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May 20

Defibrillator paddles vs defibrillator pads/ patches

I've read that the defibrillator pads enable a faster defibrillation than defibrillator paddles. What is the explanation for that? and how faster is it? if it significant why is it (paddles) still in use? 93.126.88.30 (talk) 05:19, 20 May 2017 (UTC)

Do the sections Defibrillation#Paddle electrodes and Defibrillation#Self-adhesive electrodes answer your questions? Rojomoke (talk) 08:30, 20 May 2017 (UTC)
Yes, I think so. Thank you. It took me to totally different explanation from what I thought... "Many hospitals in the United States continue the use of paddles, with disposable gel pads attached in most cases, due to the inherent speed with which these electrodes can be placed and used. This is critical during cardiac arrest, as each second of nonperfusion means tissue loss.". But it's written there that "Paddles offer a few advantages over self-adhesive pads.", and no more details or explanation which or what the advantages are. 93.126.88.30 (talk) 11:59, 20 May 2017 (UTC)
Note it also says "In hospital, for cases where cardiac arrest is likely to occur (but has not yet), self-adhesive pads may be placed prophylactically." This would seem to imply that in such cases, the pads are used instead of the paddles. It isn't however stated if the advantage here is that they are faster to use than the paddles, or offer some other advantage (like less room for error), although the fact that paddles are sometimes preferred for speed would suggest that in these cases if attaching the electrodes to the machine is slower than using the paddles, the difference is likely small enough that the other advantages otherweigh the use of paddles. Nil Einne (talk) 12:59, 21 May 2017 (UTC)

Where are the amino-acids / steroids derivatives hormones are made in?

I'v listened to this lecture on Youtube (by KhanAcademy), and mentioned that athat peptides and proteins hormones are made in the RER but he doesn't mention where amino-acids-derivatives hormones and steroids hormones made in, and it just increases my curiosity to know where they are made in... 93.126.88.30 (talk) 11:44, 20 May 2017 (UTC)

You read out article about endoplasmic reticulum, that you linked to above, you may discover that they are made in SER. Ruslik_Zero 12:17, 20 May 2017 (UTC)
There must be something very memorable about this particular term; I have a rather naughty lack of interest in biology except where it overlaps chemistry, and yet this particular term has stuck in my head ever since I first heard it in middle school. This is completely irrelevant, but this answer certainly called it back as clearly as it did the first time I heard it. ^_^ Double sharp (talk) 15:16, 20 May 2017 (UTC)
Peptide hormones are made in RER because the "rough" is ribosomes, and to be hormones, they generally need to leave the cell at the time of their creation. Note there can be exceptions - some cell surface proteins might be made in RER, but only become "hormones" when cleaved at a much later time, on some other part of the cell surface.
Modified amino acids can, in principle, be made just about anywhere. Amino acids are present throughout the cytosol and modifying them (in general) can imply a simple standalone enzyme.
Smooth ER should be important for steroid hormones because cholesterol lives in membranes, and SER has a lot of membrane. Wnt (talk) 17:27, 20 May 2017 (UTC)
The steroid hormones are mostly made from cholesterol, found in many biological membrane, include the plasma membrane of every cell in the body. A series of enzymes convert the the cholesterol into hormones like cortisol, for which the synthetic pathway is known. The amine hormones, like epinephrine, are made from amino acids; for example, in the case of epinephrine (adrenaline) there are a few enzyme-mediated steps from tyrosine. These reactions take place either in the cytoplasm of cells (tysoine through to dopamine) or inside neurotransmitter vesicles (dopamine to norepinephrine through the action of dopamine beta-hydroxylase). Klbrain (talk) 00:12, 21 May 2017 (UTC)
Thank you very much you helped me to study further by your notes! I appreciate it. 93.126.88.30 (talk) 02:48, 21 May 2017 (UTC)

Ladder paradox and electric circuit

In the ladder paradox, if the doors were doubling as electrical switches in series on a circuit, would current be able to flow? It must either flow in the ladder's reference frame with the circuit open, or fail to do so in the garage frame with the circuit closed. If current does flow, is the impedance the same in both frames? NeonMerlin 18:56, 20 May 2017 (UTC)

Err. You have lost me here. Is this anything to do with Voltage ladder? Aspro (talk) 19:27, 20 May 2017 (UTC)
No, see Ladder paradox. I'm trying to get my brain around the relativity of current flow at the speed of light (or just a fraction slower). If the ladder is the same length as the garage in the same reference frame, then current should flow for just a fraction of a second as the ladder collides with both doors. This must also happen with the moving ladder, but how do we explain away the apparent paradox? Dbfirs 19:45, 20 May 2017 (UTC)
The apparent paradox is based on the assumption that that current flow is simultaneous. Current can flow in an open circuit. For a short period of time current can flow though one door/switch whether or not the other door/switch is closed because the opened/closed information cannot propagate to the other door/switch faster than the speed of light. This is a familiar situation in electronics engineering. Take two long coaxial cables, one shorted at the far end and the other open at the far end, and suddenly apply a voltage to one end. The current and voltage at the near end of the two cables will be identical until enough time has passed for light to have traveled to the far end and returned. In real life it takes a bit longer, but that is the absolute minimum time needed to tell of the far end is shorted or open. Also see Relativity of simultaneity. --Guy Macon (talk) 19:51, 20 May 2017 (UTC)
Thank you for the clear explanation and for the link. In the case of an observer sitting on the ladder travelling at 0.99c, he sees a short garage so thinks that the current should flow for longer because both ends appear to be in contact for most of the ladder length, but if he measures the flow along the ladder at the centre, it will actually be for a shorter time than for a slower ladder. Dbfirs 20:49, 20 May 2017 (UTC)
I don't think there can really be a paradox here. If two doors can appear to do something simultaneously from some frame of reference, there is a spacelike interval between them, and no circuit flows in a spacelike interval because no electron goes faster than light. So the circuit is never completed, period. Now to be sure, you might be looking at different events on either end of the barn - like if the front end of the ladder makes a circuit and the back end makes a circuit. Then, depending on the placement of the battery and so on, maybe you see some process that involves both; but if so, then this will be a process that could happen whether both ends of the ladder complete circuits "simultaneously" or not. The comments above apply then. We're talking about a really short interval anyway for each contact, and the electron doesn't get "intel" about other things going on except via slower than light transmission. Wnt (talk) 01:03, 21 May 2017 (UTC)
In case it makes a difference, I'm not talking about the ladder itself being a conductor in contact with the doors, just there being a circuit that runs through the ceiling, floor and doors, so that it is closed when and only when both doors are closed. Guy Macon's answer above -- which seems to indicate that the circuit can be divided into two halves, and that current can flow through one half while the other half is open -- seems to be the most relevant so far. (Another way to think about it might be a DPDT switch, with the fixed terminals connected to the terminals of the power source, the 1P1T and 2P2T terminals connected to opposite ends of the load, and with the 1P2T and 2P1T terminals left open.) NeonMerlin 05:40, 21 May 2017 (UTC)
Apologies for mis-reading the circuit design. I agree that Guy's answer is most relevant. We haven't answered your question about electrical resistance in the circuit which one would expect to be proportional to the length of wire (assuming the majority runs from front to back of the garage), so one would expect it to appear reduced in the frame of the moving ladder? Will the current appear proportionally greater to fit Ohm's law? Dbfirs 06:46, 21 May 2017 (UTC)
That's a funny point. We can consider any circuit for this, without worrying about switches - even the light bulb in your room can be considered from a relativistic frame of reference, and the math ought to hold up. Now it should be clear that the number of charge carriers, and the value of each charge, is invariant under any transformation, so the total number of coulombs passing through a circuit between two recognizable events stays the same. Time dilation however means that this charge will go around the circuit more slowly. Since the carriers move, in some directions calculating the exact amount of time dilation could be tricky, but I think we can approximate that the electrons' speed is too slow to matter much. Given that approximation, note that the current appears slowed (i.e. reduced, a factor of sqrt(1 - v^2/c^2) ) regardless of which way it is moving because the whole object is subject to time dilation; however, the circuit is only shortened in one direction! By a factor (Lorentz contraction) of sqrt(1 - v^2/c^2). So when the electrons are moving front to back, the time to make the trip increases as quickly as the length decreases, and so the current is the same. Yet when they are moving along the front or back of the barn, then the time is longer for the same distance and so the current seems reduced! Now the current could be flowing in either direction perpendicular to the relativistic velocity, so the potential of left and right can't be altered one relative to another, even if there were a reason for it. It is possible that there is some aspect of relativistic electromagnetism that I don't understand that can be used to model a different magnetic field that somehow affects the current... but at the moment I'm not thinking of how to explain an apparent increase in resistance from left to right. Wnt (talk) 11:33, 21 May 2017 (UTC)
See Multiway switching#More than two locations. The speed of current is the speed of light. The ladder paradox can not apply to a circuit using simple switches. But and wire has a capacity and inductivity when current flows. Turning on a light bulb, the filament needs to be heated up before light is emitted and cooled down to stop the light emission. Speed limits of every computer are the physical properties of the used circuits. A master-slave-flip-flop (electronics) shows how dependent switching speeds are inside a circuit. If the inverter is slower than the NAND gates, the input will rush though the whole circuit at once. --Hans Haase (有问题吗) 13:10, 21 May 2017 (UTC)

