Wikipedia:Reference desk/Archives/Science/2014 April 22

= April 22 =

Which covalent bonds are broken in DNA cleavage by nucleases?
When DNA is cleaved by restriction endonucleases, meganucleases, engineered zinc finger nucleases/TALENs or Cas9 which covalent bonds are broken? Are they different for each type and if so, does that affect the repair mechanisms? --78.148.106.196 (talk) 00:47, 22 April 2014 (UTC)
 * I suggest you read the first sentence in nuclease, and our article on DNA. There is only one bond to be cleaved in DNA. Fgf10 (talk) 07:08, 22 April 2014 (UTC)
 * I think I can see five potential bonds that could be cleaved. What makes you so sure that all enzymes cleave cleanly at that point? --78.148.106.196 (talk) 10:58, 22 April 2014 (UTC)
 * Only likely (from a chemical standpoint) possibilities, given we are talking about the phosphodiester bonds of the backbone linkage, are the two P–O (those O being the 3' and 5' on the adjacent deoxyribose parts of the two DNA residues). The two O–C are comparatively harder to break. And the C5'–C4' is not at all fragile. The question of "which P–O" is a reasonable one. Assuming the phosphate remains attached to one of the two DNA residues, does it stay with 3' (breaking off the next one's 5') or vice versa? DMacks (talk) 14:56, 22 April 2014 (UTC)


 * I think it's going to be a while before I can answer my own question. I know that some of the enzymes form covalent bonds with the DNA which are then broken. Also, since the restriction endonucleases, Cas9 and the business ends of ZFNs and TALENs all derive from an evolutionary need to protect bacteria from invading phage and plasmids, I think it stands to reason that the DNA might be cleaved in such a way that it couldn't easily be ligated back together. I don't know nearly as much as I'd like to about DNA repair. Some day... 78.148.106.196 (talk) 19:46, 22 April 2014 (UTC)


 * For "which side", the answer depends on the nuclease. For example, there's Deoxyribonuclease I which cleaves such that it leaves a 5'-phosphate, and Deoxyribonuclease II, which cleaves such that it leaves a 3'-phosphate. For most restriction endonucleases (and things like zinc finger nucleases, which have their catalytic domain derived from restriction endonucleases like FokI), they typically cleave the bond so as to leave a 5'-phosphate. (For example, Meganuclease I-SceI notes that it leaves a 3' hydroxyl.) This leaves things set up such that DNA ligase can repair the break by joining the 5'-phosphate with a 3' hydroxyl. (Note that leaving a 3' hydroxyl is important if you need to continue the chain via polymerases, which use the 3' hydroxyl to displace the pyrophosphate on the NTP.) -- 160.129.138.186 (talk) 23:28, 23 April 2014 (UTC)


 * Thank you very much for this. It's not what I expected. 78.148.106.196 (talk) 20:29, 26 April 2014 (UTC)

