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: I know what you mean about "factor-label" (though I hadn't heard it called that). And indeed your ''(0.5 rad/sec) * (1 rotation/2 pi radians) * ((4 inch * 2 pi) / 1 rotation)'' ends up being pretty darn close to my ''ω · 2π · d / ¤'' ! —[[User:scs|Steve Summit]] ([[User talk:scs|talk]]) 05:37, 20 December 2018 (UTC) |
: I know what you mean about "factor-label" (though I hadn't heard it called that). And indeed your ''(0.5 rad/sec) * (1 rotation/2 pi radians) * ((4 inch * 2 pi) / 1 rotation)'' ends up being pretty darn close to my ''ω · 2π · d / ¤'' ! —[[User:scs|Steve Summit]] ([[User talk:scs|talk]]) 05:37, 20 December 2018 (UTC) |
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::I should redirect [[Factor-label method]] to [[dimensional analysis]] which has a section on it. The key difference is that your formula tends to look kind of like a random series of operations, where ideally a factor-label computation should take only a few pieces of data (ideally, none, if you phrase it just so) and everything else involves multiplication by 1 or, rarely, addition of 0. In other words, one rotation '''is''' 2 pi radians, so you know you can multiply by that ratio, and the same is true for the circumference being one rotation, and you can even say that, for purposes of this computation, 0.5 radians ''is'' one second, in the sense that every time you have one occur you have the other occur, and so inevitably, the result is equal to 1 also, in some conceptual sense: 2 inches ''is'' what happens in one second. [[User:Wnt|Wnt]] ([[User talk:Wnt|talk]]) 15:25, 20 December 2018 (UTC) |
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== Can Tapir's be ridden or domesticated? == |
== Can Tapir's be ridden or domesticated? == |
Revision as of 15:25, 20 December 2018
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December 12
Science problem: Statistical solutions vs non-statistical solutions
What does it mean when an equation has a statistical solution? Like: Statistical solutions of hyperbolic conservation laws Or: Statistical solution Navier Stokes equation Wouldn't any solution be somehow a statistical solution? Wouldn't any solution be algebraic too? Could a science problem have an algebraic solution but no statistical solution, or the other way round?--Doroletho (talk) 17:57, 12 December 2018 (UTC)
- Can you give a link to a specific article where you read this so we can have some context? --Jayron32 18:04, 12 December 2018 (UTC)
- For example: [1] or [2]. But the question is in general. What makes a solution 'statistical'? What non-statistical solutions are there? Can statistical and non-statistical approaches to a problem differ? Or one approach, as long as it's not a blunder, will always confirm the other?--Doroletho (talk) 18:48, 12 December 2018 (UTC)
- I think that means statistical in the sense of statistical mechanics. E.g. the ideal gas law is statistical. It starts with the idea of gas molecules flying in all different directions and colliding randomly. It then estimates statistical quantities like the mean kinetic energy of the molecules (i.e. the temperature of the gas) and the distribution of the energy (the Maxwell-Boltzmann distribution) without predicting the energy of individual molecules. Even in purely classical mechanics it's not possible to solve such a system exactly, but only statistically. On the other hand, if you can solve a system exactly (e.g. a simpler system involving a few billiard balls) then you can compute the statistics from the solution. 173.228.123.166 (talk) 21:22, 12 December 2018 (UTC)
- For example: [1] or [2]. But the question is in general. What makes a solution 'statistical'? What non-statistical solutions are there? Can statistical and non-statistical approaches to a problem differ? Or one approach, as long as it's not a blunder, will always confirm the other?--Doroletho (talk) 18:48, 12 December 2018 (UTC)
In what sense is a Newtonian fluid elastic?
The first sentence of Speed of sound says, "The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium." But Elasticity (physics) implies that Newtonian fluids are not elastic: "Elasticity is not exhibited only by solids; non-Newtonian fluids, such as viscoelastic fluids, will also exhibit elasticity in certain conditions..."
Yet the concept of sound compression waves seems to apply just fine in Newtonian fluid models such as the ideal gas model. Are there different senses of "elastic" at work here? For example, are ideal gases elastic media despite not being elastic substances? --Allen (talk) 20:24, 12 December 2018 (UTC)
- I thought elastic just meant kinetic energy is conserved after a collision. In that sense, gases are elastic, while viscous liquids are less so. 173.228.123.166 (talk) 21:17, 12 December 2018 (UTC)
- Thanks. I can see how that's true of elasticity of the collisions of the constituent particles. But I can't see how that relates to elasticity of the material as a whole, which according to Elasticity (physics) is "the ability of a body to resist a distorting influence and to return to its original size and shape when that influence or force is removed." So rubber would be elastic, but not a gas (at least not in any sense that's obvious to me). --Allen (talk) 22:49, 12 December 2018 (UTC)
- A guess at answering my own question: From Elastic modulus, I see that bulk modulus (which of course gases have) is one type of elastic modulus. So perhaps when people say things like what I quoted before from Elasticity (physics) (statements that imply that Newtonian fluids are not elastic), it's understood that they're only talking about Young's modulus and shear modulus. But when they refer to sound traveling through elastic media, it's understood that any elastic modulus counts. On the other hand, it seems weird to say "elastic media" in a context in which no media could possibly not be elastic. --Allen (talk) 22:49, 12 December 2018 (UTC)
- Sound waves in solids can be either P-waves (compressional waves) or S-waves (shear waves), but only the P-waves can pass through a fluid, where it moves as a pressure wave. Shear wave propagation requires elastic rigidity. Mikenorton (talk) 00:15, 14 December 2018 (UTC)
- Indeed, that's how we know that the outer core is a liquid. Here's an explanation of the longitudinal pressure waves that carry sound through air - nothing elastic there. Mikenorton (talk) 21:40, 14 December 2018 (UTC)
- Thanks! So should the first sentence of Speed of sound be changed? Or is even air an "elastic medium" in the appropriate sense? --Allen (talk) 17:24, 15 December 2018 (UTC)
- In my understanding, it should. A gas is not an elastic medium and only non-Newtonian liquids have any elasticity. The key thing is that any change in shape of an elastic material should be recoverable - to quote from our article "elasticity .... is the ability of a body to resist a distorting influence and to return to its original size and shape when that influence or force is removed". Mikenorton (talk) 18:48, 15 December 2018 (UTC)
fatty acid beta oxidation
Hi all, I'm trying to understand beta oxidation of fatty acids, I've watched a few youtube vids and looked at the wiki article on beta hydroxylation but I'm still a bit uncertain! Please note, I'm trying to learn this from a human biochemistry perspective, not any other organism. From my understanding, beta oxidation cleaves 2 carbons from the fatty acid per cycle, until it is completely used up. My understanding is that acyl-coA dehydrogenase carries out the first step of beta oxidation through the process of dehydrogenation. My question is this: If only 2 carbons are removed at a time, why is it necessary for the human body to have short chain, medium chain and long chain acyl-CoA dehydrogenases? Surely it doesn't matter how long the chain is if the body is only removing 2 carbons at a time? There must be a reason why but I don't understand it. I also don't understand if there's 3 or 4 acyl-coA dehydrogenases... Any help would be much appreciated! RichYPE (talk) 20:52, 12 December 2018 (UTC)
- Courtesy links: Acyl CoA dehydrogenase, beta oxidation. It looks like mammals have three acyl CoA dehydrogenases: short, medium, and long; very long chain fatty acids are first broken down in peroxisomes into medium-chain fatty acids, which are then sent to beta oxidation.
- When talking about biology we need to be careful about using words like "necessary". Evolution is a "blind watchmaker". Biological systems are often not "necessary" or "optimal", just "good enough". Presumably there is some selective advantage in having specific acyl CoA dehydrogenases, or else they wouldn't be conserved. Perhaps a generic enzyme wouldn't bond well to substrates. Or perhaps the different enzymes are localized to different areas of the cell, making them more efficient. Molecules have all kinds of shapes. "Removing two carbons" is a useful model for analyzing the reactions in a schematic way, but remember that this is very reductionist. When analyzing how the enzyme reaction actually proceeds, and how different enzymes interact with different substrates, we have to look at the enzyme kinetics. There might be some research on this, but I'm afraid I'm not familiar with the subject. Maybe someone else is. --47.146.63.87 (talk) 20:29, 13 December 2018 (UTC)
- There is a bit on it here, showing overlapping specificities. Quick glance I think they said there's a C-terminal domain that controls substrate specificity, and that this was upheld in [3] later on. Now the thing to bear in mind is that there are fatty acids in a cell that are doing different things, so there is some biological imperative to control which become substrates for degradation under various circumstances, I think. After all, if all fatty acids did exactly the same thing, why would there be more than one compound available to be broken down? Wnt (talk) 22:14, 13 December 2018 (UTC)
Many thanks to both of you for your responses. There must indeed (presumably) be a survival advantage conferred by having a number of different AcylCoA dehdyrogenases. Best wishes RichYPE (talk) 09:23, 14 December 2018 (UTC)
bread vs pasta
Are they pretty much the same thing, nutritionally speaking? I.e. basically mostly wheat flour with about the same carbs, vitamins, protein, etc. per ounce of dry weight? Web search shows a bunch of "health" comparisons but no nutritional ones that I saw immediately. Question came up because of an online discussion of foods to keep around the house in case of natural disasters etc. Thanks. 173.228.123.166 (talk) 21:15, 12 December 2018 (UTC)
- Instead of random websites, consider using Nutrition.gov, "a USDA-sponsored website that offers credible information to help you make healthful eating choices."