May 21

What is the exact number of hormones in human?

What is the exact number of hormones in human? Is there a short answer or it's complicated issue? By searching I found the article here: List of human hormones. Then I read it and I found that it mentions 4 hormones only under the category of amino acid derivatives while there is no mention for norepinephrine and dopamine which are also considered to be amino acid derivatives, at least according to this table and this academical site. for example. In addition, histamine as well should be there, according to the last source since it is made of glutamic acid. Could I relay on this article (wiki) when counting the exact number of hormones in human body? 93.126.88.30 (talk) 02:41, 21 May 2017 (UTC)

50.4.236.254 (talk) 03:39, 21 May 2017 (UTC)

Comment: I think Joseph Fermin gives an approximate answer, because science is an ongoing field with tentative facts. 50.4.236.254 (talk) 04:38, 21 May 2017 (UTC)
Thank you for you comment. Anyway, I made googling before putting my question here, and I saw this answer as well, but it doesn't answer my question if you look at it well. (what is the exact number which is known for the moment) in addition to some points that are not clear about Catecholamines and histamine in the context of hormone) 93.126.88.30 (talk) 05:22, 21 May 2017 (UTC)
The article talk page discusses norepinephrine and dopamine. They were not included because they were considered neurotransmitters not hormones. Definitions of hormone may vary adding to the difficulty. Rmhermen (talk) 12:43, 21 May 2017 (UTC)
  • There isn't really a definite number, because of some ambiguities in definition. For one thing, some hormones can appear in variant forms, and it isn't clear whether they should be counted as one entity or several. Also, some act only within a limited part of the body, such as the lymph or the hypophyseal portal system. Looie496 (talk) 14:30, 21 May 2017 (UTC)

Heyday of immune system

Approximately when the human immune system was able to fend off the majority of viral pathogens (including influenza) before viruses evolved to avoid and bypass it? That said, when, for example, anatomically modern humans emerged, were viruses already mutated/adapted by that time or it happened much earlier? Brandmeistertalk 07:16, 21 May 2017 (UTC)

Never. Count Iblis (talk) 08:33, 21 May 2017 (UTC)
The typical person today can fend off viral pathogens for ~70 years, give or take. Any virus that doesn't kill you is one that your body manages to either fend off, or at least fight down until it is controlled. The immune system is actually quite good at conquering most invaders. Dragons flight (talk) 10:27, 21 May 2017 (UTC)
It's safe to say that the relationship of pathogens and immune system well predates humans, since there are so many comparable diseases of other mammals. A key difference is social - humans now survive in larger communities - and above all technological - jet airplanes moving things around from one continent to another, or on the other hand of course vaccines. Influenza is quite effective at killing birds, which potentially takes us back before the dinosaur/mammal split, though I won't assert with any confidence that influenza dates back that far, only the relevant vulnerabilities. Known subtypes of influenza seem to have diverged only thousands of years ago [1] but I don't know what came before that. We have an article evolution of influenza but it is missing far more than it contains. The, uh, "good" news is that some bright technicians have been able to isolate DNA from hundreds of thousands of years ago in soil [2], so bio-warriors the world over can dig through sediments from hundreds of thousands of years ago and introduce us to whatever unknown predecessors of influenza existed; maybe we'll see how well they fare against modern immune systems. (Caveat: influenza is RNA, which ought to be much less likely to survive - DNA viruses are certainly better targets for this) Wnt (talk) 11:47, 21 May 2017 (UTC)
The arms race between the adaptive immune system and viruses began when the former first appeared in early vertebrates, about 550-600 million years ago. Ruslik_Zero 13:13, 21 May 2017 (UTC)
Perhaps even much earlier, in a sense, since bacteria have an adaptive immune system of their own -- the CRISPR mechanism. Looie496 (talk) 14:22, 21 May 2017 (UTC)
If anytime, that heyday is now, worldwide. People with better nutrition and general health seem more able to fight off diseases on their own. However, in the US and a few other places, due to poor diet and lack of exercise, we may now be headed back down. Also, artificial immunity due to vaccinations, and lack of contact with pathogens due to improved sanitation, may cause our natural immunity to decline. Note that new diseases like AIDS don't indicate that our immunity has declined. Had those existed thousands of years ago, they would have been even more deadly. StuRat (talk) 16:08, 21 May 2017 (UTC)

Power consumption of black and white screen

How does a black and white screen power consumption compare to a color consumption? --Hofhof (talk) 15:02, 21 May 2017 (UTC)

