Wikipedia:Reference desk/Archives/Science/2014 May 12

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May 12

I keep cracking my joints

It's not a medical issue! 2 doctors told me "It's nothing" and they crack their joints before me and told me "See? It's normal". Oh yea? than way only at the last two years I started having that? (Before that I never had Cracking-Joints). this phenomenon is annoying... how do I lower it's frequency? thanks! — Preceding unsigned comment added by Ben-Natan (talk • contribs) 00:35, 12 May 2014 (UTC)

If you're asking how to lower the frequency of annoying bodily phenomena, it's a medical issue. If two medically trained professionals tell you it's nothing to worry about, they're the qualified ones. Take it or leave it. Generally speaking, things get older with time. Every year brings new wear and tear. You're not whatever age you used to be. InedibleHulk (talk) 00:41, 12 May 2014 (UTC)

I agree that one should listen to one's doctor, but have we no article on the subject iteslf? One of my nephews started cracking his knuckles at 3. THe older one still can't 7 years later. μηδείς (talk) 00:48, 12 May 2014 (UTC)

Cracking joints is somewhat relevant. I suggest drinking a bottle of synovial fluid for increased vigor and vitality. Seriously though, don't listen to me. InedibleHulk (talk) 00:49, 12 May 2014 (UTC)

Smoking joints fights joint pain. You didn't say if they hurt or not yet, so this isn't advice. Right? InedibleHulk (talk) 00:59, 12 May 2014 (UTC)

I've been cracking many of my joints (not just all of my knuckles, but also both wrists, right shoulder, at least 3 cervical vertebrae, both knees, both ankles and all my toes) since I was 9 years old, and have never experienced any pain or discomfort from that. One thing I noticed is that my joints usually crack after being left immobile for a few hours -- so if cracking joints bothers you, a good way to reduce occurrence of this would be to keep your joints moving periodically. 24.5.122.13 (talk) 06:55, 12 May 2014 (UTC)

After 20 years or so of the same, I noticed the wait time between cracking and stiff enough again has drastically dropped. Only takes a few minutes to "recharge" now, and where I used to have to try, now it's just automatic. Still in my thirties, so just a very dull soreness, but the stiffness is sort of making me regret it. Both get worse in the cold.

Sort of like an addiction, it just doesn't feel as rewarding, either. Especially with my back, it used to be almost thrilling. Now, meh. But gotta keep on chasing that dragon, on account of the stiffness. Also, my hands look like owls should be perching on them in a cemetery. In my case, it actually fits nicely with my tall frame, pale skin, sunken eyes and wizard beard. But it's something you may want to avoid. InedibleHulk (talk) 04:19, 14 May 2014 (UTC)

Lifting legs and blood pressure

Hello, I am doing a project to see the how laying down and lifting one's legs higher than their heart affects blood pressure. However, I have tried researching the following, and could not get an answer. Which of the two is likelier to raise blood pressure higher: the height at which the legs are lifted, or the length of time for which the legs are lifted? We are only able to choose one of those two variables to change, and we have a "subject" with a fairly low blood pressure and we wanted to see which one is better to try. Any help would be great. Thanks. 70.54.113.147 (talk) 02:09, 12 May 2014 (UTC)

The basic intent of the system is to maintain the blood pressure in the brain/head at optimum. The sensor for blood pressure is in the neck arteries, and the brain has a built-in algorithm to compensate for sensor lag and non-optimum position. In a young healthy person, the head pressure remains constant regardless of body and limb position, but the blood pressure in the limbs will vary in order to keep the head presure constant even though the effects of gravity vary depending on how much lower than the head the body and limbs are.

In very old people, the system tends to deteriorate, due to cholesterol clogging up the neck arteries and preventing the pressure sensing from working fully.

If you are laying down, and decide to get up, there is a brain reflex that automatically raises blood pressure to compensate for gravity now being able to pull blood to the extremities. The presure sensing in the neck functions so as to fine tune the reflex and keep pressure stable over time.

The heart and circulatory system also automatically raises pressure, as well as pulse rate, as your physical activity level goes up.

