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August 8

Is IUPAC nomenclature unambiguous?

Has it been proven that the nomenclature for say organic compounds is totally unambiguous? Could you theoretically write a computer program to go from the IUPAC name to a 3d model and back again? 24.255.17.182 (talk) 06:08, 8 August 2016 (UTC)[reply]

Such programs exist (although they may not necessarily be complete). IUPAC nomenclature is demonstrably complete and unambiguous, but not necessarily unique (one chemical could theoretically have two names), since it is effectively a way of converting the structural formula into text. As long as a chemical's structure can be drawn on paper, it can be represented in IUPAC nomenclature. However, there are some weird legacy complications that come from putting old names in the IUPAC systems (see Phosphorus pentoxide - unambiguous, but technicaly wrong). SMILES or InChl are preferred for inputting the structure of compounds, since they contain the information in a much more straightforward way. (For instance, the convertor that I linked failed to render the structure of uranium hexafluoride properly, possibly because it didn't have access to information about how uranium bonds, but it rendered it properly when given the SMILES code "F[U](F)(F)(F)(F)F". It also failed to render morphine at all, since the IUPAC name includes Greek letters which the parser couldn't handle.) Smurrayinchester 10:24, 8 August 2016 (UTC)[reply]

Every time someone discovers a kind of detail that is not covered by the existing IUPAC (or SMILES, or InChI, or whatever) ruleset, the rules are enhanced to cover that case. New IUPAC recommendations and new versions or layers of InChI sometimes come out every few years. The whole purpose of these structure-based definitions is to be able to commuicate clearly (with a person, written index, computer program, etc.), so they are intentionally complete (to the known cases) and correctable going forward (by new editions of the rules). See for example the preface of the IUPAC Nomenclature of Organic Chemistry[1].

But names/etc don't usually include conformational or other transient shape details or positions of ions. Predicting those sorts of things is one of the unsolved problems in science for many types of compounds. Computerized converters are notoriously bad (and often predictably very wrong) for 3D structures because the data given to them does not contain that information. They can only convert among representations of the data in the string that is given. Lots of cases of "everyone knows what we mean" (say X to mean X and Y, or say X to mean Z) don't work when literally analyzing the strings of text. DMacks (talk) 19:16, 8 August 2016 (UTC)[reply]

No language is entirely disambiguous (Umberto Eco,Ferdinand Saussure) but neither is it impossible to make any language precise enough to achieve the necessary accuracy: Aristotle, Spinoza, Leibnitz, Isaac Newton, Ayn Rand. Even Heisenberg and Douglass Hoffstaedter agree, assuming you pay attention to what they actually say. μηδείς (talk) 15:02, 9 August 2016 (UTC)[reply]

Another IUPAC question

How sure are we that every possible organic compound has an IUPAC name? Is it possible to rigorously show this? 24.255.17.182 (talk) 06:48, 8 August 2016 (UTC)[reply]

Every possible organic compound has a possible IUPAC name; because as noted above, the nomenclature is unambiguous; there are rules one follows that connect the chemical structure of a compound with a specific IUPAC name; it's a "code" if you will, where the name codes for a specific structure, and you can work the code in both directions: given a name, you can build the structure. Given the structure, you can build the name. See IUPAC nomenclature of organic chemistry for more details. Now, not every single possible molecule has an IUPAC name for it already put together. For example, large macromolecules like proteins would make little sense to develop a full IUPAC name. There are other, more reasonable systems for describing the structure of such molecules. --Jayron 32 18:46, 8 August 2016 (UTC)[reply]

As above, the systems are self-correcting by publication of new rules when ambiguous aspects or un-known structure-types/details are discovered. For example, in 2013, IUPAC published a new edition of its Nomenclature of Organic Chemistry, the previous edition of which was from 1993. "This publication includes nomenclature principles developed for more complex substances and new classes of compounds such as fullerenes and cyclophanes."[2] DMacks (talk) 19:27, 8 August 2016 (UTC)[reply]

Friction and area of contact

On slippery conditions, the drivers are asked to deflate the tyres a bit, apparently to provide more friction. My own experience is that if the tyres are inflated more than necessary, they don't 'grip' the road very well (I tried this on my bicycle). If this because deflated tyres 'hug' the road better (more area of contact) ? I thought friction was independent of the area of contact ? That is what the Amontons' Second Law mentioned in the article Friction says too.