About the same, these days. The technology used may matter, though. With an LCD screen the backlight consumes most of the energy, and it's either always on, or varies with the scene brightness, but not with the color. Old CRT screens might have had more difference. StuRat (talk) 15:27, 21 May 2017 (UTC)
Brightness only increases the power consumption when the pixel is the lamp itself. This applies to CRT and LED (also OLED) displays. LCD/TFT use backlight. Projectors use though (TFT) or reflected light (DLP). Backlight should be based on energy efficient LED technology, today. Older models used CCFL. Compared to a light bulb, CCFL needs about 50%, LED 10 to 20%, of the bulb. --Hans Haase (有问题吗) 17:25, 21 May 2017 (UTC)
There are still millions of glass Cathode ray tube (CRT) screens in use on Television sets and Computer monitors. A color CRT contains three electron guns and less than 1/3 of each gun's cathode current contributes to the picture brightness. The remaining 2/3 of current is intercepted by a Shadow mask. In contrast a black and white CRT has no shadowmask, only a single gun and nearly all its cathode current contributes to the picture brightness. The color CRT display also consumes higher power in its Deflection yoke around the larger diameter tube neck and in circuits for Chrominance signal that the monochrome display does not need. Blooteuth (talk) 16:13, 22 May 2017 (UTC)
I'm no expert, but I would expect that the power requirements for backlit LCD monitors would also be roughly 3:1 between RGB and monochrome to achieve the same effective brightness. A monochrome LCD panel could theoretically pass 100% of the available backlight (or maybe 50% after accounting for polarization). An RGB panel would have color filters in place that would only pass a about one-third of that polarized white light through each of the red, green, or blue sub-pixels. The result would be that for the same backlight intensity, the RGB panel would only be roughly 1/3 as bright. Alternately, to achieve the same effective brightness, the RGB backlight would have to be 3 times as bright. -- Tom N talk/contrib 02:35, 23 May 2017 (UTC)
I don't follow your argument. For a white scene, all 3 of the filters would be open, so the brightness would be the same. For a scene which is all red, blue, or green, then only 1/3 of the light would get through (although reflections may allow some of the light blocked by one filter to pass through another). But, on a black-and-white set, a red, blue, or green scene would also be displayed at reduced brightness. Also note that CRT screens often were brighter than LCD screens. StuRat (talk) 02:42, 23 May 2017 (UTC)
How can the brightness be the same when in one case each individual pixel is blocking ~66% of the light while in the other case each individual pixel is blocking ~0% of the light? --Guy Macon (talk) 03:54, 23 May 2017 (UTC)
OK, I see what you're both saying now. StuRat (talk) 03:59, 23 May 2017 (UTC)

Continuity equation for fluids

Can someone explain why is valid because of conservation of mass?

How did we get from conservation of mass to conservation of mass per time? All I know is , but I don't see why to divide by .

BernoullisLawDerivationDiagram.png

יהודה שמחה ולדמן (talk) 15:43, 21 May 2017 (UTC)

This is assuming a constant volume flow rate. In the case of pipes of varying cross-sectional area all connected up as shown, that's a good method to determine the velocity. However, if the volume flow rate is changing, the calcs would need to be changed. Moreover, with a compressible fluid, you could also get pressure waves moving through the system, even if the overall flow rate is constant. So, those equations have their place, but shouldn't be used everywhere. StuRat (talk) 15:52, 21 May 2017 (UTC)
Yeah, I know we are assuming that , but why are we allowd to divide by and get .
Even so, in the case of - how can you prove that the fluid doesn't go faster than ? How? Conservation of momentum/energy? Let me know. יהודה שמחה ולדמן (talk) 18:52, 21 May 2017 (UTC)
is no divisor but a multiplier. Also you can not combine both terms cause they are not equal because . --Kharon (talk) 20:21, 21 May 2017 (UTC)
What? What is the last thing?
Still, if - how can you prove that the fluid doesn't go faster than ? Conservation of momentum/energy? יהודה שמחה ולדמן (talk) 23:57, 21 May 2017 (UTC)
@הודה שמחה ולדמן: Its average velocity won't be faster than but there definitely could exist local areas of higher velocity.--Jasper Deng (talk) 02:01, 22 May 2017 (UTC)

An easy way to derive the continuity equation is to apply the divergence theorem. The mass flux and velocity fields are parallel with constant density (i.e. incompressible flow) so the derivation proceeds similarly for both mass and volume conservation in that case.--Jasper Deng (talk) 02:10, 22 May 2017 (UTC)

  • Assuming v1, v2 are constant in time and the same over their respective cross-sections, consider the volume between A1 and A2. In a time , it gains a mass flowing in at A1 and loses flowing out at A2 (corresponding volumes are in light blue on the figure). With the steady-state assumption, the inner mass must be constant hence those quantities must be equal, and the time cancels out. (This is true for any , thus in particular you can choose a which allows you to divide.) TigraanClick here to contact me 11:15, 22 May 2017 (UTC)
I'm not convinced.
You're Assuming are constant. Why should I believe it's true? How do I know the flow through is actually and not some larger ?
Let us assume the fluid is non-viscous and non-compressible. is of course the volume, but who ever said the fluid has to flow at the same rate through both cross-sections of the pipe?
Again, what if the volume at has more energy/momentum such that ? יהודה שמחה ולדמן (talk) 12:25, 22 May 2017 (UTC)
Because the continuity equation does not apply in the absence of the steady-state assumption. Also (with the steady-state assumption), if the fluid does not flow at the same rate through both cross-sections then there is a net sink or net source in the segment of the pipe, which is not allowed.--Jasper Deng (talk) 15:18, 22 May 2017 (UTC)
Well, with the additional assumption of an incompressible flow, it does apply to nonconstant v1 and v2. Same argument, the mass balance becomes , valid for any integration time bounds (but only because the constant trapped volume means there is a constant mass as well, i.e. no sink or source), hence you can drop the integration sign (assuming some mathematical hypotheses on the continuity which should be verified in a real physical problem). TigraanClick here to contact me 15:52, 22 May 2017 (UTC)
This is part of Bernoulli's equation in fluid dynamics which is probably part of any higher technical education. Its best to learn it from ground up. Some parts may be boring at start but starting somewhere in the middle seem not working well. So there are masses of books and webpages, probably even videos, you can use to learn how Bernoulli developed it and how its applied to calculate the flow, temperature, pressure, volume, exit velocities etc. of contained fluids and gasses. --Kharon (talk) 16:45, 22 May 2017 (UTC)

May 22

Brush for painting walls

What kind of brush was used before the invention of the Paint roller and what was it's name? In Romania, the paint roller was not used before the fall of communism (1990). The painters used a big and thick brush called bidinea - the word comes from the Turkish badana. It looked pretty much like an American Wall-paper brush. I wonder if Wikipedias in other languages have an article for this item. —  Ark25  (talk) 01:05, 22 May 2017 (UTC)

Paintbrush#Decorators' brushes has some information, and a picture. Rojomoke (talk) 04:01, 22 May 2017 (UTC)
I still use them (for better coverage on rough surfaces), but just call them large paintbrushes. Dbfirs 08:52, 22 May 2017 (UTC)
  • They're still used today, made much the same as they always were. A wallpaper paste brush is easier to find and very close in style (if of the traditional style).
Masonry walls are alkaline, which will damage many paints. So "wall paint" has historically been alkaline itself, either "whitewash" or "distemper" (a very thin lime wash). This was applied with a distemper brush, a large brush itself resistant to alkali, so it was tied together mechanically, rather than the bristles glued in. Owing to their size (often 6" wide) they were usually a hollow core, i.e. the bristles were placed on two sides of a wooden centre, with two wood sides nailed or riveted on (rivets stay tight longer in wood that's being continually wetted). As these crude washes will never give a particularly smooth finish, they never needed a very high quality brush.
The manufacture of paint brushes isn't a simple "one style makes all", and some large or specialised brushes get very expensive. Japanese paper-making brushes can be a few hundred $ each, mostly because they're so rare and are handmade. French oval varnish brushes became unavailable a few years ago, when the last machine that made the ferrules was sold. Andy Dingley (talk) 10:43, 22 May 2017 (UTC)
Here's one type of distemper brush, a WWII ex-RAF one: https://hatchfive.wordpress.com/2015/06/
This one is (as described above) mechanically bound, by the bristles being string-tied bundles. As such bundles are always round (or slightly oval), a wide brush is made by putting pairs of bundles onto the handle. Andy Dingley (talk) 21:46, 23 May 2017 (UTC)

Eliminating infectious diseases by bioengineering large numbers of brainless humans with weakened immune systems?

