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March 3

Blood donation risk factors

Why do different countries, even with similar statistics for HIV risk etc have different eligibility criteria for donating blood? For example, in the US those who have had tattoos in a licensed state, are not barred from donating blood but in the uk, they are barred for 4 months despite licensing for tattoo parlours existing across the uk. Similarly, men who have sex with men have a lifetime ban in the US, whilst only a 12 month ban in the UK. Other risk factors such as those who pay for sex are not even considered a risk factor in the us, whilst there is a 12 month ban in the uk. Why are these restrictions so different? — Preceding unsigned comment added by 2.126.193.17 (talk) 11:09, 3 March 2014 (UTC)

Because different people in different places are making different decisions. --Jayron32 13:14, 3 March 2014 (UTC)

As a regular donor I can tell you it's because of the Window_period for various infectious diseases. The ban is an attempt to screen donors in that period where the disease is not detectable in a blood/serum test. This relies on people telling the truth about their sexual activities. Both systems (US and UK - and even South Africa) have risks to the recipient. When my father went for his prostate op my brother and myself donated and "banked" our blood for him. The blood is still tested but dad knows we would not lie about the screening questions. 196.214.78.114 (talk) 13:49, 3 March 2014 (UTC)

Also as a regular donor, I can tell you that in the USA there IS a 12-month deferral period for anyone who pays or receives payment for sex. 24.5.122.13 (talk) 03:49, 4 March 2014 (UTC)

It's four months for a tattoo in the UK now - see Who Can't Give Blood?. They keep changing these rules as research about the diseases improves. It wasn't long ago that your blood wasn't acceptable in the UK if you were a man who had EVER had sex with another man, but things move on. I understand that many countries won't accept your blood if you lived in the UK during the 1980s, because of fears of Variant CJD. Obviously, if such a rule was applied in the UK, there wouldn't be many people who could donate. Alansplodge (talk) 13:43, 4 March 2014 (UTC)

You've already had some good answers here particularly relating to different people making different decisions based on a variety of factors (such as differing views on the evidence). It's worth noting that the restrictions are not without controversy. Notably the Gay male blood donor controversy, particularly as the indefinite restriction in some countries is argued to be inconsitent with other deferrals for high risk groups (such as people who have been paid for sex or paid for sex or had a very large number of sex partners at some stage) which are for shorter periods in some countries. Of course as some above have indicated there is a risk of people not telling the truth so it is possible that statisticly there is a higher risk for some reason (e.g. a person by be willing to admit they engaged in MSM at some point in their life, but not recently which doesn't happen with some other grouping). Although I've also seen it suggested that by making the criteria too strong such that people feel it's unreasonably discriminatory, people might be more likely to lie. Nil Einne (talk) 15:16, 5 March 2014 (UTC)

Human body parts that we have more than 2 of?

What is the body part that we have the fewest of but we have more than 2 of?

Besides fingers and toes.

And not general body parts like "facial orifices" or "bones in the forearm". They should be discrete body parts like kidneys or femurs or hairs on the head. — Preceding unsigned comment added by 24.228.94.244 (talk) 15:40, 3 March 2014 (UTC)

Wisdom teeth? --109.189.65.217 (talk) 17:53, 3 March 2014 (UTC)

The pineal gland is historically a third eye, and was active as a light sensor in early fish. The liver usually has three lobes, but that is not vital to its function. There are all sorts of nerves, muscles and bones that come in small sets, like cranial nerves. The inner ear has three canals for balance in the three dimensions. μηδείς (talk) 18:18, 3 March 2014 (UTC)

Hmmm, that's good... except you have two ears, making it 6. :) Getting to 3 is difficult, but I'll go with ... the tricuspid valve. Except, that is, sometimes it doesn't have 3. :) For 4 there are more choices, such as canine teeth. Wnt (talk) 19:59, 3 March 2014 (UTC)

That valve is a part of the heart. The OP would need to clarify just what he means by "body part". Does a portion of a larger organ count? ←Baseball Bugs carrots→ 21:35, 3 March 2014 (UTC)

Most people have two nipples. Some have three. A case could be made that the average person has two-point-something nipples. Also apparently applies to kidneys. (Other than that, an internet search for "people have three" finds little, other than "three brains" which probably doesn't qualify.) 88.112.50.121 (talk) 21:32, 3 March 2014 (UTC)

• The human body, like most animal life, has bilateral symmetry. So most of what we have comes in pairs. --Jayron32 23:20, 3 March 2014 (UTC)

• But rather incomplete bilateral symmetry, as it doesn't apply to our liver, gall bladder, pancreas, heart, most of our digestive system, etc. StuRat (talk) 23:50, 3 March 2014 (UTC)

• The small intestine is divided into 3 parts, the duodenum, jejunum, & ileum. StuRat (talk) 23:44, 3 March 2014 (UTC)

Veins and arteries. All connected, but many tributaries. Fewest and most. InedibleHulk (talk) 23:47, March 3, 2014 (UTC)

If you toss in capillaries, that makes 3 types of blood vessels. StuRat (talk) 01:16, 4 March 2014 (UTC)

Cheeks.165.212.189.187 (talk) 15:50, 4 March 2014 (UTC)

This one is right - you have 4. :) The eyelids are more of a stretch. :) Wnt (talk) 06:04, 5 March 2014 (UTC)

I thought you would have said that about the sphincters! I think eyelids is it165.212.189.187 (talk) 14:09, 5 March 2014 (UTC)

Eyelids165.212.189.187 (talk) 15:51, 4 March 2014 (UTC)

Limbs165.212.189.187 (talk) 16:04, 4 March 2014 (UTC)

Sphincters.165.212.189.187 (talk) 16:06, 4 March 2014 (UTC)

Testicles...all manner of reproductive bits. SteveBaker (talk) 17:59, 4 March 2014 (UTC)

Giving that answer without explaining what you mean takes a lot of balls. :-) StuRat (talk) 19:37, 4 March 2014 (UTC)

Yes, but at least we now know that User:SteveBaker comes from Cape May, lucky bastard. μηδείς (talk) 19:50, 5 March 2014 (UTC)

Oxygen

What's the lowest level of oxygen humans can tolerate? Is 2% enough to guarantee death? I heard some people can tolerate like 1%, which I find bull. Money is tight (talk) 23:21, 3 March 2014 (UTC)

It's not the percentage of oxygen, it is the partial pressure of the oxygen that matters. That is, lowering the percent oxygen by raising the overall pressure (such as adding an inert gas to the breathing gas) shouldn't have a significant effect. The actual amount of oxygen you need to survive is more complex than just a single number, it can be found most easily using the Oxygen–haemoglobin dissociation curve which depends on many factors, including blood pH, partial pressure of O2 in the air, partial pressure of CO2, etc. --Jayron32 23:26, 3 March 2014 (UTC)

To be completely clear, if you doubled the atmospheric pressure, you'd only need half the usual PERCENTAGE of oxygen - and if you halved the atmospheric pressure, you'd have to double the percentage. So this question only has meaning if the atmospheric pressure is specified. We know that people can survive (albeit with difficulty - and for limited amounts of time) at the top of Mt Everest - which has a third the atmospheric pressure (and hence a third of the oxygen) than at sea level. At sea level, we have 21% oxygen - so clearly 7% (at sea level) ought to be survivable for at least some amount of time. However, as Jayron says, it's a complicated business - people who live at high altitudes can survive with much less than people who normally live at sea level, so we should expect wide variations. Also, your needs are different with exercise rather than when relaxed. There is a big difference between what you could survive for an hour at - and what would keep you alive for 5 minutes or an entire day. SteveBaker (talk) 17:57, 4 March 2014 (UTC)

How many people could live for 5 minutes without O2? Tolerance of course varies on if you get to hyperventilate first, hold your breath, whether brain damage is "tolerating" or not, and whether you were exposed to vacuum which would make it impossible to hold onto any oxygen. (your lungs might explode if you tried) Even a deep exhalation still keeps some air in the lungs. Sagittarian Milky Way (talk) 17:25, 5 March 2014 (UTC)

22 minutes is possible. Count Iblis (talk) 13:49, 6 March 2014 (UTC)

March 4

Trumpet mouthpiece discolored?