Rotary cannon and super capacitors
The M61 Vulcan compared to the Gryazev-Shipunov GSh-6-23 has a slower rate of fire and has a slower "spin up" time. The article says this is due to the GSh-6-23 using a gas operating system instead of an electrical system to cycle the weapon. I was wondering if a rotary cannon were powered by super capacitors, would they have more power, and therefore spin up faster and have a rate of fire comparable to the gas operating rotary cannon? Also on a related note, would super capacitors grant electric cars greater acceleration? ScienceApe (talk) 00:59, 22 April 2014 (UTC)
 * My guess is that you are not on the right track. The reason the electrical system would be "slower" I doubt is because of the power delivery, which is probably from the vehicle. It seem far more likely to me that it is a synchronization or motor speed issue, there’s also heat dissipation and many other factors to consider. These guns are designed with many specifications in mind, including weight, cost and serviceability, not just “rate of fire”. No doubt you COULD make a faster firing, electrically powered rotary cannon, could you do it with super capacitors? I don’t really see why you’d bother, there are perfectly serviceable high speed motors, I don’t think power delivery is the limiting factor there. As for cars, power delivery ‘’might’’ be a factor, but no doubt weight and cost are also, and I don’t think electric cars have a problem with acceleration, the major problem for electric cars is range, so weight plays a more important factor. Vespine (talk) 02:17, 22 April 2014 (UTC)
 * Have a read of the applications section of the Supercapacitor article, if you haven't already. You'll get a good idea of the kinds of situations supercapacitors are being used in, they generally supplement systems where batteries are already used or only small amounts of power are required. They might be used in electric cars, but it would be more to save the batteries from spikes in current drain which could reduce their (usually expensive) life, rather then to "supercharge" the acceleration or anything like that. Vespine (talk) 02:24, 22 April 2014 (UTC)
 * Right, as for electric cars, I would guess that the acceleration is actually limited by the computer to act more like a traditional gas-powered car. In a conventional internal combustion engine, torque increases with engine RPM and will eventually decrease at even higher RPMs (see Power band), hence the need for transmissions with multiple gear ratios. This isn't the case with electric motors. Electric motors can produce maximum torque instantly. So the actual acceleration is probably limited to keep people from doing a burnout every time they tap the accelerator. Though they certainly can if you want, the Tesla Model S has a 0-60 time of ~4 seconds, comparable to a Corvette. And then there's the EV-converted 1972 Datsun that can do it under 2 seconds, comparable to an F1 car.
 * Another benefit to using supercapacitors in vehicles though is their short charge time. They can be fully charged in minutes rather than hours. So for vehicles that stop a lot at predictable locations for short periods of time like buses, you can build electric vehicles that can run all day without needing continuous overhead wires like a trolleybus. Mr.Z-man 03:34, 22 April 2014 (UTC)
 * Why does the gas operated rotary canon fire faster/faster spin up time? Isn't it because the propellant produces more power? ScienceApe (talk) 01:49, 23 April 2014 (UTC)
 * To touch the gun part of the question, there are newer Vulcan cannon which have both an increased ROF and shorter spin-up interval; they achieve that through decreased spun mass rather than better motors (or a Tim Taylor-esque "more power" approach). Other guns (including the XM301, no image though) feature a truncated-conic (rather than cylindric) barrel array, where the muzzles are at the narrow end. This does not only (slightly) decrease spun mass, but it limits the energy taken to spin the array, because a lower linear velocity will translate to the same angular velocity. The fact that the muzzle velocity vector and the axis are no longer parallel is accounted for easily.
 * During live-fire exercises, many guns are used at a low speed setting (usually half or two thirds of battle speed), and fed training rounds, which are not only cheaper than fully combat-effective rounds, but also easier on the barrels. - ¡Ouch! (hurt me / more pain) 06:59, 22 April 2014 (UTC)
 * Most modern gatling guns operate using a DC motor to cycle the rounds (chambering, ejecting, etc.) A typical time frame for the gatling gun to reach full fire rate is about 0.4 seconds (see GAU-19).  So the super capacitor idea is kind of a moot point. Justin15w (talk) 15:30, 22 April 2014 (UTC)
 * This page states about the Soviet GSh-6-30 that
 * The charging and spin-up of the barrel group before firing was achieved with the use of a pneumatic system which included, among other things, a pair of compressed air storage tanks and a "pneumostarter".
 * From the same author, we get spin-up times of about 0.4s for both the older M61 Vulcan and its bigger cousin the GAU-12, while the GSh times are merely given as "far less". The newer M61A2 is stated to have an improved spin-up of 0.25 seconds.
 * Currently, rotary cannon seem to come in roughly five sizes.
 * 5.56mm, based on the 5.56mm NATO round, the 5.45mm Soviet, or the .22 Long Rifle. These are of very limited use due to their short range.
 * 7.62mm (NATO or Soviet). These are heavy machineguns in terms of raw firepower but not WRT range. However, the sheer volume of fire does help achieve kills at ranges which wouldn't be called "effective" for single-barrel machineguns.
 * 12.7mm MG (again, NATO or Soviet). These are the true heavy machineguns, usually found on the heavier vehicles, or as small guns on ships or attack helicopters.
 * 20 to 23mm. Most of these cannon are mounted on fighter planes or air defense platforms. The CIWS role is notable, too. Spin-up is critical in the dogfighting role, and only there. In the CIWS role, the ROF is usually stepped down on purpose, because the full volume of shells would not outweigh the lower accuracy, except at very close range.
 * 25mm and more. Old high-caliber guns are usually in the 30+ range, while most newer guns use 25mm shells with little to no loss of effectiveness. Roles are the same as the 20mm class, but better suited to overcome the low hit probabilities on maneuvering targets with a more damaging round. As noted above, the spin-up time is comparable to older 20mm weapons, but the GAUs are probably still behind the Soviet GShs. - ¡Ouch! (hurt me / more pain) 07:58, 23 April 2014 (UTC)
 * There are at least two larger calibers. The 30mm GAU-8/A_Avenger and the 37mm T249 Vigilante, although the latter never saw full production. ScienceApe (talk) 20:52, 23 April 2014 (UTC)
 * Technically, those two are in the "25mm and more" class, in the "30+ range" to be precise. ;) But thanks for providing the links.
 * According to the article, the Vigilante "had a 192-round drum magazine, which in the 3,000 rpm mode would have equated to approximately 5 seconds of fire", which indicates a really long spin-up (~2 seconds). - ¡Ouch! (hurt me / more pain) 05:51, 24 April 2014 (UTC)

Mesopotamian units of measure
Dear refence desk

please supply the reference for two specific Sumerian units which are given with precision

first the cubit (kus) of 497 mm

second the mass of the pound (ma-na) of 497.7 grams

Thank you

Roland Boucher Irvine california — Preceding unsigned comment added by 2602:306:CE07:A6D0:21F:F3FF:FECE:3122 (talk) 03:14, 22 April 2014 (UTC)
 * You already asked here three months ago for support for the cubit being 497mm, which I'm aware that you need to be true in order to support your theory (that I won't link to) that the Sumerians basically invented the metric system 5000 years before the French proposed it. The information you were given last time, that in historical reality the cubit wasn't a precise consistent length but rather varied a bit depending on the precise time and location you're talking about, is still true.  Red Act (talk) 05:43, 22 April 2014 (UTC)