You can see from all this, that what blood presure reading you get depends on how you measure it. If you measure it in the forearm in the usual way, it will not change much with changes in leg position. If you have the person hold their arm slightly up from horizontal, so that you are measuring at the same height as the neck, it should not change at all, regardless of what the legs are doing - held up as high as posible, or dangling down. If you measure blood pressure in a leg, it varies considerably with leg position. Leg presure is greatest when upright and running - as usefull side effect to having the sensor in the neck.

In young healthy persons, time has little or no effect on pressure. However, in very old people, people with certain types of brain deterioration, and people with low blood pressure, there may be a propitious drop in head blood presure when they stand up upon getting out of bed, or when getting up from a chair - for a few heartbeats anyway. This is due to the reflex I mentioned above not able to work properly. Excessive cholesterol can also cause a slow response in the neck artery sensor. It's why stenting a neck artery should not be done unless really necessary, and then only on one side. Stenting both sides, or the wrong side, will cause very unstable blood pressure with very adverse consequences as the stent prevents the sensing from working.

120.145.72.212 (talk) 02:35, 12 May 2014 (UTC)

How can a drop in head blood pressure, likely experienced as dizziness or nausea be called propitious? 84.209.89.214 (talk) 11:57, 12 May 2014 (UTC)

Perhaps 120.145.72.212 meant precipitous. AndrewWTaylor (talk) 12:13, 12 May 2014 (UTC)

No, I meant propitious. This is a cultural thing - the dictionary meaning is about things that are positive, but in my part of the World, many people use the word to indicate any change that will initiate something, be it positive or just very interesting consequences. The usual effect of a drop in head blood presure is not nausea but blacking out and perhaps falling back into the chair or bed. If severe enough, you'll faint. 121.215.85.7 (talk) 13:40, 12 May 2014 (UTC)

The Latin root of propitious is propitius "favorable, kind, gracious, well-disposed". English language Wikipedia has no good reason to promote so-called "cultural" malaproprisms arising in isolated areas when to do so both contradicts reliable English dictionaries and will mislead an OP who by IPs appears to be as distant from your part of the World as Canada is from Australia. 84.209.89.214 (talk) 19:20, 12 May 2014 (UTC)

Note the condition being referenced above is orthostatic hypotension. Wnt (talk) 20:33, 12 May 2014 (UTC)

Work done

I always thought that work done is the force x distance moved by force. (I.e. Axial work done =Fd or Torsion=T*theta etc so why is it that when equating to strain energy it becomes Fd/2? The result is Fd/2=F^2L/AE. Clover345 (talk) 13:55, 12 May 2014 (UTC)

Because the general formula for the work done by a force is the integral of the force over the distance moved (in the direction of the force) i.e. the integral of F.dr in vector terms. This simplifies to force times distance if the force is constant. In the case of strain energy, the force is not constant, but is assumed to be proportional to d (i.e. Hooke's law is assumed to hold). Hence you get Fd/2 where F is the final force at distance d. Gandalf61 (talk) 14:23, 12 May 2014 (UTC)

thanks for the answer. Is another way to look at this dU/dF? I find calculating dU/dF also gives me Fd/2 but I'm not sure if this is a coincidence? Clover345 (talk) 14:51, 12 May 2014 (UTC)

When we write physics using a slightly more general formulation, we define force as the gradient of the potential energy field. So, $-\nabla U={\vec {F}}$. If you play with the partials, you'll find that your algebraic "coincidence" works only for certain forms of energy field. Nimur (talk) 16:00, 12 May 2014 (UTC)

Resting eyes by closing

If the [human] eyes are closed, does the lens change focus to infinity ("resting" the eyes)? --129.215.47.59 (talk) 14:36, 12 May 2014 (UTC)

Answered recently. 84.209.89.214 (talk) 18:44, 12 May 2014 (UTC)

The previous answers were not entirely intelligible. The focus of the eyes due to accommodation in the absence of anything to focus on (such as in total darkness), is believed to be typically closer than infinity (see e.g. [1]. A closely related question is that of dark vergence. --catslash (talk) 20:23, 12 May 2014 (UTC)

What is the current scientific position on the issue of race supported by American anthropologist and biologists?