- If you're looking for suggestions about best practices to prepare for natural disasters, consider reading Ready.gov, including their suggested food preparation tips.
- In particular, they recommend "dry cereal or granola" - but neither bread nor pasta make the top of the food supplies list. Remember, in a natural disaster, fresh clean water may be unavailable, and if it's really scarce, it's best to save it for drinking (not for boiling noodles). Bread is a fine food, and works well in an emergency, but compared to alternatives, it's harder to stockpile bread in large quantities.
- Nimur (talk) 21:29, 12 December 2018 (UTC)
- Thanks. The online person had suggested storing stuff like flour in order to bake bread, rather than storing actual bread. That sounded kind of inconvenient compared to pasta because of the long baking time. I'm not trying to do the prepper thing myself but I do have a fair amount of water and canned stuff on hand, just in case. I think it would be ok to re-use the water after cooking pasta. It's also supposedly possible to cook it by soaking it in cold water for 1.5 hours, followed by a quick (1 minute) boil, saving a lot of fuel. I might try that just to see if it works. 173.228.123.166 (talk) 22:05, 12 December 2018 (UTC)
- A lot depends on the type and magnitude of disaster that you're preparing for. In many conditions, water and power won't be scarce; but it all depends on what happens and where.
- Here's a good free book for residents of the San Francisco Bay area: Putting Down Roots in Earthquake Country. It answers questions like... "Why should I care? Why should I prepare? What should I do?"
- Plan for at least one gallon of water, per person, per day, with a three-day supply - and that's just for drinking. Anything else you want to do - like cleaning, bathing, cooking, washing, ... or handling a messy medical emergency, no matter how minor - and you'll want to stockpile a lot more water. This is the single easiest, cheapest, and most useful thing you can do to prepare yourself: even in a mega-disaster like the so-called "Big One" earthquake, you aren't going to starve before help arrives - but you might get thirsty and you'll probably get dirty.
- Nimur (talk) 23:06, 12 December 2018 (UTC)
- Yeah here in the bay area, quakes are the main concern. I'll look at that book. Thanks. 173.228.123.166 (talk) 01:01, 13 December 2018 (UTC)
- Thanks. The online person had suggested storing stuff like flour in order to bake bread, rather than storing actual bread. That sounded kind of inconvenient compared to pasta because of the long baking time. I'm not trying to do the prepper thing myself but I do have a fair amount of water and canned stuff on hand, just in case. I think it would be ok to re-use the water after cooking pasta. It's also supposedly possible to cook it by soaking it in cold water for 1.5 hours, followed by a quick (1 minute) boil, saving a lot of fuel. I might try that just to see if it works. 173.228.123.166 (talk) 22:05, 12 December 2018 (UTC)
- Most Italian-style dry pasta is made from durum wheat flour (usually semolina flour) which is higher in protein and contains more iron, vitamin B, selenium, and iron than common wheat. The types of starches are also different, as Durum wheat also has more amylose (30% vs 22-26%) and less amylopectin than common wheat, and amylose takes longer for your body to convert into sugar. That said, there are some rustic breads that are made from durum wheat, and some pastas (incuding most asian wheat noodles) that are made with common wheat. --Ahecht (TALK
PAGE) 19:50, 13 December 2018 (UTC)- The comparison also depends on whether the flour used to make the pasta/bread is wholegrain or refined, and whether the bread is leavened or unleavened - leavened bread provides greater availability of zinc than unleavened.[4] According to this slightly irreverant article, zinc deficiency is linked to irritability and belligerence in males, thus Edward de Bono suggested that conflict in the Middle East be tackled by making zinc-rich yeast extract available there, to supplement the zinc-deficient unleavened bread normally consumed in the region. PaleCloudedWhite (talk) 20:32, 13 December 2018 (UTC)
December 13
The need for cooler roads
I thought about it and realized that painting an asphalt road with white paint is effectively adding more work on road maintenance. Wouldn't using concrete be a cheaper alternative?--Arima (talk) 07:44, 13 December 2018 (UTC)
- According to this site, asphalt has the advantages of being quicker to complete, cheaper to build (although the shorter lifespan may negate this in the long term), easier to repair, and safer. From personal experience they are also more pleasant to drive on (they have a slight give in them that makes for a smoother ride, although apparently this results in worse fuel economy). Iapetus (talk) 09:09, 13 December 2018 (UTC)
- Yes, the disadvantages outweigh the advantages Concrete#Roads. 86.131.187.141 (talk) 11:11, 13 December 2018 (UTC)
- It isn't paint. It is asphalt with some stuff in it to make it white and a good surface coat. Dmcq (talk) 13:42, 13 December 2018 (UTC)
- If you happen to be near an oil refinery, asphalt is essentially free. They try to maximize gasoline and diesel fuel but they often end up with more natural gas and asphalt then there is a demand for. --Guy Macon (talk) 17:44, 13 December 2018 (UTC)
- In the UK, concrete was the surface of choice for motorways (limited access highways) but is being replaced by asphalt because it is so noisy. "Motoring groups say driving on concrete is so unpleasant that some drivers believe there is a fault with their car" - End of the road for concrete on the motorway. Asphalt getting too hot is rarely a problem here though. Alansplodge (talk) 18:24, 13 December 2018 (UTC)
- And of course the noise is even more of an issue for the residents, since the road won't be driving away any time soon. Though our article on concrete roads claims there is some kind of "diamond grinding" technique to reduce it, which might be worth investigating further. Wnt (talk) 22:22, 13 December 2018 (UTC)
- In the UK, concrete was the surface of choice for motorways (limited access highways) but is being replaced by asphalt because it is so noisy. "Motoring groups say driving on concrete is so unpleasant that some drivers believe there is a fault with their car" - End of the road for concrete on the motorway. Asphalt getting too hot is rarely a problem here though. Alansplodge (talk) 18:24, 13 December 2018 (UTC)
- If you happen to be near an oil refinery, asphalt is essentially free. They try to maximize gasoline and diesel fuel but they often end up with more natural gas and asphalt then there is a demand for. --Guy Macon (talk) 17:44, 13 December 2018 (UTC)
- In parts of the world where it regularly gets below freezing, concrete cracks and spalls due to freezing water and the application of road salt. Some warmer parts of the United States have used concrete roads, but as someone that used to live there, it was not fun to drive on. Sunset Boulevard in Los Angeles was paved with concrete in the 1920s, but since it was such a busy road and concrete takes so long to cure (unlike asphalt, which can be drivable within hours), it wasn't repaved until 2010 and driving on it was always very rough (and the 2010 repaving used asphalt). Similarly, several of the highways in California are concrete, but since concrete is smoother and less water permiable than asphalt, they had to cut "rain grooves" in the roadway to prevent hydroplaning, but these grooves can cause groove wander, where the car tends to wander to follow the grooves. --Ahecht (TALK
PAGE) 19:18, 13 December 2018 (UTC)- I think you've hit it on the head. Laying concrete takes way too long compared with asphalt. That's find when you're building a new highway, but an exasperating inconvenience when you're re-paving. ←Baseball Bugs What's up, Doc? carrots→ 01:55, 14 December 2018 (UTC)
- Why are JFK's 8,400 and 10,000 foot runways asphalt while its 12,079 and 14,511 foot runways are concrete? Are the drawbacks of concrete less for (super)jumbo jets than for cars? Sagittarian Milky Way (talk) 19:54, 13 December 2018 (UTC)
- Advisory Circular 150/5320-6F, which I found from the airport design and engineering standards website, "provides guidance to the public on the design and evaluation of pavements used by aircraft at civil airports."
