Wikipedia:Reference desk/Archives/Science/2009 November 18

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November 18

Fluoroantimonic acid

Is fluoroantimonic acid only considered the strongest acid because no one has discovered any stronger acid, or is it physically impossible for a stronger acid to exist? --75.50.50.76 (talk) 02:40, 18 November 2009 (UTC)

I;m not sure what you mean by "physically impossible". Acid strength is determined relatively; it is determined by how the acid reacts with other substances. Consider the model Brønsted–Lowry reaction:

HA + B^- <---> A^- + HB

The strength of two acids HA and HB are determined relative to each other such that if HA is the stronger acid, then the equilibrium above will favor the forward reaction, while if HB were the stronger acid, the equilibrium would favor the reverse reaction. There is a "standard acid equilibrium constant" known as K_a, which is calibrated for B to be water, and HB to be hydronium; however in practice HA and HB could be any two acids. Thus, to compare acid strength, you just compare how the equilibrium lies, and you can literally "rank" every acid against every other acid based on how well it reacts compared to each other. The acid at the top of this list is fluoroantimonic acid; there is nothing about the structure that would make a stronger acid actually impossible, its just that for every known substance so far, fluoroantimonic acid is the one at the top of the list. Hypothetcially, any number of substances could be discovered/isolated/created which may end up being even stronger; there is no theoretical upper limit to acid strength, since its not an absolute value; it is only a relative value. --Jayron32 04:30, 18 November 2009 (UTC)

The article calls it "the strongest known acid", so that just means nobody discovered a stronger acid. — DanielLC 16:32, 18 November 2009 (UTC)

Another possibility is that at least one of our articles is wrong (since they are mutually exclusive): the article Helium hydride ion also describes its subject as "the strongest known acid". I'm surprised this has not already been raised, since Graeme Bartlett just mentioned helium hydride on this page. -- Scray (talk) 01:59, 19 November 2009 (UTC)

It could be that helium hydride is only being obtainable in near-vacuum gas-phase conditions ("HHe+ cannot be prepared in a condensed phase"), whereas fluoroantimonic acid is something that can exist at "standard" lab conditions. So fluoroantimonic acid is the strongest known acid that you could put in a flask on a lab bench, whereas helium hydride is the strongest acid which exists under any condition. -- 128.104.112.237 (talk) 17:22, 19 November 2009 (UTC)

Different definitions of acid are producing different answers. Most would think of acid as a liquid, rather than a gas. I suspect there may be quite a range of stronger acids yet to be discovered. Some other gas phase ones that are strong but not strongest could be trihydrogen cation CH₅⁺. Perhaps they could exist in the appropriate fluid as well, like liquid hydrogen or methane. And then there is the case of the pure proton. It is the most extreme case, but is it an acid? Graeme Bartlett (talk) 21:07, 20 November 2009 (UTC)

Need derivations of the solar insolation inside earths atmosphere and latitude at any point

Best method of learning about insolation is incoming solar radiation.in-incoming,sola-solar,tion-radiation I need derivations of all insolation related parameters like latitude, declination and insolation itself...please help —Preceding unsigned comment added by 220.225.125.246 (talk) 03:53, 18 November 2009 (UTC)

There isn't much to derive; it's just trigonometry. The insolation is the solar constant times the cosine of the angle that the sun makes with the zenith. That angle depends on both the latitude and the day-of-year/time-of-day. We have a nice diagram of this effect in the insolation article. We also have a formulation for the position of the sun as a function of time and date. (If you want to derive this, it will be a matter of geometric projection of the orbital parameters of Earth - the math is not complicated, but there is a lot of it). Earth's orbit explains these parameters; ironically, they are derived by the inverse process - measuring the sun's declination and reprojecting it back to an orbital specification. So, you might as well take the solar declination formula at face-value, since it is basically an empirical (observation-based) formula.

