Wikipedia:Reference desk/Archives/Science/2012 December 6

= December 6 =

heavy cream in a blender
I blend heavy cream for 1 minute on high on a really good 1000W blender with the top open -- I can see the liquid sloshing around. I stop the blender. When I look inside the jar, the cream is essentially solid. When I try blending the cream again, it no longer sloshes around. What's the mechanism? John Riemann Soong (talk) 01:28, 6 December 2012 (UTC)


 * Not sure of the chemistry, but we say "the cream has set". StuRat (talk) 01:30, 6 December 2012 (UTC)


 * It seems that it could not set while it was kept in constant motion. Bus stop (talk) 01:35, 6 December 2012 (UTC)


 * Agglomeration (sticking together) of the butterfat particles? 24.23.196.85 (talk) 01:48, 6 December 2012 (UTC)


 * The article on butter (which is essentially what you've made) covers it, as does churning (butter). You've inverted the emulsion from oil-in-water to water-in-oil.  Acroterion   (talk)   02:43, 6 December 2012 (UTC)


 * Well, it's not butter, it's still soft. I wonder why it can't set while it's in motion, and why this setting is not reversible. If I constantly blend water at -5C, won't ice eventually form? Or would I create supercooled ice? John Riemann Soong (talk) 03:50, 6 December 2012 (UTC)


 * You may not have completed the inversion, but butter is commonly made in small batches in a food processor. As noted farther down, incorporating air stiffens it up too, and there are degrees of similarity between whipped cream and whipped butter.  Butter made in that manner, since it's unsalted, will taste bland.  The action of the processor will keep the buttermilk suspended: in finished butter it would be pressed out and the mass rinsed.  Acroterion   (talk)   15:49, 6 December 2012 (UTC)


 * I don't know about -5°C, but keeping it moving should prevent ice from forming down to some temperature. The molecules need to remain adjacent long enough for the ice to form. StuRat (talk) 04:40, 6 December 2012 (UTC)


 * I wonder how low before ice would form anyway. If we had a blender that sloshes the water quickly enough and is not itself affected by extremely low temperatures, what's to stop us having a blender mixing liquid water that's 20C below freezing?  Or 40C below?  Or ...  Is it purely a function of how fast the blender can be made to go? --   Jack of Oz   [Talk]  07:59, 6 December 2012 (UTC)


 * You just made whipped cream: "Cream containing 30% or more butterfat can be mixed with air, and the resulting colloid is roughly double the volume of the original cream as air bubbles are captured into a network of fat droplets. If, however, the whipping is continued, the fat droplets will stick together destroying the colloid and forming butter; the remaining liquid is buttermilk". Don't any of you guys ever cook? --Saddhiyama (talk) 13:39, 6 December 2012 (UTC)


 * Yes, but your answer does not address the OP, which asked why the whipped cream only sets when still. StuRat (talk) 17:34, 6 December 2012 (UTC)


 * reminds me of simple liquefaction, no?GeeBIGS (talk) 02:46, 7 December 2012 (UTC)


 * More like the reverse. StuRat (talk) 05:33, 7 December 2012 (UTC)


 * obviously I meant while the blender was still on the only thing keeping it from solidifying was the rigorous agitation.GeeBIGS (talk) 06:17, 7 December 2012 (UTC)


 * There are other substances that behave differently when still and when disturbed. See Thixotropy and its See also list. Bus stop (talk) 05:42, 7 December 2012 (UTC)


 * I don't see how it doesn't answer the OP. The blender mixes air into the cream. The first round of blending the cream it will obviously slosh, but with the blending it will turn into whipped cream. This is what the OP witnessed. With the second round the cream has set and will not slosh around. What is so difficult to understand about it? It is not unusual to prepare whipped cream in a blender. Sorry, but I do sense unfamiliarity with cooking in general, and whipping cream especially from the responsens to this thread. It is no great mystery. --Saddhiyama (talk) 00:40, 8 December 2012 (UTC)


 * Yes, the observation is common, but that's not what was asked about. They asked what the mechanism is, which involves chemistry.  Thus we need to provide a chemistry answer.   If they asked why water freezes at 32°F/0°C, would you have answered that it freezes because it becomes solid ? StuRat (talk) 00:43, 8 December 2012 (UTC)