As pointed out here:

"How do the principles of virulence help to explain the enigma of invertebrate immunity? One answer is that invertebrates don’t need an acquired immune system because they never had it. The parasitic agents of invertebrates have not coevolved with acquired immunity so their virulence is calibrated to the coevolved innate immune system. The proposal here is that contrary to widely held views of practicing immunologists, the immune system is not evolutionarily selected to prevent infection in an absolute sense. Rather, it is selected to make one individual slightly more resistant or at least different than others of the same or related species. The adversary of any individual is not really the world of parasites, they are truly undefeatable, it is his or her neighbor. A zebra doesn’t have to outrun the lion, just the slowest member of the herd."

This then suggest that we could benefit from creating vast numbers of biological systems that can be infected by the same microbes that infect us, but with weaker immune systems. On the long run this would cause the microbes to evolve to infect these systems instead of us. Or you could say that since the relevant competition is between us, we put ourselves collectively in the winning position by creating a large number of losers. This will then work because we're then going to keep the losers alive artificially. For ethical reasons one would then be led to creating brainless humans, or just large volumes of human tissues that can be kept artificially alive in some way. Count Iblis (talk) 01:30, 22 May 2017 (UTC)

No, this logic is flawed. It would only work if regular humans no longer interacted with each other, but only with the brainless easy prey-humans, in which case you don't even need them, just keep the regular humans isolated from each other. Anywhere a large pool of regular humans existed, microbes would still evolve to take advantage of them. Those microbes have no way of "knowing" that easier prey exist elsewhere, and no way to get there, in any case, so they will continue to evolve to take advantage of their current hosts. StuRat (talk) 01:36, 22 May 2017 (UTC)
I don't think StuRat's argument is valid, so long as the prey-humans have enough contact with real humans to transmit disease to them. The prey-humans would fulfill a role broadly similar to that of plants in crop refuges, which someone should probably blueify. See eg [3] and [4]. Adrian J. Hunter(talkcontribs) 10:58, 22 May 2017 (UTC)
As some day it may happen that a victim must be found, I've got a little list... The OP's biological application of Chamberlain's Appeasement method against microbes should ensure "Health For Our Time", what could possibly go wrong? But it really doesn't matter whom you put upon the list, For they'd none of 'em be missed. SdrawkcaB99 (talk) 11:06, 22 May 2017 (UTC)

I can think of a more practical context for a related idea in the context of antiviral drugs for influenza. Drugs like oseltamivir can result in resistance, the prevalence of which is limited by the larger number of people who don't have immediate resort to a doctor to wave the magic pen unlocking their access to the drug, who presumably serve as a sort of susceptible reserve population like the GMO-free band around a Roundup Ready cornfield. Stockpiling antiviral medications for pandemic influenza implies such decisions with the limited stockpiles. But I haven't actually looked into what has been worked out about all this scientifically... Wnt (talk) 14:01, 22 May 2017 (UTC)


Thinking a bit more about this.... According to the article one would explain the fact that young people rarely get diseases like pneumonia while it's not unusual for old people to get it, not by saying that young people we have such good immune systems but because old people bring down the average that the microbes have adapted to. If immunity in old people were to decline less, then infectious diseases would occur at similar rates as they do now but it would be more evenly divided between old and younger people. One may then postulate that the aging process could have been an outcome of natural selection. Not only would biology not have come up with a fix for the aging processes, it may actually have fine tuned it in such a way as to steer away diseases from the young in an optimal way. Count Iblis (talk) 01:02, 23 May 2017 (UTC)

Why epinephrine is discriminated from nor-epinephrine and dopamine by the definition of hormone?

Why epinephrine is discriminated from nor-epinephrine and dopamine by the definition of hormone? I've read the talk on the article "list of hormones" and I saw that the same answerer there, made two different comments... in one he said: "The catecholamines such as epinephrine, etc. are classified as neurotransmitters, not hormones." and later after someone comment on his answer he said "Epinephrine has significant distant effects from the adrenal glands where it is synthesized and therefore it is legitimately classified an hormone... The rest of the catecholamines such norepinephrine appear to act primarily as neurotransmitters and therefore have not been added to the table.". and I really didn't understand his discrimination between epinephrine, norepinephrine and dopamine. I would like to understand it. 93.126.88.30 (talk) 12:44, 22 May 2017 (UTC)

Neurons send neurotransmitters at the synapse between the axon terminals and dendrites. Hormones may use the circulatory system or the extracellular fluids to deliver signals, near and far. Sometimes, the boundary between a hormone and neurotransmitter is not very clear. 140.254.70.33 (talk) 13:24, 22 May 2017 (UTC)
Epinephrine and the other catecholamines are all made by neuroendocrine cells. The difference in whether a specific catecholamine is described as a neurotransmitter or a hormone is its primary function.
Quoting from our article neuroendocrine cell,

"Neuroendocrine cells are cells that receive neuronal input (neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release message molecules (hormones) to the blood."

Neurotransmitters provoke the production of hormones - they are the body's signal to produce hormones and drive organs and other groups of cells into action. However, both general classes of biomolecules can act centrally on the brain and also in the neuroendocrine system distributed throughout the body. The extent to which this happens wasn't always understood when molecules were being classified as one or the other. "Neurotransmitters" tend to have a much shorter duration of action in the body than "hormones", which is probably what the author of that article was getting at. loupgarous (talk) 23:30, 23 May 2017 (UTC)

Histamine is considered as hormone or not?