Hey everyone, this may be a chemistry-related question. So, I play a Vincent Bach 1.5C mouthpiece, and lately I've noticed the inside is beginning to turn a light shade of bronze or gold, it seems, from the normal silver. A similar incident has happened before, where the inside of another mouthpiece had actually turned blue/purple/black. What's caused this? And I've heard that something can be done about the color change by using aluminum foil, baking soda, and hot water. Someone else said to use toothpaste. Can this work? Thanks. 64.229.204.125 (talk) 00:02, 4 March 2014 (UTC)

Do you know what the metal is ? Sounds like some type of patina has formed, and the first method sounds like using electrolysis to remove it, while the toothpaste method is just using abrasion to remove it. If it really is silver, then silver polish might work (although silver tarnish is typically not that color). StuRat (talk) 01:21, 4 March 2014 (UTC)

"Silver Plated", according to Bach: 351 1.5C Trumpet Mouthpiece - Silver Plated 71.20.250.51 (talk) 05:56, 4 March 2014 (UTC) —(Also available in Gold-plate:)

Could be that the silver plate is wearing through, beginning to show the brass underneath - assuming you clean regularly with a brush or whatever. The bluish-black is the normal silver oxide "patina". 71.20.250.51 (talk) 06:06, 4 March 2014 (UTC)

According to our articles, silver tarnish actually consists of silver sulfide, not silver oxide. Silver sulfide is black, so I agree with 71.20.250.51 that it sounds like with your current mouthpiece, the silver plating is wearing off, exposing the underlying brass.

The Vincent Bach mouthpiece manual says "If the silver plating of your mouthpiece has worn off, the mouthpiece should either be refinished or discarded. Caution: Exposure to raw brass can lead to an allergic reaction or poisoning." You'd have to use your own judgment as to whether that admonishment still applies if the only place the silver plating is wearing off is inside the mouthpiece's throat and/or backbore. Red Act (talk) 06:26, 4 March 2014 (UTC)

(WP:OR warning) I played trumpet (poorly) for years. On the rare occasion I get it out, my mouthpiece looks just like you describe. As others suggest, it's (probably) just the plating wearing off if it looks bronze-y. If it's the purple/black, it can be cleaned off with toothpaste. I've used mine (with plating worn off inside) with no problems for years. But I don't play every day (nor even every year). If you do play often, it's probably worth getting a new mouthpiece, they are not very expensive. SemanticMantis (talk) 16:28, 4 March 2014 (UTC)

Do they make stainless steel mouthpieces ? If so, they might be less expensive and require less maintenance. Hopefully they are either solid stainless steel or at least have a thicker layer than won't wear through. StuRat (talk) 19:33, 4 March 2014 (UTC)

Hmmm, would different expansion rates in the metals foul up the music? (Unless it's a stainless steel horn, that is) Wnt (talk) 05:58, 5 March 2014 (UTC)

Pro musicians will say that the choice of metal affects the timbre. 12.217.87.18 (talk) 13:01, 5 March 2014 (UTC)

DNA testing and meiosis

How does making a match in a DNA test work when one of the samples is haploid? For example, if testing sperm from a crime scene, how does one prove it matches to a suspect if the sperm is meiotic/haploid/gametic, while presumably a DNA sample obtained from a suspect is mitotic/diploid/somatic. (Pardon all the slashes there, but I'm not 100% on which is the best terminology.) Thanks in advance, 76.168.226.134 (talk) 00:47, 4 March 2014 (UTC)

An individual sperm is haploid, but a sample of multiple sperm contains all the possible choices and so tends to look a lot like diploid DNA. If you had a really small one-sperm sample you'd be missing some markers, which could happen if you lost some of the DNA of a single cell sample to degradation also. So it isn't a big practical difference. Wnt (talk) 00:59, 4 March 2014 (UTC)

The probability of a sample of m sperm cells containing both copies of all of n chromosomes is (1 - (1/2)). For humans, n=23, and with 10 sperm cells the probability is already more than 95%. Icek (talk) 15:05, 4 March 2014 (UTC)

Well, remember that there is chromosomal crossover in the pachytene stage of meiosis, so chromosomes aren't inherited as units. However, the same analysis still applies for n markers on your actual test, provided they are not in linkage disequilibrium (i.e. a large number of centimorgans distance between them). I assume that any forensic tests will choose fully independent markers -- unless the agents are being really overzealous and starting to look into the health or mental propensities of their suspects rather than merely trying to identify them. Wnt (talk) 15:55, 4 March 2014 (UTC)

Missile Command

Some here remember that fabulous advance in video game technology known as Missile Command, in which players stopped incoming missiles (which were slowly moving, later not slowly moving, lines) that would reach and destroy your cities unless you set off immense explosions in their path with your own missiles. The scenario is seemingly very straightforward, and somewhat documented, - by exploding nukes in space you could use them as effective anti-ballistic missiles. This led to a disastrous space test in 1962 known as Starfish Prime and others in Operation Dominic that disabled various satellites including the original Telstar. (Before the invention of intercontinental ballistic missiles around 1957 I suppose it would always have made more sense to send ordinary fighter aircraft to stop an incoming plane) On account of the unwanted side effects, the Partial Nuclear Test Ban Treaty was enacted in 1963 to ban the space tests.

Despite the lack of testing under peacetime conditions, it makes me wonder: how long was a Missile Command scenario actually possible, or even a mainstay of nuclear strategy? I would think that with the advent of multiple independently targetable reentry vehicles the time to calculate the intercept would decrease, and the number of shots required would increase (almost like the game). Perhaps more significantly, having some manner of second stage propellant on board would seem to suggest to me that these were no longer ballistic missiles, but moving targets. The MIRV article says that they were first deployed in 1970; the SALT I entry seems to suggest more like 1968; in any case, the advent of MIRVs and the agreement of arms reductions seem closely linked. So I would hypothesize that Missile Command is what caused nuclear arsenals to swell to the point of "being able to destroy the planet N times over", and that reduction in the arsenals was an almost unavoidable aspect of that no longer working. Mutual Assured Destruction may have been coined in 1960, but it would seem to be a reality once "Missile Command" becomes impossible. (But if the ABM treaty proposed in 1967 and enacted in 1972 hadn't been made, could they have gone that way? I'm not sure when SDI/BMDO/Missile Defense Agency stopped being bravado and became plausible, if they really have)

Anyway, I've tried to answer my own question, but I'll pose it to you anyway: how long was "Missile Command" a real strategy? Did the various treaties I've described emerge as the nearly inevitable consequences of advancing technology, rather than as any reflection of the political 'thaws' and 'reforms' between the then superpowers? And does it still have a place in our thinking, and an effect on the number of missiles in our arsenal, even today? Wnt (talk) 15:48, 4 March 2014 (UTC)

Well, the Stanley R. Mickelsen Safeguard Complex was active for about a year in 1976-76. The Russian A-135 anti-ballistic missile system still exists, though it's an open question how useful it really is. (I recall wasting a lot of quarters in Missile Command machines, coincidentally in the mid-1970s, so it was clearly an au courant concept at the time). Acroterion (talk) 16:15, 4 March 2014 (UTC)

Hmm. I'm thinking this is the "exception that proves the rule". Not just the exception to the ABM treaty - with coverage of 100 nuclear missiles in a small area, MIRVs wouldn't apply very much to this either, because they'd all have to go the same place more or less. Wnt (talk) 17:18, 4 March 2014 (UTC)

The US basically abandoned the "set off nukes in space" method of defense in the late 1970s when the LIM-49 Spartan was removed from service. That, plus the shorter-range Sprint were part of the Sentinel Program developed in 1963. But that program was never actually built, except for a significantly scaled-down version that became Safeguard. So it was a real strategy, but only for a few years. Treaties were part of the reason it was killed, but there were also technical, strategic, and cost issues. The current US system, Ground-Based Midcourse Defense relies on a direct impact with the target. Mr.Z-man 17:36, 4 March 2014 (UTC)

The Nike Hercules were deployed in 1958 and only deactivated in 1988 in Europe although all the nuclear variant ones were removed in the late 70s from the U.S. 75.41.109.190 (talk) 17:55, 4 March 2014 (UTC)