As far as other parameters - are you working with a more sophisticated atmospheric absorption model? Usually, the effect of longer travel through the atmosphere is negligible compared to the geometric projection (that I described above), but you can read about optical depth if you want to try to apply that formula and estimate the total amount of gas between the sun and a particular location on earth. For this approach, you would also need an atmospheric profile model - you can look at Earth atmosphere to get some background. I believe there are several reference atmosphere profiles published by NOAA and the American Geophysical Union; I will try to dig up some links. Nimur (talk) 04:55, 18 November 2009 (UTC)

The "U.S. Standard Atmosphere" and the Jacchia's thermosphere profile are hosted at NASA Goddard's website; I know people who have used COSPAR parameters for pressure as a function of height; you can use these to estimate atmospheric absorption if you dare... this approach may require some integral calculus... but as I mentioned above, if you actually calculate it out, you will find that the optical depth variation with angle is negligible for most purposes. This is because the "longer paths" traced out end up traversing through the (sparse) high atmosphere. If you don't like these atmospheric profiles, our article has a list of other common atmosphere models; or you can create your own simple gas-density model based on the hydrostatic equilibrium approximation (e.g. exponential fall-off with height). Nimur (talk) 05:00, 18 November 2009 (UTC)

Dog Whisperer

On the Dog Whisperer last night (I'm in Edmonton, Canada), a woman is bound via wheelchair because of a 3 last name disease (Cabot is a name?). It affected her brother, mother and I think their dad. They died. Her leg muscles don't work that well, I think. What is this disease?174.3.102.6 (talk) 05:31, 18 November 2009 (UTC)

Charcot-Marie-Tooth disease? --TammyMoet (talk) 11:02, 18 November 2009 (UTC)

Season 5 (the current season), episode 20, "Mad Dogs", which first aired on September 11, 2009,[1] does involve a brother and sister who live together, who both have Charcot-Marie-Tooth disease.[2] Red Act (talk) 19:47, 18 November 2009 (UTC)

what canadian animal stores apples in the fork of branches ?

This fall, I've found apples (presumably fallen for nearby apple trees) stuck in the fork of branches in trees near my south-eastern canadian home and I'm wondering what kind of animal stores fruits that way ? The apples seem damaged on top, but sides and bottom look undamaged to me. They are placed 2 to 4 meters (6 to 12 feet) high in the forks of main branches of young trees, quite near the trunk. The tree trunks are 5 to 8 cm (2 to 3 inches) in diameter at that height and the branches are about 1 to 2 cm (3/8 to 3/4 of an inch) in diameter at the fork. The apples are all quite solidly stuck. 207.96.217.70 (talk) 05:45, 18 November 2009 (UTC)

Occam's Razor gut reaction - someone is placing those there intentionally to feed birds & small mammals. Just a guess, though. 218.25.32.210 (talk) 07:38, 18 November 2009 (UTC)

I would guess a squirrel. Normally they bury things, but an apple is too big, plus maybe it knows it would rot. I would guess it went to eat it, and got distracted, or in a fight and lost it. If it then got stuck it was unable to remove it, so just ate the top. Just a guess. Ariel. (talk) 09:49, 18 November 2009 (UTC)

A squirrel is a good guess, as they are known to "squirrel" things away. However, I'd guess that the apples were intentionally stored. I agree that they won't last as long as nuts, so they probably eat them within a few days. The damage on top is probably just from where they carried it with their mouths. StuRat (talk) 11:46, 18 November 2009 (UTC)

Knife against bone / nails on a chalkboard

I've read the Wikipedia articles about nails on a chalkboard and psychoacoustics, but have another twist I'd like to share/ask about on the SciRefDesk. When I'm cutting apart raw chicken thighs, occasionally the knife will strike the bone at an angle. I find this kinetic sensation even more immediately revolting than nails on a chalkboard. I'm feeling physically uncomfortable right now just thinking about it. Note that I've never had any bad experiences with knives, been stabbed, lost a loved one to a stabbing, or suffered any other sort of event that might provoke an unusual psychological reaction to this particular situation. I've also noticed that at work when I slice the plastic seals off new watercooler bottles with scissors I feel vibrations/textures/whathaveyou quite similar to the knife into bone, and then of course the corresponding involuntary physical reaction is also quite similar. The nails-chalkboard example is an auditory stimulus, but mine with the knife is a kinetic stimulus - is this a personal quirk, or is there an equally well-recognized phenomenon for particular vibrations and such? Thank you! 218.25.32.210 (talk) 07:07, 18 November 2009 (UTC)

I'm not sure if it's related, but vibrations at a certain frequency (the resonant frequency ?) cause me searing waves of pain. One example was when I tried to hit a baseball with a wooden bat and hit it outside of the "sweet spot". StuRat (talk) 11:40, 18 November 2009 (UTC)