I've come across different websites saying that most biologist and anthropologist in the U.S. are convinced that race is not a biological or genetic concept. However, based on my Google searches, there are also a number of notable scientists who oppose the idea that there are no different human races.119.95.198.47 (talk) 16:46, 12 May 2014 (UTC)

Here's a recent Nature paper on the issue from the genetics perspective, titled 'Genetic variation, classification and race' [2]. It's freely accessible, and a short read.

“ there is no scientific support for the concept that human populations are discrete, nonoverlapping entities. ”

- is one of their final conclusions. The key is "nonoverlapping" -- most humans who are familiar with people of different recent ancestry (e.g. Sub-Saharn Africans, Northern Europeans, South-east Asians, etc.) will recognize that there seem to be some differences. However, what the science shows, is that there is just as much genetic variation within these groups as between these groups. The conclusion is that, from a purely genetic perspective, distinct human "races" like "white" or "black" are not well supported by evidence. See also our articles Race_and_society, Race_(human_classification)#Modern_debate, and Race_(human_classification)#Social_constructions. Anecdote: my friend grew up in Brazil, and was considered "white". He was very shocked when he moved to Idaho, and was considered "black". Genetics aside, there is definitely as strong cultural component to how we use race in everyday language and social interactions. SemanticMantis (talk) 17:18, 12 May 2014 (UTC)

This sounds like an argument of semantics. Different pedigrees of dog can breed and different strains of bacteria can exchange genetic material yet no-one would suggest that dog breeds and bacterial strains are "not well supported by evidence". I've never used the word race to mean "non-overlapping" populations and I think pretending differences don't exist instead of accepting or embracing difference is about as ridiculous an undertaking as accepting that 2+2=5 or that the chocolate ration went up to 20 grams. 129.215.47.59 (talk) 17:27, 12 May 2014 (UTC)

The problem is that race and genetics do not easily correspond. For example, two neighboring ethnic groups in Africa may have greater genetic differences between them than, say, someone from the Maya and the Chinese would, and yet the understood "racial" classification system would consider the two Africans to be of the "same" race, and the Mayan and Chinese to be of different races. Ultimately, racial groups are largely based on a superficial classification system which is fluid and based on who is doing the classifying. --Jayron32 17:33, 12 May 2014 (UTC)

Right. Nobody is claiming that there is no such thing as e.g. black people. What is being claimed is that our current understanding of genetics indicates that "the race of black people" is not a grouping supported by genetics. SemanticMantis (talk) 17:43, 12 May 2014 (UTC)

But that's because the things that define races (even if they are nebulous) are superficial anatomical ones and nothing to do with the functions of the plethora of enzymes and structural proteins in the brain, heart, skeletal muscle, liver, kidneys, gut etc etc. I think if you looked at the genes that governed those superficial characteristics, you'd find that genetics does support the existence of races. 129.215.47.59 (talk) 18:00, 12 May 2014 (UTC)

With all due respect, we don't care much for opinions here, our goal is to provide references that can be of use in answering questions. Please refrain from speculating on the reference desk, in accordance with our guidelines [3]. Consider also that your opinion is not consistent with the findings of the paper I linked above, and that has been published in one of science's most prestigious and competitive journals. Finally, note that if we search for "scientific" evidence to support our prior notions, and ignore scientific evidence that contradicts our prior notions, we are no longer truly following the path of scientific inquiry. SemanticMantis (talk) 19:33, 12 May 2014 (UTC)

The article you linked to seems to say the opposite of what you're suggesting. When they look at most genes, yes there is as much variation within a race as between races but there are still many genes 10-15% for which there is a distinction. Thus, as per figure 2, when comparing a sufficiently large number of alleles there are in-fact clear distinctions between races. Importantly, they aren't saying that there is more variation within a group than between groups. They're saying that there are more genes for which there is as much (not more) variation within groups as between groups. 78.148.106.196 (talk) 22:26, 12 May 2014 (UTC)

I agree entirely with your final two sentences. I'm pretty confused about the first. I did say "just as much genetic variation", not "more variation" above. Anyway, I encourage readers to read the paper for themselves. There are far more subtleties there than I can address here. SemanticMantis (talk) 16:51, 13 May 2014 (UTC)