 * It's also important to understand (and accept) that the levels of precision you're expecting did not exist in those days, even in places and timeframes where the units employed were consistent. AlexTiefling (talk) 12:27, 22 April 2014 (UTC)


 * Bah! This theory has bogus written all over it!
 * The meter was originally defined as 1/10,000,000th the distance from the pole to the equator - there is no way the Mesopotamians knew the size of the earth to any kind of precision - so it can only be coincidence that their definition of the cubit is remotely close to a half meter.
 * As we've already pointed out, there are countless different standards for the "cubit". Finding one specific place where it just happens to come out within a percent or two of a half meter isn't a great way to come up with a theory.  What about all of the other places where it WASN'T close to a half meter?
 * If they had intentionally defined the pound in terms of the cubit in the same manner that the kilogram is defined in terms of the meter (1 kg = the mass of 1 liter of distilled water at 4 degC = 1/1000th of a cubic meter of water at 4 degC), they'd have had to choose a temperature at which to make that definition. The only way for your theory to make sense would be if they (like the French) chose the temperature of 4 degrees Centigrade to measure the density of water.  Since Mesopotamia is a fairly hot region of the world, with very few mountains high enough to reach those low temperatures, it seems highly unlikely that they'd have happened to pick the exact same temperature as the French at which to define their system.  It's stretching the bounds of plausibility to breaking point to imagine that they'd have done that.
 * If you'd picked a HALF meter as your standard unit of length, then the natural unit of mass would be 1/1000th of a cubic cubit of water. That 'natural' measure would be EIGHT times less than the kilogram - not half a kilogram as you claim they used for their pound.  A cubic volume of water weighing 1 meospotamian pound would be some very odd size in mesopotamian cubits...actually, an irrational number.
 * For all of these reasons, your ideas are very, very busted. The relationships you think you've found are nothing more than a coincidence.


 * SteveBaker (talk) 02:12, 23 April 2014 (UTC)

Hunger hormones VS Appetite hormones
As I know, it is now clear for scientists that Hunger and Appetite are 2 different things - Hunger will always bring appetite but appetite won't necessarily bring hunger.

My question is: Are there any documented Neurochemicals that deals specifically with hunger and others that specifically with appetite? Thanks Ben-Natan (talk) 04:08, 22 April 2014 (UTC)


 * I don't think your distinction between Hunger and Appetite is standard in the literature, but if you are using "Appetite" to mean the act of eating, then yes, there are biological distinctions. It isn't true, though, that hunger always causes eating.  Unfortunately we don't yet have a very deep understanding of either the neurochemistry of hunger or the mechanisms that drive eating.  Our article on hunger (motivational state) gives a rather sketchy overview of what is currently known.  The hormones leptin and ghrelin seem to be pretty directly related to the sensation of hunger. Looie496 (talk) 14:02, 22 April 2014 (UTC)

Buoancy
If no water is displaced when an object is put in it, then how do objects like leaves float when they do not displace water and thus the water does not have buoyancy force? — Preceding unsigned comment added by 122.150.66.107 (talk) 04:52, 22 April 2014 (UTC)


 * Anything that floats on water displaces water. See Archimedes principle. Leaves don't displace very much, because they don't weigh very much. --50.100.193.30 (talk) 05:43, 22 April 2014 (UTC)


 * Perhaps surface tension has something to do with it. Richard Avery (talk) 05:59, 22 April 2014 (UTC)


 * Yes, surface tension has two effects: firstly, for an unwetted leaf, it moves the displacement of water away from the leaf (but the same amount of water is still displaced, it's just not as obvious); secondly, at the boundary of the container, it allows displaced water to form a meniscus above the edge of the container, preventing overflow. Thus the displacement of a small mass of water is not clearly observable.  The displacement will be more clearly visible if you use heavier leaves and add a surfactant (e.g. detergent) to the water next time you try the experiment.    D b f i r s   09:07, 22 April 2014 (UTC)

Giant bumblebee
Here in Central California this spring, there seem to be quite a few GIGANTIC black bumblebees (I'd say at least 1.5-2 inches in length, and about the same in wingspan), which I hadn't noticed in previous years. Are these native to California, or are they an invasive species? Are they any more dangerous than ordinary bumblebees (more aggressive/more toxic/etc.)? Thanks in advance! 24.5.122.13 (talk) 05:54, 22 April 2014 (UTC)


 * Black bees tend to be Carpenter bees, and there are many species of them. Some can get quite large: see .  If I had to guess, you have seen some kind of carpenter bee, but more specific than that, it would be hard to say.  -- Jayron  32  12:40, 22 April 2014 (UTC)


 * There's also the black-chinned hummingbird there. The ones in our article don't look all that black, but here's one that does:, which can be mistaken for a giant bee, as it makes a similar sound and also hovers as it goes from flower to flower.  (While in motion, the wings are a blur, so you can't see the obvious difference between bird wings and insect wings.) StuRat (talk) 13:38, 22 April 2014 (UTC)