Am I missing something here ? Are there some other forces in play ? - WikiCheng | Talk 08:49, 8 August 2016 (UTC)[reply]

Yes. Amontons' second law is a typical case of an idealised "applies only to spherical cows in a vacuum" type of law. In other words, it applies to sliding friction of smooth, dry, homogenous surfaces. For a real vehicle, none of these assumptions are true. First, the tire patch on the ground does not, ideally, move against the ground at all. So it's not a case of sliding friction, but one of static friction. Note that static friction is typically much higher than dynamic friction, so you very much want to avoid crossing into the other domain. One advantage of less inflated tires is that they are more flexible, i.e. they reduce load spikes which might push the vehicle over the static friction load. Another significant effect is that your car not only can slide over the surface, but can abrade it. For a thought experiment, consider a flat road partly filled with spherical marbles. If your tire is under high pressure, it will just ride on top of the marbles, and experience nearly no friction. If, on the other hand, it is only partially inflated, it will "sag" down and meet the firm ground, thus providing some friction. Now mentally replace the ideal surface with the road, and the marbles with e.g. gravel. --Stephan Schulz (talk) 09:47, 8 August 2016 (UTC)[reply]

[EC] In addition to the classical frictional forces (which in the real world do not correspond to the theoretical ideal - see our article Tire load sensitivity), less inflated tyres will flex a little more and therefore heat up a little more, in turn making them even more flexible. The higher temperature might well improve road adhesion (depending on further factors like the physical and chemical nature of the road surface), and their increased flexibility would also enable them to conform a little better to smaller and larger irregularities in the surface: the former would improve momentary grip, and the latter (functioning as a component of the suspension) would help to keep the tyre in contact with the surface for longer. As followers of Formula 1 and similar motor sports will be aware, this is a complex area – any F1 engineers reading this? {The poster formerly known as 87.81.230.195} 2.123.26.60 (talk) 09:51, 8 August 2016 (UTC)[reply]

Stephan... sorry I didn't understand. I understand that it is the case of static friction but why is the static friction more when the tyre is deflated ? As for your example, if the road is full of marbles, even a partially inflated tyre can ride on the marbles (if the marbles are closely placed), right ? Let us say we take two vehicles. One with regular tyres and another with tyres made of a different type of rubber which compresses more (thus providing more contact area with the road) but has the same coefficient of friction as the regular tyre. If both the vehicles are braked from the same speed, will the vehicle with the special rubber stop earlier ? - WikiCheng | Talk 10:15, 8 August 2016 (UTC)[reply]

With regard to the static vs. dynamic friction: Assume your car is in the regime where the static friction is large enough to keep the car on the road, but the dynamic friction (which is lower) is not. When the car gets a small extra bump, and the tires are very inflexible, that bump is passed unattenuated to the tire/road interface, causing the car to start sliding, i.e. moving into the dynamic regime. If the tire is more flexible, the bump will lead to a deformation of the tire first, i.e. the force is spread over a longer time, possibly avoiding crossing the static friction limit. As for the second part: Yes, of course you can play with the parameters to break the thought experiment. It's a simplification. Also see 2.123's comment. --Stephan Schulz (talk) 10:29, 8 August 2016 (UTC)[reply]

Hmmm... I think I understand it. Just another question: would it be more difficult to drag a mass (assume cuboid) of rubber on a surface, if the area of contact is more ? Is this scenario different from a car with rubber tyres ? - WikiCheng | Talk 10:53, 8 August 2016 (UTC)[reply]

Note also that deflating tires can be dangerous. Tigraan^{Click here to contact me} 11:29, 8 August 2016 (UTC)[reply]
- Especially while the car is moving. Watching the recent bicycling races on TV, they were talking about swapping out bikes and tires for more or less deflation depending on circumstances within a given course. That's pretty easy to do when you've got a team car nearby to carry the extra bikes and tires. It doesn't work so well on a car in the wilderness. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:44, 8 August 2016 (UTC)[reply]

Can the stomach adapt to eating large quantities of food in a relatively short period of time?