Sounds similar to the reason I can't file my nails. It's giving me shivers just to think about it. So, you're certainly not the only one experiencing this, but I don't think it can be universal: it is were, other people wouldn't be able to file their nails. 86.142.231.220 (talk) 15:22, 18 November 2009 (UTC)

Absolutely subjective -- as a periodontal resident, I cut bone, soft tissue and teeth with no revulsion whatsoever -- and it's not like the first 3 times I did it, I was cringing. Pretty much the only thing I can't handle is watching someone vomit -- it makes me gag. DRosenbach ^{(Talk | Contribs)} 15:45, 18 November 2009 (UTC)

My wife files her nails almost every day, but I can always give her a shiver by giving one of her fingernail a light scratch with one of mine. She says that it makes her teeth hurt. (?!) Kingsfold (talk) 19:39, 18 November 2009 (UTC)

Directed graph condensation terminology

The strongly connected components of a directed graph form a poset. What is the terminology for lesser, greater, minimal, maximal components?

For example, some websites (strongly connected components made of pages (nodes)), can't be reached from the rest of the internet (have no hyperlinks coming back in). So in a sense this site is locally maximal, or "outgoing only". But what is the actual term? -Craig Pemberton 08:41, 18 November 2009 (UTC)

We have a glossary of graph theory which might help you, which suggests k-edge-connected graph may be what you are looking for. This is more a question for the Mathematics desk though. I'm not sure I really understand your question though, a poset does not have to be strongly connected and just because a graph is strongly connected does not make it a poset. They are two different things. SpinningSpark 10:06, 18 November 2009 (UTC)

In the context of Markov chains, one calls the strongly connected components "communicating classes" and calls them "closed" if they are minimal in the reachability ordering (i.e. you can't get out of them) and "open" if they are not. I don't know a term for maximal classes, or anything for minimal classes in more general graph theory. Algebraist 12:41, 18 November 2009 (UTC)

Accuracy of Sonar?

I was just thinking, sonar works by sending out pings and then timing the time it takes to come back from different directions right? Is there a reason why you couldn't make sonar goggles, for seeing in total black conditions or something, or even being able to see through things sound can go through, or is there an inherant low resolution to it? What about radar? Gunrun (talk) 10:46, 18 November 2009 (UTC)

I would think one problem would be sonar-absorbing materials, like cloth, which would look "black". Not sure about the resolution, although bats manage to catch insects with it. On the other hand, they will also go for a tossed rock, so that supports the "inherently low res" theory. StuRat (talk) 11:34, 18 November 2009 (UTC)

See Medical ultrasonography, Gynecologic ultrasonography and lot's of images. The wavelength of 100 kHz ultrasound in air is .343mm. Since you can't see sound, sonar goggles would have to contain a 2-D phased array of microphones, a processor to convert the microphone signals to an image, and a screen to display it on. Include a Wheelbarrow to carry all that stuff. Cuddlyable3 (talk) 11:45, 18 November 2009 (UTC)

There has been research into enabling the blind to form an image through transmitted sound, some of these devices working on a scanning principle. It is called sonification, our article is not very informative, here's a magazine article. The theoretical resolution in the audible band is going to be on the centimetre scale - you could find the beach but not individual grains of sand. SpinningSpark 13:28, 18 November 2009 (UTC)

The Make Magazine blog recently had a link to this : DIY sonar visualizer with Processing + Arduino. But, of course, that's 2d. APL (talk) 15:29, 18 November 2009 (UTC)

8 or 9 abreast seating in Airbus A300 & A330 airliners.

What is the percentage of Airbus A300 & A330 airliners operating with 9 abreast seating, rather than the standard Airbus 8 abreast seating ?

