the same as required for going up a 33.3° ramp. In general, for a step of height

s

and wheel radius

r

(half the diameter), when the wheel makes contact with the step, the axle is still at a horizontal distance

d={\sqrt {r^{2}-(r-s)^{2}}}

from the edge of the step. The angle between the vertical and the line from the axle to the point of contact is then

\alpha =\mathrm {arctan} {\frac {d}{r-s}}.

The effort of moving forward is the same as on a slope perpendicular to the line from axle to point of contact, that is, a slope at angle

\alpha

. If

s

is small compared to

r

,

\alpha

can be approximated by

\alpha \approx {\sqrt {\frac {s}{r}}}\times 81^{\circ }.

--Lambiam 21:58, 27 May 2020 (UTC)[reply]

It's important to realise that people use wheelchairs for a wide varieties of reasons. Some paraplegics have better upper body strength and capability than most able bodied people, and could cope with a 5 cm step with no difficulty at all. Others can be closer to being tetraplegic, and while they can use a manual wheelchair successfully on flat, hard surfaces, they would find a 2 cm step an insurmountable barrier. Of course, one needs to consider whether we are talking about manual or motorised wheelchairs, or among the latter, those specifically designed to navigate stairs.

An alternative and possibly more important question to me is, why on earth does a 2 cm step exist? Surely a sloping ramp of some sort would be more appropriate for all of us there. Architects and builders seem to have an obsession with putting stairs everywhere, even when a sloping ramp would be easier and cheaper to install. I call it the "Gone With The Wind" syndrome, a subconscious desire to create some sort of dramatic statement with a staircase. HiLo48 (talk) 23:59, 27 May 2020 (UTC)[reply]

Whether you meant 2 cm or 5 cm, possible reasons include: incorrect matching of outdoor paving with as sidewalks with building floor levels; settling of the substrate below the paving materials; incorrect adjustment of the train floor height according to its current load; reduced horizontal clearance between the train and the platform (by placing the bottom of the train floor is slightly above the platform). It's not all about an architectural obsession with stairs. --76.71.5.208 (talk) 00:51, 28 May 2020 (UTC)[reply]

Whoops. Sorry. I meant 5 cm. I'm a mature aged, metricated Australian who tends to do instant cm<->in conversions to keep up with what Americans usually write here, but this time I didn't need to. My mistake. But yes, I agree, small steps can occur "by accident", but even a lot of them could be smoothed out if society thought and cared more about wheelies. HiLo48 (talk) 01:33, 28 May 2020 (UTC)[reply]

2cm could keep water outside a threshold of a building even for nominal ground-floor entrance, or in the gutter rather than spreading up a driveway (roadway design standards seem to be 1–2 inches for that latter context). DMacks (talk) 02:23, 28 May 2020 (UTC)[reply]

But it could be a sloping ramp, rather than a step. HiLo48 (talk) 03:19, 28 May 2020 (UTC)[reply]

The step could be that of the kerb separating a roadway from the pedestrian pavement. --Lambiam 07:59, 28 May 2020 (UTC)[reply]

The equivalent ramp slope only indicates the initial force required to start moving over the step, not a sustained effort. Another measure is the speed of approach for which your momentum will carry you over the step. If I'm not mistaken, it is given by

v={\frac {r}{r-s}}{\sqrt {2gs}},

in which

g

is the gravitational acceleration. For a step of 5 cm, this comes out at 4.3 km/h, a walking pace. The jolt will be considerable but not insufferable. A small ramp, even if as steep as 45°, will significantly reduce the jolt. --Lambiam 07:50, 28 May 2020 (UTC)[reply]

I'm in awe of your calculations Lambiam, but you seem to have forgotten that on a standard manual wheelchair, the wheels making first contact with the step will not be the big 24" ones but the much smaller caster wheels at the front (unless you are going backwards). So it's necessary to tip the wheelchair back to lift these caster wheels over the step before the big wheels make contact. There are users who can perform this manoeuvre unaided, but there are also those who cannot. Just ramming the step at high speed tends to result in the forward ejection of the user, as the small wheels come to a full stop. Wheelchairs (except sports models) have a projecting piece at the back which an assistant can push down on with their foot to lift the front wheels when negotiating steps or kerbs.

The obvious solution to this conundrum is a rubber threshold ramp. The linked site says: "4mm may not seem much to someone not in a chair, but it presents a physical barrier that stops you entering if you are in a chair". They start at GBP 15 (USD 18). [4] Alansplodge (talk) 11:09, 28 May 2020 (UTC)[reply]

For my first calculation I had not forgotten about them, having observed that wheelchair users typically have become adroit at popping the caster wheels by giving the chair a quick backward jolt, a skill acquired mainly by practice.^[5] Doing that at just the right moment while approaching a threshold at 5 km/h is indeed more difficult – although, I expect, not out of reach for most wheelchair users. --Lambiam 14:49, 28 May 2020 (UTC)[reply]

BTW, I doubt the claim that a 4 mm threshold constitutes a problematic barrier. --Lambiam 16:43, 28 May 2020 (UTC)[reply]

