Science desk
< July 26	<< Jun \| July \| Aug >>	July 28 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

July 27

Why do bullet-proof vests expire ?

I heard on the news that the Detroit Police Department is wearing expired vests. So, what makes them lose effectiveness over time, or is it just a case of the manufacturers only having tested them after so long, so only guaranteeing them for that period ? StuRat (talk) 00:33, 27 July 2013 (UTC)[reply]

High temperatures and UV rays from the sun can slowly degrade polymers such as Kevlar; however, the expiration date is set conservatively, with a large safety margin (i.e. the vests expire well before degradation reaches a serious level). 24.23.196.85 (talk) 01:10, 27 July 2013 (UTC)[reply]

This paper discusses some of the aging effects (due to use and environmental exposure) of one kind of balistic fabric material. Over its lifetime a vest in real daily police use will be exposed to a range of environmental attacks (sunlight, heat, sweat, gasoline, spent firearm propellant, tear and pepper spray and their solvents, cleaning materials, damp and mildew, and general rough handling). 146.90.36.204 (talk) 01:13, 27 July 2013 (UTC)[reply]

You'd think that they could protect it from UV light with a protective cover layer opaque to UV. StuRat (talk) 01:30, 27 July 2013 (UTC)[reply]

Part of the protection that Kevlar provides is the weave of the fabric. A spinning bullet interrupts the weave, causing the force to be spread over a larger area. Over a period of years the fibers in woven Kevlar fabric start to separate, reducing the ballistic protection provided by the vest.OsmanRF34 (talk) 01:21, 27 July 2013 (UTC)[reply]

Very true; in fact, the specs say that if the vest is hit by a bullet, even once, it's no longer usable and must be sent back to the manufacturer for repair. 24.23.196.85 (talk) 01:35, 27 July 2013 (UTC)[reply]

Really? I'm surprised. Safety harnesses (see below) are single use only, at least in Ontario. Once the fabric wears out or the harness experiences a drop, the harness is trashed. If I was in a situation where I needed to wear kevlar, I don't think I would get a warm fuzzy feeling about seeing a "reconditioned" tag on it... Matt Deres (talk) 01:42, 27 July 2013 (UTC)[reply]

A similar instance occurs with safety harnesses, which actually come stamped with an expiration date. Besides UV light and the other factors mentioned above, marking the material with permanent ink or marker also degrades the fabric. Matt Deres (talk) 01:39, 27 July 2013 (UTC)[reply]

Before we veer into some crazy side discussion about safety harnesses... I think Stu's question makes a lot of sense. Obviously after a vest is shot you're not going to use it again. Similar to how once a bike helmet is used it won't be used again. The real question Stu's asking is whether they really age. I suspect the answer is largely no (or not very quickly). That some reporter dug up the expiration date on bullet proof vests and then tied that into the Detroit bankruptcy doesn't surprise me either. If you really wanted to get into it, I say you email the author of that piece with what you're really getting at Stu: "Does this actually matter or is this just some neat rhetorical device?" Shadowjams (talk) 08:12, 27 July 2013 (UTC)[reply]

I'm not sure what's crazy about it; they're both safety devices made out of fabric that have to withstand tremendous forces in order to be useful. They're also very rarely put to the test (as it were), which makes them prime candidates to get put on the agenda when it comes to cutting costs. I bring them up not simply due to the similarities, but also because they're much more widely used and because I've had experience with them that might be relevant. For example, the individual strands of fabric wear out fairly easily considering how sturdily made the harnesses are; if they can wear out to the point of being unsafe without experiencing a drop or even getting up to the stamped code date, then you really do have to wonder about how good the vests are if they've been held onto past the code date. Matt Deres (talk) 13:13, 27 July 2013 (UTC)[reply]

Comparing a harness that has to support a distributed weight over long periods to a material that has to absorb a smaller force over a very short period is vastly different. I don't have the materials-engineering vocabulary to articulate it exactly, but the differences between the two applications is dramatic; "both safety devices made out of fabric" is about as far as the similarity goes. Shadowjams (talk) 17:27, 27 July 2013 (UTC)[reply]

Yeah, nothing crazy about Matt's post. The question can dwell into expiration of fabric. I'd even add that regarding climbing ropes, I seem to recall from an old Chouinard Equipment catalog that five year old unused, properly stored ropes, when subjected to the standard UIAA test (a factor two fall) broke on the first attempt! I wouldn't trust my life to them. The same applies to bullet proof vests. BTW, a harness doesn't have to support a distributed weight over a period of time. It is a security device that mostly doesn't get stressed.OsmanRF34 (talk) 17:37, 27 July 2013 (UTC)[reply]