The Nike Hercules was intended for interception of supersonic bombers and short-range ballistic missiles -- it was never capable of intercepting an ICBM. 24.5.122.13 (talk) 07:21, 5 March 2014 (UTC)

Missile Command, the game wasn't detailed enough to tell what kind of missiles you were intercepting or at what altitude. 75.41.109.190 (talk) 18:15, 5 March 2014 (UTC)

Wasn't there a version specifically titled "Patriot Command"? 24.5.122.13 (talk) 03:15, 6 March 2014 (UTC)

Explaining the existence of molecule with MO

Hi there,
Can somebody explain me why Ethane and Ethylene exist according to the MO theory?
Thank you — Preceding unsigned comment added by 77.125.103.142 (talk) 16:17, 4 March 2014 (UTC)

Because when you figure out the molecular orbitals of those molecules, you end up with more electrons in bonding orbitals than in antibonding orbitals. But that is why anything exists in MO theory. If you want some references that show how all of the molecular orbitals in Ethane work this page showed up using a simple Google search for "Molecular orbitals of ethane" and a similar page here for ethylene (ethene). In the future, if you try typing variations on your question into search engines like Google, you can often find web pages such as this. --Jayron32 17:17, 4 March 2014 (UTC)

Hospital evacuations

How are critical parts of hospitals such as intensive care units or operating theaters evacuated if there is an immediate fire hazard near them. Surely they can't just stop during a surgical procedure. 194.66.246.65 (talk) 17:23, 4 March 2014 (UTC)

Generally, such facilities are designed to permit a strategy of "defend in place" (Google that phrase with 'fire' and/or 'hospital' and you'll get a multitude of discussions and plans). Buildings are designed with physical firebreaks, and staff are trained in procedures, to allow critical areas to be isolated from their surroundings in the event of a fire or other incident, for long enough (fire doors are often rated for two or more hours) to allow firefighting and outside rescue. TenOfAllTrades(talk) 17:48, 4 March 2014 (UTC)

Hospitals, nursing homes and similar facilities are always subdivided by fire and smoke barriers so that the occupants of one section of the building can be evacuated into another section on the same level. Theyt're also required to have sprinklers. The next time you're in a hospital, look for the bazillion cross-corridor fire doors, usually held open with a magnetic latch until the fire alarm is activated. In ICUs and ORs there are additional requirements, but if a fire breaks out in an ICU, for instance, the patients will be evacuated. The monitors have batteries, and the beds usually have an oxygen cylinder on board for such a contingency. In an operating room it would depend on the medical procedure that's being performed to determine how feasible evacuation might be. Nearly everything in an OR has wheels and back-up power. Acroterion (talk) 04:36, 6 March 2014 (UTC)

Water bond angle, snake oil

Hi, I recently came across this charmingly over-the-top snake oil site "water cures anything" . Now, let me make it clear I don't believe any of the numerous health claims, but this bit caught my eye:

My question: is it possible to change the bond angle of a given water molecule? Or, failing that, is it possible to select from a mixture of angles, to produce a sample with a different distribution than the original? I see that water model has a table with values ranging from ~104.5 to ~109.5, but it's not clear to me if these are just different model outputs for the same water, or if the distribution of bonding angles varies that much in nature. Or does the angle of a specific molecule vary depending on its environment? Thanks! SemanticMantis (talk) 18:21, 4 March 2014 (UTC)

The answer is no. There's going to be some difference in the bonding angle from any one water molecule to another because of molecular vibration, see here for a rather easy to understand discussion of the modes of molecular vibration. However, it isn't possible to "select" water that is at any one bond angle within the normal range of vibrational motion, nor is anything written above anything except unmitigated bullshit. Pay it no mind. It is completely nonsensical, excepting that it uses sciency-sounding words. It's the chemistry equivalent of someone randomly pasting words from a foreign language together and hoping the resulting sentence fools people who don't speak it into thinking it is really French or something. --Jayron32 18:50, 4 March 2014 (UTC)

Thanks. To clarify: any samples of relatively pure water (at same temperature and pressure) will have indistinguishable distributions of bonding angles, correct? Is the 104.45 given at properties of water just a mean for standard temp and pressure? If so, does heavy water have the same mean (our article says it has a larger dipole moment than regular water)? SemanticMantis (talk) 19:03, 4 March 2014 (UTC)

Yeah, that's pretty much it. Heavy water may have slight differences in bonding angles, and probably for the reasons you note, but as Wnt's (hopefully satirical) account below shows, it still doesn't add any credence at all to the silliness above. Someone learned a few words in HS chemistry, and is hoping the consumer remembered the same words, and is banking that the consumer doesn't have any real understanding of the principles involved here. --Jayron32 21:54, 4 March 2014 (UTC)

In the VSEPR model, water has a bond angle because electron pairs more or less "push" the hydrogen atoms (and associated electron pairs) together. I'd expect that, say, the water in David Banner's superpowered urinary tract might, under harsh green gamma irradiation, have some of its electrons pushed to far, far outer shells, where they would no longer be subject to the Pauli exclusion principle with those interacting with the hydrogen atoms, or one another, and thereby all of them could more or less muddle around the nucleus symmetrically, leaving the hydrogen atoms to wander more or less opposite one another. The caveat being that outside of comics that would be a very short-lived state, and the water would return in very short order to normal (unless it reacted to form hydrogen peroxide, ionized to a hydroxide ion or something). I don't know if anyone has actually observed hydrogens moved apart in excited water molecules or not. Wnt (talk) 21:43, 4 March 2014 (UTC)

Ok, I knew it was nonsense as a whole, but I was a little confused about whether bonding angles can ever change (outside of vibrational modes). Thanks to Wnt and Jayron, I'll call this a wrap. SemanticMantis (talk) 22:10, 4 March 2014 (UTC)

Why do people appreciate panoramic views?

Most people will consider a panoramic view to be much better and more pleasant than, say, a street scene in which nothing far away can be seen and everything is at base level. Particularly, why do humans find valleys with a view, or any sort of terrain which has different topographic elevation and provides a good view, appealing compared to flat surfaces or areas where not much can be seen in the distance?

It's also worth noting that castles and other fortified structures have often been built on hilltops, or at least on higher ground, that provide a good view around as throughout history this would have been valuable in terms of defence.

In terms of evolutionary psychology, what would the reason for this be? Would the ability to see everything from a vantage point provide a sense of comfort in that it would mean that a person would be secure from threats from other humans and dangerous animals? — Preceding unsigned comment added by Lookbackbeforeyouleavemylife (talk • contribs) 18:57, 4 March 2014 (UTC)

Yes, being able to see trouble coming is useful, but having places to hide may be even more important. A primitive man standing out on an open plain would be a sitting duck. Also, even if cornered, say by a pack of wolves, having part of the approach blocked off would limit the directions from which he could be attacked, and perhaps his spear would be able to ward off attackers from that direction.

Another consideration is protection from the elements. Even a natural windbreak would make a storm more survivable. StuRat (talk) 19:15, 4 March 2014 (UTC)

IIRC, there's a section on this in Pinker's How the Mind Works, but I seem to have mislaid my copy. --ColinFine (talk) 20:22, 4 March 2014 (UTC)

According to this research, humans prefer the view of savannahs .

Link is to blog post, see also peer-reviewed articles cited therein. SemanticMantis (talk) 20:53, 4 March 2014 (UTC)

• Was it Pinker who said we prefer wooded meadows with a gurgling stream, indicating fresh water, cover, shade, and hunting opportunities? This seemed to be a trope in evolutionary psychology in the nineties, going along with blue and green being the most popular colors. See landscape art and evolutionary aesthetics. μηδείς (talk) 18:32, 5 March 2014 (UTC)

I think there are cultural norms involved too. Arguably England's finest views are to be found in the mountains of the Lake District National Park. Before the Picturesque movement extolled its virtues and Wordsworth wrote about the daffodils, the Lake District had been described as "very terrible" (Celia Fiennes, 1698) and "barren and frightful" (Daniel Defoe, 1727). Alansplodge (talk) 17:17, 6 March 2014 (UTC)

As far as the strict definition of 'panoramic view' I think it's appealing because human eyes have a wide peripheral vision, thus your brain is taking in more information that whatever you're focused on. But i'm not sure of the connection to topography or distance. El duderino 07:04, 7 March 2014 (UTC)

Laser hair removal

A few years back, I was told that laser hair removal works, but that it only works for hair that is dark in color (and not white or gray or blonde hair). Has any more modern technology changed that? Or is that still the case? Does anyone know anything about this? I will appreciate any info. — Preceding unsigned comment added by 75.44.113.200 (talk) 19:34, 4 March 2014 (UTC)

For a start, laser hair removal says "Coarse dark hair on light skin is easiest to treat," and "At this time, is the only permanent option for permanent removal of all types of hair, including very fine and light-colored hair." So... according to our article, laser removal is less effective on light or fine hair, but it doesn't say that it is ineffective. SemanticMantis (talk) 21:00, 4 March 2014 (UTC)

One obvious thing to try is first dying the hair black, then laser treating it. However, I suspect that the dye doesn't make it's way down to the roots. And the object is to heat the roots to kill them, without heating the surrounding skin enough to cause burns.