 * No, they asked about the "mechanism", I see no question about chemistry. I believe my quote from the article about air bubbles and fat etc is a sufficient answer about what exactly happens when cream thickens from this kind of behaviour, especially compared to your original answer, which just provided the extremely insightful reply that it is called "the cream has set". I am sorry to break it to you, but you dreamed up the bit about chemistry. And yes, regarding your last question. --Saddhiyama (talk) 00:47, 8 December 2012 (UTC)


 * Mine was not intended to be answer, just providing the proper terminology, in case they want to do Google searches. StuRat (talk) 06:06, 10 December 2012 (UTC)

how can waterproof camera lenses work both underwater and above water?
If the refractive indices of the media are different, I don't get how one could design a lens to robustly work and focus both above water and under water. John Riemann Soong (talk) 04:20, 6 December 2012 (UTC)
 * If an underwater lens housing has a flat glass port then refraction at the glass/water interface is not an issue for narrow-angle lenses. For wide-angle lenses a dome port is designed to correct distortion, and there are wet-coupled accessory lenses that can be added or removed underwater during a dive. See Underwater photography. SkylonS (talk) 08:56, 6 December 2012 (UTC)

Which substance in urine smells like wheat?
--Inspector (talk) 05:02, 6 December 2012 (UTC)
 * The body excretes nitrogen in the form of Urea which, itself odorless, gives off ammonia in the presence of water. Ammonia has a pungent odour which may be what the OP perceives as the smell of Wheat. Otherwise there are specific diseases and foods that give particular urine odour, see the referenced section.SkylonS (talk) 08:45, 6 December 2012 (UTC)
 * Hmmmm... my assumption is that most odorants of relevance in wheat will be big hydrophobic molecules that come out in wheat germ oil; if you can smell some of this you can determine if this is the scent you're thinking of.  It is possible to "refine" wheat germ oil to remove the odor.  This suggests some obvious experiments which I expect have already been done.  Maybe  touches on it, probably not, haven't read the article.  Searching "wheat germ oil" on PubMed actually yields a nice set of old free articles going all the way back to 1935, which is nice, but I don't think any of them say what either.  I suspect the research into these basics might have been done even well before that, but I backed off and went to Google, and after a few tries wheat, scent, compound got me a paper about rice scent  which says "For example, oxidation products l-octen-3-ol, 3-methylbutanal, 2-methylbutanal, hexanal, 2-hexenal, 2-heptenal, 2-nonenal, and decanal were identified as key aroma compounds in 12 barley cultivars based on odor thresholds in water (Cramer et at 2005). In whole meal and white wheat flour, (E)-2-nonenal, (E,Z)- and(E,E)-2,4-decadienal, 4,5-epoxy-(E)-2-decenat, and 3-hydroxy-4,5-dimethyl-2(5H)-furanone were odor-active based on AEDA (Czerny and Schieberle 2002). Most of these compounds are also odor-active in rice."  Which, first off, tells us what we should know already, that the smell of wheat will be complex and depend on multiple compounds in a ratio to distinguish it from rice, and second, points us to a reference.  Which, as luck will have it, is not listed at the end of the paper, but NCBI quickly gives up  when prodded with the two names.  But I won't even go in there because we already have enough chemicals - the question now is, what might be excreted unaltered?  That's a big project and I don't think I'll actually do it, but for example 1-octen-3-ol comes up in  as being present in bovine urine - and physiologically active in attracting flies to cattle.  There's lots of fun to be had here, and some opportunities for fresh research. Wnt (talk) 17:24, 6 December 2012 (UTC)


 * when I eat Cheerios my p smells like Cheerios.GeeBIGS (talk) 00:38, 7 December 2012 (UTC)
 * Do your nuts smell like honey? μηδείς (talk) 01:54, 7 December 2012 (UTC)
 * how did you....? Is that some kind of medical condition?GeeBIGS (talk) 02:04, 7 December 2012 (UTC)
 * OK, now that's funny. -- Jayron  32  03:37, 7 December 2012 (UTC)
 * @SkylonS: Once I added some sodium hydroxide solution into urine and heated for a while, and there is the smell of ammonia, but the yeast-like smell persists. So it is definitely two different smell.--Inspector (talk) 14:49, 8 December 2012 (UTC)

kitchen water vs. bathroom water from the tap
I know people who won't drink water from a bathroom tap. I wonder if it is different from the water that comes out of the kitchen tap. — Preceding unsigned comment added by 98.207.158.141 (talk) 07:16, 6 December 2012 (UTC)