This question is partly to resolve whether 9 abreast seating constitutes an "unusual layout" in A300/A330 aircraft, and therefore that fact should be included on the Monarch Airlines page. --JustinSmith (talk) 14:11, 18 November 2009 (UTC)

Why not just say that the seating is 9 abreast and don't make any commentary on it at all. No need to analyze whether it is unusual or non-standard or anything, just make the statement and let the reader reach his own conclusions about how he should feel about 9 abreast seating. --Jayron32 15:40, 18 November 2009 (UTC)

Some contributors to the Monarch Airlines Talk page do not consider that the 9 seats abreast is relevant, saying it`s not an unusual layout. For my part every other A300/A330 I`ve been on has 8 abreast seating. I`m prepared to accept 9 abreast seating may not be unusual if someone is able to tell me what percentage of these aircraft have each layout. Why argue about an objectively provable fact ? --JustinSmith (talk) 18:36, 18 November 2009 (UTC)

The number of seats is based on the type of seats used. I haven't been on an Airbus, but I have flown very often. Identical aircraft owned by two different airline companies commonly have different seat configurations. The cheaper company will cram in more seats. The more expensive company will use less (wider) seats. It is a normal aspect of commercial aircraft, not an odd feature. -- kainaw™ 18:55, 18 November 2009 (UTC)

For most aircraft models, the way that one airline "crams in more seats" is to reduce the spacing between them (seat pitch). But information of this kind, whether seat pitch or number of seats in a row, seems automatically notable to me as people may find it useful to decide on how to value the price difference between airlines. I like the suggestion to mention the fact without comment as to how rare or common it is. But that discussion belongs on the article's talk page or a general talk page about air travel or similar, not here. The question for the RD was how rare or common it is. This, I can't help with. --Anonymous, 20:15 UTC, November 18, 2009.

I would agree with Kainaw that seat pitch varies significantly between different airlines, but the number of seats abreast doesn`t. Not on most of the airliners I`ve been on, apart from Monarch obviously. For instance on Airbus widebody planes Economy is always 2+4+2, Premium Economy 2+3+2 and Business 2+2+2. As far as I`m concerned variation in this is unusual. --JustinSmith (talk) 09:40, 21 November 2009 (UTC)

Why do you feel more tired after a few hours sleep than when you went to bed?

It seems that people generally feel more tired when they have only had a few hours sleep than when they went to bed. No doubt they would have felt more tired if they hadn't gone to bed at all, but why would you feel more tired than when you went to sleep? I guess it must be simply a perception, perhaps influenced by different hormone levels at night and during the day. QuickSnow (talk) 14:12, 18 November 2009 (UTC)

• REM sleep is crucial to rest.
• The air quality where you repose may be compromised, if you wake up groggy. Vranak (talk) 14:23, 18 November 2009 (UTC)

"No doubt they would have felt more tired if they hadn't gone to bed at all" - actually, with me, that's hardly ever true. Usually after I have gone to sleep, for just a short period, or, for a longer period, I wake up feeling much more tired than if I had actually stayed up, even if I had stayed up a full 8 hours. So I reject this hypothesis :p Rfwoolf (talk) 14:34, 18 November 2009 (UTC)

To the contrary, when I have had to work for more than 24 hours, and get a few hours sleep, I feel far more alert than before the sleep. Even a 1 hour nap helps. Chalk it up to individual differences. Part of it may be how lousy you feel for a short while when you first wake up. Edison (talk) 14:38, 18 November 2009 (UTC)

Ditto. As a chronic insomniac, there is, for me, a huge difference between 4 hours of sleep (which ain't great, but is something) and zero hours of sleep. The former can actually get me through the day; the latter will get me until about 5pm and then I'm zonked (unable to concentrate, agitated, physically and mentally exhausted). --Mr.98 (talk) 22:53, 18 November 2009 (UTC)

It is called Sleep inertia. That article may be of some help. --Tango (talk) 14:51, 18 November 2009 (UTC)

Very interesting. Thanks. QuickSnow (talk) 01:31, 19 November 2009 (UTC)

An object at rest tends to stay at rest unless acted upon by an outside force. — DanielLC 16:28, 18 November 2009 (UTC)

When I had keep crews working for 3 days in an emergency, I would send some home after 24 hours for 8 hours, perhaps allowing them 5 or 6 hours of actual sleep, allowing for travel, family interaction, etc. A worker who got 5 hours actual sleep on 3 consecutive nights was demonstrably more safe and productive than someone who went 36 or 48 hours with no sleep. It may feel rough to wake up after a short sleep, but an hour or two later there is all the difference in the world. In the American Civil War Nathan B. Forrest's smaller Confederate force in one battle pursued a larger Union force for several days. Forrest had his men take turns napping, while the Union forces were constantly at battle stations. In the final battle, after which the Union forces surrendered, the Union forces were falling asleep in combat. Edison (talk) 05:16, 19 November 2009 (UTC)

health care

what type of national health care does New

Zealand have? —Preceding unsigned comment added by 76.210.215.156 (talk) 16:02, 18 November 2009 (UTC)

Have you read our article on health care in New Zealand? — Lomn 16:29, 18 November 2009 (UTC)

Small passive solar water heater

Hi all,

If I wanted to build a small solar water heater for just a gallon or so of water, would there be any reason not to allow the water itself move through the solar collector by convection, looping around and around?