Another solution would be if there is a frequency of light that is absorbed by even light-colored hair roots, and not skin, but I suspect there isn't such a frequency, or it would be widely used by now. StuRat (talk) 21:25, 4 March 2014 (UTC)

Shrink-fitting

Hi, You might think I should know this, as I'm an engineering student, but I just can't figure it out. We haven't learnt it in my university syllabus, and lecturers are useless. I am making a wheel assembly for a Formula Student car and I am trying to get a shaft into a bearing. The bearing is a 3207RS bearing (http://simplybearings.co.uk/shop/Bearings-Angular+Contact+Ball+Bearings/c3_6/p500893/32072RS+Budget+Rubber+Sealed+Double+Row+Angular+Contact+Ball+Bearing+35x72x27mm/product_info.html - this gives dimensions in the 'Technical' tab), and the shaft is made of mild steel. I want to shrink fit the shaft with liquid nitrogen, to give a really tight fit so it doesn't fall out of the bearing as that will mean that the wheel will fall off.

So, I am trying to find out how much bigger the diameter of the shaft should be than the hole in the bearing, when at room temperature. I don't know how to calculate that, so I'm using this website: http://www.amesweb.info/InterferenceFit/InterferenceFit.aspx It can't be too small, otherwise the shaft will slip out under force, and not too big otherwise there will be too much interference force and the bearing might break or something.

For the website...

For the First Part

The torque transmitted is maximum 1100 Nm. I calculate axial load to be 1400Nm, using how this guy did it: http://www.physicsforums.com/showthread.php?t=655431 -- the car will be maximum 300kg, travelling at max 31.3m/s, with a steering angle of 30 degrees.

Coefficient of friction, I don't know, because I don't know what metal the bearing uses. This (http://hypertextbook.com/facts/2005/steel.shtml) has varying figures for steel on steel. I'm guessing 0.15 is right. Operation temperature is 21 degrees (room temp) And maximum rpm is 2000, I'm estimating. Engagement length is 27mm.

For the Second Part

The outer diameter of the bearing is 72mm; I don't know why that matters, but we seem to need it. The inner diameter of the bearing is 35mm. I don't know what the deviations are - let's guess 0.05mm.

I don't know what to put for stress concentration factor. Could you help? The website auto-provides 2, so that seems fine to me I can't get another value.

As I don't know the metal of the bearing, I'll guess it's also steel (but it's probably not). According to Misplaced Pages, "The density of mild steel is approximately 7.85 g/cm3 (7850 kg/m3 or 0.284 lb/in3) and the Young's modulus is 210GPa (30,000,000 psi)." Wiki also gives Yield strength as ~250MPa and Poisson's ratio as 0.27. Coefficient of thermal expansion is approximately 0.0000117m/mC (from here:http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html).

For the Third Part

The shaft is 35mm diameter when cooled with liquid nitrogen, and a little bigger when at 21 Celsius so that there is a tight fitting. (how much that little bit needs to be is what I'm trying to figure out) There is no inner diameter for the shaft as it is not hollow.

Then the exact same as before: "I don't know what the deviations are - let's guess 0.05mm. I don't know what to put for stress concentration factor. Could you help? The website auto-provides 2, so that seems fine to me I can't get another value. As I don't know the metal of the bearing, I'll guess it's also steel (but it's probably not). According to Misplaced Pages, "The density of mild steel is approximately 7.85 g/cm3 (7850 kg/m3 or 0.284 lb/in3) and the Young's modulus is 210 GPa (30,000,000 psi)." Wiki also gives Yield strength as ~250MPa and Poisson's ratio as 0.27. Coefficient of thermal expansion is approximately 0.0000117m/mC (from here:http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html)."

For the Fourth Part

I don't think we want a clearance.

For the Fifth Part

I don't know what this is. The website gives 6 for everything, so I'll go with that.

Could you please help with this? I realise it's a lot, but I just don't know how to do it. I don't know if I'm putting the right numbers in, and I don't really understand the numbers that come out. I just need to know what diameter the shaft needs to be manufactured to.

Thanks so much for any help that you can provide!! 143.210.123.79 (talk) 20:29, 4 March 2014 (UTC)

3) I suggest you do a test with liquid nitrogen and calipers, to determine the exact diameter at both temperatures. (Use leather gloves and safety goggles when pouring liquid nitrogen.) StuRat (talk) 21:32, 4 March 2014 (UTC)

Does your project have faculty advisers (it should...) Ask them about it, if you can. Or, if you don't get any other responses here in a day or two, try again next week. Lacking that, take the empirical route, as Stu suggests. SemanticMantis (talk) 00:06, 5 March 2014 (UTC)

My university does have advisors, but they are useless. My supervisor is an electrical engineer, so he knows nothing about mechanical engineering, even though my whole project is mechanical. I was told that last year someone used +0.03mm and it worked, so I'll just do the same. Thanks! 143.210.123.80 (talk) 11:57, 5 March 2014 (UTC)

Ah, well, in that case, perhaps you could ask an unaffiliated mech.Eng professor, or look for specific Mech.Eng forums to post on. The only other advice I have is to make a few prototypes for that part before you settle on the final. You don't want the competition part to also be a test case! SemanticMantis (talk) 15:10, 5 March 2014 (UTC)

Or they can try to make the competition part right off, but, if something goes wrong in manufacturing or testing, consider that to have been a prototype, evaluate what went wrong, make adjustments, and try again. Of course, this requires allowing time for several tries. StuRat (talk) 19:12, 5 March 2014 (UTC)

There is no time (Easter holidays in 4 weeks after which there are exams, which is ablout the time it takes machinists to get things made) and the Department won't provide much money, so there's no room for making prototypes. I'm just going to cross my fingers and hope for the best! 143.210.123.60 (talk) 20:57, 5 March 2014 (UTC)

Ah, the spirit of studenthood! Good luck, sorry we (apparently) have no MechE types to weigh in at this time. SemanticMantis (talk) 21:25, 5 March 2014 (UTC)

Future power supplies

Sir/Madam, As an example solar power & wind power & other renewables processes will not generate electricity 24/7, at the present time coal fired power stations are used as a back up, what do you see as a viable way to replace these Coal fired power stations? — Preceding unsigned comment added by Discspray (talk • contribs) 22:19, 4 March 2014 (UTC)

We don't really do opinion and speculation on the reference desk, but you might be interested to read our articles on Nuclear power and Tidal power. This combination, with seawater pumped up to a reservoir at off-peak times to generate extra power at peak times should provide a viable alternative to coal, but see also Combined cycle#Fuel for combined cycle power plants Dbfirs 22:43, 4 March 2014 (UTC)

Why just seawater ? Fresh water can be pumped up to higher reservoirs, too, and that has several advantages:

1) Less corrosive on the pipes and less sediment will accumulate.

2) Available many places where seawater is not.

3) Can also be used as drinking water, with the higher reservoir providing better water pressure. In places where pumps must be used to deliver fresh water anyway, this makes even more sense.

4) Seawater can contaminate the nearby soil with salt, killing any plants that live there (or replacing them with salt-tolerant plants).

5) Many "freshwater reservoirs" already exist, along with generation capacity. For example, water could be pumped back up behind Hoover Dam when demand is low, or back up Niagara Falls (beyond the generators).