 * Without pics it's hard to ID conclusively, but Xylocopa_varipuncta is the largest bee native to California. Females are metallic black, males are gold and fluffy. They can reach over an inch long, and can easily seem to reach 2" while on the wing. In general, actual bumble bees (bombus) are not very aggressive. Carpenter bees can be aggressive, but mostly to other bees. This in part comes from different social structures. Most carpenter bees are solitary, and each male will defend an area from other males, while hoping to attract mates. People often think they are a threat, as they will aggregate around e.g. wooden picnic shelters that they have prepared nesting sites in. The bees will then fly at incoming animals to investigate. So, you might feel threatened if one of these comes flying at your head, it actually poses no threat, it just wants to make sure you're not a rival bee. Males can't sting, and you can swat them away with impunity. For fun, you can wad up a bit of paper or aluminum foil and chuck in the direction of such an aggregation. The bee will quickly track that object, and follow it away from you. Since they don't live together in a large colony, they do not cooperatively defend anything, so it is almost impossible to get a swarm of them chasing you (unlike certain wasps, africanized honeybees, etc.) Anyway, just wanted to give you the general info that carpenter bees are nothing to be afraid of. Yellow jackets, however, are horrible jerks, and will seriously ruin your day ;) SemanticMantis (talk) 15:46, 22 April 2014 (UTC)


 * Looks like Xylocopa californica to me, because it was not shiny, although the one I saw up close was at least 1.5 inches long. Anyway, from what I gather, they ARE native to California, and the best way to deal with them is to just leave them alone, right? 24.5.122.13 (talk) 22:16, 22 April 2014 (UTC)
 * Right! The only "problem" they cause if if a whole bunch of them decide to try to nest in the eaves of your house or garage. Even then, mostly just an annoyance, but enough small holes can structurally damage over time (see e.g. here ) If you are getting "buzzed" by them often, just ignore, swatting at them may trigger aggression. SemanticMantis (talk) 15:04, 23 April 2014 (UTC)


 * Well, first of all, it has been proven that bumblebees can't fly, so you needn't worry these are bumblebees. On the East Coast, I always notice the carpenter bees come out first, then the bumblebees, then the honeybees.  Control is easy when necessary, they sell specifically designed insecticide cans with long straight spray streams with which you can easily reach their nests at a distance.  There's no need to kill them unless they are excavating the wooden beams of a valued structure. μηδείς (talk) 17:10, 23 April 2014 (UTC)
 * ...Bumblebee (under "misconceptions"). --&mdash;  Rhododendrites talk  |  19:07, 23 April 2014 (UTC)

Do Jews have some African/Black DNA?
All the people that I know are Jewish ethnicity/race have some of the same ethnicity-specific features as Black people including a cloud of big curly hair that grows out like an afro, very full lips and according to a funny book about true stereotypes, a giant penis. Since Jews have some of the features that are unique to African people does that mean they have some African/Black DNA? — Preceding unsigned comment added by 68.8.106.52 (talk) 05:12, April 22, 2014


 * We all have African DNA. HiLo48 (talk) 09:44, 22 April 2014 (UTC)


 * More specifically "In paleoanthropology, the recent African origin of modern humans, frequently dubbed the "Out of Africa" theory, is the most widely accepted model describing the geographic origin and early migration of anatomically modern humans." -- Jayron  32  10:52, 22 April 2014 (UTC)


 * That's correct, but the "Out of Africa" event occurred over 50,000 years ago. Since then there has been limited genetic mixing between sub-Saharan populations and non-African populations, so the question actually does make some sense at that level.  The bigger problem is that there is really no such thing as a "Jewish race", or anything like it, at a biological level.  Modern-day Jews derive from a pretty diverse mixture of backgrounds.  In any case, as a group, it's extremely unlikely that they have higher levels of sub-Saharan-derived DNA than other people from the same parts of Europe or Asia. (I think the question is probably trolling but I have answered it seriously anyway.) Looie496 (talk) 13:51, 22 April 2014 (UTC)
 * The question may be related to Ethiopian Jews in Israel. Ruslik_ Zero 19:32, 22 April 2014 (UTC)


 * For the hair bit: obviously not all Africans have the same hair type, nor do all Jewish people. Afro has some discussion and references. SemanticMantis (talk) 22:12, 22 April 2014 (UTC)