Please don't make me try and find some numbers.... but you can't possibly compare the tensile strength of a 3" strip of fabric taking a 300 lbs weight falling, let's say 15 feet, to a few centimeters of fabric taking a 700 J force? Imagine shooting a handgun at a seatbelt, or through one of the climbing harnesses you're talking about. That's the point I'm trying to make. [you're probably right though about the aging factor... but it'd be nice to have some refs on it]. Shadowjams (talk) 19:50, 27 July 2013 (UTC)[reply]

Ok, thanks all. It sounds like they really do expire. That makes them even more expensive, if they have to be replaced every few years. I wonder if it's possible to make a more robust version. StuRat (talk) 06:06, 30 July 2013 (UTC)[reply]

Hatchet wasp "attacking" spider?!

Earlier today as I was walking through the garden I saw something very unusual: a small spider being chased around by a (much smaller still) hatchet wasp. For about a minute or so I watched the spider jump about frantically as the wasp followed in hot pursuit (it seemed to land, or at least come very close to landing on the spider several times). I haven't been able to find anything detailing this sort of behavior, but then again the research on these things seems pretty scant. Can anyone with insect knowledge shed some light on this? 70.112.97.77 (talk) 00:58, 27 July 2013 (UTC)[reply]

I can think of three possibilities:

1) It's trying to kill and eat the spider.

2) It's trying to kill or drive off the spider to protect itself or other hatchet wasps from the spider.

3) It wants to lay eggs in it.

It might take a study to determine which is correct. StuRat (talk) 01:27, 27 July 2013 (UTC)[reply]

See Spider wasp. — Quondum 01:51, 27 July 2013 (UTC)[reply]

Yep, that's it! Thank you. 70.112.97.77 (talk) 08:29, 27 July 2013 (UTC)[reply]

So, #3 it is, although it seems they only lay one egg per spider. I will mark this Q resolved. StuRat (talk) 00:23, 28 July 2013 (UTC)[reply]

Thanks, Stu. I'll try to remember to tag it next time! 70.112.97.77 (talk) 03:05, 28 July 2013 (UTC)[reply]

No, see this first! 24.23.196.85 (talk) 01:52, 27 July 2013 (UTC)[reply]

Black widows and snakes battling it out in his basement ? I think it might be time to move. :-) StuRat (talk) 20:37, 27 July 2013 (UTC) [reply]

Or to hire a professional exterminator. 24.23.196.85 (talk) 07:52, 28 July 2013 (UTC)[reply]

Resolved

Hero shrew article - what does this mean?

In WPs Hero shrew article, it says "The hero shrew's unique anatomy allows it to bear the weight of a 160 lb (73 kg) human without harm". Does this mean that a human can stand on the thing's back, or ride it around (the article doesn't say how big this critter is), or what? I'm struggling to visualize... --Kurt Shaped Box (talk) 10:18, 27 July 2013 (UTC)[reply]

From the article on shrews:"All shrews are comparatively small, most no larger than a mouse. The largest species is the Asian house shrew (Suncus murinus) of tropical Asia, which is about 15 cm long and weighs around 100 grams." Thus I would think it means one can step on it and not kill it like one would other creatures that size or smaller. -Modocc (talk) 10:44, 27 July 2013 (UTC)[reply]

If you read the cited source, you'll see exactly what they're talking about - a ritual dance involving jumping on the shrew. I was skeptical at first, so I independently verified it with the 1917 Bulletin of the American Museum of Natural History, Volume 28, which is available for free from their website.

“

The natives of these regions, especially the Mangbetu, who are well acquainted with this shrew, first called our attention to its abnormally strengthened back-bone by their performances upon captive specimens... Whenever they have a chance they take great delight in showing to the easily fascinated crowd its extraordinary resistance to weight and pressure. After the usual hubbub of various invocations, a full-grown man weighing some 160 pounds steps barefooted upon the shrew. Steadily trying to balance himself upon one leg, he continues to vociferate several minutes. The poor creature seems certainly doomed. But as soon as his tormentor jumps off, the shrew after a few shivering movements tries to escape, none the worse for this mad experience and apparently in no need of the wild applause and exhortations of the throng.