 * It's the same until it gets to the faucet, but the kitchen tap is more likely to have an aerator and screen on it, both of which can improve the quality a bit (not as much as a charcoal filter, though). StuRat (talk) 07:43, 6 December 2012 (UTC)


 * No it's not the same. One may revisit water but it is always different water. The philosopher Heraclitus 535 – c. 475 BCE had this insight which Simplicius of Cilicia 490 – c. 560 expressed as 'Panta rhei, everything flows'. SkylonS (talk) 08:30, 6 December 2012 (UTC)


 * In many houses in the UK, the bathroom tap is fed from a header tank in the attic, so the water has possibly been contaminated there. This is why many people prefer to use a kitchen tap for drinking-water because this is almost always supplied from the incoming mains supply.    D b f i r s   09:09, 6 December 2012 (UTC)


 * Agreed, although for some years it's been compulsory for new houses to have mains feed to all cold taps. There are stories of people finding dead pigeons in their tank, so you wouldn't want to be drinking that. Alansplodge (talk) 09:28, 6 December 2012 (UTC) Alansplodge (talk) 09:26, 6 December 2012 (UTC)


 * Some people have even found Barcelonan pigeons in their water tanks. This elusive species is known for its unusual call, which sounds like "oink, oink". Dominus Vobisdu (talk) 09:40, 6 December 2012 (UTC)


 * I do believe you're having a tin bath. Alansplodge (talk) 13:26, 6 December 2012 (UTC)


 * They don't speak cockney in Torquay. They speak English. They learn it from a book. Dominus Vobisdu (talk) 21:42, 6 December 2012 (UTC)


 * Funny, I only drink from the bathroom tap. Because of the way the pipes flow, the water from my bathroom is more likely to be very cold (which I prefer). The kitchen sink pipes are next to the dish washer and other heat sources and the sink itself is more frequently used for hot water as well. Matt Deres (talk) 18:02, 6 December 2012 (UTC)


 * Yes, and bathroom water might actually be better, in certain houses, if the pipe leading to the kitchen tap is rusty, for example. StuRat (talk) 18:04, 6 December 2012 (UTC)


 * Given people's habits of peeing in bathroom sinks, but not usually the kitchen sink, I could see germphobics having a problem with this. Doesn't bother me. μηδείς (talk) 20:54, 6 December 2012 (UTC)


 * Do you have a source for "people's habits of peeing in bathroom sinks"? HiLo48 (talk) 21:44, 6 December 2012 (UTC)
 * Contributing to the plural of 'anecdote', whatever it may actually be: by the time I'm at the bathroom sink and needing to pee, I'm no longer concerned about having to settle for a sink. &mdash; Lomn 22:17, 6 December 2012 (UTC)


 * What do you do when the wife's already on the toilet and your daughter's in the shower? Pee out the window?  Having witnessed otherwise I would never assume a public bathroom sink, men's or women's, had not just been peed in by a drunken busboy or three. μηδείς (talk) 01:53, 7 December 2012 (UTC)


 * When we had to turn off the water to the house to make plumbing repairs, I peed in a jar and dumped it out in the garden. I wonder if it killed any slugs. :-) StuRat (talk) 07:24, 7 December 2012 (UTC)
 * @ Medeis, why the perpetuation of the myth that urine contains "germs"? Urine in the huge majority of the population is virtually sterile. When urine contains pathological bacteria or viruses the producer of the urine will probably be aware of it by well known signs and symptoms. Caesar&#39;s Daddy (talk) 08:25, 7 December 2012 (UTC)
 * ... but will they inform the next user of the sink? I find it surprising that anyone would consider the above practice in any but the most desperate of situations!    D b f i r s   16:46, 7 December 2012 (UTC)
 * I think this will be my last comment on this thread--I'd just be doubtful that someone who peed in the sink--and the busboys who worked in a restaurant I used to frequent which had only two commodes were known to do that--would also be too fastidious about washing their hands after crapping. μηδείς (talk) 17:17, 7 December 2012 (UTC)


 * Greywater recycling (3049288568).jpg Sometimes greywater is used for flushing in toilets (even directly without purification). ···V ani s che nu「m/Talk」 14:25, 8 December 2012 (UTC)

take off and landing on moon or planets
How can escape the moon or planet landing module from those gravity field?--Akbarmohammadzade (talk) 07:41, 6 December 2012 (UTC)