By African/Black I mean someone the same racial ethnicity as (and this probably sounds offensive but) Flavor Flav. I know about the "Out of Africa" event and that all people are descended from people from the entire continent of Africa. But there are a lot of different types of African-nationality individuals, from this to this to this. But the hair texture of 100% Black people is unique to people of specifically Negro ethnicity (which is maybe what I should have written instead of African). Take the afro or the flat-top for example. Those specific TEXTURES are examples of natural Black hair and no one else on Earth has hair that grows that way, not even if they want it to, unless they have some Negro ancestry I would think. So I would like to get this question answered if you'll give it another try. Looie469, are there any charts or graphs or studies that could prove your view or mine? And I've read before that there is a difference between people who are religiously Jewish and ethnically Jewish. I guess not? — Preceding unsigned comment added by 68.8.106.52 (talk) 23:30, 22 April 2014 (UTC)
 * That type of hair is known as Afro-textured hair, or sometimes as "nappy hair", and it actually isn't true that only Africans and their descendants have it. As Afro-textured hair explains, there are several other populations who live in equatorial regions who have similar hair. Looie496 (talk) 03:42, 23 April 2014 (UTC)

Those articles were very interesting! Thanks for enlightening me. — Preceding unsigned comment added by 68.8.106.52 (talk) 06:11, 23 April 2014 (UTC)


 * I had never heard of Flavor Flav, so I just checked our article on him. In every photo he's wearing a hat, so I still don't know what his hair looks like, and the article says nothing about his ethnicity. HiLo48 (talk) 06:31, 23 April 2014 (UTC)

Here HiLo, you can see a bit of his hair in this photo: http://images.starpulse.com/pictures/2009/03/22/previews/Flavor%20Flav-PRN-035552.jpg This guy doesn't look Black to you? Well, we're all entitled to our own opinion. Haha. Thanks. — Preceding unsigned comment added by 68.8.106.52 (talk) 07:01, 23 April 2014 (UTC)

Baseline fitness
Other than age, what influences natural baseline fitness levels. Without any training, anyone's fitness regresses but what influences the speed of regression and also where the regression stops, other than age. Clover345 (talk) 10:13, 22 April 2014 (UTC)
 * Physical fitness is a nebulous concept, and not easily quantifiable. There are aspects of fitness (such as the ability to complete certain physical tasks, like run a certain distance, etc.) or things like Basal metabolic rate or Body Mass Index or other such measures which are sometimes used as proxies for fitness, but none of those singularly captures what it means to be "fit".  -- Jayron  32  12:34, 22 April 2014 (UTC)
 * Hormone levels are important, in particular testosterone in males. Looie496 (talk) 14:47, 22 April 2014 (UTC)


 * Physical activity an important factor here. If you exercise regularly from a young age onward, you should not notice a significant decline in physical fitness until you start to hit old age. Only if you are an olympic class top sporter who exercises a few hours per day will this be different. A typical 60 year long distance runner who has run for most of his life can perform just as well as when he was 30; exercising regularly will have helped to preserve the physical fitness he had decades ago. Count Iblis (talk) 15:07, 22 April 2014 (UTC)

Urinalysis
Do chlamydia and gonnorhea urine tests involve the same test as a urinalysis looking for white cells, blood etc? If not, why don't they do it along with the std test since they have a sample of urine anyway sand it can't be that expensive to do both? Do these test literally only detect chlamdia and gonnorhea? 90.205.212.104 (talk) 14:14, 22 April 2014 (UTC)
 * According to http://std.about.com/od/gettingtested/qt/Urine-Testing-For-Gonorrhea-And-Chlamydia.htm, the standard method of testing is to test for the presence of bacterial DNA in the urine. That's quite different from the method of looking for blood cells. Looie496 (talk) 14:46, 22 April 2014 (UTC)
 * They could also test for LSD metabolites, gold particles, and Borg nanoprobes. The questions invovled are reasonable suspicion of risk, cost-benefit analysis, and informed consent.  μηδείς (talk) 16:57, 22 April 2014 (UTC)
 * Would borg nano probes and gold particles come out in urine? 90.205.212.104 (talk) 18:27, 22 April 2014 (UTC)

Does (or did) any country's military or police use the Desert Eagle pistol?
Question as topic. I thought at one time that the Israeli military and US Special Forces used them, for two examples - but I've been told not. I have been told that most Desert Eagles will only ever be fired at targets, tin cans and water jugs by private owners, because they're not actually that useful, in terms of effectiveness to the military and police, despite being a .50 caliber pistol. Also that they're basically 'more form than function'. Thanks. — Preceding unsigned comment added by 87.112.139.143 (talk) 21:43, 22 April 2014 (UTC)
 * This impressively outsized handgun quickly attracted the attention of Hollywood. The Desert Eagle debuted in "The Year of the Dragon," a 1984 action flick staring Mickey Rourke. Since then, it's been featured in some 400 to 500 motion pictures and TV films, including Arnold Schwarzenegger's "Eraser" and "The Last Action Hero." Whenever a script calls for a wicked-looking, thoroughly intimidating handgun, the Desert Eagle still gets the nod - says "American Handgunner". I observed one in the holster of a petite policewoman rushed on duty at Venice Airport hours after transatlantic air transport was shut down following the WTC Attacks. Nella lettura di questa mia cara, mi scusi se glielo dico ciò che difficilmente la si può sollevare solo il look ridicolo. JustAnotherUploader (talk) 22:09, 22 April 2014 (UTC)