- The concrete runways are more rigid than the asphalt; this may affect aircraft performance for certain very large aircraft. For KJFK, the airport directory lists the Pavement Classification Number (PCN) for each runway. This number is affected by surface material and many other parameters.
- Airport designers usually spend a lot of time studying pavement; there are entire books on how to design and build runways. Here's an entire listing of resources on airport engineering and here's a sub-page just about pavement. Oh, if I could only count the hours I've whiled away, talking pavement with airport engineers,...
- From the extraordinarily thorough research paper, Porous Portland Cement Concrete: The State of the Art: "The rising costs for petroleum-based products will make portland cements more competitive with asphalt binders...", and closes by saying "commercial applications of porous concretes should be closely monitored for cost and performance data." So there you have it...
- Nimur (talk) 00:43, 14 December 2018 (UTC)
What building is the most number of rooms long?
Sagittarian Milky Way (talk) 15:26, 13 December 2018 (UTC)
- Hilbert's Hotel. DroneB (talk) 16:14, 13 December 2018 (UTC)
- What about Earth buildings from the part of spacetime before about 2019? Sagittarian Milky Way (talk) 16:55, 13 December 2018 (UTC)
- Do you mean "room" as a unit of linear measurement? Bus stop (talk) 17:33, 13 December 2018 (UTC)
- Yes, so a motel 1,000 meters long with rooms every 2 meters would be 500 rooms long and probably only about 2 wide and 1 to a few high. Sagittarian Milky Way (talk) 17:57, 13 December 2018 (UTC)
- The great wall of china has an really large number of towers. If you could get an estimate of how many towers and how many rooms in each, it might be the clear winner. --Guy Macon (talk) 18:02, 13 December 2018 (UTC)
- It's only sort of a building, vast majority of volume and length was never hollow so you couldn't walk between towers in it, only on it. Must've been a huge amount of man-hours to build though, maybe a room every ~10 meters between towers would've actually been cheaper (but less cannon resistant). Sagittarian Milky Way (talk) 18:26, 13 December 2018 (UTC)
- The great wall of china has an really large number of towers. If you could get an estimate of how many towers and how many rooms in each, it might be the clear winner. --Guy Macon (talk) 18:02, 13 December 2018 (UTC)
- Yes, so a motel 1,000 meters long with rooms every 2 meters would be 500 rooms long and probably only about 2 wide and 1 to a few high. Sagittarian Milky Way (talk) 17:57, 13 December 2018 (UTC)
- Do you mean "room" as a unit of linear measurement? Bus stop (talk) 17:33, 13 December 2018 (UTC)
- What about Earth buildings from the part of spacetime before about 2019? Sagittarian Milky Way (talk) 16:55, 13 December 2018 (UTC)
- --Guy Macon (talk) 17:51, 13 December 2018 (UTC)
- Prora. 100 hotel suites in each building, plus. Heaviside glow (talk) 20:45, 13 December 2018 (UTC)
MIT's Infinite Corridor runs through several buildings so I don't know if something like that "counts". I'm sure there are longer such corridors but that would expand the possibilities. Mall of America is one building though, I think. 173.228.123.166 (talk) 20:48, 13 December 2018 (UTC)
- ...or maybe having many small rooms is the key. I wonder how big the largest capsule hotel or self storage unit is? --Guy Macon (talk) 21:42, 13 December 2018 (UTC)
- That's true--the definition of 'building' can be complicated. A hospital I visited not too long ago was built in several stages over the course of several decades with the different additions variously abutted to existing structures and/or linked by bridges and underground corridors. The bulk of the facility was tied together by a single long, straight, publicly-accessible hallway below grade; looking on Google Maps it's at least a quarter mile long. Is that one building or a dozen? TenOfAllTrades(talk) 14:32, 14 December 2018 (UTC)
I don't know for sure, but my bet is that whatever building it is is home to a great many shelf companies. Wnt (talk) 22:23, 13 December 2018 (UTC)
- The Palace of Versailles has 700 rooms. The Winter Palace in Saint Petersburg "has been calculated to contain... 1,500 rooms" but that sounds like a bit of a guesstimate to me. The "main facade" is 150 m long, but I'm not sure if that includes the two substantial wings which must almost double the frontage. Alansplodge (talk) 11:11, 14 December 2018 (UTC)
- And a bit more recently, the main holiday apartment block at Prora, a Nazi holiday resort for the working classes, had a frontage of 4.5 km (2.8 mi) and could accommodate 20,000 people. According to this article, the whole resort had 10,000 rooms, although how many were in the main block is unclear to me. Our article says "all rooms were planned to overlook the sea, while corridors and sanitation are located on the landward side. Each room [was] of 5 metres (16 ft) by 2.5 metres (8.2 ft)".
- If only the Nazis had stuck to the package holiday business.... Alansplodge (talk) 14:04, 14 December 2018 (UTC)
- ...can't they just count the rooms instead of calculating them? shoy (reactions) 14:06, 14 December 2018 (UTC)
- You would have thought so. Alansplodge (talk) 16:36, 14 December 2018 (UTC)
- One can count them. That's a different question than asking if one did count them. Can is not a synonym of did. --Jayron32 18:01, 14 December 2018 (UTC)
- Indeed, but User:Shoy's post was an idiomatic way of asking why they did not. Perhaps they have more pressing problems in Russia. Alansplodge (talk) 17:26, 15 December 2018 (UTC)
- One can count them. That's a different question than asking if one did count them. Can is not a synonym of did. --Jayron32 18:01, 14 December 2018 (UTC)
- You would have thought so. Alansplodge (talk) 16:36, 14 December 2018 (UTC)
- So which is the building which has (from a point within it) the greatest distance to an external wall, and what is that distance? Appearances can be deceptive - from the street Waltham Forest College looks like a huge monolith but it's actually built round two courtyards which can be accessed externally. Some apartments in Portugal have a sala interior which is a rather grand name for a small windowless bedroom. The Royal Courts of Justice (which is a building so large that judges have remarked that litigants often get lost) features enclosed open areas to let in daylight (for the purposes of this question the walls to these areas are "external"). 2A00:23C0:7903:B200:6D35:1B0:E2BB:692D (talk) 10:48, 16 December 2018 (UTC)
- The Aalsmeer Flower Auction? Sagittarian Milky Way (talk) 14:09, 16 December 2018 (UTC)
- So which is the building which has (from a point within it) the greatest distance to an external wall, and what is that distance? Appearances can be deceptive - from the street Waltham Forest College looks like a huge monolith but it's actually built round two courtyards which can be accessed externally. Some apartments in Portugal have a sala interior which is a rather grand name for a small windowless bedroom. The Royal Courts of Justice (which is a building so large that judges have remarked that litigants often get lost) features enclosed open areas to let in daylight (for the purposes of this question the walls to these areas are "external"). 2A00:23C0:7903:B200:6D35:1B0:E2BB:692D (talk) 10:48, 16 December 2018 (UTC)
Bacitracin
Is the antibiotic bacitracin effective against the bacteria Clostridium tetani aka tetanus?--User777123 (talk) 22:15, 13 December 2018 (UTC)
- Tetanus infections are treated with tetanus antitoxin but it's nasty. Normally (at least here), people are supposed to get a tetanus vaccination in grade school, then get a booster shot every 10 years iirc, as routine health maintenance. 173.228.123.166 (talk) 04:51, 14 December 2018 (UTC)
- According to this it is effective against other three clostridia species. It is likely to be effective against Clostridium tetani as well but has likely been never tested in this role. On other hand there are reports that some bacteria can be resistant. Ruslik_Zero 14:13, 15 December 2018 (UTC)
December 14
Elephant rescue?
In India, are elephants used for rescue work? If so, what are their usual tasks in this line of work (e.g. removing debris, rerailing train cars, bringing supplies, towing disabled vehicles, or maybe something else)? 2601:646:8A00:A0B3:0:0:0:718C (talk) 06:24, 14 December 2018 (UTC)
Why is He2 so large?
Someone here above asked "What's the largest diatomic molecule?" and the answer according to Helium dimer is He2.
Why would He2 be so large, larger than O2, Cl2 etc., given than these atoms have much larger nuclei, more electrons etc.?
Apologies, I am not a chemist and thought you could use the atomic number to have an good idea of the relative sizes of atoms, and a double-atom molecule should be roughly double the size of each atom? I must be missing something, if someone could be kind enough to explain. --Lgriot (talk) 12:26, 14 December 2018 (UTC)
- Oops, I read further the Helium dimer article and it says it is "due to its extremely long bond length".