Histamine is considered as hormone or not? In our article here (histamine) as well as in the list of hormones- no mention about its definition as hormone but on other places it's considered as an hormone as well. Wikipedia normally doesn't take a side but just share with the readers the different opinions objectively. For example in this encyclopedia it's considered as an hormone.93.126.88.30 (talk) 12:43, 22 May 2017 (UTC)

If you look up hormone, there are two problematic aspects to the definition the article gives: hormones are secreted by glands, and they are transported by the circulatory system. This is a typical definition but in biology definitions tend to creep. Are individual immune cells secreting histamine from wherever part of a "gland"? Well, is a gland defined by a visible morphology, or is anything that secretes a hormone a gland, making the definition tautological? Then there is the circulatory system aspect. Localized inflammatory reaction does involve the circulatory system. But I think usually when people think of a "real hormone", they mean something that you can take out a vial of blood from whatever site is most convenient to measure it, even if the nurse has to pull out a junkie's penis hunting for a suitable vein. But histamine's effect is, by design, local, and the overall blood level, if there is one, likely doesn't represent the intensity of its effect where it matters. There are some relevant concepts that come up if you look at endocrine, exocrine, paracrine, apocrine etc. Wnt (talk) 14:11, 22 May 2017 (UTC)
Based on encyclopedia.com's ridiculous article about homeopathy I wouldn't trust it for anything.
I agree with Wnt that a hormone, as I understand the term, is a molecule that works by endocrine signalling, meaning it's transported throughout the body rather than acting locally (paracrine signalling). Histamine's best-known effects are local. But histamine has many effects, some of them apparently systemic (eg), so perhaps it acts as a true hormone in addition to its better-known effects. Also, even peer-reviewed articles sometimes speak of "paracrine hormones" (pubmed), so there's inconsistency (or at least sloppiness) in how people use the term "hormone". Adrian J. Hunter(talkcontribs) 14:25, 22 May 2017 (UTC)

How can gunpowder propel something over Mach 1?

If it doesn't burn faster than the speed of sound? Sagittarian Milky Way (talk) 15:32, 22 May 2017 (UTC)

Newton's second law. --Jayron32 15:35, 22 May 2017 (UTC)
  • I think the question is misguided. Gunpowder does indeed burn "slowly", i.e. it makes no detonation, but that defines how fast the combustion front can move, which has no direct relation to the speed of a projectile. The latter is propelled, not by the combustion front itself, but by the heated gases trapped (e.g. in a gun's rifling), hence Jayron32's taciturn answer.
However, there is still a nontrivial question: as the projectile accelerates and reaches the speed of sound, the heated gases are no longer at pressure equilibrium. Said otherwise, the projectile will "outrun" the pressure that is trying to accelerate it, and hence the acceleration should stop at the speed of sound (actually, a bit slower). But then it is the speed of sound in the heated gases, higher than at room temperature (the projectile will move in the air). TigraanClick here to contact me 15:45, 22 May 2017 (UTC)
Oh right. Duh. Sagittarian Milky Way (talk) 16:05, 22 May 2017 (UTC)
The important thing is that there is no law of conservation of velocity, but there is a law of conservation of momentum... --Jayron32 16:19, 22 May 2017 (UTC)
There is no law on how many percent of the momentum goes to the bullet, though. If a rifle's illegally sawed off to a 2 millimeter barrel the bullet will not leave as fast.[citation needed] Sagittarian Milky Way (talk) 16:55, 22 May 2017 (UTC)
Again, I have no idea how to respond when you write such confusing statements and non-sequiturs. It makes it difficult to help you find answers to your questions. --Jayron32 17:01, 22 May 2017 (UTC)
What he wrote is pretty straightforward - if we suppose that the expanding gas were to stop at a certain velocity, then the bullet would carry only a fraction of the momentum and the remainder would stay with the gas as it leaves the gun later on. Really, F=ma seems more of a non sequitur since the whole issue is whether the gas can keep up with the bullet to apply any force to it past a certain point.
On second thought, I find myself wondering as I think about that is whether "speed of sound" is relevant to the expanding gasses in the gun barrel. I mean yes, it can limit the speed of disturbances in air, but those gasses aren't moving through air, they are the medium. If the gun barrel were perfectly smooth, then we can think of the gas right behind the bullet as stationary in its own frame of reference! Maybe what we need to be looking at is how a bullet or any other kind of fast-moving piston can move down a bore faster than the speed of sound in the air in that bore. How does the air get out of the way faster than the speed of sound? (It's time for me to look up shock wave and start from scratch, because I don't know this!) Wnt (talk) 18:18, 22 May 2017 (UTC)
Air is highly compressible, so can just compress until the end of the barrel is reached, then move to the sides and decompress. Of course, this will create a bang. StuRat (talk) 19:19, 22 May 2017 (UTC)
Which bang is known as the bullet crack. It is also called a bullet bow shockwave; but in article one thing I do not get is when and where a witness will not be able to hear the cracking sound produced by the bullet. --Askedonty (talk) 19:38, 22 May 2017 (UTC)
Infact is seems Mach 6-9 is typical for explosions. See article Detonation velocity. --Kharon (talk) 17:02, 22 May 2017 (UTC)
@Kharon: Not all explosions are detonations. See also high explosive, which gunpowder is not. TigraanClick here to contact me 09:35, 23 May 2017 (UTC)
The essential item in this discussion is the law of conservation of momentum. Confined in the gun barrel, the pressure behind the bullet as the gunpowder (smokeless powder, for high-velocity rounds, actually) charge imparts momentum to the bullet from the expanding gases, which at the point the bullet leaves the bore of the gun barrel is what propels the bullet forward. A mathematical summation of the forces acting on that bullet would show the cumulative force on that bullet over the amount of time it takes for the gases to push the bullet out of the barrel, plus the wavefront of the escaping gases pushing the bullet afterward.
The difficulty the OP had was imagining the bullet being pushed by an unconfined wavefront of expanding gases, outside the barrel, when most of the velocity and momentum imparted to the bullet are transferred during a time interval when the bullet is being pushed through the barrel, some of the gases's energy is dissipated as heat from friction between the barrel bore, the lands of the rifling, and the bullet, heat from the powder's combustion inside the barrel but mostly the linear transfer of energy from the expanding gases against the barrel and the bullet.
As the bullet "gives" and the barrel doesn't, most of that pressure moves and accelerates the bullet for that fraction of a second it's in the barrel to make it travel faster than sound. The momentum imparted to the bullet's mass is where most of the force of the exploding powder and its violently expanding gases goes while the bullet is in the barrel.
Since it is imparted over a period of time, more of that momentum's pushing the bullet away from the explosion than it would be if the bullet were just placed at the edge of a charge of gunpowder detonated outside a gun barrel - the barrel "focuses" the momentum mostly toward the bullet, so it moves faster than the molecules of the expanding gas. loupgarous (talk) 00:13, 24 May 2017 (UTC)

What kind of voltage regulator is this?

The voltage regulator article discusses many different kinds of voltage regulator. What [kind of regulator is this] so that I can read about that one specifically? Does it turn all the unwanted energy to heat? So 1 A @ 5 V from a 12 V supply would give 7 W heat? Thanks! ----Seans Potato Business 22:25, 22 May 2017 (UTC)