When reading or hearing the anecdotes of obese people, I find that obese people have a huge appetite. They are always hungry. Do these obese people naturally have huge appetites for food, or is taking on bigger portion sizes of food the result of the interaction between the individual and environment? 66.213.29.17 (talk) 17:22, 8 August 2016 (UTC)[reply]

Obesity#Causes has some information on the causes of obesity. Regarding appetite, the article at Hunger (motivational state) may have some information which may help you find your answer. --Jayron 32 18:27, 8 August 2016 (UTC)[reply]

Obese people do not typically gorge on huge quantities of food. You can train your body to tolerate gorging on massive quantities of foods. It's not healthy to do, of course, but competitive eaters are masters at this. Take e.g. Matt Stonie and note that he is quite far from being obese "He weighs 130 pounds (59 kg) and is 5 ft 8 in (1.73 m) tall." He can eat as much as Michael Phelps eats in an entire day in less than 45 minutes. Count Iblis (talk) 18:54, 8 August 2016 (UTC)[reply]

I'm not sure if this relates to obese individuals, but as for the stomach adapting, see Competitive eating and associated pages like Joey Chestnut. Competitive eaters apparently train for their events. Cannolis (talk) 23:50, 8 August 2016 (UTC)[reply]

Do colors look hard to distinguish??

How come violet and magenta are easy to distinguish, but yellow-green (the intermediate color between yellow and green) and green are not?? Georgia guy (talk) 18:14, 8 August 2016 (UTC)[reply]

Who says so? ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:16, 8 August 2016 (UTC)[reply]

Color vision is a tricky thing, and varies greatly between individuals. As noted at places like Contrast (vision), and Visual acuity, the ability to discern colors (even among people without actual color blindness) will vary greatly. That is, while YOU have the experience regarding drawing distinctions between yellow-green and green, that doesn't mean everyone does. There's even some research which shows that your ability to distinguish colors may be influenced by cultural phenomena as well. If I had to make a guess (and this is a WAG) why this may be so (if it even is true), it may have something to do with the responsiveness of the three main types of Cone cells most people have. You'll notice the "center" of the peaks in the graph at the top of that article are not evenly spaced; it may mean that distinguishing differences involving variations may have a biological reason. But that is a guess. --Jayron 32 18:23, 8 August 2016 (UTC)[reply]

This is purely OR, but I would suggest that in extremely bright environment (noon on a summer day) you try closing your eyes alternatively, and look at some multicolored surface like an indoor-outdoor rug. If you are like me, the view will differ from each eye. In my case, the view from the right eye seems more orange, that from the left more blue. When I took Biology 101 in college, I scored 30/30 on the color vision test, and my professor failed me, even though my co-students said I had not falsified my results. It turns out women often have better color perception than men. See tetrachromacy. μηδείς (talk) 22:15, 8 August 2016 (UTC)[reply]

For what it's worth, I'm an XY with normal color vision and I've also noticed that my color perception is bluer in one eye and oranger in the other (at least sometimes), so I think that's not related to tetrachromacy. -- BenRG (talk) 01:50, 9 August 2016 (UTC)[reply]

The unclarity was mine, I did not mean to suggest that the blue/orange difference was itself the same as tetrachromacy, but that it might have other causes, yet be responsible for an increased colour sensitivity. μηδείς (talk) 14:41, 9 August 2016 (UTC)[reply]