- So I have another question, does this molecule look like this
"O-------O"
rather than like this"OO"
, if we were able to shine light on it? (I know light's wavelength is too large, but if we had some other way to 'see it'). In short, is it very large, but also very thin? --Lgriot (talk) 12:33, 14 December 2018 (UTC)- If you tried to illustrate the appearance of a chemical bond, what would you like it to show? The simplest I can thing of is the electron density. This is what you might observe if you could irradiate the molecule with gamma rays. In the case of this "molecule", the atoms of helium repel each other when close, but when further apart London dispersion forces attract the electrons from one atom slightly to the other. For helium, this is the weakest of any atom. You won't see a long thin stream of electron from one nucleus to another. Instead you will see two balls of electron density, more concentrated around the nuclei, but slightly deformed so that they are slightly lopsided, with a bit more density towards the other atom. So rather than thin, I would call it very distended and pretty wide. Other possibilities to illustrate could be the phase of the matter wave, or the magnetism due to spin and rotation, but I won't guess at that. Don't forget that most molecules are always vibrating and rotating, so they are not just like this: O-O, but in constant motion OO O-O O--O O-O OO ... Graeme Bartlett (talk) 11:24, 17 December 2018 (UTC)
My feeling is that this is more or less a trick. A "Van der Waals molecule" is basically two molecules (or nonbonding atoms) of ultra low pressure gas stuck next to each other (i.e. condensed into a liquid). Surely two adjacent atoms of liquid helium have more right to declare a bond with each other! I am somewhat curious if the Xe2 "Van der Waals molecule" is really smaller (it definitely does exist but I don't know the bond length) but calling this a 'diatomic molecule', as we do in the article, seems like it is meant to mislead those who don't know better. Wnt (talk) 15:28, 14 December 2018 (UTC)
- To the contrary, it's not really a trick, it's that our categories for these things are human-created boxes to aid and simplify our own understanding and comprehension of the phenomena involved (i.e. what is called a "model" in science) and all models are wrong. In this sense, the notion we have of things like "molecules" and "covalent bonds" and "intermolecular forces" are useful tools we have to categorize different ways atoms organize themselves, but in reality, all that really exists are electrons and nuclei. Electrons are attracted to various nuclei at a certain strength, and how those electrons and nuclei organize themselves based on the strengths of those attractions is how we classify various structures. Whether or not we call something a "molecule" is an us-problem, not an atom-problem. The reason why He2 (and Van der Waals molecules in general) represent a categorization problem is that they represent an "edge case" where we have clearly stable multi-atom structures that traditional bonding models (valence bond theory or VSEPR or molecular orbital theory or whatever) fail to predict or account for. What we call a molecule that our models predict shouldn't exist, but yet still does, is we qualify it as a "Van der Waals molecule" or call it a trick or a fake, but it isn't any less "real". After all, it is an observable phenomenon. It's the realest thing we have. What's fake is our models. Models are always "fake", they're just "useful", except that there is always some limit outside of which they become not useful. If a model doesn't verify an observable phenomenon, then it isn't the observable phenomenon which is wrong (and the model is not necessarily bad, universally, just bad for this application). --Jayron32 16:32, 14 December 2018 (UTC)
- This isn't so much a matter of models as definitions. We know that molecules stick together in larger clumps. The question is, do we call a water dimer a molecule? Do we name it H4O2? Do we say that H2O is chemically unstable because it forms water clusters? A water dimer, or larger water clusters, actually have hydrogen bonds and so are far more stable than He2, I might add. Wnt (talk) 12:20, 15 December 2018 (UTC)
- Yes, but the definition you use depends on your context. To an astronomer, oxygen is a heavy metal. When someone says "what is the longest bond between atoms" we need to first establish what we mean by "bond". Are IMFs considered bonds? Like I said, it's all bonds. What particular organization of atoms we define as a "molecule" is all we're asking here, and any limit we set in that definition is arbitrary. We have to set some definitions, so long as we recognize we can always find edge cases that will challenge whatever parameters we set in our definition. Which is fine, but we need to recognize that these definitions, useful as they are, are still not immutable laws of nature, they're just sometimes useful categories we create to simplify the world and give ourselves a common language to talk about it. And language is ALWAYS ambiguous.--Jayron32 22:14, 15 December 2018 (UTC) --Jayron32 22:14, 15 December 2018 (UTC)
- Thanks for the background explanation. Reading further, the length of a an He2 molecule 103.9 pm, versus a single He atom having a radius of 28 pm. This means the molecule is on average 1.855 times longer than 1 single atom. So I imagine/visualise the atoms still "touching", but not being embedded into each other, like O2 is, as represented here: [5]--Lgriot (talk) 16:53, 17 December 2018 (UTC)
- Yes, but the definition you use depends on your context. To an astronomer, oxygen is a heavy metal. When someone says "what is the longest bond between atoms" we need to first establish what we mean by "bond". Are IMFs considered bonds? Like I said, it's all bonds. What particular organization of atoms we define as a "molecule" is all we're asking here, and any limit we set in that definition is arbitrary. We have to set some definitions, so long as we recognize we can always find edge cases that will challenge whatever parameters we set in our definition. Which is fine, but we need to recognize that these definitions, useful as they are, are still not immutable laws of nature, they're just sometimes useful categories we create to simplify the world and give ourselves a common language to talk about it. And language is ALWAYS ambiguous.--Jayron32 22:14, 15 December 2018 (UTC) --Jayron32 22:14, 15 December 2018 (UTC)
- This isn't so much a matter of models as definitions. We know that molecules stick together in larger clumps. The question is, do we call a water dimer a molecule? Do we name it H4O2? Do we say that H2O is chemically unstable because it forms water clusters? A water dimer, or larger water clusters, actually have hydrogen bonds and so are far more stable than He2, I might add. Wnt (talk) 12:20, 15 December 2018 (UTC)
December 15
Adaptive springs, all steel wheels
Are cars with no rubber tires, made of a tough and hard steel, on the horizon? I was thinking that with computer sensing of bumps and potholes, the springs , maybe with the support of quick reacting solenoids, could give an acceptable ride for some applications.Rich (talk) 21:02, 15 December 2018 (UTC)
- What would they do to the roads? ←Baseball Bugs What's up, Doc? carrots→ 21:57, 15 December 2018 (UTC)
- (Edit conflict) In addition to the smoothness of the ride, you would also have to consider the friction between the steel wheels and the road (probably less than with rubber tires) and the increased wear on the roads caused by stiffer and harder tires.--Wikimedes (talk) 22:03, 15 December 2018 (UTC)
- Computer sensing of bumps and potholes only goes so far. If the mass of the wheels/tires was zero, a clever enough active suspension could pull up the wheels going over bumps and push them down going over potholes while keeping the car smooth and level. Alas, the mass of the wheels/tires isn't zero, and thus when you push the wheel down you push the rest of the car up. This is where pneumatic rubber tires shine: essentially the contact patch acts as an additional suspension spring with close to the ideal of a zero-mass wheel. There are further complications: See unsprung mass. --Guy Macon (talk) 22:23, 15 December 2018 (UTC)
- Think about how smooth a rail is compared with a road. Also, they won't grip. Greglocock (talk) 22:29, 15 December 2018 (UTC)
- Yes, see slippery rail. Alansplodge (talk) 19:42, 17 December 2018 (UTC)
- There are experimental wire mesh wheels, which are mostly "empty" space and shouldn't weigh more than a normal wheel and tyre. The LRV is probably the most famous vehicle fitted with them. --TrogWoolley (talk) 14:04, 17 December 2018 (UTC)
- NASA shows off Mars rover tires that bounce back into shape (November 2017). Alansplodge (talk) 19:45, 17 December 2018 (UTC)
- The future is already here - see @ 2:08. PaleCloudedWhite (talk) 19:56, 17 December 2018 (UTC)
- NASA shows off Mars rover tires that bounce back into shape (November 2017). Alansplodge (talk) 19:45, 17 December 2018 (UTC)
December 16
Galileo pulse meter
According to this book, Galileo invented a pulse meter based on a swinging pendulum. By adjusting the length of the string of the pendulum, the user could alter its speed to match their heart rate. That particular speed would correlate with a particular length of string, and there was a scale measuring how long the string was at a particular time, thus telling you your heart rate.