See, this is where the better concept to work with is ethnicity and memetics rather than race and genetics. Race is based on superficial characteristics, such as skin tone. If you really want to learn about people's cultural connections and treat cultural groups as distinct cohesive populations, race and genes are a lousy way to do it. Memetics, a non-biological idea, which acts as a sort of "stand in" for genes when looking at how cultures develop, evolve, and become distinct from each other in humanity, is a pretty decent construct to work in. Then one can speak of cultural groups and memes rather than races and genes, which do not correspond in useful ways. --Jayron32 19:28, 12 May 2014 (UTC)

In the old south, allegedly, some of the "white only" signs had the footnote "no albinos". Race tagging is about more than skin tone. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:50, 12 May 2014 (UTC)

In the language known as English, the phrase "such as" means "one example, but not the only possible example" and does not mean "an exact synonym" or "the only example possible". --Jayron32 20:56, 12 May 2014 (UTC)

True. I just think back to MLK's famous statement about wanting to be judged by the content of one's character rather than the color of one's skin. And he was oversimplifying. Race prejudice is about a lot more than just skin color. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:27, 12 May 2014 (UTC)

Agreed. But many of the other markers - such as accent - are not genetic at all. AlexTiefling (talk) 23:29, 12 May 2014 (UTC)

Yes. But keep in mind that accents evoke all manner of stereotypes and prejudices that are by no means confined to race. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:39, 12 May 2014 (UTC)

I'm sure you're right, but I'm not sure how I'd recognise something that was 'confined to race', since part of the point is that the concepts of race, culture and ethnicity overlap to a significant extent. AlexTiefling (talk) 23:41, 12 May 2014 (UTC)

Accent is often thought of, rightly or wrongly, as an indicator of economic class and education level than anything else. If you didn't know Obama was black, and heard him only on the radio, you wouldn't necessarily think he's black. The lack of a stereotypical black accent hasn't stopped the right wing from railing against him with less-than-subtle race-oriented remarks. Whereas if you heard someone talking with an obvious southern white accent, you might automatically think "redneck", despite the fact that many with southern twangs are well-educated and unprejudiced. Accent can be part of the prejudice package, but it's not a requirement. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:52, 12 May 2014 (UTC)

The term is vague, ambiguous, and emotionally charged. For these reasons it is of little use to geneticists and other biologists, but important to sociologists. Anthropologists use it in some parts of their work, and avoid it in others. Jim.henderson (talk) 17:21, 13 May 2014 (UTC)

Google Maps, the Red River and the Texas/Oklahoma border

Google Maps and Google Earth show the Texas/Oklahoma border as deviating from the Red River by several hundred meters, usually to the north, where the state line nominally follows the south bank. Is this because the river's course has changed since it was surveyed, or is it because of an error in Google Maps? (It is known, for example, that Maps distorts the WGS84 ellipsoid into a sphere, and that some of its satellite photos are off-register compared to the elevation data. But the latter wouldn't explain why there's no southward bend in the state line where Groesbeck Creek joins the Red River at 34.368805°N 99.613165°W / 34.368805; -99.613165.) NeonMerlin 17:58, 12 May 2014 (UTC)

Likely it has. Border irregularities of the United States does not specifically list those examples, but notes many other examples where a river has changed course and left land from one state on the "wrong side" of the river. See, for example, Kaskaskia, Illinois. This is especially true in the midwestern and great plains regions of the U.S., where the rivers run over wide flood plains and tend to meander and change course rather dramatically in relatively short periods of time (decades or centuries). By legal convention, land can lost to a river changing course through the gradual change of the course of the river, but if the river suddenly abandons its old channel and takes a new one some distance away (as in a rincon or oxbow lake), the land it "jumped" around stays with its former owner. This process is known as Avulsion (legal term), and can explain why some state boundaries don't exactly follow the modern borders of rivers they are supposed to. --Jayron32 19:17, 12 May 2014 (UTC)