 * The Moon only has 1/6th Earth's gravity, so it takes a lot less fuel to reach escape velocity from it. The lunar landers tended to have just enough held in reserve.  Mars has over 1/3 Earth's gravity, so more fuel would be required.  This is one complication making manned trips there difficult.  Venus has over 9/10ths Earth gravity, and a thick atmosphere to blast through, but no human would want to land there, so it's not an issue, unless we want a robotic probe to grab samples and return with them. StuRat (talk) 07:47, 6 December 2012 (UTC)


 * You might also wonder about the possibility of obtaining fuel on the planet or moon. Some do have methane, which would burn nicely, provided we also had oxygen.  However, planets rich in methane tend to be oxygen-poor.  So, we'd need to bring oxygen (or some other oxidizer) with us. StuRat (talk) 08:02, 6 December 2012 (UTC)

Then how could Russian module return from hot and dense atmosphere of Venus?--Akbarmohammadzade (talk) 08:12, 6 December 2012 (UTC)


 * The lander must reach a certain Escape velocity to break free from the planet or moon's gravitational field without further propulsion. The referenced article gives a formula for escape velocity. SkylonS (talk) 08:14, 6 December 2012 (UTC)


 * No Russian modules have returned AFAIK. See Observations and explorations of Venus. They go in, but they don't come out. Clarityfiend (talk) 09:52, 6 December 2012 (UTC)

And about landing on moon ,it has not atmosphere then non of helicopter or umbrella system work there.--Akbarmohammadzade (talk) 12:31, 6 December 2012 (UTC)


 * By "umbrella system" you probably mean a parachute. Since that won't work on the Moon, they use retrorockets, instead. StuRat (talk) 17:31, 6 December 2012 (UTC)


 * Correct. DMacks (talk) 13:04, 6 December 2012 (UTC)


 * Unexpectedly, and unfortunately, it has been found that almost all planets bearing intelligent aliens are tilted on their side, meaning star ships can only land on their north poles. Luckfully, transporter beams can rotate an object by 90, or 270 degrees, allowing people to materialize on the side of the planet. μηδείς (talk) 16:40, 6 December 2012 (UTC)


 * That must be some kind of a joke, but I don't get it, and the OP, with a limited grasp of English, certainly won't. StuRat (talk) 17:28, 6 December 2012 (UTC)


 * If anyone else finds this problematic they are invited to hat it or remove the small section. I have always worried about poor Captain Kirk trying to land sideways on a planet. μηδείς (talk) 20:50, 6 December 2012 (UTC) Let no one


 * OK, I get it now. You aren't talking about the planet being tilted, but rather the Enterprise's orientation being tilted relative to the surface below it. StuRat (talk) 05:31, 7 December 2012 (UTC)


 * Well, that does raise a question I hadn't really considered before. Obviously, the way the Enterprise is depicted orbiting is for a nice dramatic effect. But if such a ship really existed and was in orbit around a planet, what would its likely attitude be? The space shuttles, as far as I knew, generally flew "upside down", that is from the astronauts' standpoint the earth was over their heads, with deep space underneath them, until it came time to descend. Would an Enterprise likely do the same thing? ←Baseball Bugs What's up, Doc? carrots→ 10:16, 7 December 2012 (UTC)


 * I expect it would depend on the spacecraft's design. There's not much sense in speculating on the Enterprise - you can make up enough babble to explain whatever you want it to do... For example: flying "sideways" provides a natural gravitational gradient accros the warp nacelles - the ship takes advantage of this curvature of spacetime to allow slow but free partial regeneration of the dilithium matrix. Using the inertial dampers to achieve this same effect in the absense of a gravitational well would require more energy than would be saved by the regeneration. When orbiting with Starboard to the planet, the warp conduits must be specifically shut off to prevent a parasitic drain, or the shields must be recalibrated to counteract the effect. 209.131.76.183 (talk) 13:13, 7 December 2012 (UTC)


 * Well, sure, you can make up anything you want, but I was just wondering what would make realistic practical sense if such a craft existed. Maybe it wouldn't matter, as they don't need to worry about "windows", what with their sensors showing them everything. ←Baseball Bugs What's up, Doc? carrots→ 00:55, 8 December 2012 (UTC)