 * The absence of scales in the pictures in our article make it hard to judge its size. DuncanHill (talk) 22:15, 22 April 2014 (UTC)
 * Demi Moore for scale (?) here, from this thread discussing the gun . (I did look around for actual scales or size comparison, but couldn't find anything else that made it look big). SemanticMantis (talk) 23:57, 22 April 2014 (UTC)
 * With a barrel length of 10-15 inches depending on model, it IS pretty big for a pistol. 24.5.122.13 (talk) 05:34, 23 April 2014 (UTC)


 * I fired one once, caliber .50 Action Express, and nearly knocked myself over, though I'm a good bit heavier than Demi Moore. In hindsight I wasn't properly braced, but in combat you can't count on that anyway. —Tamfang (talk) 09:23, 23 April 2014 (UTC)

Why are "double red cell" blood donations by females subject to a higher weight requirement?
While looking at the Red Cross blood donation elgibility rules for something else, I noticed here that double red cell donations by females are subject to a higher weight and height requirement than those by males. Why is this the case? I initially thought that this was a mistake, but after some more searching, this doesn't seem to be the case. To me it doesn't make much sense, since one would think that quite a sizeable number of females wouldn't be heavy or tall enough, and given that men are generally taller/heavier. (While perhaps there are many "average" Americans who might fit the requirements, it for instance seems clear to me that from personal experience, this might disqualify a lot of Asian American women.) Morningcrow (talk) — Preceding undated comment added 22:45, 22 April 2014 (UTC)


 * On average, women have a lower proportion of blood per unit of body weight (in part due to their higher average body fat content) than men. Roughly speaking, men have approximately 75 mL of blood per kilogram of body weight, whereas woman only have about 65 mL per kilogram. Women also average a lower hematocrit (volume fraction of red blood cells in whole blood) than men.
 * The purpose of the requirements isn't to ensure that equal numbers of men and women are able to donate, but rather to ensure that it is safe for donors to do so. TenOfAllTrades(talk) 22:57, 22 April 2014 (UTC)


 * I figured it wasn't because of an interest in having equal numbers (otherwise it definitely wouldn't have made sense), but had no idea. Thanks - this more or less explains everything. 23:06, 22 April 2014 (UTC) — Preceding unsigned comment added by Morningcrow (talk • contribs)

Direct current from stored torsion (e.g. wind-up battery/battery charger)
Hi, I'm interested in the general physics and engineering at play here, but I'll put the two specific questions up front:

1. Are there any devices on the market that let you turn a crank to store energy in a spring, and then release that energy slowly to power a small electronic device (or charge a battery)? I'm aware of crank-powered flashlights and and chargers, e.g., but those don't store mechanical power; they seem to universally require high speed, low-resistance turning for long periods of time, even to power a small radio.

2. Assuming negative to the above, are there any physical problems with the idea, or is it more a matter of engineering/materials/market limitations?

Elaboration on the idea: wouldn't it be more convenient to turn a crank with much more resistance, over a shorter period of time, with less dependence on rotation speed? E.g. if I wanted to charge a flashlight with a one-hour charge while camping, I'd rather work to wind a very stiff spring for a minute or so, than spin those little cranks quickly for about 10 minutes, perhaps repeatedly.

I'd think that a torsion spring could be rigged up to drive a small dynamo, and the resulting current could either run a device directly, or charge its battery, or perhaps even both, depending on needs. But then I get a little lost through all the details. Surely there's a lot of linkages, gearing, and other mechanics to worry about. We have to consider current/voltage demands, the spring constants of the material, the size of lever arms and rotors, etc. In summary, is it foolish to think that a device could do this? If not, could something like this be cobbled together from mostly of-the-shelf components? Weight, size and conversion efficiency don't matter much for the last bit, but of course they would for a hypothetical product. Thanks for any suggestions, SemanticMantis (talk) 23:35, 22 April 2014 (UTC)


 * Compressed air is used to store energy. Count Iblis (talk) 00:38, 23 April 2014 (UTC)


 * See Human power for a practical example of something that does exactly this. AndyTheGrump (talk) 00:44, 23 April 2014 (UTC)


 * Ah, thanks! I hadn't heard/seen any specific mention of spring storage before! SemanticMantis (talk)


 * I have such a wind-up radio. The idea is that it's ready for emergency use, while a battery-powered radio always seems to have dead batteries whenever the emergency occurs, and even if you have spare batteries, acid may have leaked out of the old batteries and destroyed the radio.