Most of the instructions I've seen suggest using antifreeze in the tubes, and then having tubes loop back inside the tank warming the water, but I wonder if this extra step is necessary for such a small system.

Any thoughts? Thanks! — Sam 63.138.152.155 (talk) 16:12, 18 November 2009 (UTC)

I agree that such a system seems like total overkill for 1 gallon. Instead, just get a 1 gallon glass water bottle and place it on a reflective surface, say aluminum foil, on top of an insulator, like Styrofoam. On a sunny day, this should get the water warm. Don't leave it out when it's below freezing and/or dark, or it could freeze and break the bottle (but then again, a broken 1 gallon glass bottle isn't exactly a disaster). If you can arrange a large inverted cone of aluminum foil with the water bottle at the bottom, you might even get the water hot. If you add some dark food coloring to the water it will get somewhat hotter, but that depends on how you want to use it. Here's some images of solar ovens to give you a few ideas: [3]. StuRat (talk) 16:20, 18 November 2009 (UTC)

You should paint the bottle black so it absorbs heat, rather than put in on foil that will reflect heat. --Tango (talk) 18:44, 18 November 2009 (UTC)

Sounds like Sun tea. 75.41.110.200 (talk) 18:51, 18 November 2009 (UTC)

Take a look at concentrating solar power for ideas about how to build solar energy concentrators, both large and small. The simplest can be built easily by lining a convex-shaped object (like a large bowl or tube) with foil; and placing the object you want to heat (hot dog) at the focal point of this mirror. You can paint that object dark colors to help it absorb heat; the rest of the apparatus can be foil-lined to increase the energy collected and concentrated. If you are good at plumbing, you can run water through the hot-spot and then plumb it to where ever you need the heated water. Here's an article with a photo of such a device. Nimur (talk) 19:20, 18 November 2009 (UTC)

I disagree on painting the bottle black, because this will generate heat at the surface, and much of it will radiate back off the bottle. The goal is to create the heat in the interior. This can be accomplished by having a black object in the center and/or by having the fluid itself absorb solar energy by being a dark color (but not so dark that all the heat is absorbed right at the surface of the bottle). StuRat (talk) 23:08, 18 November 2009 (UTC)

We have a very detailed article on Solar water heating. It describes several schemes of increasing complexity; the simplest one(s) are passive and use just water in a single open loop (no closed loops, no antifreeze, no forced circulation). I think that's what you are looking for. --Dr Dima (talk) 01:26, 19 November 2009 (UTC)

Thermosiphoning will cause cold water to sink and hot water to rise, maintaining circulation through a solar water heating system. A collector might include serpentine (curving back and forth) copper tubing, soldered to a steel back plate, all of that painted flat black. Facing the sun should be a pane of glass. Behind the steel should be good insulation to reduce loss. The assembly should be sealed against air leaks. Expect about 1 kilowatt per square meter of solar energy input in full sun, if the collector is oriented at an angle equivalent to the latitude. If it goes below freezing at night, the collector will freeze and burst after radiating off all the energy you collected during the day, so it would need provisions for draining at night and pumping the water up in daytime. I have collected solar energy with a simpler system consisting of a black garden hose lying on a roof, connected to the top and bottom of a storage tank. Hot water rises, cold water falls, and warm water results. There are actually places in the world where it rarely goes below freezing, and an antifreeze loop would seem to create unnecessary expense and complexity there, but you get what you pay for. Edison (talk) 05:08, 19 November 2009 (UTC)