”

— J.A. Allen, American Museum of Natural History, 1917

It turns out that there were a series of expeditions to the Congo between 1917 and 1919 by the museum. More reports are also available on the same website. I have found them to be fascinating reading. They are providing excellent context for my next major literary foray, The Crime of the Congo, a non-fictional work that I had already queued up from Project Gutenberg as I have completed reading most of that author's fiction.

For the record, after reading of the shrew on Wikipedia's main page on Friday morning, I researched this material extensively before I left to work; and the Armored Shrew became the topic of our lunch-hour conversation. Like many of my conversations, it was received with much incredulity. Nimur (talk) 15:08, 27 July 2013 (UTC)[reply]

Even so, WP:EXTRAORDINARY.--Shantavira|^{feed me} 16:30, 27 July 2013 (UTC)[reply]

Would explain the dutch name: Pantserspitsmuis, meaning "armored shrew". Ssscienccce (talk) 20:47, 27 July 2013 (UTC)[reply]

Thanks for the info. If someone just told me about that 'shrew dance', I'd probably just assume that they were testing my gullibility. It does sound fairly Boratian, doesn't it? But then again, these strange local customs do actually exist - e.g. the one where they throw a goat from a bell tower and then get drunk - or the one where they run around on a field, fighting each other over a sheep carcass and then get drunk - or the one where they chase a rolling cheese down a v.steep hill and then get drunk. I'd assume that they all get drunk after the shrew thing too? --Kurt Shaped Box (talk) 15:55, 28 July 2013 (UTC)[reply]

Having read quite a bit about Africa, particularly material written in the latter half of the nineteenth and early twentieth century, I would not entirely rule out that the publication is embellishing details. Much historical writing about Africa is exaggerated, or fictitious; or partakes in scientific racism, or other cultural insensitivity; many encounters with local people portray them as "savage" (whether for political ends or for the purposes of entertainment). One hopes that these unfortunate portrayals were extinguished by the enlightened time of this 1917 expedition. One hopes that respectable naturalists refrain from exaggeration or cultural insensitivity, and sticks to strict observationalism. Yet, when one studies literature and science written by Europeans and Americans from that era on the topic of sub-Saharan Africa, one is left with the distasteful impression that perhaps not every writer attains these enlightened goals. (We know, from even a cursory study of history, that racial equality and respect was not a universal reality in America or Congo in 1917).

I have never been to Congo, nor met a Mangbetu who danced on a shrew. All I can truthfully say is, the American Museum of Natural History archived the writings of its member, one J.A. Allen, who wrote in 1917 that during travels to the Congo, he met more than one person from the Mangbetu group, and from other areas, who stepped on the shrew. He drew very detailed diagrams of the shrew, and took a few photographs; yet no photograph depicts anyone stepping on a shrew.

Our encyclopedia's threshold for inclusion is verifiability, and not veracity. Nimur (talk) 01:57, 29 July 2013 (UTC)[reply]

Diode?

If we place a P and N type semi conductor closely (touching) as shown in fig. Will it works as a diode? Pls Explain?

http://i.stack.imgur.com/n5UzB.jpg

— Preceding unsigned comment added by Rahuloof (talk • contribs) 06:45, 27 July 2013 (UTC)[reply]

Yes, see the article on p–n junctions for why. --Modocc (talk) 12:08, 27 July 2013 (UTC)[reply]

Although the lead of that article does indicate that this construction "severely inhibits its utility". — Quondum 02:24, 29 July 2013 (UTC)[reply]

Although that sentence is tagged with {{citation needed}} for over 3 years. It looks sort-of supported by the linked grain boundary article, but not in all aspects:( DMacks (talk) 08:47, 29 July 2013 (UTC)[reply]

Audio physiological effects

Which impairs hand-eye coordination and causes cardiac dysrhythmia, volume or pitch, or a combination of both? Plasmic Physics (talk) 13:24, 27 July 2013 (UTC)[reply]

Bee

Can anybody identify this bee? It was taken just outside the Cambridge Bay Airport terminal so it is obviously some sort of Arctic bee. Thanks. CambridgeBayWeather (talk) 15:08, 27 July 2013 (UTC)[reply]

Looks like a carpenter bee. They resemble small bumble bees. μηδείς (talk) 15:17, 27 July 2013 (UTC)[reply]

Possibly a Bombus bifarius nearcticus, see examples near the top of the page, but there are so many bombus species. Mikenorton (talk) 07:58, 28 July 2013 (UTC)[reply]