My question is; how did the user insure that the pendulum maintained a constant momentum so that variations in speed could only come from variations in the length of the string? As I picture this in my mind, I imagine that the user would have just a few seconds to measure the patient’s pulse and adjust the pendulum accordingly, before the measurements became inaccurate. Puzzledvegetable (talk) 00:04, 16 December 2018 (UTC)
- I don't know what your source exactly says but a pendulum has a nearly constant period and not speed. When the motion slows down due to friction and air drag, the period doesn't. Pendulum says: "The period depends on the length of the pendulum and also to a slight degree on the amplitude, the width of the pendulum's swing." If the source said speed then it probably meant period. PrimeHunter (talk) 00:30, 16 December 2018 (UTC)
- @PrimeHunter: Thanks for explaining. I found a very helpful article online that clarifies how a pendulum works. According to explainthatstuff.com: "If there were no friction or drag (air resistance), a pendulum would keep on moving forever. In reality, each swing sees friction and drag steal a bit more energy from the pendulum and it gradually comes to a halt. But even as it slows down, it keeps time. It doesn't climb as far, but it covers the shorter distance more slowly—so it actually takes exactly the same time to swing. This handy ability (technically called isochronism, which just means "equal amounts of time") is what makes a pendulum so useful for timekeeping." Puzzledvegetable (talk) 01:29, 16 December 2018 (UTC)
- Note that a metre is (almost exactly) the length of a pendulum beating seconds. 2A00:23C0:7903:B200:6D35:1B0:E2BB:692D (talk) 10:14, 16 December 2018 (UTC)
- @PrimeHunter: Thanks for explaining. I found a very helpful article online that clarifies how a pendulum works. According to explainthatstuff.com: "If there were no friction or drag (air resistance), a pendulum would keep on moving forever. In reality, each swing sees friction and drag steal a bit more energy from the pendulum and it gradually comes to a halt. But even as it slows down, it keeps time. It doesn't climb as far, but it covers the shorter distance more slowly—so it actually takes exactly the same time to swing. This handy ability (technically called isochronism, which just means "equal amounts of time") is what makes a pendulum so useful for timekeeping." Puzzledvegetable (talk) 01:29, 16 December 2018 (UTC)
- That's for the historical reason that on 8 May 1790 – The French National Assembly decided that the length of the new metre would be equal to the length of a pendulum with a half-period of one second. These days the second is defined as exactly the time for light to travel 299792458 metres in vacuum. DroneB (talk) 15:13, 16 December 2018 (UTC)
- Huh? The why does our Metere arcticle say
- "The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole"?
- Also see History of the metre. which says
- "As a base unit of length, many scientist initially favored the "seconds pendulum" (a pendulum with a half-period of one second), but this was rejected when it was discovered that it varied from place to place with local gravity. A new unit of length, the metre was introduced - defined as one ten-millionth of the distance from the North Pole to the equator."
- --Guy Macon (talk) 15:38, 16 December 2018 (UTC)
- There is no contradiction here, as reading our Metre article, specifically Metre#Timeline will make clear. DroneB (talk) 19:55, 16 December 2018 (UTC)
- I don't think that comment, with its useful links, needed to be in small type. However, it does not really address the issue Guy raises. More below. --76.69.46.228 (talk) 07:27, 17 December 2018 (UTC)
- My choice of tiny type was for politeness. Specifically I posted a timely correction to my learned colleague Guy after he reacted to my post with an exhalation "Huh?" coupled with a misspelled link to a non-existing article that, had it only been found and read, would have revealed the "Huh?" to be a non-question. Regardless what "issue" IP User 76.69.46.228 thinks remains unaddressed here Guy Macon's [posted claim that the meridional definition of metre dates from 1789] can be challenged. I deduce that it is an error of false precision generated when Guy copied the globe image from History of the metre article with its underlying text hastily paraphrased from that article's lede summary, neglecting the detailed article content that supports 1791. DroneB (talk) 17:30, 17 December 2018 (UTC)
- I don't think that comment, with its useful links, needed to be in small type. However, it does not really address the issue Guy raises. More below. --76.69.46.228 (talk) 07:27, 17 December 2018 (UTC)
- A meter today is the distance the light travels during 1s/299792458. And the speed of light is 299792458m/s Doroletho (talk) 23:24, 16 December 2018 (UTC)
- There is no contradiction here, as reading our Metre article, specifically Metre#Timeline will make clear. DroneB (talk) 19:55, 16 December 2018 (UTC)
- Huh? The why does our Metere arcticle say
- That's for the historical reason that on 8 May 1790 – The French National Assembly decided that the length of the new metre would be equal to the length of a pendulum with a half-period of one second. These days the second is defined as exactly the time for light to travel 299792458 metres in vacuum. DroneB (talk) 15:13, 16 December 2018 (UTC)
Guy states above the reason why the "seconds pendulum" definition was rejected. According to The Measure of All Things by Ken Alder, these objections were already known in 1790 when the National Assembly was working on the law, so Talleyrand had wording put into it that said the length of the pendulum should be determined at 45° "or whatever other latitude might be preferred", with the French Academy of Sciences to establish a commission to execute the plan. Therefore the law of 1790 was more a plan for a definition than an actual definition, and I think this is why the 1791 definition is usually considered the first one.
Note incidentally that the "seconds pendulum" definition was rejected because it would be tied to a particular location and therefore might not be accepted worldwide, but the "quarter-meridian" definition that replaced it was also tied to a particular location, as it was specifically based on the meridian passing through Paris. But as Alder points out, the commission's chairman, Jean-Charles de Borda, provided a list of good reasons for this. To actually determine the length of the meridian, they would have to survey a sizable chunk of it, through country that was already well surveyed at the time, with both ends of the chunk at sea level, and passing through the latitude of 45°. Which meant a meridian through France from the English Channel to the Mediterranean. And when they actually sent a team to survey it (which is what the book is mainly about), the result was internationally accepted. --76.69.46.228 (talk) 07:27, 17 December 2018 (UTC)
- I would put it a different way. It seems to me the key issue is whether there is any connection between the seconds pendulum definition and the later meridional definition. From what I can tell, neither the metre article nor the history of the metre article say there was say there was. The fact that some people came up with the seconds pendulum definition (or whatever you want to call it) is irrelevant if there was no concern about trying to approximate that definition with the new definition, and nothing in our articles suggest there was. While the fact these are similar may seem awfully coincidental, the fact that their definition was "one ten-millionth of the distance from the North Pole to the equator" and their decimalisation outlook and considerations of what would make a useful length makes it a lot less clear it was anything other than a happy accident. In other words, when the meridional definition was being designed, the designers could have said, 'we don't give a damn what definition people came up with before, we just want to choose something which works well'. By comparison, since then the redefinitions have generally attempted to keep the same length. I'm sure you're right that this is mostly because it was never an official definition but a working plan that was abandoned. So they had no real reason to try and approximate the older plan/definition, but later they actually had metre that people had been using for a while so changing the actual length in any significant way would have been insane. And all this means Guy Macon was right to question the claim being made. Nil Einne (talk) 15:51, 19 December 2018 (UTC)
What happens to an electrical device when it gets the wrong power?
Too much, and you can burn a fuse (or whatever). But what if you get too little, like connecting a 220V device in a 110V power source or one source with too much variation? Or what happen by the wrong Hz? I imagine my microwave oven, which sets its clock through the power, won't show the right time, but what else? What's the worst that can happen in terms of damage?--Doroletho (talk) 23:19, 16 December 2018 (UTC)
- It's complicated. There are many sorts of device, there are many failure modes. In general, "inefficient" operation leads to excess heat dissipated in the machine, and that can cause overheating and an insulation failure.
- A 60Hz motor or transformer will have sheet iron laminations in its armature. The losses here increase with lower frequencies, so although many 50Hz machines of similar power work just fine throughout Europe on 50 Hz, an imported US motor designed badly for 60 Hz with no margin beyond that, will overheat. This is a common failure in imported workshop machinery.
- A powerful 12V motor, such as a car engine starter motor, won't have enough power to turn the engine fast enough to start if its run under-voltage. This - mostly the tendency to run the starter for too long - is a classic example where a motor (normally only designed for brief use) will easily overheat.