 * In the Star Trek: The Animated Series opening credits, the ship does appear to be flying sideways (they had extremely low budget animation). StuRat (talk) 01:03, 8 December 2012 (UTC)


 * This xkcd comic (wow, two very relevant links to xkcd in one day!) illustrates the situation in a very easy to understand graphic. It's one thing to see the numbers; it's something else entirely to see such a clear visual representation. Matt Deres (talk) 01:22, 7 December 2012 (UTC)


 * That is one cool effing graphic! μηδείς (talk) 01:44, 7 December 2012 (UTC)


 * Indeed; I have a poster of that up in my office. Matt Deres (talk) 12:23, 7 December 2012 (UTC)

Is it necessary to use ballistic missile to return from mars with 1/3 gravity strange of own Earth? --78.38.28.3 (talk) 06:16, 8 December 2012 (UTC)


 * I don't quite understand the question. Are you asking if the launch vehicle needs to be aerodynamic to launch from Mars ?  That's a function of the thickness of the Martian atmosphere, which is only indirectly related to the gravity there.  Also, if you launch slowly enough, aerodynamics is less of a factor, but gravity is more of a factor.  I'm sure being aerodynamic would help, but not sure how much. StuRat (talk) 06:39, 8 December 2012 (UTC)

I heard that there are two (descent engine) ( ascent motor ) for landing on moon and take of from it, how can ascent motor take the module of from its gravity field specially direct above without falling ?--78.38.28.3 (talk) 09:17, 8 December 2012 (UTC) (Slightly cleaned up - ouch)
 * 78.38, you read too much into "descent" vs. "ascent motor".
 * During descent, slowdown uses so much fuel that the craft is considerably lighter when taking off than when it started descent. The difference is so dramatic that they can use a smaller motor for ascent, leaving the bigger descent stage behind; that's cheaper than reusing the heavier descent motor for ascent.
 * And (OR) a rocket motor can not simply go full blast when it is facing a surface. So, there's a nonnegligible chance that the descent motor has already taken some damaged on the final feet down, and would take even more damage when taking off. So they use a different motor, which has not taken any damage and/or can take more abuse for ascent. Whether the LEM can take off is just a matter of the forces involved: weight (mass times local g) vs. thrust. Although the ascent motor has the lower thrust rating, it can take the LEM off the ground, because it is lighter than when it started descent. - ¡Ouch! (hurt me / more pain) 17:39, 8 December 2012 (UTC)

plant morphology
what type of structures are found in Bryophyta and what are they used for? — Preceding unsigned comment added by 113.199.156.26 (talk) 10:03, 6 December 2012 (UTC)
 * Read Bryophyta. DMacks (talk) 13:05, 6 December 2012 (UTC)

showing who is online.
Here we can give a line of information that who is online ,why don't you do it ?--Akbarmohammadzade (talk) 12:34, 6 December 2012 (UTC)
 * I think you are referrring to the "Users currently online" feature of many online forums. That seems to me it would't "fit" with the way wikipedia reference desks works (ie with edits and so on as opposed to "posts"). Other wikipedians will probably be able to explain why wikipedia has chosen not to add this feature. — Preceding unsigned comment added by 80.254.147.164 (talk) 13:18, 6 December 2012 (UTC)
 * Wikipedia cares about privacy, but does not care about tracking users to serve them an ad. OsmanRF34 (talk) 14:59, 6 December 2012 (UTC)
 * Wikipedia is not very safe in many ways about privacy, but this feature is burdensome to implement. For the closest thing, look at the "contributions" for a user e.g. Special:Contributions/Wnt and compare the time of the last edit to the actual time.  You can get this from the menu at left under "contributions" when you are on someone's user or user talk page. Wnt (talk) 17:33, 6 December 2012 (UTC)