 * I also recall that on the TV show M*A*S*H, Radar O'Reilly would wind up the army phone before using it. StuRat (talk) 11:59, 23 April 2014 (UTC)


 * I am not 100% certain, but I think the 'winder' was only to signal the other telephone or switchboard. See here, and here which says this type of phone was in use from ≈1937, WW2 through to Korea, and "into the 1980s".--220  of  Borg 13:47, 23 April 2014 (UTC)


 * Yes - the phone doesn't have a battery - but the base station does. You wind the crank to produce enough energy to ring the bell at the base station - then they connect you to a battery at their end that provides the power for the duration of the call. SteveBaker (talk) 16:30, 23 April 2014 (UTC)


 * Steve, many years ago we used old Army surplus field phones WW2 vintage, to talk over pairs of wires as from an electrical substation to an industrial customer's electrical room where there was no land line phone and celphones were not around yet and they were too cheap to give us handheld radios. Each fieldphone had a local battery to power the talk circuit (carbon mic in the handset, battery transformer like old country wall phones). The crank indeed provided the ringing voltage. Phone company phone systems switched to having a battery only at the central station in the early-mid 20th century. The field phones might also work with a central battery. I just know they worked with other field phones with only a copper pair connecting them and that each had a battery.  Edison (talk) 20:38, 24 April 2014 (UTC)


 * Yes. Search for images of the old Freeplay flashlight.  I was given one a long time ago, perhaps in the early 1990s, which had a clear plastic case so you could see the mechanism.  The handle wound a very wide, substantial spring from one spool to another, taking about 20-30 seconds. When switched on, the spring wound back over a period of about 3 minutes, either operating the light or charging a small NiCd battery.  I don't have the bandwidth for any images, but the Freeplay site does have a small support page for their discontinued Freeplay 2020 flashlight, which includes the warning, "The spring is powerful and dangerous.  Do not remove any covers unless you are an experienced engineer.", so that sounds like the mechanism I remember.  If I ever make it back to my storage unit, I think I will dig it out and look into converting it to LED. -- ToE 12:49, 23 April 2014 (UTC)


 * Yea, 3 minutes is pretty much useless. With it converted to LED hopefully you can get 10-20 minutes out of it.  But how exactly does one convert a flashlight from incandescent to LED ?  I have some large flashlights that take 4 D-cells each, and I'd love to convert those to LEDs. StuRat (talk) 16:50, 23 April 2014 (UTC)


 * If they're Maglites I'm certain they sell drop-in LED replacements for their incandescent bulbs, though can be rather pricey! Some related info here. Googling "led torch conversion kit" got useful hits like Tektite. 220  of  Borg 00:37, 24 April 2014 (UTC)


 * Wow. $30 for a single LED bulb ?  That's like 10X what I would be willing to pay, and more than the flashlights cost when new. StuRat (talk) 05:08, 25 April 2014 (UTC)


 * I don't understand the advantages here. I understand that you'd like to apply more torque for a shorter period of time - but that's just a matter of gearing.  The only real questions are:


 * Is there some efficiency difference between storing energy in a spring versus a battery?
 * Is there some cost benefit in one mechanism versus the other?


 * My gut feel is that the spring doesn't discharge energy at all uniformly. Very high quality mechanical clocks go to a lot of trouble to even out the energy produced by the spring over time. All of that extra 'stuff' seems unnecessary if you use a battery to store the energy instead.  Assuming the thing you're driving needs a uniform energy input, I think batteries will do a better job.  NiCd and NiMH's have a fairly uniform voltage over most of the discharge cycle.
 * Cost is harder to estimate - but in our modern age, moving parts are generally avoided in favor of solid state stuff - so eliminating the spring seems like it would save money.


 * SteveBaker (talk) 01:42, 23 April 2014 (UTC)


 * Steve, I agree with most of what you say. I don't think this would be objectively better, or more efficient than batteries. Moving parts do wear down, etc. I thought maybe with some clever circuitry it wouldn't matter if the spring output was uneven. Shouldn't a system of RLC circuitry be able to buffer the mechanical input, so that the electrical output is more even? E.g. a moving average. Don't conventional battery chargers already do a bunch of "smart" controlling of the charging process to deliver current and voltage at different rates? To clarify, my interest is for things like camping, hiking, biking, etc. As for storage, I guess a large capacitor with high-resistance gearing might make the spring a worthless complexity.  So maybe I should think about how to get bigger capacitors and higher gearing onto one of those cell phone chargers I linked above? SemanticMantis (talk) 02:45, 23 April 2014 (UTC)


 * This reminds me ... it's been a few years since we discussed energy budgets for automatic watches: has anyone found a reliable source that quantitatively discusses comparative energy-budgets for digital and mechanical watches? Nimur (talk) 04:55, 23 April 2014 (UTC)


 * Well, how about this: There is an extremely comprehensive source for Seiko watches HERE, From a quick look at that document, the analog quartz watches mostly used between 0.8 and 2.0 microAmps for the movement and 0.3 to 0.8 microAmps for the "circuit block"...although there are lots of outliers in some of their models.  So we're talking between one and three microAmps for a typical quartz/Analog watch.  This delivers a battery life between one and five years.   Surprisingly, the digital watches seem to use a lot more power - around 1.0 to 10 microAmps for the "movement" (presumably that's watchmakerspeak for "display") and between 0.8 and 3.0 microAmps for the "circuit block" - so maybe 2x to 5x more power consumption than analog watches.  But with a similar battery life quoted for each...so probably there are more/bigger batteries in the digital models.  A watch battery holds around 250 milliamp-hours of charge - and at 1.2 volts, that's 200 milliWatt hours or 720 Joules of energy.