6) If the average generation capacity is more than is needed, this excess fresh water can help to replenish lakes and reservoirs at low levels as a result of drought and overuse. StuRat (talk) 19:06, 5 March 2014 (UTC)

You are correct, of course, but I specifically mentioned seawater because of the tide factor. By pumping up at high tide and generating at low tide, an apparent storage efficiency of more than 100% can be achieved, though that would be possible only with appropriate timing of demand, and distribution of power between areas with different tide times. I was thinking specifically of the UK with a good spread of tide times over a relatively small area. In non-coastal America, freshwater pumping makes more sense. Dbfirs 07:02, 6 March 2014 (UTC)

Also, note that we currently do not generate all our power exactly as when it is used -- that would be very difficult! See energy storage for more information on how we can balance supply and demand for power. Many of those methods are fairly sustainable and have low environmental impact. SemanticMantis (talk) 23:29, 4 March 2014 (UTC)

Supposedly a Tesla project and competing efforts promise cheap batteries in the future that will equalize power usage day and night. But I don't trust these PR-like news stories, and after a few decades of disappointments I don't trust any battery until many years after it is in mass production... Wnt (talk) 01:33, 5 March 2014 (UTC)

There are some radically different approaches to the problem:

• Nuclear fusion is unfortunately not available yet, but once it is, it would make a great backup. As of now, the only kind of fusion we can use is the natural kind, and modern nukes for some very special purposes.
• Mixing several methods. When there's no wind, maybe the sky is clear and photovoltaics are useful. Hydro and tidal power mix quite well, too. You can use tidal energy when the tides allow, and keep the hydro plants as a reserve for times when the tidal power generators can't provide any power. Where photovoltaics are not profitable (read: almost everywhere), biofuel is a potential application.
• If you have a surplus, there are literally dozens of ideas on how to use them. The most practical one right now is simple, too: using the power to pump more water into the reservoirs of hydro plants. Other energy storage ideas work on a smaller scale, for example, flywheels.
• Use fission and coal if you really need.

Of course, these methods aren't mutually exclusive either. - ¡Ouch! ( / _{more pain}) 06:44, 5 March 2014 (UTC)

I lived in Denver, Colorado, USA until late last year, a place where soi-disant "progressive" politics has made it the law that wind and solar ("renewable") energy are to be the preferred source of electric power in the state's power generating grid. This, in my humble opinion, has MUCH less to do with the economics and physics of power generation or the ecology of Colorado than it does with the political patronage of wind power investors, and finding something to do with millions of dollars' worth of solar cell parts now being expensively stored in and around Denver.

However, Denver's almost ideally suited to be a test site of at least one storage technology. A huge reservoir, the Cherry Creek reservoir, is located on a bluff overlooking much lower land in suburban southeast Denver. A lower reservoir could be dug on that land, the idea being to use Denver's spare water as a gravitational storage battery -- in times of peak demand, let the water flow down to the lower reservoir through hydroelectric turbines, then in the daytime or during high winds, use surplus power to pump water back up to the higher reservoir. That solves one pressing issue with wind and solar power - they somehow never seem to be present exactly when needed. However, it fails to address the other pressing issue (ignored by the Colorado Democratic Party) - the dismal cost inefficiency of solar and wind power - one of several issues that precipitated referendums in Northern Colorado counties on secession from the autocratic central government in Denver. loupgarous (talk) 19:08, 6 March 2014 (UTC)

Colorado could also benefit from oil shale-based synthetic fuel technology (having a veritable cornucopia of the stuff in the Green River formation); however, pollution control could be a problem. 24.5.122.13 (talk) 07:26, 7 March 2014 (UTC)

March 5

metamorohic rocks may have colored bands

m — Preceding unsigned comment added by 172.1.205.91 (talk) 00:00, 5 March 2014 (UTC)

Yes, they may. --Jayron32 00:00, 5 March 2014 (UTC)

Metamorphic rock has a few pictures; ask if you want something more specific. Wnt (talk) 05:56, 5 March 2014 (UTC)

The colored bands arise where the metamorphic rock has been formed from sedimentary rock which has been "cooked" by the heat of the earth. Sedimentary rock has bands in it because it forms in different layers over many millions of years. The layers reflect different conditions on earth at the time they were formed. RomanSpa (talk) 14:49, 5 March 2014 (UTC)

The bands represent layers of different chemical composition. Exactly what colours predominate (the particular minerals that are formed) depends on the composition, the confining pressure (the effect of the overlying rock layers) and the temperature at the time of metamorphism - see also greenschist, blueschist, whiteschist for just some of the variety. Mikenorton (talk) 18:00, 5 March 2014 (UTC)

Why to sit on the Internet?

To whoever closed this discussion: Please sign the top of your hatting, or add a signed comment saying that you closed it. We have agreed on the talk page that this is best practice. SemanticMantis (talk) 16:05, 5 March 2014 (UTC)

The closer is IP 140.254.227.87 i.e. the Ohio State IP editor who asks questions about Christian traditions etc. Nil Einne (talk) 22:30, 5 March 2014 (UTC)

He's the one identified as not a troll on the talk page, the one who complains about female operators on the computer desk. Given consensus, Nil, I suggest you not criticize him any further, he's to be given all leeway on the chat forum. μηδείς (talk) 01:33, 6 March 2014 (UTC)

'Any further'? Also I don't see any complaints about female operators on the computing desk. Simply a few simple questions relating to female telephone operators which could not logically be intepreted as complaints about said operators. Nil Einne (talk) 11:39, 6 March 2014 (UTC)

Okay, complaining in the context of female operators, that they have wombs, are kept in glass cages, and only get paid 1/4 what men do. Not complaints about the women as individuals. μηδείς (talk) 17:23, 6 March 2014 (UTC)

Are "Summa Technologiae" and "Profiles of the Future" outdated?

Dear Ladies and Gentlemen

I have read "Summa Technologiae" by Stanislaw Lem and "Profiles of the Future" by Arthur C. Clarke. Are these works outdated? Do they still have some interesting ideas that could become real or do these books lack much of modern days physics? Are there more modern books that cover a possible future much better?--92.105.189.138 (talk) 16:50, 5 March 2014 (UTC)

Clarke's general ideas on the exploration and exploitation of the solar system are still interesting, although he has probably got many of the details wrong. His descriptions of the celestial mechanics of this rely on Newtonian physics, which remains an excellent tool for describing the solar system in general terms. I recall that Clarke also predicts a far future development of immortality. This is certainly regarded by many people, notably transhumanists, as an attainable and desirable goal, though the process by which this will be achieved remains unclear. I haven't read the Lem book, so can't comment on that. Generally, books predicting the future get it wrong, for the obvious reason that each book only describes one possible future, but there are infinitely many possibilities. Such books are nevertheless worth writing and reading: by writing them, the author gives his contemporaries a vision to aim at, and a direction in which to exert their efforts to improve the life of mankind; by reading them decades or centuries later, we learn how people at that time thought about the universe, and what they valued, and may be able to make useful judgements about where things went "right" and where they went "wrong". Like science fiction generally, predictive literature of this kind tells us about the time in which it was written, but (like history generally) we can try to learn from it. RomanSpa (talk) 17:35, 5 March 2014 (UTC)

I wikilinked the works/authors in the original post for our convenience. :) Wnt (talk) 19:02, 5 March 2014 (UTC)

Thank you very much for your responses. @RomanSpa: do you have a book tip or so for me? I am asking, because I am looking since ages for a modern good book that deals with possible futures. — Preceding unsigned comment added by 92.105.189.138 (talk) 19:45, 5 March 2014 (UTC)

There are some interesting references in the article on futurology. RomanSpa (talk) 23:23, 5 March 2014 (UTC)

Hell, the Revelation of John doesn't seem outdated. No privacy, weapons that can strike anywhere in the world, drones armed with long term incapacitating "less than lethal" weapons, people needing modifications to their heads to get a job ... there can't have been any generation in history that this stuff made half as much sense to as it does to us. Wnt (talk) 00:44, 7 March 2014 (UTC)

Unopened long life juice

What would happen if you put unopened long life juice in the fridge, then took it out the next day, put it in the next day and repeated the cycle? Would it alter the taste? — Preceding unsigned comment added by 82.40.46.182 (talk) 18:47, 5 March 2014 (UTC)

It would certainly limit it's life. If any bacteria are present, they will grow during the warm periods. If they've managed to completely sterilize it, this won't happen, but other deterioration can happen.