Thanks. I thought this might be a bit difficult. It turns out the bee was dying and when I came back to work it was still there and is now dead. So I've taken some more pictures of the bee but now upside down. They are at the above link. I've also kept the bee in a plastic bottle in case there is something else I can do to id it. I wouldn't bother but there does not seem to be too many pictures of bees from up here. I noticed that there are something like 12 - 15 species of bee in the Arctic. Also based on the underside pictures and second paragraph it's probably not a carpenter bee. CambridgeBayWeather (talk) 12:09, 28 July 2013 (UTC)[reply]

No clue what it's called, but I see them frequently enough (including today) in my southern Northern Ontario fields. So it's not strictly Arctic. Or maybe I'm seeing a slightly different bee, but to my layman eyes, it looks the same as your pictures. Not the most helpful answer, but might help you narrow it down. RIP bee. InedibleHulk (talk) 23:40, 28 July 2013 (UTC)[reply]

flare

why dont/where /how do we use flare coming out of an oil refinary.Why do we waste such an enermous amount of energy for free.We must think of uitilixzing it somehow.Has anyone thought of using it to generate energy in steam power plant.Please suggest me some reaing on this topic. 175.101.60.14 (talk) —Preceding undated comment added 15:51, 27 July 2013 (UTC)[reply]

Gas flare and Associated petroleum gas are the relevant articles. The answer to your question is "because it's cheaper just to burn the gas than to transport it anywhere useful", but your point about wastage is valid, and discussed in the articles. Tevildo (talk) 16:28, 27 July 2013 (UTC)[reply]

The flair at an oil well is mainly waste - the flare at a refinery is a safety device. Rmhermen (talk) 19:24, 27 July 2013 (UTC)[reply]

I have to think that if they properly valued both the cost of the pollution they are generating/global warming and the fuel value lost to future generations, then this would be seen as the waste it truly is. Unfortunately, the company doing that doesn't have to pay those costs, and doesn't care if others do. This is one argument for the proposed cap and trade system. StuRat (talk) 20:02, 27 July 2013 (UTC)[reply]

[Dons ex-refinery employee hard-hat]. As Rmhermen correctly said, refinery flare flames are safety devices: specifically, they're pilot lights.

Oil refineries produce from crude oil a number of commercially saleable liquids and gases. These processes leave a relatively small residue comprising a mixture of inflammable gases which would cost more to purify than they could be sold for.

This mixed residue is called refinery gas [what, no Wikilink?], and most of it is used to produce power and heat for operations in the refinery itself. However, the complex and sensitive nature of various refining processes means that the volume of refinery and other inflammable gases being produced is subject to occasional surges which the normal procedures and equipment cannot absorb.

These excess gases have to be vented to the atmosphere (otherwise, kablooie!) and since they may in sufficient concentration be toxic to the refinery workers and residents in surrounding areas (who may predominantly be refinery workers' families) they have to be burnt. They are directed up the flare stacks, ignited by the pilot flame, and make an alarming but harmless roar. In the refinery I used to work at and still work within earshot of, this happens less that half-a-dozen times a year at most.

Refinery flares are therefore in general no different to the millions of household gas water heaters and central heating boilers which also have a permanent pilot light lit.

If someone else would care to crunch some numbers, I suspect it could be shown that the flares' contribution to pollution is utterly insignificant compared to that caused by the utilisation of oil and gas that we all directly or indirectly benefit from. I don't say that this is a good thing, and I'd be the first to welcome better viable alternatives. {The poster formerly known as 87.81.230.195} 90.213.246.168 (talk) 21:09, 28 July 2013 (UTC)[reply]

Long reverberation time in stone-walled cathedrals

Are thick-stone-walled cathedrals intentionally built with a long reverberation time, or is the echo-like feel of entering the cathedral a product of its big and sturdy architecture? Also, where can a person find a stone-walled cathedral in the Midwestern region? Sneazy (talk) 16:51, 27 July 2013 (UTC)[reply]

You won't find any purely stone buildings in the Midwest but there are many mainly stone (with some cement and limited structural steel) ones. The Basilica of the Sacred Heart, Notre Dame and Rockefeller Memorial Chapel come immediately to mind. More common is steel framed stone like at about 3 minutes in this video: [1] Rmhermen (talk) 19:49, 27 July 2013 (UTC)[reply]

Doesn't have to be thick-walled, for example: plaster walls, painted brickwork and 3 mm plywood panels have about the same low sound absorption coefficient (http://www.engineeringtoolbox.com/accoustic-sound-absorption-d_68.html http://www.sengpielaudio.com/calculator-RT60Coeff.htm).