- There can also be problems with induction motors, where 'slip' becomes excessive. If the motor is adequately powerful to run at the correct speed, then it works fine. But under-voltage it may lack power and so run too slowly. This has a secondary effect where the 'slip', a difference between the physical speed of the rotor and the speed of the rotating field, becomes excessive. This increases the induced current in the rotor (induction motors transmit power to their rotors by induction, not through brushgear) and so the rotor may burn out. Normally a small reduction in speed acts as a feedback mechanism, so that the increased slip increases torque and causes the motor to safely speed up to its designed speed - however too much of this drops out of the controllable region and becomes a failure. Andy Dingley (talk) 23:31, 16 December 2018 (UTC)
- Your microwave clock is probably controlled by a crystal, not the mains, unless you have an analog clock on it! I run my immersion heater off a 110V transformer in my 240V off grid house to reduce the peak load on the batteries, but it would take four times as long to come up to temperature. Greglocock (talk) 23:40, 16 December 2018 (UTC)
- I had exactly that problem at a performance last week. My amplification for my upright bass, which had been very reliable previously, failed utterly; I was getting no sound. Had to play unamplified, not as planned. Turns out the problem is I had two wall warts in my system, one rated 12v 500ma, the other rated 9v, 1000ma. Of course, they had identical connectors. The device that wouldn't work -- a little tube preamp -- wanted the 9v 1000ma. Nothing died; but no sound. I hate wall warts. You should too. --jpgordon𝄢𝄆 𝄐𝄇 05:10, 17 December 2018 (UTC)
December 17
Newest species
What is the newest animal species on Earth? Not the most recently discovered, but the most recent species that diverged from another species in the same family? Thank you. → Michael J Ⓣ Ⓒ Ⓜ 22:43, 17 December 2018 (UTC)
- See Species problem. There are many different ways to define what a species is, and there is no precise agreed-upon definition even among biologists. When considering geographically separated populations, deciding when these become separate species is quite arbitrary. Besides that, the genetic makeup of many species has never been studied, so there is also insufficient data to really answer this question. - Lindert (talk) 22:52, 17 December 2018 (UTC)
- The 5 entries in the Category:Speciation events may be of interest. As Lindert suggests, our knowledge in this area is limited to what we have observed, which can only be a small fraction of all species that exist, and it can be quite difficult (and take a long time to determine) when studying a hitherto unseen species to determine how long it has existed. There is also the question of whether species created by artificial techniques such as hybridization count or not. {The poster formerly known as 87.81.230.195} 90.217.251.247 (talk) 00:15, 18 December 2018 (UTC)
- Indeed, this is true; a hybrid species created by deliberate human action (oranges and grapefruits) might be disqualified, but there are plenty of temporarily deliberate/later reconsidered or accidental hybrid species like killer bees and these little guys. Even within species there can be separate groups whose hybridization causes real-world problems [6]. I do think that the mass extinction that is ongoing, with all its losses to posterity, will not plague people so much as evolution's vengeful creativity. But I can think of at least one more or less "new species" that is not a hybrid (that I know of), that being the mutant strain of Caulerpa taxifolia taking over the oceans. But it is hard to get a new species a brand new name, no matter what! I don't know if it's been thought of by those who do such things. Wnt (talk) 04:13, 18 December 2018 (UTC)
- There's some discussion of this in an article on gizmodo.com. Rojomoke (talk) 09:33, 18 December 2018 (UTC)
- Ah -- Haplochromine cichlids of Lake Victoria, a favorite example of recent speciation (of real accepted species, that is). I vaguely recall thinking there was still a bit of room for skepticism about "cheating" in the sense that multiple precursor species might have hybridized in the past, leaving the adaptive radiation to progress with some reservoir of pre-made pieces, but it is hard to exclude such things from any speciation event, and I haven't tried to chase down what came out of that idea any time recently. Wnt (talk) 14:16, 18 December 2018 (UTC)
- There's also the case of Procambarus virginalis, thought to have formed around 1995: [7]. Not everybody agrees it's a separate species. PiusImpavidus (talk) 15:11, 18 December 2018 (UTC)
December 18
"Static discharge" circuit tech
I have a sense that electrical technology has progressed unimaginably in the past two centuries, yet with some notable exceptions the application of "static electricity" and "static discharge" has not (what I'm thinking of is more, perhaps much more, than about 4000 volts, with amperage so low as not to be more than a minor annoyance to humans). I know that static electricity circuits can be made easily for a lark [8], I know there's the ionocraft and even Van de Graaf generators used for X-rays and particle accelerators, but are there reliable, precise static electricity power supplies, perhaps pulsed, that you could safely touch (not high tension electric lines!), are there solenoids powered by static electricity, have people built relays and complex logic circuitry that uses it, and above all, can you generate an arbitrary waveform of ultra high voltage electricity (subject to the need to have some way to restrict total amperage) using small cheap components? Wnt (talk) 14:27, 18 December 2018 (UTC)
- Are there reliable, precise static electricity power supplies, perhaps pulsed, that you could safely touch? Yes. They are called "hipot testers" and "ESD testers" and you can adjust the (high) voltage and (low) current up and down. Many of them put out a waveform with a rise and fall that mimics a static electricity shock.
- Are there solenoids powered by static electricity? No. Too little energy.
- Have people built relays and complex logic circuitry that uses it? Relays are just solenoids that activate switches, so no on that one. Modern logic circuits use transistors, which need to be protected from static electricity. Older vacuum tube and relay logic isn't so sensitive to damage, but there is too little erergy in static electricity to run either.
- Can you generate an arbitrary waveform of ultra high voltage electricity (subject to the need to have some way to restrict total amperage) using small cheap components? If you want to include DC in your definition of "arbitrary" then no. If you are OK with AC only, just build a cheap low voltage arbitrary signal generator and step up the voltage with transformers (a cheap signal transformer will give you a signal suitable for driving a neon sign transformer). Or you can wind your own high voltage transformer if you really want cheap. --Guy Macon (talk) 17:34, 18 December 2018 (UTC)
Converting angular to linear velocity
I have a question about determining the linear (tangential) velocity of a point rotating with some angular velocity at a distance d from a center, and more specifically, the metrological aspects thereof.
As everyone knows, it's simple enough:
- v = ω · d
as long as the angular velocity ω is expressed in radians-per-unit-time.
So my questions are:
- What if ω is not in radians?
- How do you resolve the unit discrepancy?
With respect to #2, as my high school physics teacher taught me lo these many years ago, it's not enough to get the right numeric answer, you've got to get the units right, too. And when you use the expression above, with angular velocity in a unit using radians, you end up with a result in units of (distance · radians) / time, while you presumably intended to get a conventional velocity expressed as distance/time. So how to get rid of the radians?
I have an answer, but I've never seen it expressed this way. I believe that a more explicit, unit-agnostic version of the expression would be
- v = ω · 2π · d / ¤
where the 2π term is in there because we're obviously dealing with the circumference of a circle somehow, and where ¤ is the number of angle units in a circle: 2π if you're using radians, 360 if you're using degrees, or 1 if you're using full rotations (as in RPM).
So the question is not so much "does this work?". (It does, with the two 2π terms nicely canceling out if you are using radians.) Rather, does anyone else formulate it this way, and has anyone ever identified a "constant" (basically another name for 1) like my ¤? (I belatedly notice that the indispensable units program does have "circle" as a unit, which is heartening.)
This is basically a simpler version of the debates that play out when comparing MKS to the various flavors of CGS. (As it says at Advantages and disadvantages, "A key virtue... is that 4πε0 is replaced by 1".) So what I think I'm asking here is, if the 2π and ¤ terms drop out (are replaced by 1) in the "simpler" radian-time-distance system, what's the more-complicated system that radian-time-distance is simpler than, and how is angular-to-linear velocity conversion expressed there? —Steve Summit (talk) 15:28, 18 December 2018 (UTC)
- I'm confused why you don't just convert angular velocity to radians-per-unit time if it were in some other unit. That seems like a trivial operation. --Jayron32 15:30, 18 December 2018 (UTC)
- +1 on that. One thing to bear in mind is that radians and degrees are formally unit-less, they are ratios. What Steve's teacher said is true but unhelpful in this case. That is, if you do a dimensional analysis of an equation in radians, it'll give exactly the same answer as a dimensional analysis in degrees. You could even argue that the circle constant is already indicated by the choice of angular measurement. Greglocock (talk) 19:30, 18 December 2018 (UTC)
Note the bold Wikilink when I state your equation:
- v = ω · d
where:
- v = Tangential velocity
- ω = angular velocity
- d = distance from center of rotation
Dimensional analysis insists that an equation have the same dimensions on its left and right sides, which is the first part of "getting the units right". In this equation the units are those of v, the speed, which is LT-1. That means "length/time" and you may meet that again in the form dx/dt called velocity, the derivative of position with respect to time if you take an interest in Calculus. You are free to select the actual units as long as they are consistent, as in these two examples:
- v metres per second = ω radians per second · d meters (using metric units)
- or
- v feet per hour = ω radians per hour · d feet (using imperial units)
- ...or any other units of time and distance used consistently will work.