For example, the juice may oxidize and turn a darker color. This is especially true of tomato juice. There may be more leaching of chemicals from the container. The juice may tend to separate. All these things would happene eventually if left in the fridge, but would take longer. StuRat (talk) 19:28, 5 March 2014 (UTC)

I'm fairly sure the OP is talking about juice which doesn't need to be refrigerated and of a time frame where you would normally expect it be perfectly safe if stored outside the fridge. In other words, I don't think a limit on the 'life' (meaning the time it can be safely drinked without fear of microorganism caused illness) is a relevant factor unless you have some reason to think the microorganism growth rates will be higher than under normal non refrigerated conditions, they are more likely to release toxins or similar, or that the seals will break. Nil Einne (talk) 22:26, 5 March 2014 (UTC)

In biology labs, where many exotic enzymes are stored, people worry a lot about freeze-thaw cycles, but basically ignore refrigerate-thaw cycles. I really wouldn't expect anything different from the average of the usual effects of putting something in the refrigerator and taking it out. But, this being biology... you never really know until you do the experiment - for example, conceivably, there could be some resistant spore that responds to the temperature cycles by germinating, etc. But some things are so unlikely I can't imagine testing them. Wnt (talk) 23:43, 5 March 2014 (UTC)

And that's when this happens. InedibleHulk (talk) 02:00, March 6, 2014 (UTC)

Angiogenesis inhibitors

According to Dr. Joel Fuhrman (3 Steps to Incredible Health! on PBS), mushrooms prevent cancer tumors by preventing blood vessels from growing which supply the tumor with nutrients. In that case, wouldn't mushrooms inhibit wound healing and be dangerous for children, similar to thalidomide ? Or is this danger only in utero ? If so, does the placental barrier protect the fetus from the angiogenesis suppressant effect of mushrooms ? StuRat (talk) 19:23, 5 March 2014 (UTC)

Fascinating lead -- this turns out to be Phellinus linteus, used in traditional Korean medicine, and also in Japan and China. I'd just about despaired of finding traditional herbal remedies that could tackle cancer because of the disconnect between therapy and results, and this might be a pleasant exception - however, I haven't yet tracked down proof that ancient practitioners were using it specifically for cancer. It is a rare mushroom, not the usual culinary fare, and I see nothing in PubMed on teratogenicity - though there may be useful lore from more traditional herbal physicians on the issue. But I would indeed think that anyone taking enough for its antiangiogenic effects to be relevant, by comparison to thalidomide, would need to be very wary of the possibility of other effects also. Wnt (talk) 20:38, 5 March 2014 (UTC)

Stu's question is phrased in terms of (general?) "mushrooms," while the paper you cite lists several specific families and species, and even some specific compounds that have anti-tumor effects (e.g. "β-(1 → 3) linked glycan" for Phellinus spp.) Your assessment seems correct, I just want to be clear that the action in humans of one compound in one fungus doesn't say much of anything about any other fungus, let alone how the therapeutic dose might compare to a dietary/culinary "dose". SemanticMantis (talk) 21:23, 5 March 2014 (UTC)

True. "In general", eating mushrooms will kill you. Wnt (talk) 22:30, 5 March 2014 (UTC)

He didn't specify which mushrooms have this effect. So, is it limited to certain species, or is that just the ones they happened to study ? StuRat (talk) 23:08, 5 March 2014 (UTC)

Ah, well he really should have specified! Anyway, the article Wnt links to above seems to be freely-accessible, and it contains a list of families/species with "anticancer potential" (I haven't read the whole article, but unless it says otherwise we shouldn't assume that all of those are "anticancer" effects are due to a specific antiangiogenic mechanism). Here is the figure with some photos and scientific names . I didn't see any "no effect" fungi listed, but those kind of negative results are not popularly recorded/published (it's a shame). This article is a summary of dozens of other articles, those source articles may mention briefly some "no effect" fungi. In absence of evidence otherwise, I personally would assume that any medical properties and compounds are completely unique to at least a genus, if not species. Mycology is crazy, and, in my limited experience, there are more exceptions than rules. SemanticMantis (talk) 00:09, 6 March 2014 (UTC)

In general, I'm prone to be persuaded that those showing anti-cancer activity in mice may be anti-cancer agents, with the caveat that many medical treatments that have absolutely dramatic effects in mice turn out to be less than satisfactory in human trials (notably endostatin, perhaps angiostatin) so that is not a guarantee, and some mouse models of cancer treatment are even less persuasive than others. (very large doses, testing tumor cells that are foreign to the mouse, testing in immunocompromised mice, etc.) Those with tests just on cell lines... well, you can show anything in the right cell line. But if a mushroom is edible anyway, and not expensive, there's little downside to trying it provisionally. There were different mechanisms for the mushrooms listed, but those referencing mice were Phellinus, Agaricus, Grifola, Hericium, Cordyceps, Inonotus obliquus, Funlia, Antrodia. The authors mention a history of anti-cancer use in herbal treatment for Inonotus -- short of genuinely scientific controlled medical studies on humans, this is to me the most hopeful indication. Wnt (talk) 07:21, 6 March 2014 (UTC)

Thanks for the answer so far, but nobody has yet addressed why such 'shrooms aren't harmful to children, who need to grow new blood vessels as they grow. StuRat (talk) 15:55, 6 March 2014 (UTC)

@ user WNT - thalidomide isn't particularly dangerous to children. Chemie Grünenthal, thalidomide's developer and first marketer, successfully sold thalidomide as a sedative for young children in the late 1950s-early 1960s (as "Contergan Saft"). But when given to pregnant women, thalidomide deforms fetuses rather reliably, causing horrible somatic injuries (such as missing limbs) in the newborn. You've confused "developing fetuses" with "children" there. Thalidomide has actually been studied to treat one condition specific to children, chronic bullous dermatosis of childhood.

Actually, thalidomide has some intriguing uses in the treatment of Hansen's Disease ("leprosy") and a number of rare cancers and immune disorders, and is used to treat leprosy/Hansen's Disease (its only approved use - all other uses are, at present, "off-label," accounting for the US$300 million market for a drug only approved to treat one comparatively rare condition) allaying considerable human suffering. As Paracelsus said, the poison is in the dose (either too much of a good thing, or a drug used in the wrong patient population). Thalidomide isn't a particularly evil drug; some motion sickness remedies, and some stomach acid blockers used for the elderly (such as misoprostol) have just as much teratogenic potential, or even more. loupgarous (talk) 19:25, 6 March 2014 (UTC)

@ StuRat, this is "original research," but I'm betting that the antiproliferative effects of thalidomide and the chemicals in some mushrooms are amplified in the fetus as opposed to a post-partum child (though I'd be very nervous about feeding any of those things to a child younger than about six, a stage when the connections in the nervous system aren't completely formed in the child). It's a matter of how much of the chemical is available to act on how much neural and other tissue - the fetus is dramatically more vulnerable to angiogenesis inhibitors compared to a child just on the basis of its comparatively smaller mass, and because so many crucial developmental stages occur in utero, not post partum. loupgarous (talk) 19:53, 6 March 2014 (UTC)

I said teratogen; I assumed StuRat meant dangerous for children who would suffer the effects after birth. When dealing with serious potential teratogens (which include some drugs presently on the market) any woman needs to seriously consider the risks should she inadvertently become pregnant, and any man should avoid situations where a woman might handle or otherwise become contaminated by the pills. Wnt (talk) 00:39, 7 March 2014 (UTC)

Structural engineering question

I often come across different supports and member types in structural engineering but its all very theoretical. So what are some practical real world (outside the lab) examples of the following?

- Roller support
- pin support
- fixed end support
- beam - works in bending and shear
- bar - works in axial load only
- tie - resists tensile forces

What do they actually look like? I only ever see these in a lab or on a diagram and its quite hard to find anything on google. Clover345 (talk) 22:04, 5 March 2014 (UTC)

Of course, in the real world, any type of support will provide some resistance to other types of forces, too.