Note that the satellite view and the Maps view differ — I looked upstream and downstream for several miles from the Groesbeck Creek confluence, and in most spots, the line was within the watered area. Looks like a Maps error, at least in part. But yes, rivers often move a bunch, and on both banks too; Grand Tower Island isn't that far below Kaskaskia, and it's on the other "wrong" side of the Mississippi. See our article on the 1820 US Supreme Court case of Handly's Lessee v. Anthony, although a 1980 decision by the Court has substantially weakened Handley's Lessee. You may find this article from the Southeast Missourian rather relevant to your interests. Nyttend (talk) 04:03, 13 May 2014 (UTC)

On the USGS topo map, your marker is on the "indefinite boundary" – and the main channel of the Red River is shown some distance north, near where Google and Mapnik have the boundary. The river must have meandered considerably in recent decades. —Tamfang (talk) 00:10, 15 May 2014 (UTC)

A/C efficiency

It seems to me that A/C would be more efficient if it cycled on and of, giving the coils time to cool off to close to the ambient temperature in between cycles. Is this correct ? Does any A/C unit try to take advantage of this ? Obviously there are times when the A/C needs to run full-out to provide sufficient cooling, but if you only need a bit of cooling, this approach might work, say by kicking on when the temp is 1 degree above the temperature setting, and back off when it reaches the setting. I also understand that restarting too soon after it stops is bad for A/C units, but a timer could prevent that.

I'd also expect A/C to be more efficient while it's raining, with all that free cool water to cool the coils, but the logistics of only running the A/C when it rains would require some type of chilled water system to implement. StuRat (talk) 18:07, 12 May 2014 (UTC)

Is it conceivable that an air conditioner manufacturer somewhere has not heard of the invention called a Thermostat ? 84.209.89.214 (talk) 19:37, 12 May 2014 (UTC)

(EC) I'm confused. Does not a thermostat do what you describe? It's 85F where I live today. My home thermostat is set for 78. The air conditioner runs until indoor temp is 78, then rests until indoor temp reaches ~79. The cycle seems to be on for ~15 minutes, off for ~25 minutes today, but of course the details will depend on insulation, size of house, temperature differentials, etc. Note that there are many types of thermostats, and some models allow for more control over the duty cycle. The Nest Labs thermostat has been attracting attention recently, in that it claims to achieve very high efficiency through artificial intelligence and machine learning. SemanticMantis (talk) 19:42, 12 May 2014 (UTC)

The only situation I can think of where a thermostat won't stay pretty close to the optimal (in terms of power use) control assuming that the hysteresis band is tuned reasonably is for initial start up or a step change in demand, which are both essentially the same. A thermostat will kick on full blast to cool the area down to the setpoint, where technically you may be able to get better efficiency by running it at a lower duty cycle over a longer time period if you don't mind the slower temperature change, for example cooling off while you're out of the house and reaching the setpoint right as you get home. It's hard to say just how much of a difference it would make, if any. Katie R (talk) 20:04, 12 May 2014 (UTC)

Good point. That idea slowly cooling until the set point is reached about when you get home is what the Nest is supposed to do by "learning" your habits (I swear I don't work for them, nor do I even own one. But I'm pretty intrigued by the possibility of saving money/energy with better algorithms!) SemanticMantis (talk) 20:26, 12 May 2014 (UTC)

I guess what I'm asking about is the "swing setting" (how far above the set point the thermostat has to be to kick the A/C on, and how far below it has to be to turn it off). Central air units often have an option to adjust that swing range on the thermostat, but I haven't seen it on portable window units. So, what is the swing on those units ? Is it small, to take advantage of ambient cooling of the coils ? StuRat (talk) 22:43, 12 May 2014 (UTC)

The whole point of all the coils and fins is to take advantage of ambient cooling, and your scheme wouldn't do much good when one most want cooling. If you think there is a problem I'd have thought you'd be better off just making sure the radiator is in the shade, has good air circulation and is cleaned of any dust and lint. If it has a big slow moving fan I don't think you're going to get anything much better except with a bigger system. Dmcq (talk) 11:51, 13 May 2014 (UTC)

If we're only considering window AC units, I agree that their control schemes leave a lot to be desired. I suspect a clever hobbyist could rig up an arduino or similar such controller to program all sorts of more interesting and efficient behavior. Though "proving" the efficiency increase would be tough outside of a laboratory context. SemanticMantis (talk) 16:48, 13 May 2014 (UTC)