The the specs say it includes "thermal shut-down control" does imply that it's converting excess energy into heat. (I find that they rarely say so outright, as this is obviously an inefficient and non-ideal way to do things.) StuRat (talk) 23:06, 22 May 2017 (UTC)
I think the way to approach this question is to begin with simple conservation of energy, as we do when first considering power transformers, gear trains etc. If the output is 1 A at 5 V, that is a power of 5 W. Assuming simple conservation of electrical energy, the input would be 12 V and 5/12 A (or 0.417 A). Power transformers and gear trains operate with a certain amount of the input energy leaving the system as unwanted heat, and it would be the same with this voltage regulator. Assuming 20% of the input energy is lost to heating of the surroundings leads to the conclusion that the input current will be 1.2 times 0.417 A (or 0.5 A). Input power will be 6 W and output power will be 5 W, so energy will be lost to heating of the surroundings at the rate of 1 W. Dolphin (t) 23:35, 22 May 2017 (UTC)
(edit conflict)::See 78xx. User:StuRat is correct in that the 78xx series dissipates excess power as heat, but his reasoning is a bit off. For example, switching regulators (which do not work by dissipating excess power as heat) often include thermal shutdown capability. Note any voltage regulator that is less than 100% efficient will dissipate some energy as heat and therefore may be susceptible to thermal overload. Physics being what it is, all regulators are less than 100% efficient. Shock Brigade Harvester Boris (talk) 00:55, 23 May 2017 (UTC)
I'm rather surprised that overheating is a concern for those. Of course, some heat will be generated by other methods, but it seems far less likely to be more than the device can handle, not being on the same order of that from a device where 100% of the unused energy goes to heat. StuRat (talk) 01:06, 23 May 2017 (UTC)
Clicking on the datasheet option and looking at the block diagram, it's apparent (if you're used to looking at this sort of thing) that the series pass element is Q17 and R11 and most of the waste heat will be dissipated in these two elements. So for output of 5 V and 1 A, about 7 W will be dissipated in Q17 & R11. A small amount of power will be dissipated in the rest of the regulator. Jc3s5h (talk) 00:26, 23 May 2017 (UTC)
The device is a linear series regulator and is mentioned here. Jc3s5h has answered correctly; the small amount of power additional to the 7W dissipated in Q17 & R11 is (Id x 12) W where Id is the quiescent current to ground, about .004 to .006 A according to the data sheet. Dolphin's answer incorrectly supposes conservation of energy but the device is only 41.5% efficient and wastes 58.6% of input energy as heat. Thermal protection is provided in case someone fails to provide the Heat sink that the device needs. Blooteuth (talk) 01:18, 23 May 2017 (UTC)
You are looking for a 7805 linear voltage regulator in TO-220 package. The 1.5 amperes may be the shortcut peak current. The 78xx series have some special variants like 78L05 or 78S05. Theres a 100 mA variant in TO-92 package. There are 2 and 3 ampere variants avail. Note: Never clamp linear regulators in parallel. Such circuits cause output voltages on the input voltage level and oscillating output similar to a noise amplifier or function generator. Indeed the heat is the power drop in the regulator device 7 W = (12 - 5 V) * 1 A. This are 7 watts heat spread over a necessary heat sink. This series usually require 3 volts more input than output to generate a proper output voltage level. A socalled low drop regulator needs less volts to operate properly. The maximum input voltage is 37 volts. When the heat drop exceeds its maximum, have another circuit before to generate 8 volts for the 5 volts regulator. Depending on the application, a stepdown converter, also called buck converter which is a switching mode voltage regulator. It might be more energy efficient, but for some applications too noisy on the output. For this issue, have the linear regulator behind the buck converter. Switching mode voltage regulators can reach an energy efficiency up to 97 %. Another example are Class-D amplifiers, using an audio input as reference voltage. German Wikipedia has an overview of switching mode voltage regulators, see de:Schaltregler. Most linear regulators have 3 pins. A switching mode regulator needs an extra input pin to sense the output. Depending on the type a voltage divider can be installed there to make the circuit generate another output voltage. --Hans Haase (有问题吗) 17:39, 23 May 2017 (UTC)
Dolphin's answer is completely wrong, and StuRat's is not helpful. Series regulators of the LM-7812 and LM-7805 type (which is the type the OP asked about) regulate the output voltage by wasting the excess. If you connect an LM-7805 to a 12 volt DC source, it will provide a regulated 5V output. Very little energy will be dissipated in the device when no load is placed on the output, since the device consumes only a few milliamps itself. When connected to a 12 volt supply and 1 Amp is being drawn by the load, the device is dropping 7 volts @ 1 Amp, which is indeed 7 Watts, answering the OP's question. The data sheet in the link does not mention the device's power dissipation, but these devices are typically rated at 5W, and at 7W it should therefore be mounted on a suitable heatsink. The alternative is to drop the input supply to 10V, when 5W will be dissipated. I don't recommend going lower, as these devices have a threshold of about 2.5 volts above their stated output, and operation becomes erratic when the supply drops below that threshold. Akld guy (talk) 20:40, 23 May 2017 (UTC)
The words of the datasheet "If adequate heat sinking is provided..." require the designer to do a proper thermal calculation. For the TO-220 package, Table 2 Thermal Data indicates we need a heatsink with thermal resistance no more than Rthj-amb - Rthj-case = 50 - 5 = 45 °C/W. If the ambient temperature is 25° then we must not dissipate more than (150 - 25)/50 = 2.5 W because 150° is the absolute maximum junction temperature. Our need to dissipate 7W therefore demands a bigger heatsink, certainly one with no higher thermal resistance than 12.86 °C/W. However a responsible engineer would not settle for a design on the edge of self destruction and would lay in a healthy safety margin against eventualities such as increased ambient temperature and poor ventilation. His options include choosing a bigger heatsink with lower thermal resistance, choosing the 2-bolt TO-3 package for better heatsinking and reducing the device dissipation with an external dropping resistor as shown in the datasheet Fig. 32. (If the load happens to be a 5 ohm resistance and the supply is exactly 12V then a 7 ohm Resistor could substitute for the whole regulator.) Blooteuth (talk) 01:06, 24 May 2017 (UTC)

May 23

Chemistry - Chloro-amphetamine

Hello, If "Chloro-amphetamine" as well as any of its ortho/meta/para-substitutions are to be called "the CA group". Chemistry speaking, will it also include the chemical "DOC"(2,5-Dimethoxy-4-chloroamphetamine)?

Thanks for the help and sorry for my grammer, Dave. — Preceding unsigned comment added by 79.181.165.55 (talk) 09:32, 23 May 2017 (UTC)

I did a quick web search and saw a paper using the term here. Actually the term is "4-CA group", for 4-chlorinated amphetamine derivatives. Two examples they give there are 4-chloro-N-methylamphetamine (CMA) and 4-chloro-amphetamine (4-CA). Arguably, 2,5-Dimethoxy-4-chloroamphetamine is also a 4-chlorinated amphetamine, therefore chemically (since that's what you ask) part of the 4-CA group.