Beams: You have those in the ceiling of a house, for example (they are likely to be uncovered in the basement or garage or perhaps in the main rooms of a rustic house). Those wooden beams provide resistance to bending and shear, but also provide resistance to compression, tension, twisting, etc.

Roller supports: I've seen those on assembly lines, say where the line ends in a chute. That allows gravity to feed the items down the chute with minimal friction, and prevents scratching you might get if the item slides down a low friction surface (say with a grain of sand in between). StuRat (talk) 22:40, 5 March 2014 (UTC)

] SemanticMantis (talk) 00:41, 6 March 2014 (UTC)

Yes, it's a roller conveyor, but isn't that a type of roller support ? StuRat (talk) 16:00, 6 March 2014 (UTC)

I suppose it depends on how specific you want the definitions to be. Usually, I see "roller support" to mean situations where the structural member isn't supposed to leave the roller (outside of failure), like the bridge example below. But I guess that isn't strictly part of the definition. Indeed, if you have a box on a roller conveyor, then is entirely supported by rollers, even if it is only supported by each roller for a short period of time. SemanticMantis (talk) 22:00, 6 March 2014 (UTC)

Pin supports: How about a wheelbarrow axle/wheel ?

Bar: I hadn't heard it called that before, but a concrete column might qualify. Those support a compression load well, but are very poor at any other type of loading. StuRat (talk) 22:40, 5 March 2014 (UTC)

Perhaps the best example of a pin support is a door hinge. Justin15w (talk) 23:02, 5 March 2014 (UTC)

A roller support allows for lateral movement of a structure. Google "roller support structure" for a picture. A fixed end support is just that - an end of the structure tied or bolted into the ground. For the tie, think about the ceiling beams in your house - the ties are the horizontal beams that connect the walls to each other to prevent the walls from falling over outward. They provide tensile support. See http://web.mit.edu/4.441/1_lectures/1_lecture13/1_lecture13.html for explanations, diagrams, pictures. Justin15w (talk) 22:47, 5 March 2014 (UTC)

Steel bridges are a good example for roller supports, you don't want to crack a support when the bridge expands in the sun. Also pins are commonly used to tie steel members together in trusses. Dmcq (talk) 23:20, 5 March 2014 (UTC)

This page from an MIT course has some decent diagrams and examples (Sorry, just realized Justin15w just game the exact same link :). If I'm understanding correctly, isn't the cantilever on a Crane_(machine)#Fixed a "fixed end support" system? You can see them all over most big cities. Also, if you are in a course discussing such matters, ask your instructors for real-world examples! That is their job, and they are probably more expert than a bunch of strangers on the internet ;) SemanticMantis (talk) 00:31, 6 March 2014 (UTC)

March 6

Soil Quality

What is the maximum value of Sodium Adsorption Ratio (SAR)? — Preceding unsigned comment added by Desalengineer (talk • contribs) 22:15, 6 March 2014 (UTC)

If you look at the second reference (New Mexico State University: Irrigation water classification systems) in the article on Sodium Adsorption Ratio, you'll find Figure 1 on page 3. This shows how to estimate the US Salinity Laboratory Classification on the basis of SAR and Electrical Conductivity (EC x 10^6). The left axis of this chart is SAR, and you'll see that it goes up to a maximum of about 30. This suggests that 30 is the maximum that you're likely to encounter in most circumstances. Note, though, that because of the way the ratio is calculated, there is no theoretical maximum value. From www.salinitymanagement.org I notice that "As a rule, water that has an SAR below 3 is safe for irrigating turf and other ornamental landscape plants. Water that has an SAR greater than 9, on the other hand, can cause severe permeability problems when applied to fine-textured soils (for example, a silty clay loam) and should be avoided." I also randomly looked at some USGS reports, and on the basis of this very small sample it looks likely that values above 3 are comparatively infrequent. RomanSpa (talk) 22:35, 6 March 2014 (UTC)

Vaccinations

If a person is vaccinated from a certain virus, can they still transmit that virus even if they don't become I'll from it themselves as a result of the vaccination? I.e. can someone not vaccinated from the virus become ill as a result of close contact with te vaccinated person, who also came into contact with the virus. 82.40.46.182 (talk) 22:35, 6 March 2014 (UTC)

As you will see from our article on vaccination, there are four types of vaccine currently in use. These are: inactivated vaccines, attenuated vaccines, virus-like particle vaccines and subunit vaccines. Since the virus in an inactivated vaccine has been killed, there should generally (so long as the virus has been correctly inactivated) be no risk of infection or transmission. Since virus-like particle vaccines and subunit vaccines don't use the viruses themselves, there is no risk of infection or transmission for those kinds of vaccines either. This only leaves attenuated vaccines. These work by breeding a very mild strain of the virus in a sterile environment; when this is delivered as a vaccine it stimulates the immune system in the usual way, giving protection against not just the mild strain, but also against nastier strains. When you are vaccinated with an attenuated vaccine you have a mild form of the virus inside you, and this can potentially infect other people, but this is a good thing, because it means that they also develop immunity against the mild strain, and thus also against its nastier siblings. The resistance you build to the mild form protects you against the nasty form. There is a tiny possibility that the virus in an attenuated vaccine might change into something more virulent, due to the normal mutation process that is going on with viruses all the time, and under those circumstances you might infect someone else, but this sequence of events is very unlikely - from the references in the article, we can see that in the case of a polio vaccination campaign there was a partial reversion to virulence that affected only four patients in a country of nearly 100 million people, and even then the problems that arose were significantly less than the original disease. Note that for vaccination to work you have to have a functioning immune system, of course, which is why immunocompromised people should talk to the doctor before being vaccinated. If you are going to be vaccinated with an attenuated vaccine, and are regularly in contact with immunocompromised people, the best thing to do is to talk to your doctor, and ask about the particular vaccine you are considering. In general, though, the answer to your original question is: there is, in some cases, a tiny possibility that someone might become ill as a result of close contact with someone who has been vaccinated with an attenuated vaccine, but the probability is so small that except in the special cases just mentioned it's not worth losing any sleep over. As always, note that Misplaced Pages is not a medical professional. If you have particular medical concerns, your doctor will be able to give you appropriate guidance. RomanSpa (talk) 23:58, 6 March 2014 (UTC)

Sure, it's possible, but very unlikely, because most human-infectious viruses can't survive very long outside the human body. For influenza, it's usually a matter of hours, unless frozen or specifically preserved. To elaborate on RomanSpa's good answer and refs: suppose I got a flu shot, and you have this year's flu. You sneeze in my face while you are infected, and then I leave that room and immediately start kissing RomanSpa. I would be protected from the infection taking hold in me via my vaccine, but it is conceivable that I could transmit the virus to RomanSpa, because my vaccine won't due anything to combat a lingering contamination on my face. In this (obviously contorted) example, I could act as a vector_(disease) to infect RS, even though I am vaccinated, and you have had no contact with RS. But, all that doesn't really matter in the aggregate. If enough people are vaccinated, herd immunity kicks in. SemanticMantis (talk) 00:28, 7 March 2014 (UTC)

For clarity, I should just say that SemanticMantis would have to take me out for a nice meal first, or at least get me very drunk. RomanSpa (talk) 01:37, 7 March 2014 (UTC)

I'd say yes. First, the vaccine can simply be ineffective or only partially effective - for example, I've heard figures around 70% for the antique influenza vaccines we're still using. Somewhere between effective and ineffective there ought to be some people who get just some symptoms and shed just some virus.

Another option is that (as suggested above) the attenuated vaccine can get out of control -- this is the case with the polio vaccine, which periodically would start mini outbreaks in the vaccinated populations. Once, public health officials had imagined that they would wipe the virus out "any year now", and had written volumes on how to decide exactly when to switch over from the more effective live vaccine to the non-risky killed vaccine. Then the U.S. sent a team to do polio vaccination while collecting information about Osama bin Laden, the Pakistanis stopped the program, the remaining virus in those hostile regions spread to Israel and various other places around the Mideast... now they're going back and revaccinating people with the live vaccine. Wnt (talk) 00:35, 7 March 2014 (UTC)

Yes this can happen and I know a victim of it. The polio vaccine is live and can be spread to carers who change the nappies of children who have recently been vaccinated: parents in the UK are now advised to wash hands well after changing dirty nappies to prevent this. As I said, I do know someone who caught polio from changing his granddaughter's nappy. --TammyMoet (talk) 10:35, 7 March 2014 (UTC)

March 7

How do whales control their depth?