- To your questions:
1. What if ω is not in radians? - Angular velocity ω has to be a unit of angle rotated per time. It provides only the T-1 part of the dimension of the right side of the equation. These values are equal: 1 radian/second = 1/(2π) rotations/second = 57.296... degrees/second (approximate) so you are free to convert between them.
2. How do you resolve the unit discrepancy? - There is none when you realize that the equation is giving tangential velocity which must be thought of as conventional velocity at a point in time. Of course you don't get rid of the given fact that the motion over any finite time is not straight but in a curve, but that does not dictate a change of units. An object such as a bullet tumbling through the air has both straight-line and tangential velocities which one can add together if one wants to estimate the air resistance due to friction. DroneB (talk) 21:08, 18 December 2018 (UTC)
- So the real issue here turned out to be that I had never thought of angles as dimensionless. I really thought they were more "real" units. (I just about argued with @Greglocock: for asserting that they were unitless ratios.)
- @Jayron32: asked why I couldn't just trivially convert my angles to radians if necessary. That's fine if you're working with ordinary numbers. But I'm working with a system which deals with formally unitful quantities wherever possible. (It is somewhat akin to Boost.Units. The intent, among other things, is to avoid bugs like the one that doomed the Mars Climate Orbiter mission.) With this system, it's considered wrong to extract values as particular units and work with them as ordinary numbers -- that defeats the purpose of carrying the units around. With this system, I would have had to write (and the co-worker who originally wrote the code did write) the equivalent of
angular_rate.extract(radian/sec) * dist.extract(meter)
- and this resulted in an ordinary number which had to be converted back to a unitful quantity by explicitly re-tagging it as meters. In our system, it's almost always wrong to extract raw values, perform math on them, and then convert back to quantities. I instructed the co-worker to replace
double v = angular_rate.extract(radian/sec) * dist.extract(meter)
vel = quantity(v, meter/sec)
- with
vel = angular_rate * dist
- But this then resulted in a dimensional mismatch, since it computed something with units of angle · distance / time. Our analysis of this failure (an analysis which, embarrassingly enough, tended to contradict my usual argument that unitful quantities are always easier and more convenient to use) resulted in the question I asked here. But it looks like the real bug is that our system treats angles as a distinct unit, incommensurate with scalars.
- Thanks for everyone's answers. —Steve Summit (talk) 11:37, 19 December 2018 (UTC)
@Scs: I don't think it's wrong to give angles units. I am personally a big fan of making up units as necessary to annotate a calculation -- for example, when diluting a solution, you can put "mL conc." and "mL dil." and the calculation can be audited with units (converting molar into mol/ml whatever explicitly), whereas otherwise it is "just ratios" and people can make the most embarrassing errors if they're in a hurry. In your case, you can address this explicitly with the factor-label method, making the equivalences:
1 full rotation = 2 pi radians 1 full rotation = 2 pi d (which may be in inches or whatever; note "d" is a radius as you defined it)
So if we start at, say, 0.5 rad/sec rotation and a 4 inch radius,
v = (0.5 rad/sec) * (1 rotation/2 pi radians) * ((4 inch * 2 pi) / 1 rotation) = 2 inch/sec.
The impulse to skip the units is generally treacherous, because the intuitive process you want to do can generally always be written in units and otherwise you're just doing algebra in your head for the heck of it. Even some of the hard ones nobody does with units will give way to just a little thought, for example you can add (273 K - 0 C) to any computation because you can add zero to anything, and preserve units instead of making a special case. You could also specify some really weird things like + ln( mol/L) in certain calculations, though that takes more adjustment to fundamental definitions.
As an aside, I should say that by far the most interesting case to apply any of this to is Planck's constant. It is the fundamental unit of action, energy times time, but it has the units of angular momentum, but for light it is the linear momentum per wavelength, i.e. divided by frequency i.e. times time. Any path to an intuitive understanding of quantum mechanics has to depend on all that making sense in a metrological way, which seems to depend on an understanding of moment of inertia and mass... let's just say I haven't gotten very far as of yet with that. ;) But I feel sure it's important. Wnt (talk) 15:16, 19 December 2018 (UTC)
- I know what you mean about "factor-label" (though I hadn't heard it called that). And indeed your (0.5 rad/sec) * (1 rotation/2 pi radians) * ((4 inch * 2 pi) / 1 rotation) ends up being pretty darn close to my ω · 2π · d / ¤ ! —Steve Summit (talk) 05:37, 20 December 2018 (UTC)
- I should redirect Factor-label method to dimensional analysis which has a section on it. The key difference is that your formula tends to look kind of like a random series of operations, where ideally a factor-label computation should take only a few pieces of data (ideally, none, if you phrase it just so) and everything else involves multiplication by 1 or, rarely, addition of 0. In other words, one rotation is 2 pi radians, so you know you can multiply by that ratio, and the same is true for the circumference being one rotation, and you can even say that, for purposes of this computation, 0.5 radians is one second, in the sense that every time you have one occur you have the other occur, and so inevitably, the result is equal to 1 also, in some conceptual sense: 2 inches is what happens in one second. Wnt (talk) 15:25, 20 December 2018 (UTC)
Can Tapir's be ridden or domesticated?
Can a Tapir be domesticated and ridden?Naraht (talk) 22:34, 18 December 2018 (UTC)
- By any chance, is this a veiled question about biblical literalism? If so, you might find this publication relevant: ..."the issue cannot be resolved by reference to normal scholarship," a statement that can be considered in isolation as about the most intelligent thing that can be said about the issue. (Please don't construe this as an endorsement of any of the madness or other conspiracy-theories associated with the horse/tapir topic).
- In actual fact, tapirs are not commonly domesticated in any part of the world.
- Nimur (talk) 01:31, 19 December 2018 (UTC)
- Where are tapirs mentioned in the Book of Mormon? ←Baseball Bugs What's up, Doc? carrots→ 01:42, 19 December 2018 (UTC)
- As far as I can tell, they are not, by that name. However it does mention "horses" and "asses", neither of which are native to the New World, though there were horse species that survived possibly as late as 7600 years ago (according to Horse#Taxonomy and evolution). Apparently some Mormon exegetes identify these with tapirs or deer. That's just from a brief search; I don't remember ever hearing of this before, and I'm not sure I got it all right. --Trovatore (talk) 04:11, 19 December 2018 (UTC)
- Joseph Smith might not have known there were no horses in the western hemisphere until our ancestors brought them over. ←Baseball Bugs What's up, Doc? carrots→ 04:28, 19 December 2018 (UTC)
- From the Mormon point of view, that hardly matters; the angel Moroni would presumably have known. --Trovatore (talk) 04:30, 19 December 2018 (UTC)
- Were there ever tapirs in the western hemisphere? ←Baseball Bugs What's up, Doc? carrots→ 05:27, 19 December 2018 (UTC)
- Most tapirs are in the western hemisphere. Iapetus (talk) 10:39, 19 December 2018 (UTC)
- D'oh! ←Baseball Bugs What's up, Doc? carrots→ 13:06, 19 December 2018 (UTC)
- If you go to the Creation museum you can see Jesus riding a dinosaur so I'm sure it's possible ;-) Dmcq (talk) 10:59, 19 December 2018 (UTC)
- Probably more fun than riding a tapir. ←Baseball Bugs What's up, Doc? carrots→ 13:06, 19 December 2018 (UTC)
- Since humans wiped out New World horses in the first place, there's no scientific way to rule out that some royal family might have kept some in captivity or on an isolated preserve/island etc.; I don't believe in the Book of Mormon to make that jump on its behalf but it can't be called unscientific if someone does, just a different choice of what historical sources to credit without other corroborating evidence. Wnt (talk) 15:23, 19 December 2018 (UTC)
- Is there any indication of why they would have done that? ←Baseball Bugs What's up, Doc? carrots→ 19:10, 19 December 2018 (UTC)
- Of course not. I don't think they did do it. The point is though, if somebody in Nebraska hits a 2500-year-old horse skeleton with his shovel tomorrow, scientists will be skeptical, but they're not going to run around in circles yelling it's impossible. Some explanation would be devised. Wnt (talk) 04:24, 20 December 2018 (UTC)
- Your alter ego said they did. ←Baseball Bugs What's up, Doc? carrots→ 04:59, 20 December 2018 (UTC)
- Of course not. I don't think they did do it. The point is though, if somebody in Nebraska hits a 2500-year-old horse skeleton with his shovel tomorrow, scientists will be skeptical, but they're not going to run around in circles yelling it's impossible. Some explanation would be devised. Wnt (talk) 04:24, 20 December 2018 (UTC)
- Is there any indication of why they would have done that? ←Baseball Bugs What's up, Doc? carrots→ 19:10, 19 December 2018 (UTC)
- Most tapirs are in the western hemisphere. Iapetus (talk) 10:39, 19 December 2018 (UTC)
- Were there ever tapirs in the western hemisphere? ←Baseball Bugs What's up, Doc? carrots→ 05:27, 19 December 2018 (UTC)
- From the Mormon point of view, that hardly matters; the angel Moroni would presumably have known. --Trovatore (talk) 04:30, 19 December 2018 (UTC)
- Joseph Smith might not have known there were no horses in the western hemisphere until our ancestors brought them over. ←Baseball Bugs What's up, Doc? carrots→ 04:28, 19 December 2018 (UTC)
- As far as I can tell, they are not, by that name. However it does mention "horses" and "asses", neither of which are native to the New World, though there were horse species that survived possibly as late as 7600 years ago (according to Horse#Taxonomy and evolution). Apparently some Mormon exegetes identify these with tapirs or deer. That's just from a brief search; I don't remember ever hearing of this before, and I'm not sure I got it all right. --Trovatore (talk) 04:11, 19 December 2018 (UTC)
- Is bad web typography a Sin? Andy Dingley (talk) 11:35, 19 December 2018 (UTC)
- It's a multitude of Sin's. ←Baseball Bugs What's up, Doc? carrots→ 05:00, 20 December 2018 (UTC)
- Where are tapirs mentioned in the Book of Mormon? ←Baseball Bugs What's up, Doc? carrots→ 01:42, 19 December 2018 (UTC)
December 19
How many floating point operations and TB of storage would be needed for a good car crash test?