I think Jayron is on the right track with Cone cell.. Have a look at the color spectrum here and compare it to the peaks int the cone cell article. I think it's quite easy to see why "red and green" appear more similar than "red and blue", i think it's less to do with individual difference or color blindness. Vespine (talk) 22:52, 8 August 2016 (UTC)[reply]

Interesting... I find violet and indigo difficult to distinguish sometimes (some things I call violet others call deep blue/indigo). But that chart shows that color distinction between blue tones is more accurate. EvergreenFir (talk) 00:37, 9 August 2016 (UTC)[reply]

The web page you linked doesn't show a spectrum but rather an sRGB hue wheel which includes spectral and non-spectral hues. The surrounding text is inaccurate. Here's a picture of cone response curves plotted on a spectrum. -- BenRG (talk) 01:50, 9 August 2016 (UTC)[reply]

Sorry yes it's not a 'spectrum' but the sRGB wheel precisely illustrates my point, being that "the Hue value places each of the six primary colors 60 degrees from its neighbors." On THIS wheel you can see the distance between blue and red and green and red is the SAME, which makes it SEEM like those colors should be equally discernible, but the link YOU linked, which is similar to what is found in the Cone cell article reveals that in fact that the red and green cones are FAR closer in sensitivity than the red and blue. In fact even GREEN and BLUE are close than red and blue. Vespine (talk) 03:14, 9 August 2016 (UTC)[reply]

Another possibility is that you're using brightness and/or saturation as a secondary clue. That's especially likely if you're viewing maximally bright/saturated color samples on a TV or computer monitor. -- BenRG (talk) 01:50, 9 August 2016 (UTC)[reply]

These Macadam ellipses are really cool, but is there better data available that paves the entire color curve with them? I think that would be really a useful and interesting resource - also, it would be really useful if someone has devised a "homonculus" where this CIE color curve has been distorted to make all the ellipses circles of equal size throughout. Wnt (talk) 12:59, 9 August 2016 (UTC)[reply]

One term that can help you find research papers is the phrase "just-noticeable difference," which is a measure of most people's ability to distinguish between individual sensory inputs. Most color spaces (and engineered systems based on those color spaces) are designed so that small changes in the color space coordinate fall below the threshold of noticeable difference. Psychologists and scientists study the JND using controlled experiments to precisely measure the change in visual input that a test subject notices as a "different color."

Using these kinds of terms will help you to find technical papers and computer software to help you render JND as an ellipse on a color-space plot; or in any other useful display format. For example, have a look at the technical research references in, e.g., the Lab color space article, or the articles for any other of your favorite color coordinate systems.

Generally, if your color coordinate is directly linearly related to the "wavelength of light," then the perceptual behavior as you perform a linear sweep across the coordinate space would appear to be a non-uniform change in perceived color. This is because the human perceptual response to color, just like the response to light intensity and many other quantifiable parameters of the human color vision system, is very nonlinear.

All of this science is sometimes obfuscated because natural human language words for color are often used very imprecisely - and subject to cultural and situational biases.

Nimur (talk) 13:07, 9 August 2016 (UTC)[reply]

Thanks for the clue. I didn't really know what CIELUV and CIELAB were about, and didn't appreciate the idea of "perceptual uniformity". Looking just a little further I see there is a HCL color space that is supposed to be more uniform; even so, it seems like all these spaces are subject to the Procrustean rule of the flat plane or space, when perhaps something non-Euclidean is in order, and they rely on formulas, when I have the sense that biology is not always that good at math... Anyway, there are some practical routines here that might be good for coming up with individual palettes. I find myself daydreaming of finding a way to identify the RGB part of a three-dimensional HCL color space, put it as an empirical data file, and write a Lua script for Wikipedia modules to pick a long set of colors as a sort of 3D maze solution that fits within it. But I have a very good inkling I'm not really going to get it done; I've left an embarrassing number of half- or tenth-done script ideas lying around already. Wnt (talk) 23:05, 9 August 2016 (UTC)[reply]