I cant seem to find any information that is informative on this and not just speculative.

Also how did avocado trees survive if the animals that transported their seeds died out?

70.210.72.203 (talk) 03:24, 7 March 2014 (UTC)

Lungs, according to this The compressability of the air in whale's lungs help keep them neutrally buoyant at any depth. As they dive, their lungs compress, and thus have a smaller volume, which makes the whale more "dense" and thus maintains buoyancy at the lower depth, where the water is also more dense. In this way, the whale's lungs work akin to a swim bladder in fish, which is interesting because swim bladders are homologous to lungs in land animals. Regarding the Avocado; the modern farm-raised avocado survives because of human cultivation. It has been modified extensively from its wild ancestors. Like maize, it is unlikely that cultivated avocado would survive without humans to keep them going. Avocado have been cultivated for like 10,000 years, so there was time for humans to take over when the animals they coevolved with died out. --Jayron32 03:34, 7 March 2014 (UTC)

Interesting. thank you, I came across this article Sink or Swim and it seemed really interesting but I couldn't figure out how to find the whole article and not just the abstract.

also how long must the avocado plant have survived without the animals that spread it's seeds being alive before human intervention?

70.210.72.203 (talk) 03:43, 7 March 2014 (UTC)

It doesn't have to be any time at all. It's quite likely that the original seed-spreading animals which co-evolved with the wild avocado (as noted in the Misplaced Pages article avocado were still alive 10,000 years ago. That same Misplaced Pages article notes that there's evidence of human cultivation of Avocado 10,000 years ago. I'm just speculating, but it's clear that something like that happened... --Jayron32 03:46, 7 March 2014 (UTC)

You're right, I had assumed that the giant sloths that ate the seeds came well before human cultivation.70.210.72.203 (talk) 03:49, 7 March 2014 (UTC)

Although according to Avocado#History, the original, undomesticated variety still survives. Rojomoke (talk) 05:19, 7 March 2014 (UTC)

Passing an avocado stone may be difficult, but nothing beats the experience. Have you tried it? μηδείς (talk) 04:10, 7 March 2014 (UTC)

Regarding the whales: Jayron's answer is all bassackwards, because water is practically incompressible and does not become "more dense" as he said. The real answer is, whales control their depth with their fins, just like a submarine -- all that their lungs do is make their buoyancy approximately neutral so they don't sink (same thing as with a fish's swim bladder). 24.5.122.13 (talk) 07:39, 7 March 2014 (UTC)

Air is not water, dearie. —Tamfang (talk) 09:15, 7 March 2014 (UTC)

Here's Jayron's comment, for your reference: "As they dive, their lungs compress, and thus have a smaller volume, which makes the whale more "dense" and thus maintains buoyancy at the lower depth, where the water is also more dense." (emphasis mine) And don't you DARE call me "dearie"!!! 24.5.122.13 (talk) 10:20, 7 March 2014 (UTC)

It's a burden being right all the time. In case you don't wish to click that, Dearie, "So, the density of ocean water increases and increases as you go to the bottom of the ocean. The deep ocean is layered with the densest water on bottom and the lightest water on top." --Jayron32 10:47, 7 March 2014 (UTC)

Yeah right -- the increase in density from top to bottom is no more than 0.5% AT MOST, which is not nearly enough to matter, moron! 24.5.122.13 (talk) 11:25, 7 March 2014 (UTC)

IP 24.5: Don't insult other users, this is not Yahoo Answers.

On top of this, 0.5% doesn't look like much, but only the air inside the lungs is more compressible, so it could be enough. OTOH, since the blood and all the solid matter that make up the whale is slightly compressible, too, I can't say for sure. - ¡Ouch! ( / _{more pain}) 11:42, 7 March 2014 (UTC)

• This article also discusses the role of lungs in maintaining neutral buoyancy in various sea animals. It should be noted that calling me a "moron" doesn't make that article disappear. --Jayron32 12:15, 7 March 2014 (UTC)

If that graph in the link is right, that's only 3 parts in a thousand density difference, mostly near the top of the ocean (which is also where the air would be compressed, but even much faster). From it looks like a bottlenose dolphin has a tidal volume of 10 liters (which is close to all of its lung capacity due to its adaptation) and from the article it weighs about 250 kg, so that is 1/25 of the animal's weight in buoyancy that is lost. So it would seem that the lungs very much overcompensate for the density increase of seawater, which is essentially negligible. Worse, the density change is in the "wrong direction", dragging the dolphin down when it is deep. As a rule, swim bladders change in volume quite slowly to mediate a long-term change in neutral buoyancy - but whales, diving all the way down and back in a short time, need to move much faster than they would sink as dead weight. Wnt (talk) 15:22, 7 March 2014 (UTC)

The reaction between concentrated HNO₃ and some metals

Can you give me the balanaced chemical equation of rection between concentrated HNO₃ and Fe, Co, Al, Cr which should produce the metals' oxides. But I don't know what the other products are. Is it NO or NO₂ like the reaction with copper? --G.Kiruthikan (talk) 14:32, 7 March 2014 (UTC)

Teaching myself General Relativity

I am thinking about teaching meyself GR with the only the internet and maybe a few texts I can buy in an ebook format and no expert guidance. Is this a achivable goal? How much time should I set aside for it? And what resources (Ideally available online on a digital format) would you recomend for it?Diwakark86 (talk) 15:28, 7 March 2014 (UTC)

This depends heavily on your background. Are you familiar with linear algebra and calculus? If so, you should be able to teach yourself basic GR, albeit with difficulty. Check out Introduction to General Relativity by Lewis Ryder and Leonard Susskind's lecture series at Stanford University: . --Bowlhover (talk) 16:03, 7 March 2014 (UTC)

The answer to that question is going to depend a lot on where you're starting from, and where you're hoping to get to. MIT offers an open courseware version of their course in general relativity 8.962, which I expect would give a reasonable working knowledge of the topic. If you were to actually take the course, you'd be looking at three hours per week of lectures, times fifteen weeks, plus probably at least as much time spent on assignments and homework to try to apply what you had learnt—call it a hundred-plus hour commitment all told.

However, that presupposes that you are prepared to take a graduate-level course, and that you already have a reasonably firm grasp of the prerequisite material. From the syllabus,

The course catalog lists Differential Equations (18.03), Linear Algebra (18.06), and Electromagnetism II (8.07) as prerequisites. Students should also be familiar with Lagrangians and action principles, Green's functions, and numerical analysis (some homework assignments require the numerical solution of systems of differential equations).

There's some pretty heavy mathematical lifting involved in GR. (Note that the prerequisite courses specified each have their own list of prerequisites as well.) There's a lot of foundational knowledge that you would be expected to have to build on. You haven't specified what level of knowledge and experience you're coming in with, so it's hard to say how long it would take you to get up to speed.

The other side of the question is what you are hoping to do with it. General relativity is something that physicists can study and work with (and argue over) for entire careers; there's a very, very long road between knowing something about GR and knowing most of GR. TenOfAllTrades(talk) 16:12, 7 March 2014 (UTC)

Here are: a simple introduction, a too-simple introduction and several videos about GR. 84.209.89.214 (talk) 16:58, 7 March 2014 (UTC)

Good free of charge comprehensive lectures. This is a good text, but it won't work for you if you don't have the rght attitude toward studying. He already gves a hint of that in the introduction when he writes: "I see no reason to shield students against the phenomenon of changes of convention and notation. Such transitions are necessary whenever one switches from one �field of research to another. They better get used to it.". The advantage of this text of just 72 pages over most other texts is that while you only need the same minimal knowledge to read other the introductory GR texts, if you complete these lecture notes, you will be able to do more advanced calculations, e.g. compute the amount of gravitational radiation emitted by a rotating object from first principles. Other lecture notes may also lead to the same end result but they typically comprise of hundreds of pages. Count Iblis (talk) 17:55, 7 March 2014 (UTC)

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