Pocket self-contained "GPS"
I mean, instead of connecting to a satellite, you could calculate your position with a gyroscope and an accelerometer, and you'll know where you are. I'm sure that would drag less battery power. Was it ever implemented in consume electronics? OsmanRF34 (talk) 14:12, 6 December 2012 (UTC)
 * What you're talking about is an inertial navigation system. They have many uses, but also substantial limitations, and I'm not aware of any being developed for the consumer electronics market.  They're certainly small enough, but GPS is also small, cheap, reasonably battery efficient (I've no idea the relative power requirements of the two, but neither is excessive), and more error tolerant.  Currently, INS tends to be used for "backup in the event of GPS blackout", but that's not a scenario the average consumer really worries about.  Even then, you're talking INS on top of GPS rather than instead of. &mdash; Lomn 14:29, 6 December 2012 (UTC)
 * There was, in fact, one developed for consumer use in cars, before GPS was deployed. I remember seeing an article about it sometime in the last year.  I can't remember the name, and the article, at a quick scan, doesn't seem to mention it.  Maybe someone else remembers more specifics? --Trovatore (talk) 17:44, 6 December 2012 (UTC)


 * Clarifying: note that a GPS receiver isn't "connecting" to a satellite. There's only a radio receiver listening to satellite signals.  That's why I note above that GPS is reasonably power efficient.  Contrast with something like a satellite phone, which does have to make those transmissions.  Alternately, consider cell phone specs for "talk time" (transmitting) vs "standby time" (not transmitting).  So for both GPS and INS, you're looking at power for computation and display, but not transmission.
 * Yes, that's the name,INS, I couldn't remember it. I suppose ships and aircrafts absolutely need a backup system to the GPS. I was talking of INS instead of GPS, not on the top of it. I don't know how efficient GPS is, maybe it's doing a lot for little battery consumption, but in a pocket device like a smartphone, GPS can't be operated for more than 24 hours. Take also into account that GPS has its accuracy artificially reduced, can be jammed, and doesn't work well in places like caves and tunnels. So, at least in some circumstances INS would be superior to GPS. OsmanRF34 (talk) 14:49, 6 December 2012 (UTC)


 * Although when I see the picture to the left, I see that INS won't be of any use in its present form for most civil uses. OsmanRF34 (talk) 14:50, 6 December 2012 (UTC)


 * GPS' "selective availablility", the means of artificially increasing position error, has not been used in 12 years. New GPS satellites do not even possess the capability.  Additionally, the constellation is currently being upgraded to possess the same jamming-resistant attributes that military receivers currently field.  Could it still be jammed on top of that?  Sure, but I submit that if that's the case, you've got bigger problems than losing turn-by-turn direction.
 * As for the "24 hours usage" bit, the key reason for that is continuously processing position updates. This isn't strictly necessary; it's of course straightforward to only turn a GPS on when you need a position fix, and that allows you to stretch your battery life almost indefinitely.  INS, on the other hand, requires that continuous processing or it's meaningless.  Under power-conservation scenarios, GPS will nearly always beat INS. &mdash; Lomn 15:00, 6 December 2012 (UTC)
 * GPS can be jammed locally, and apparently this is fairly hard to trace, though there are some (ahem) trying to co-opt people's cell phones to get them involved in the hunt, supposedly voluntarily. Wnt (talk) 17:39, 6 December 2012 (UTC)
 * Yes that's a place where one really would like to have an inertial navigation system as well as it would help detect and track down such interference quite easily. Dmcq (talk) 11:52, 7 December 2012 (UTC)
 * In fact thinking about it, if you could get approximate positions using the local cell phone masts then after driving around a little with an INS one could probably work out ones position fairly accurately, if the system just assumes one is not moving if there is no bumping then it wouldn't start drifing when you stop. Dmcq (talk) 14:33, 7 December 2012 (UTC)