Aluminum borohydride

When researching sources to create the article aluminum borohydride, I found a source stating that aluminum borohydride reacts violently with water, even reacting with the trace amounts of moisture in the air. However, another source I found states that aluminum borohydride has an NFPA-R rating of 0, meaning that it doesn't react with water. Which source is correct? ----J4\/4 <talk> 17:31, 18 November 2009 (UTC)

I'm pretty sure all borohydrides are quite reactive with water, so I would guess that the source which indicates a reactivity rating of 0 is probably in error. I would find additional sources before proceeding, however, just to be sure. --Jayron32 20:53, 18 November 2009 (UTC)

According to the Riedel/Janiak, Anorganische Chemie, 7. Auflage, de Gruyter 2007, p. 577 Aluminum Borohydride is a covalent compound that is liquid at 25°C. It therefore should hydrolyze quite easily to give B(OH)3 and H2. Regards --91.6.28.130 (talk) 18:46, 20 November 2009 (UTC)

Evolution of separate sexes

Sexual reproduction obviously has a survival advantage, in that it increases genetic diversity, thus allowing species to adapt faster. However, this doesn't explain the evolution of separate sexes. After all, genetic diversity would increase faster if any organism could reproduce with any other organism. Why did separate sexes evolve despite this? ----J4\/4 <talk> 18:26, 18 November 2009 (UTC)

I'm having difficulty comprehending the question. Maybe I'm just dense, or maybe there's a faulty assumption or two contained in the question. Vranak (talk) 18:40, 18 November 2009 (UTC)

If organisms reproduced by a variant of sexual reproduction, where any organism could produce an offspring, which would contain the DNA of both parents, with any other organism, genetic diversity would increase more that under the artificial limits on diversification created by having each organism only able to reproduce with approximately half of the other organisms of that species. ----J4\/4 <talk> 18:43, 18 November 2009 (UTC)

I'm still baffled by the scientific lingo. I used to understand this stuff, but I'm glad I've forgotten. Vranak (talk) 19:42, 18 November 2009 (UTC)

To simplify as much as possible: "Why don't we all have both male and female parts". Some animals do indeed work like this, but they tend to be "lower orders", not mammals and birds, for instance. The advantage of different sexes is that it provides a way to have a division of labor with each gender customized for it's role. Males, not having to carry babies, can be faster and fight more, while females can concentrate on rearing the kids. StuRat (talk) 23:01, 18 November 2009 (UTC)

For a novel, decidedly nonscientific perspective, I would suggest that life would be so boring as to verge on the intolerable if there were no sexes. Video games and comics books just aren't that compelling much past the age of 14. Something needs to keep our interest, so why not a separate gender that is both like yet unlike? Doesn't totally cover it for everyone I know. Vranak (talk) 05:09, 19 November 2009 (UTC)

This is a big question that a lot of evolutionary biologists have worked on. Our article, Evolution of sexual reproduction, should give you a good introduction. --Tango (talk) 18:46, 18 November 2009 (UTC)

Actually, having just skimmed through that article, it doesn't seem to address this exact issue. It probably should... --Tango (talk) 18:53, 18 November 2009 (UTC)

Sexual reproduction allows for specialization. There are further advantages to sexual reproduction than just genetic diversity. Specialization can take on a wide variety of forms. Survival is enhanced by sexually divergent members contributing advantageous characteristics that it would be more difficult to develop by evolutionary means in an all-in-one organism. Bus stop (talk) 19:14, 18 November 2009 (UTC)

I'm pretty sure this ^ is the essence of the answer, although it might help to also ask why the gender balance is roughly 50:50. Let's consider a hypothetical species in which every individual is fully hermaphrodite and can reproduce with any other individual. Assuming they are otherwise mammal, one of these individuals will have to gestate the offspring. Let's say that the normal situation is that individuals both impregnate and are impregnated. While an individual is pregnant, they can still impregnate others although they cannot be impregnated themselves. And towards the end of gestation, they are less capable of impregnating others. Once the child has been born, the individual who gave birth to them has to devote resources to caring for them. The individual who also contributed to this child's DNA can choose whether they invest resources in them or not, but they are also responsible for a child they gave birth to.

Let us imagine that an individual is born who does not allow themselves to be impregnated, or physically cannot be impregnated. They can still pass on their DNA by impregnating others, but they never have to devote resources to a developing child within themselves, nor do they have times when they cannot impregnate others, nor do they have to devote resources to any given child. They might increase some of their children's survival chances by devoting resources to them, but they might equally spend all their resources begetting as many children as possible. This individual has a clear advantage over the hermaphrodites.