To calculate the 60dB reverberation time of a room, you use Sabine's formula, t=0.161 V/A with V=volume in m³ (or 0.049 V/A if measuring in cubic and square feet), A is equivalent absorption surface = S₁.a₁ + S₂.a₂ +... for all the surfaces, S = surface area in m² and a is the absorption coefficient of the material.

Carpet, foam, curtains will absorb higher frequencies better, window glass will mainly absorb low frequencies.

The formula explains the long reverberation: low absorption materials and a large space. If you double the length, width and height then the time will also double. Since the main material is so low absorbing, changes to small areas can make a big difference, For example, if all surfaces have a=0.02, covering just 3% with carpet having an absorption coeff of 0.6 will halve the reverberation time.

Long reverberation was preferred in Catholic churches. It fitted the music, specially the chants. After Vatican II, that changed, because the liturgies were no longer conducted in Latin. Before, it didn't matter whether the people could hear clearly what was being said from the altar, since most didn't speak latin. But after Vatican 2, people were supposed to listen to what was said, and the reverberation became a problem. (http://www.catholica.com.au/misc/ChurchAcousticDesign.pdf) Ssscienccce (talk) 22:53, 27 July 2013 (UTC)[reply]

Interesting... Is this also true for Eastern Orthodox churches (where the liturgies are conducted in Old Church Slavic, and the chants are much like the Catholic ones in the musical sense)? 24.23.196.85 (talk) 07:59, 28 July 2013 (UTC)[reply]

Well, here's what I found:

installing a sound system in a Russian Orthodox church in the Volga: "One of the most awkward factors for the acoustic engineers here was the reverberation time, with RT60 beneath the central dome extending to several seconds, severely compromising intelligibility. "
The reverberation time of the Hagia Sophia (Greek-orthodox) was measured by popping a balloon (singing, especially religious chants, is not allowed there) and Stanford University used the results to recreate the acoustics at Bing concert hall for a concert by the Cappella Romana vocal ensemble of Portland, OR.
Hagia Sophia featured an eleven second reverb, the Alexander Nevsky Memorial Cathedral in Sofia a bit less with nine seconds, according to this CD of Slavonic Orthodox Lithurgy.
Acoustics of the Russian Tent-Shaped Temples:

"The reverberation time RT60 is very high in the largest TST. The volume of the Church of the Ascension at Kolomenskoje is equal to V=1700 m3. That is about 2 times less than in 5-dome Trinity Cathedral in Kostroma (V=3200m3 [1]). And the values of RT60 in AK are about 2 times larger" (the text is hard to interpret at places, I think some Russian expressions were translated verbatim, like "two times less" when "half" is meant)
Such values of RT60 depend greatly on the inner attire of TST. For example, IC has the volume of V=1670 m3 that is very close to the volume of AK. But the values of RT60 in IC are much smaller than in AK. This is caused mainly by the large wooden icon-stand that was installed in IC. During the measurements in AK in this church there was no icon-stand and the room was completely empty with only several wooden benches on the floor. Still shorter reverberation time was estimated in more lower TST with exedrae on the sides of the tetrahedron. For example, in IF RT60 is equal to 1.4-1.7 s at middle frequencies. In this case the huge icon-stand with 8 rows of icons and various wooden decorative structures as well influenced greatly on the sound absorption in the church.
It should be noted that there is no large increase of RT60 at the low frequencies in IC and IF that is typical for 5-dome churches and cathedrals. This fact deals with the large surface of windows in IC and IF that absorb sound energy at the low frequencies. The surface of windows in AK is much smaller in comparison with IC and IF.
The problem of speech intelligibility is of the great importance in Russian Orthodox churches. In general the speech intelligibility in TST is better than in 5-dome churches.

Don't know if this answers your question in any way... Ssscienccce (talk) 11:24, 28 July 2013 (UTC)[reply]

Thanks! You've confirmed my hypothesis, as well as my own personal observations re. the acoustics in the Church of the Assumption in the Kremlin. 24.23.196.85 (talk) 07:33, 29 July 2013 (UTC)[reply]

See Is There an Acoustical Tradition in Western Architecture? by Marc CRUNELLE. Alansplodge (talk) 21:06, 28 July 2013 (UTC)[reply]

If sperm from a white man, black man, chinese man and indian man were inserted into a woman?

closing trolling per talk consensus

The following discussion has been closed. Please do not modify it.