 * I'm not sure that helps to illuminate this question though! As I explained above, our OP's problem lies mostly with the gearing on the crank rather than the technology used to store the energy...and perhaps we can use this example to illustrate the problem:


 * I recently bought a rather nice antique grandfather clock. It has chimes that go off every 15 minutes, a second hand and a moon-phase dial - and it runs for about a week using the gravitational potential energy of three (roughly 1kg) weights lifted through about a meter.  This is a better way to store and retrieve energy than a spring because the force exerted by the falling weights is exactly constant over the entire week...but obviously it's a lot less convenient for portable devices!


 * Anyway, I can wind the three weights up to the top of the clock using the hand crank provided in about 5 seconds per weight. It takes quite a bit of force to turn the crank - but it's a reasonably comfortable rate to crank at - and a longer crank arm would probably allow you to crank at this rate for a considerable amount of time.  Now, we know that for gravitational energy, E=mgh, so when I'm winding it up, I'm providing about 30 Joules of energy over 15 seconds of work - so I'm providing about 2 Joules/second or 2 Watts while I'm cranking it.  When fully wound, my clock stores about 30 Joules of gravitational potential energy - about 24 times less than that 720 Joule watch battery!   Which means that this gigantic clock could hypothetically run for 24 weeks - close to 6 months - on a battery that will power a quartz watch for two years.   So this gigantic mechanical monster from the 1800's is only 4 times less efficient than a beautifully engineered modern watch - and it ticks loudly enough to be heard in the next room, chimes every quarter hour loud enough that I can hear it from anywhere in the house and keeps a gigantic pendulum swinging backwards and forwards against all of the friction generated by 100 year old bearings and gears!


 * I find that rather impressive - (or to put it another way, those fancy watches are surprisingly inefficient devices!).
 * SteveBaker (talk) 14:11, 23 April 2014 (UTC)


 * I'm not so sure that a watch battery can power a grandfather clock for 24 weeks. Assuming your math is all correct, you're just comparing energy, and ignoring all the details of how a battery would transmit mechanical power, at what efficiency/loss, etc! Which was kind of the information I was looking for here, but the other way around. Still, interesting comparison, thanks. SemanticMantis (talk) 15:10, 23 April 2014 (UTC)


 * 30 Joules is 7 calories - so the clock consumes one calorie per day and by winding it every week as a form of exercise, I'll lose a whole pound of body fat after about 10 years of clock-winding. :-)  SteveBaker (talk) 16:26, 23 April 2014 (UTC)
 * Fascinating work Steve! I'm feeling nitpicky today, though!  You claimed an exact property: "the force exerted by the falling weights is exactly constant over the entire week..." but that assumes the little g gravitational constant is exactly the same.  In fact it changes slightly with respect to the height of the weight above the Earth's surface.  (Not to mention the gravitational attraction of objects other than Earth!)  I'll readily admit that the effect is negligible, but because you use the phrase exactly, I had no choice but to nitpick!  Nimur (talk) 21:43, 24 April 2014 (UTC)


 * You forgot to factor in proton decay in the weights ! :-) StuRat (talk) 05:08, 25 April 2014 (UTC)


 * ...or more mundanely, the gradually increasing uncoiled length of the cables suspending the weights slightly increases the pull that they exert on the clock mechanism. That effect might even be measurable.  So, OK, if you're being super-nit-picky, I shouldn't have said "exact" - but the sense of what I was trying to convey matters here.  The amount of power produced by a coiled spring as it unwinds is hugely variable compared to the almost constant amount of power produced by a falling weight.


 * The analogy with a car engine is useful here. An internal combustion engine has wild variations in the efficiency with which it converts gasoline into useful work depending on the RPM that it's turning at.  Hybrid vehicles get the good mileage that they (mostly) do because they can run the engine at its optimum RPM all the time and use it to charge a battery that can provide spurts of high power when needed.


 * A similar thing happens with human-powered machines. We can only turn a crank at some limited ranges of speed, torque and duration - and to drive something directly from that input shares the same problem as a car engine.  So using the crank to charge a battery is a good way to optimize the energy usage.  Springs can be used (like a battery) to store energy - but they lack the ability to release that energy in a uniformly controlled manner without resorting to more complexity with mechanisms like a fusee.   The falling weights in a grandfather clock are an elegant way to release stored mechanical energy in an almost uniform way - but clearly that's inconvenient in general.  We have a six foot tall cabinet in our hallway - effectively doing the job that a wristwatch can do almost as well.


 * Finding a compact, lightweight and cheap way to store and release mechanical energy is therefore a difficult problem. Skirting it by converting the mechanical crank turning into electicity to charge a battery is a reasonable way to handle the problem. Springs are probably the second most convenient way - although they do suffer from this lack of uniformity of energy output over time.   A flywheel is another possibility - but they tend to be either very heavy or they have to spin at such high speeds that they are in perpetual danger of flying apart and doing enormous damage in the process. SteveBaker (talk) 13:59, 25 April 2014 (UTC)