But if more than 50% of the individuals in a species use this tactic, it starts to be an advantage to be the one bearing the children. The individuals who have taken the male route cannot reproduce with each other: they need to find a mate who can bear young if they want to reproduce. Those who can bear children can pick their mate more carefully, because they have a choice of willing partners: they need to choose carefully because they invest more in each child. And, to maximise their chance of grandchildren, it's in their interest to bear both types of children: those who can bear children, and those taking the male route. A 'male' should also beget both 'male' and 'child bearing' children to maximise grandchildren. Since the quirk that produced the 'male' individuals is almost certainly carried by the 'males', as long as the 'males' are only passing it on to about half their children, it is in the child-bearing individuals' interest to reproduce with the 'males' rather than with other child-bearing individuals. And thus, we have two genders.

If the males passed on the tendency to be male to every child, or females carried a recessive gene, two copies of which rendered a child male, you might expect some hermaphroditism to remain. But in an XY situation, two genders emerge. 86.142.231.220 (talk) 19:30, 18 November 2009 (UTC)

this old article discusses the evolution of Anisogamy--Digrpat (talk) 19:21, 18 November 2009 (UTC)

It's all about the gametes (sperm and eggs) and finding "a selective advantage of large, immobile gametes (eggs) ... An assumption that eggs produce a pheromone sperm attractant leads, by established physical principles, to a more than sufficient advantage of large egg size. Without pheromones, combinations of increased target size and weaker increased zygote fitness or increased gamete longevity also provide sufficient selection."[4] Also see [5][6][7]. Fences&Windows 01:40, 19 November 2009 (UTC)

There are two strategies for gametes: get big so it'll survive longer, or get smaller and try to impregnate the big ones. Imagine Reason (talk) 04:20, 19 November 2009 (UTC)

That's right. There is a division of labor at the gamete level. Some of them are big (female) to provide enough of a head start for the new organism while the other ones are mobile (male) to increase the chances that they will find each other. Dauto (talk) 13:53, 19 November 2009 (UTC)

ATSC standards

I am trying to understand how television channels use the ATSC standards to broadcast their information. The RF channel is 6 MHz wide, but is there usually only one signal that is modulated over the entire 6 MHz? I know that there was two different carriers for video and audio under NTSC, and that DTV is supposed to be able to make possible additional features to broadcast television, like what is available to cable and satellite TV, but I can't make out from the articles whether a TV station would be able to use something like "sub-channels", within their allotted bandwidth. How does ATSC account for this? Thanks —Akrabbim^talk 19:05, 18 November 2009 (UTC)

I have received subchannels over-the-air, so I know they exist. I don't know how it's done. --Jc3s5h (talk) 21:48, 18 November 2009 (UTC)

Specifically, in the US I've seen three subchannels per frequency. StuRat (talk) 22:52, 18 November 2009 (UTC)

I guess my question can be rephrased as this: Does the ATSC have different carriers on the same channel for different content like the NTSC had with audio and video, or do subchannels and the like come from one compressed signal on one carrier, spanning the entire channel. I have hooked up an antenna to a spectrum analyzer, and it looks like there is approximately equal power across all 6 MHz of the channel, but I can't tell if there is information split between more than one carrier or not. Thanks for the responses so far. —Akrabbim^talk 04:57, 19 November 2009 (UTC)

The subchanels are multiplexed in the digital realm, not the rf realm. The rf signal contains a single mpeg transport stream. Inside that stream are the various subchannels and audio for them. There is no separate rf band for audio, or for any of the subchannels. Ariel. (talk) 06:33, 19 November 2009 (UTC)

Fantastic, thank you. —Akrabbim^talk 13:22, 19 November 2009 (UTC)

Vacuum-Forming PMMA (Acrylic)

Can PMMA (Lucite, Acrylic, Plexiglas, Perspex) be heated and vacuum-formed using a partial-vacuum machine?

Will it discolor? Will it separate ("sweat") into subcompounds? Will the addition of semi-transparent aluminum film on the acrylic substrate have an adverse effect on the forming process?