Is it a lot? Then a container ship crashing into another or something complex like that would be computationally expensive as hell. Or are auto regulators just crashing them in real life out of caution? Sagittarian Milky Way (talk) 00:09, 19 December 2018 (UTC)
- You are talking about simulating car crashes? Why would anyone do such a thing? The expense would be far higher than crashing a car would be, even if computing resources were free. Just entering in the data for the shape and thickness of every part would be far more than a car. Then there is the liability cost; even if the simulation was perfect, would a jury believe that with the lawyers on the other side trying to confuse them? Would every jury believe it? --Guy Macon (talk) 01:37, 19 December 2018 (UTC)
- In fact, IIHS wrote this year-2000 op-ed on the topic of the NHTSA Driver Simulator: Advanced driving simulator is costly, value is questionable.
- Of course, computer modeling is important, but actual crash tests are also important.
- Nimur (talk) 01:44, 19 December 2018 (UTC)
- For a few years I was employed under the same manager as was responsible for computer analysis of car crashes. The results are good enough these days that the physical tests are done to verify the CAE runs. The calibration for the airbags etc is done from the sensor signatures generated by the CAE test. Yes it is lots of data. Yes the computer tests take a while to run on supercomputers. The fidelity of the model used is a mesh size of about 1-3mm. The main programs used are LS Dyna and Nastran. The physical tests that are done by NHTSA and IIHS and NCAP and so on are to verify that the auto manufacturers are telling the truth and to provide an absolute number. The error is rarely more than 0.5 stars. These are crashes into concrete blocks, car vs car is rarely modelled. A test has to be reliable, repeatable, and, representative. Car vs car scores 1/3, car vs block scores 2/3. In practice car vs block seems to be pushing the casualty rate the right way. Greglocock (talk) 04:43, 19 December 2018 (UTC)
- How expensive are individual runs? Could you test those two, car vs. standard guardrail, car versus highway divider, car versus moose, car versus jogger etc. and start getting composite pictures of overall safety? (I realize that economically the goal would be to get the best score on the test and never mind the real world, but I mean, potentially...) Wnt (talk) 15:28, 19 December 2018 (UTC)
- Not amazingly expensive once you have the model. I don't know if they use the same models when they develop pedestrian safety, I'd have thought so. I'm not sure how to implement a composite picture other than the current approach which gives a single figure score made up of several categories. Greglocock (talk) 21:31, 19 December 2018 (UTC)
- How expensive are individual runs? Could you test those two, car vs. standard guardrail, car versus highway divider, car versus moose, car versus jogger etc. and start getting composite pictures of overall safety? (I realize that economically the goal would be to get the best score on the test and never mind the real world, but I mean, potentially...) Wnt (talk) 15:28, 19 December 2018 (UTC)
Why do we shake when we're angry?
Many people shake when they're angry. I looked for this question on Google and I saw many answers that said that it's because the activation of the sympathetic system (by the amygdala) which cause to flow of adrenaline. But I really didn't find the relation between the adrenaline and the shaking of the body muscles. So assuming that the shaking is from the adrenaline, I'm asking what's the relation between them? --ThePupil (talk) 00:12, 19 December 2018 (UTC)
- Increased blood pressure might also figure into it. ←Baseball Bugs What's up, Doc? carrots→ 00:43, 19 December 2018 (UTC)
- See The effect of adrenaline on the contraction of human muscle -- one mechanism whereby adrenaline increases the amplitude of physiological tremor.[9] --Guy Macon (talk) 01:56, 19 December 2018 (UTC)
December 20
Do fish drink?
Well, do they? Is it accurate to speak of a fish drinking? If they don't drink, what do they do, exactly? FreeKnowledgeCreator (talk) 03:55, 20 December 2018 (UTC)
- Googling "do fish drink" yields many items. This one from Quora, for example, says saltwater fish drink and freshwater fish don't.[10] ←Baseball Bugs What's up, Doc? carrots→ 04:57, 20 December 2018 (UTC)
Mercury and gallium
Mercury has a melting point of -39°C while the boiling point is 357°C; gallium has a melting point of 30°C and boiling point 2203°C. Is Mercury's low melting and boiling points are due to the same reasons? If so, how the boiling point of gallium so extremely high relative to its melting point? PlanetStar 05:51, 20 December 2018 (UTC)
- Gallium and mercury are not in the same column of the periodic table. 62.49.80.34 (talk) 09:49, 20 December 2018 (UTC)
- More to the point than simply not being in the same column is what that means. Transition metals with unpaired electrons tend to have higher boiling points (there is some simple discussion of that here. Gallium has one unpaired electron in its 4p orbital, while mercury has zero unpaired electrons. These unpaired electrons allow for the formation of stronger interatomic forces with neighbors, leading to a higher boiling point for gallium. Certainly this also has an impact in the melting point differences as well, but other factors may also come into play there (crystal structure, perhaps?). --OuroborosCobra (talk) 14:33, 20 December 2018 (UTC)
How many pregnancies at one time?
At any one time, roughly what percentage of women in the world aged between 18 and 50 are pregnant? -- Jack of Oz [pleasantries] 07:48, 20 December 2018 (UTC)
- Googling that subject, the following US information turns up, which isn't a full answer, but it's interesting.[11] ←Baseball Bugs What's up, Doc? carrots→ 11:52, 20 December 2018 (UTC)
- Current rate of fertility (average number of children per woman) is current about 2.35 - which would mean each woman being pregnant for 21.15 months - which would probably need to be increased to allow for those pregnancies which do not result in a live birth. Over the 32 year span suggested (which may be too short, as in many parts of the world women start having children much younger than 18) that would mean the average woman spending just under 18% of the period pregnant. Logically, that would also mean that at any given time about 18% of them should be pregnant - perhaps 20% is you allow for the miscarriages, still births and abortions. Wymspen (talk) 13:01, 20 December 2018 (UTC)
Tuned response function.
I have interest in the equation that gives the amplitude of the current (or the voltage across the resistor) in a voltage exicted series RLC circuit but using only the parameters: Q and omega0,(Where Q is the quality factor of the complete circuit and omega0 is the undamped resonant radian frequency of the circuit). I need the equation to be a function of omega (the driving frequency),omega0 and Q only please. I have searched all over the web and all the books i can find. Yet I cannot find the formula for the amplitude against radian frequency. This is not homework but a personal interest. Its too hard for any homework and doesnt seem to be covered anywhere. Help!!!213.205.242.154 (talk) 23:45, 19 December 2018 (UTC)
Can someone ask that sparky spinner to look at this, He.she seems to be very clever.213.205.242.154 (talk) 23:47, 19 December 2018 (UTC) --213.205.242.154 (talk) 12:49, 20 December 2018 (UTC)