That said, putting oxygens on the back end of an amphetamine may chemically be a substitution, but practically, oxygenation is the relevant difference between derivatives of the amino acids phenylalanine, tyrosine, and L-DOPA. It's not that neat to explain here since (like other 4-CA group) it has a chlorine where tyrosine and DOPA have a hydroxyl, and an oxygen at a 2-position that doesn't come up much in nature that I can recall. The archetypal contrast where the aromatic ring is concerned is between meth and MDMA; it can make a difference. Anyway, all this sort of chemical logic is questionable under the best of circumstances - every compound is pretty much its own experiment, and there's no actual guarantee that what happens from a new one will make much sense a priori, not unless you have a structure for every possible catecholamine receptor and can model them all simultaneously, and I doubt if that would work either. Wnt (talk) 18:39, 23 May 2017 (UTC)

Sugar in baked beans

Apparently baked beans make people fart because the sugar in them are not digestible by humans. Does that mean even though a can of baked beans lists it as having 20g of sugar none of that sugar actually enters the blood stream and is all used up by bacteria producing gas in the large intestine? Or do the nutritional values on food packaging only list sugars that can be digested? — Preceding unsigned comment added by 59.44.40.209 (talk) 13:13, 23 May 2017 (UTC)

It is not the added sugar, which is in the form of sucrose, glucose, or fructose; all of which are highly digestible. There are different sugars in the beans, specifically sugars such as stachyose and raffinose, which make it to your intestines and make you fart. These sugars are in the beans themselves, and can make you fart even in applications that don't involve added sugar, such as chili. --Jayron32 13:16, 23 May 2017 (UTC)
I remember watching a food-related documentary, Michael Pollan's In Defense of Food, and one section mentions that the indigestible sugars known as dietary fiber are food for the bacteria, which then secrete gaseous waste products, which make you expel flatulence. 140.254.70.33 (talk) 15:13, 23 May 2017 (UTC)
Not exactly the same thing, while both dietary fiber and sugars are both carbohydrates, "sugars" are usually taken to be relatively small, either mono-, di-, or tri- saccharides, fiber is a very long-chained polysaccharide. Dietary fiber could also be the source of gas, depending on a person's intestinal flora. --Jayron32 18:46, 23 May 2017 (UTC)
As to whether those sugars are included on the label, there may not be a universal answer. It may depend on the actual methods and devices used, such as a calorimeters. According to [5], there's only 1 g of "sugar" in a 177 g sample, so I have to think they are excluding most of the indigestible sugars. StuRat (talk) 16:15, 23 May 2017 (UTC)

Acidity and preservatives

Often, recipes instruct newbie cooks that lemon juice can prevent the discoloration of the avocado or apple. The fruit is still edible, but the ugliness of the color will probably be less appetizing. In regards to acids and bases, can vinegar and very dilute hydrochloric acid work just as well? How acidic does the acid have to be to avoid burning or dissolving the esophagus? What about bases? Can bases be used to preserve foods too? Instead of lemon juice, what happens if sodium bicarbonate is coated on food? 140.254.70.33 (talk) 14:59, 23 May 2017 (UTC)

I would avoid using HCl, since, as a major constituent of stomach acid, it smells "like vomit" to most people. StuRat (talk) 15:43, 23 May 2017 (UTC)
I highly doubt that. HCl ... well, you shouldn't smell it, but it just has a sharp acid smell. People scrub concrete with moderately dilute solutions of it. And when you mix it into anything that is a weak base that can be neutralized, like a food, it will just be chloride ions and a more protonated something. Many of the pills people take contain HCl to turn drugs to the hydrochloride for isolation; recreational alkaloids like cocaine and morphine also use it. Wnt (talk) 18:43, 23 May 2017 (UTC)
For many who have vomited, or been near somebody else who has, that smell is distinctive, especially when mixed with food, and will trigger unpleasant memories. StuRat (talk) 19:49, 23 May 2017 (UTC)
That's probably so because the limbic system, which contains the hippocampus and amygdala, is so close to the olfactory nerve. The sense of smell is a good danger detector. 140.254.70.33 (talk) 20:57, 23 May 2017 (UTC)
"Can bases be used to preserve foods too?" - yes, see Food_preservation#Lye. SemanticMantis (talk) 16:44, 23 May 2017 (UTC)
Note that the example of a browning apple isn't bacterial decomposition, but rather oxidation. I don't know if bases would prevent that. We need a chemist (and I don't mean a pharmacist, for any BrE speakers). StuRat (talk) 16:48, 23 May 2017 (UTC)
I just did a web search and found [6]. This looks well written and plausible. Polyphenol oxidase is indeed the cause of browning - note that polyphenols are a plant compound fairly analogous to melanin - perhaps even homologous, over some vast evolutionary distance, but I don't know any way to prove that. Plants use polyphenols like insects use melanization, to make a pigment and simultaneously protect against infection. Oh, anyway, note that it gives primary credit to ascorbic acid, though bringing the enzyme out of its optimal pH range also helps. Vitamin C is an archetypal antioxidant, and all these polyphenol/melanin processes involve taking aromatic compounds and reacting them with so much oxygen that they turn into a hyper chemically active mess (compare the reactivity of phenol, quinone, etc.) that congeals into an extended polymer. Wnt (talk) 18:46, 23 May 2017 (UTC)

History of Allergy Testing in the 20th Century

Can anyone point me in the direction of information about the history of allergy testing. I've looked over Radioallergosorbent test, which was a little bit helpful but not quite enough. Specifically, I'm seeking to verify the feasibly of an allergy test for meats (chicken, lamb, etc.) that was conducted in the late 1950's-early 1960's. What sort of allergy testing technology existed in that time period for food allergies? Thanks! Helene O'Troy - Et In Arcadia Ego Sum (talk) 15:19, 23 May 2017 (UTC)

You really don't need any technology. Just feed the person a tiny sample of the item, and see if they react. If not, give them slightly more, until you get to a normal dosage (meal). If they still show no reaction, then they aren't allergic to that item. The "give them a tiny amount" part might need to be even tinier amounts in ancient times, though, due to a lack of medical ways to respond to an allergic reaction. StuRat (talk) 15:46, 23 May 2017 (UTC)
This does make sense. However, I'm trying to verify a story about someone "getting a test when I was a teenager" that resulted in a positive allergy to lamb meat. I'm looking for information about what a doctor's procedure would have been in this situation. Helene O'Troy - Et In Arcadia Ego Sum (talk) 16:01, 23 May 2017 (UTC)
Try searching for variants on allergen restriction or elimination diet. There were published diet schedules that eliminated possible allergens in a systematic way. I remember seeing one of these a long time ago and, oddly, in the diet schedule I read through, lamb was one of the last things to be eliminated as it was regarded as having low allergenic potential. Odd the things one's brain saves away for a rainy day. 2001:8B0:1625:41F:0:0:0:36 (talk) 23:37, 23 May 2017 (UTC)
There a skin test method where samples of the substances in question are adhered to the skin with something like the "dot" bandages, remain there for some time, and any skin reaction is seen as a sign of an allergy. However, I'm not sure this method would catch all food allergies and reactions, such as lactose intolerance. Skin tests, including pricking the skin, have been around since the 1860's, so a century before your inquiry: [7]. Note that an allergy to lamb is sufficiently rare that they wouldn't be likely to test for it proactively, but only to verify the problem once somebody becomes ill after eating lamb. (Personally, if eating item X made me sick, I wouldn't bother with tests to verify this fact, I'd just avoid eating it again.) StuRat (talk) 16:21, 23 May 2017 (UTC)
This is reviewed at skin allergy test. I've heard more argument than usual about the accuracy of these tests, but without doing a lot of research I'm not prepared to summarize it in a neutral way. Wnt (talk) 18:24, 23 May 2017 (UTC)

Feynman Lectures. Exercises PDF. Exercise 4-14 JPG1JPG2

. . .

...

4-14. Smooth, identical logs are piled in a stake truck. The truck is forced off the highway and comes to rest on an even keel letgthwise but with the bed at an angle Θ with the horizontal. As the truck is unloaded, the romoval of the log shown dotted leaves the remaining three in a condition where they are just ready to slide, that is, if Θ were any smaller, the logs would fall down. Find Θ.