I'm working on forming some custom visors for movie prop helmets and am trying to find a suitable material. The final shapes of these visors precludes the use of commercial tinting after the forming process, and the tint will burn off, discolor or tear if applied to the plastic sheet before heating and forming.

Thanks, Nullexe (talk) 23:03, 18 November 2009 (UTC)

I have heated tinted perspex with a hot air gun until plastic (fairly soft), then formed it by hand/vice etc into the required shape. It did not discolor. It did not decompose. I would put the semi transparent film on after forming if poss. This will avoid wrinkling of the film under heating. I see no reason why it could not be formed on a vacuum m/c. Just be careful you dont get it too hot and dont expect small radii.--79.75.63.71 (talk) 15:15, 19 November 2009 (UTC)

PMMA is used extensively in dentistry and to increase setting rate and quality, pour-ups are placed into pressure pots with warm water. I don't exactly know what you mean by the use of a vacuum in your question, but better setting occurs under positive pressure, so perhaps to a worse extent under negative pressure. DRosenbach ^{(Talk | Contribs)} 16:52, 19 November 2009 (UTC)

@DRosenbach-I'm referring to vacuum forming, which uses a rigid form to hold a sheet of plastic over a heat source until extremely malleable, then overlays that sheet onto a positive mold (a "buck") and then negative pressure is applied underneath the buck, which pulls the plastic down forceably around the buck. The plastic is then allowed to cool and harden, and when detached from the buck, produces an almost-identical copy of the buck. With the helpful input of you and 79.75.63.71, I'll be attempting some test pulls here soon and will update this topic with results. Nullexe (talk) 20:43, 24 November 2009 (UTC)

Do you expend more energy with intense exertion over a smaller exertion for a longer period of time?

Per the title, say I:

• Run hard ten miles and it takes me thirty-five minutes (seven minute miles); or
• Jog ten miles and it takes me one hour (twelve minute miles); or
• Leisurely stroll ten miles and it takes me two hours and five minutes (twenty-five minute miles).

My impression is that I burn more energy with high exertion even if I complete the same task no matter that at lower exertion I take more time at it. Is my impression correct? Is it wrong? What are the comparisons in energy expended? And why does one burn more energy than another, if indeed that is the case? Purely from musing on it, it seems to me the only reason one would expend more energy than another is efficiency of the movement, breathing, respiration and so on, rather than something inherent about expending energy faster verses slower, but even with that hypothesis, I don't even have a guess as to whether slower verses faster is more energy efficient.--108.1.106.132 (talk) 23:05, 18 November 2009 (UTC)

Bear in mind that you also burn energy when doing nothing (basal metabolic rate), so, the longer you take to travel the ten miles, the more energy will be burnt up that way. So, for that reason I'd say the stroll would burn the most energy total. However, if you add in resting time afterwards to make the total time the same in each case, then that changes the equation. In that case the fastest run (and rest afterwards) would burn the most calories, since running is less efficient. StuRat (talk) 23:23, 18 November 2009 (UTC)

I don't think "time" here is the only factor—muscles use energy differently depending on exertion and time (see the chart on Aerobic exercise), and so I don't think StuRat's reasoning works out. That being said, I have read studies that implied that using up a lot of energy vigorously does trigger your body into seeking out more calories, and thus a lot of people, after hitting the gym, consume far more calories than they would have otherwise, even more than were burnt exercising. For that reason I have heard it recommended that light, regular exercise (e.g. walking regularly, rather than running) can be more productive if the overall goal is weight-loss, as it avoids less over consumption. --Mr.98 (talk) 00:51, 19 November 2009 (UTC)

You'll also burn more calories the longer you keep your heart rate up. That's why swimming or playing squash for 30-60 minutes will likely burn far more calories than a game of baseball. It's also part of the reason why marathoners are thin as paper whereas sprinters actually have some fat and muscle on them (I'm simplifying, there are other reasons as well). ~ Amory (u • t • c) 01:42, 19 November 2009 (UTC)

For a fixed distance, running burns more energy per mile than walking, because the body moves up and down a lot more during running. (The exception is if you're walking really fast -- the movement is so inefficient then that switching to a slow jog at the same speed can reduce energy output.) Also the faster you run, the more energy you burn per mile, but the difference is not all that large until you hit the point where you are getting close to maxed out. Looie496 (talk) 20:08, 19 November 2009 (UTC)