... what colour would the child be? --Sïleïni (talk) 20:49, 27 July 2013 (UTC)[reply]

Oocyte activation by the first sperm prevents most zygotes with two fathers, and in any case polyspermy that results in aneuploidy is certainly lethal in humans. Therefore, you get a random race. I am not aware of any systematic research into whether sperm from any particular race might have a competitive advantage - there's no great reason to suspect it, and while you could measure times in vitro in an artificial environment, there's no way to conduct the experiment with any real validity without actually creating zygotes in a human woman, which would never get approval (by the woman, let alone the IRB). If the sperm were delivered naturally, rather than as an artificial mixture, there is some possibility that orgasm would cause the woman's sexual preference among the men to translate to a higher chance of conception by his sperm (essentially, by muscular movements that give his sperm an advantage) but I'll admit that idea is not widely accepted. Wnt (talk) 21:20, 27 July 2013 (UTC)[reply]

Do you have any source to this orgasm - fertilization link? It seems akin to the link rape - no pregnancy link. OsmanRF34 (talk) 22:11, 27 July 2013 (UTC)[reply]

See [2], etc. The rape - no pregnancy claim was obviously not literally true per se, but it did have some percentage of truth to it, massively amplified by wishful thinking among those with a certain point of view. Wnt (talk) 23:46, 27 July 2013 (UTC)[reply]

Indeed you can't get a child with two fathers. Though there is a rare chance of superfecundation where twins can be born of two different fathers. This is rare in humans, though more common in fellow mammals such as dogs and cats - members of a litter of street cats or dogs can have obviously different characteristics. 88.112.41.6 (talk) 21:39, 27 July 2013 (UTC)[reply]

As my Grandmother said, it doesn't matter what color, as long as it comes out Catholic. μηδείς (talk) 21:40, 27 July 2013 (UTC)[reply]

Hat as trolling per Summit, Tevildo, self. μηδείς (talk) 22:25, 27 July 2013 (UTC)[reply]

I guess you guys are just more important than the three of us answering here, so I'll defer to your "consensus", but at least leave me the opportunity to answer the request for source? Wnt (talk) 00:26, 28 July 2013 (UTC)[reply]

Why do people feel tired after eating ?

I can understand a huge meal doing this, as most of the body's energy must be used for digestion, but I've observed this for small meals, too. Does the body release insulin in anticipation and cause blood sugar to drop early in the digestion process ? (Not a medical question, just asking about the normal digestion process.) StuRat (talk) 22:22, 27 July 2013 (UTC)[reply]

I think I know one reason. If you are eating way past your bedtime, regardless of the meal size, then you might still feel tired due to your circadian rhythm reminding you that it is bedtime. Sneazy (talk) 22:53, 27 July 2013 (UTC)[reply]

If blood sugar levels were to drop, you would feel hungry after the meal. I know how this feels, I eat about 4000 Kcal per day but when travelling and eating in restaurants it's difficult for me to eat this amount. It can then happen that when I start to eat I will feel more hungry than before; when I am done eating and completely full, I can be extremely hungry. I then need to wait a few hours and eat again. But I won't feel tired (unless I would force myself to eat a normal diet). Count Iblis (talk) 23:47, 27 July 2013 (UTC)[reply]

At that many calories per day, I assume you are either very active or overweight. StuRat (talk) 23:49, 27 July 2013 (UTC)[reply]

I weigh 58 kg, I do 50 minutes of fast running per day, but I have had a fast metabolism for most of my life also before I was exercising at this level. I can tell you that low blood sugar after a meal is a strange feeling that most people will never experience. On the one hand your belly is completely full, on the other hand it feels like you are starving. Count Iblis (talk) 00:02, 28 July 2013 (UTC)[reply]

Read Parasympathetic nervous system --Kharon (talk) 01:28, 28 July 2013 (UTC)[reply]

We have an article about the phenomenon: Postprandial somnolence. -NorwegianBlue^talk 16:43, 29 July 2013 (UTC)[reply]

Good link, thanks. (I had no idea "postprandial" meant "after eating".) StuRat (talk) 06:00, 30 July 2013 (UTC)[reply]