If there's a reader here who uses a wheelchair, maybe they could comment. 4 mm is .15 inches, which isn't much. Just going from a bare floor to a carpeted area could be more than that. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:55, 28 May 2020 (UTC)[reply]

4 mm would not be a barrier to most, but it would be to some. HiLo48 (talk) 23:41, 28 May 2020 (UTC)[reply]

Assuming a 4-inch diameter for the caster wheels, the momentum will carry you over a 4 mm barrier even at a moderate speed of 1.1 km/h (0.68 mph). Section 303 (Changes in Level) of the Architectural Barriers Act Accessibility Standard (ABAAS) of 2006 allows vertical changes in level up to 6.8 mm to remain unbevelled.^[6] --Lambiam 11:10, 29 May 2020 (UTC)[reply]

Thanks, it seems the 4 mm claim is hyperbole. However, that must mean that a step of 50 mm is not permitted in US federally funded buildings. Alansplodge (talk) 11:48, 29 May 2020 (UTC)[reply]

The threshold ramp link mentions doors with thresholds. If it's high enough, you could trip on it just from normal walking, so that could certainly be a problem for a wheelchair. 4 mm is small, as is 6.8 mm - but 50 mm or 5.0 cm would be close to 2 inches, and that could definitely be a struggle in a wheelchair. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:36, 29 May 2020 (UTC)[reply]

May 28

Complex compounds

Hi everyone, today, I will ask question about color of two complexes:

CoBr₂(urea)₁₀ (1), and
FeCl₃(thiourea)₆ (2).

I don't think that (2) will have light pink color, compound (1) is ... (I don't know), :(. If you can, please tell me what is exactly color of these compounds. Thanks for very much (Sorry if you don't understand, because my English is not good). --Ccv2020 (talk) 10:36, 28 May 2020 (UTC)[reply]

Mechanics

Why is the information surrounding automobiles and automobile repairs so poor everything is theory nothing is real life the information on automobiles in Wikipedia is very much on the weak side or anything mechanical for that matter you need to step it up — Preceding unsigned comment added by 24.192.193.98 (talk) 13:18, 28 May 2020 (UTC)[reply]

Everything here was contributed by volunteers. If it doesn’t meet your lofty standards, perhaps you could volunteer too. Brianjd (talk) 13:27, 28 May 2020 (UTC)[reply]
Can you give us a specific example? ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:25, 28 May 2020 (UTC)[reply]

Note that Wikipedia is an encyclopedia, and doesn't offer "how-to" instructions (see:WP:NOTHOWTO). Wikihow (unaffiliated) does. 107.15.157.44 (talk) 21:40, 28 May 2020 (UTC)[reply]

May 29

Looking for a source for Cotyledon tomentosa synonym

Hi, I am working on expanding Cotyledon tomentosa, and I am having trouble finding a source for an obscure taxonomic synonym. The SANBI website [8] lists that Cotyledon ladismithensis Boom may be a synonym for C. tomentosa subsp. tomentosa, but I haven't been able to track down this particular taxonomy, especially since the same name was used by Poelln. and is now a synonym for C. tomentosa subsp. ladismithensis. Here, Boom refers to Boudewijn Karel Boom, so I looked through the worldcat list of publications and narrowed my search to two works,

An article: Boom, B. K. Notes on nomenclature for succulent plants. (In Dutch.) Succulenta 11:141-145. Nov. 1959.
A book: Notities over succulenten [9]

The first article I was able to find in an archive here: [10] and even though I cannot read Dutch, this article does not mention C. tomentosa at all. So I'm assuming that the original SANBI Red List listing is referring to the book, but I have not been able to find this book anywhere online. In addition, I haven't been able to find any secondary sources listing C. ladismithensis Boom as a synonym for C. tomentosa. Any assistance in finding either the book or a secondary source listing Boom as a synonym would be greatly appreciated! Sn1per (talk) 05:23, 29 May 2020 (UTC)[reply]

Another possible book: [11] mentions "The fourth and last part [of the books Flora Der Cultuurgewassen Van Nederland] will deal with the assortment of succulent plants grown in special collections" but I can't find any evidence of this part actually existing. Sn1per (talk) 05:40, 29 May 2020 (UTC)[reply]

I have found the original source! So the worldcat entry for the book gave the crucial insight that the book is actually "Articles reprinted from Succulenta, 1957-1958." And since I found the Succulenta archive, I searched the 1957 and 1958 archives and found the article. Indeed, it is "Codyledon ladysmitiensis Poellnitz" by B. K. Boom, in Succulenta 1958, issue 2, and the description of the article is much closer to subsp. cotyledon than subsp. ladysmithiensis. Cheers, Sn1per (talk) 15:15, 29 May 2020 (UTC)[reply]

May 30

Rocket launch speed is very slow and constant?

If I walk slow , I reach destination at slow. Rocket launch speed is very slow and constant?

Will it reach the destination without obstacles? Ram nareshji (talk) 02:30, 30 May 2020 (UTC)[reply]