The band on the window units is hard-set to a value chosen by the engineers that designed it to match the size of area it is rated for. They test the designs in environmental testing chambers, allowing the unit to cool a chamber that is either sized to match the rated area or programmed to simulate it by running it's own temperature and humidity controls, while other half of the unit sits in another chamber that is programmed to simulate various outdoors temperature and humidity conditions. The engineers hook up instrumentation to measure whatever aspects of the unit they want to test, and work out controls that optimize the performance. Obviously, there are situations where tweaking the factory-set values could improve things, but for the most part they are already very well tuned for the setup they are sold for. That's why it's important to buy a unit that is sized and rated for the area you want. Going smaller or larger will result in suboptimal control of your room's environment, and presumably worse efficiency. The same thing goes for central A/C units, but like you've noted they tend to be more tweakable. Sorry I don't have a reference for this - the company I work for builds environmental testing chambers, and has sold custom units to air conditioning manufacturers, so I know the sort of testing that at least some of them are doing. Katie R (talk) 11:54, 14 May 2014 (UTC)

Do window A/C units have any "intelligence" to them ? I've seen some with an "economy" setting, but I'm not quite sure what that includes. Would it detect when the temp is near the set point and run shorter A/C cycles at those times ? StuRat (talk) 19:14, 14 May 2014 (UTC)

Question about Penis Transplantation

Is it possible after a certain number of years, can a doctor/surgeon still be able to transplant a penis. I'm asking this because it is not stated in the article: Penis Transplant.

Penis transplantation is indeed an article! I am mystified what sort of "psychological trauma" would demand removal though ... should look into that, something sounds fishy. In general my impression is that motor and sensory nerves to severed parts remain functional as far as they go [4] though I don't really understand how this works at the cellular level. Also, apparently people can actually have a "phantom erection" like a phantom limb! [5] None of this actually answers the OP's question because this is biology and you really never know until you try. Wnt (talk) 20:29, 12 May 2014 (UTC)

Don't try. Evan ^{(talk|contribs)} 21:10, 12 May 2014 (UTC)

Can the regulars here who are finding this such fun please tell me why this isn't a medical advice question? The Rambling Man (talk) 21:12, 12 May 2014 (UTC)

Because we're not telling anybody whether (s)he in particular needs a penis transplant (though I imagine it doesn't take much of a doctor to make that diagnosis!) Wnt (talk) 21:30, 12 May 2014 (UTC)

Agreed, and as long as it's not medical advice, we should attempt to answer both member and non-member questions. StuRat (talk) 00:06, 13 May 2014 (UTC)

Unless I dismember, I think such member-raised questions are covered by the policy of Kainaw's criteria. Why the latter was castrated without any discussion here where it is used stumps me. μηδείς (talk) 21:24, 13 May 2014 (UTC)

@Evanh2008: why do you say "Don't try"? I saw some yammering about 'ethics' in the article too, and I'm afraid I don't grasp it. Is there some sort of religious-ish attitude that the borrowed penis would be adulterous...? Wnt (talk) 18:12, 13 May 2014 (UTC)

First moment of area calculations

I've asked this question before but I'm still not sure which area to use when calculating things like first moment of area, shear flow etc. My understanding is that it should be the area above the neutral axis (or below as they should be equal) but when I actually do the calculations, this doesn't seem to be the case. Clover345 (talk) 21:21, 12 May 2014 (UTC)

The answer completely depends on why you are calculating this term. The moment is a purely mathematical tool; it's just a special type of weight function. You may calculate the area pre-multiplied by anything - but the application dictates whether that calculation is useful. Once you reach problems of a certain complexity, you'll probably replace the "area" with an integral, whose boundaries are defined according to a boundary function (or its approximation). Again, the problem will dictate what those boundaries are - and therefore, which area you are calculating.

For example, if I were solving equations related to fluid flow, I would want to construct the generalized fluid flow equations. Some people describe those equations as a superposition of several moment integrals, where each term represents a linearly-independent physical property. We integrate over a volume, or apply Stoke's theorem to integrate over an area.

You might have a dramatically different formula; your class or textbook or professor might prefer a different form of equations; you might have a different application altogether. Nimur (talk) 00:46, 13 May 2014 (UTC)