—  R. B. Leighton , Feynman Lectures on Physics. Exercises
...

We number the logs, as shown in the figure. Consider such a virtual movement of logs, in which the log 3 is moved up the body, as a result of which the log 2 fails between the logs 1 and 3. Let the center of the log 3 be displaced a short distance BB '= Δx. The center of the log 2 will then take the position of C ', and we may assume that the log 2 first moves perpendicular to the body by a distance CK = Δy, and then to the right parallel to the body by a distance KC' - LL '= (AB'-AB) / 2 = Δx / 2. With such movements, the center of the log 3 will rise upward by a distance Δh3 = Δx sin Θ, and the center of the log 2 will drop down by Δh2 = Δy cos Θ - (Δx / 2) sin Θ. But the change in the potential energy of the logs with such displacements must be zero, whence Δh2 = Δh3 (here it is taken into account that the masses of all logs are the same). Substituting for Δh2 and Δh3 their values, we obtain
Δy cos Θ - (Δx / 2) sin Θ = Δx / 2
or
tan Θ = (2Δy)/(3Δx)

We compute Δy. It follows from the right-angled triangles ACL and AC'L 'that CL = R√3 , and .We neglected, as in the previous problem, the terms of the type Δx2, as we extracted the square root, we used the approximate formula , which is valid for x << 1. Thus, Δy = CL - C'L' = Δx/2√3 , and tan Θ = 1/3√3, Θ = 11°.


—  MEPhI , Solutions (Google Translate)

There is second solution from MEPhI (earlier date of publication) which says that upper and rightmost logs must replace each other heights to keep potential energy the same before and after the falling . So Θ = 30 °.
I have made some drawing https://s.sender.mobi/u/image/2017/5/23/lUnPk5ylV/-.PNG
From it, it is clear that angle Θ = 15 °.

It is obvious that the 2nd MEPhI solution is wrong, but I can't find a mistake in the 1st one. Why then doesn't my answer coincide?

Username160611000000 (talk) 19:55, 23 May 2017 (UTC)

The potential energy solution is what came to mind immediately for me, but there's a catch. If the logs fall you have three logs lined up on the bottom (i.e. the dotted log in the diagram is real). If we arbitrarily define the center of the corner log as 0 potential energy, then the one next to it has PE = D sin theta * M, where D is the diameter of a log and M is the mass. But that's present in both configurations and we need speak no more of it. The dotted log has PE = 2DM sin theta by the same logic. The top log looking to fall has energy DM sin (theta + 60) = DM sin theta cos 60 + DM cos theta sin 60 = 2DM sin theta at equal potential energy. Given cos 60 = 1/2 and sin 60 = sqrt(3)/2 then DM sin theta * (2-(1/2) = sqrt(3)/2 DM cos theta, or tan theta = sqrt(3)/(2(3/2)) = 1/sqrt(3), so theta = 30. It's a great solution, except for one catch, which is that the logs don't actually have to reach their potential energy minimum without jiggling (which is why wise physicists do not climb up into crashed logging trucks to admire the beauty of their equations). Wnt (talk) 20:52, 23 May 2017 (UTC)
(edit conflict) Looking at your drawing, I think your method would be correct if log B and log D moved equal distances, i.e. if moving B 0.1 mm parallel to AB would move D also 0.1 mm along DC. However, I don't think that's the case, but log D would slide slower than B would. - Lindert (talk) 21:28, 23 May 2017 (UTC)
Yep. According drawing if the log D goes 1 unit along its trajectory the log B goes √3 units along its trajectory. So we must choose Θ such that √3 Sin(Θ) = 1 Sin(30 - Θ). Username160611000000 (talk) 04:52, 24 May 2017 (UTC)

Have any planes, ships or train cars had cables to increase stiffness or strength?

Older airliners were aluminum cause steel's too heavy so would steel-cable reinforced aluminum have been viable? Or one of those strong polymer cables/wires like Kevlar? Sagittarian Milky Way (talk) 20:30, 23 May 2017 (UTC)

  • Older ships used cables to secure some structures. See rigging for sailboat examples. Here's a later steamship example: [8] (you may need to zoom in to make out all the cables). The German battleship Bismarck also had some support cables: [9].
    Many ancient Egyptian ships were stiffened by one or more cables (though neither steel nor polymer) running lengthwise above the deck, at least according to Björn Landström's interpretation. He shows them used e.g. in a ship from the time of Sahure (3000 BCE), and in ships built 1500 years later by Hatshepsut (likely not in person ;-), both in sea-going ships for an expedition to Punt, and in giant Nile ships used in the transport of obelisks. Egypt never had good timber, so they had to assemble ships from small pieces - ok for compressive loads, but horrible for tensile loads. --Stephan Schulz (talk) 21:37, 23 May 2017 (UTC)
  • I haven't found any pure locomotive examples, but I can imagine a system that needs to raise and lower components off a rail car using cables. Locomotive-based weapons systems come to mind, but I haven't found any of them using cables to do this, yet. A train car with it's own crane is a case I did find: [10], but I'm not sure if that's what you're looking for. StuRat (talk) 20:36, 23 May 2017 (UTC)
Our article says that ancient Greek triremes were stiffened with cables called hypozomata which "would act as a stretched tendon straight down the middle of the hull, and would have prevented hogging". CodeTalker (talk) 01:13, 24 May 2017 (UTC)
You may be interested in learning about the principles of tensegrity. Most of the examples thus far are in that vein. I think you may be thinking more in terms of reinforcement like reinforced concrete or ripstop nylon. I don't think I've ever heard of aluminum being reinforced with steel cable in that manner, though I too would be curious to see if anything like that has been tried. SemanticMantis (talk) 01:57, 24 May 2017 (UTC)
  • And some newer aircraft such as the Boeing 787 Dreamliner make extensive use of composite materials, which use things like Kevlar or carbon fiber in a plastic matrix. -Arch dude (talk) 04:50, 24 May 2017 (UTC)
Thor Heyerdahl built two ships from papyrus, called Ra and Ra II, hoping to prove that the ancient Egyptians could have sailed to the Americas. When the first one fell to pieces, he added "a tether that acted like a spring to keep the stern high in the water while allowing for flexibility". Rojomoke (talk) 04:47, 24 May 2017 (UTC)
I read the Kontiki book half a century ago, but I think his early 1940's rafts were built of a balsa wood and with a bamboo deck.lashed together with hemp. (If all else fails, smoke the rope). . Later he made rafts of the reeds from which papyrus was made. Edison (talk) 05:15, 24 May 2017 (UTC)
Small quibble: Kon-Tiki was 1947, in part using WW2 surplus equipment, and with some of the participants being ex-WW2 fighters. In the early 1940s, much of the world had different priorities.The Ra expeditions were in 1969 and 1970, and while one might argue either way, I think the boats would be offended to be called rafts ;-). --Stephan Schulz (talk) 06:44, 24 May 2017 (UTC)
Many modern ultralight aircraft use cables extensively as part of the structure. Roger (Dodger67) (talk) 07:17, 24 May 2017 (UTC)

May 24

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