Jump Starting a Car
I had to jump start my wife's car yesterday and it got me thinking. The instructions clearly warn you to connect positive to positive and negative to the engine block or chassis, not negative to negative which could cause an explosion. What I can't understand is why that would be. The negative terminal is just a ground (earth) terminal, right? So wouldn't it be the same electrically to connect to either the negative terminal or the frame? Also, why is shorting across the terminals so bad? Please explain as simply as possible, while I consider myself reasonably well educated electrical circuits and electricity have always seemed like magic to me! Tobyc75 (talk) 19:03, 6 December 2012 (UTC)
 * The final connection, which closes the circuit, is prone to creating a large spark. Car batteries can potentially outgas (hydrogen, I think?) which could result in that spark triggering a fire.  Thus, the last connection should be made away from the batteries.  Now, the positive connections basically have to be made on the batteries, so they're out for being last.  The negative connection, though, can be made elsewhere on the frame (and many modern cars have a specific "connect negative here" bolt elsewhere in the engine compartment.  As for shorting across terminals, short circuits tend to spark and create a lot of heat (since batteries aren't actually perfect conductors).  We've already covered why spark is bad, and heat just makes that situation worse. &mdash; Lomn 19:28, 6 December 2012 (UTC)
 * And, as I understand, the first connection has to be positive pole of the bad battery to the positive pole of the good battery, right? Would this connection, if made the other way round, also be able to cause a spark? OsmanRF34 (talk) 19:51, 6 December 2012 (UTC)
 * The two positives should be connected first, but can be safely connected in either order. The first connection will put the other end of the positive cable at +12 V (or near enough; the dead battery likely still has some voltage), but since the cars are insulated from each other, "+12 V to what?" applies and there's no real effect.  You can touch the far end of the positive cable to basically anything that isn't the original car with zero effect.  Once you connect the second end of the positive cable, the two ends flopping around are both on the negative cable, disconnected from everything, and so still can't do any damage.  Contrast with making your first two connections to the terminals of a single battery -- now you can easily get a big spark (and short the battery) by letting the two floppy ends touch.  I've done it by accident, and it's rather impressively scary if you're not expecting it.  As best I can tell, the biggest reason to specify "Dead Positive, then Live Positive, then Live Negative" is so that you don't need to walk to the other car between the second positive connection and the first negative connection.  For that matter, I think even which negative is battery vs frame (or if you put both on the frame) should be irrelevant electrically; it's just a matter of reducing spark risk. &mdash; Lomn 21:39, 6 December 2012 (UTC)
 * Connecting the black jumper cable between the negative terminal of the "live" car's battery and the chassis of the "dead" car works better than the other way around because it creates a circuit with lower resistance.  The starter motor is usually (always?) grounded to the engine block, and during normal operation the return path for the starting current is via the battery-to-chassis ground cable.  The starting current passes through neither battery-to-chassis grounding cable when jump starting the recommended way, but passes through both battery-to-chassis grounding cables when doing it the other way around. -- ToE 19:11, 12 December 2012 (UTC)
 * Alright, thanks for the excellent response. I hadn't even thought of producing hydrogen and that causing the explosion.  I definitely saw a big spark when I connected the final clamp to the engine, so that would have been bad. Tobyc75 (talk) 20:44, 6 December 2012 (UTC)


 * IMPORTANT SAFETY NOTE: Cars of American and Asian design origin are standardised on battery negative connected to earth (chassis).  However, cars made in Britain were often positive to earth.  Never use jumper leads on veicles that have differing nominal battery voltages (eg never use a 24V truck to start a 12V car & vice versa; never use a 12V car to start a 6V volkswagen - push it instead).  Wickwack 124.182.154.215 (talk) 01:57, 7 December 2012 (UTC)


 * Good points about safety, though positive-earth cars are now rare in the UK. Most modern batteries are semi-sealed, so the concern about hydrogen ignition is probably unnecessary.  I've jump-started many vehicles just by using the battery terminals, and observed that rescue services no longer follow the advice to connect to the frame.  There's no harm in continuing the practice though, for maximum safety.    D b f i r s   10:13, 7 December 2012 (UTC)


 * What about Indian or European cars? Are they positive or negative to earth? OsmanRF34 (talk) 18:41, 7 December 2012 (UTC)


 * I think the last positive-earth vehicles anywhere in the world were manufactured in the late 1970s because negative-earth became a world-wide standard around then. There is possibly some exception somewhere, particularly if some manufacturer is still using an early 1970s British design, but I don't know of any.    D b f i r s   23:28, 7 December 2012 (UTC)
 * My father's Morris Minor actually had a notice in the engine compartment warning that it was negative-earth. This was in the early 1960s in the UK. I've never actually seen a positive-earth car; my first car was a Hillman Minx from 1969, which was negative-earth. Alansplodge (talk) 21:27, 9 December 2012 (UTC)


 * "I've never actually seen a positive-earth car" I don't believe you, grins. TR4 for example. Greglocock (talk) 03:39, 10 December 2012 (UTC)

Accessing the journal Science from home via institution
Hi, Do you know if it is possible to access the journal Science with an institutional login from my home computer? Most other journal websites (Science Direct/Elsevier, Nature etc) allow me to do this by pressing "Institutional login" (or similar) and logging in using my university credentials, but I can't find it on Science. The article in interested in can only let me login if I have personal (purchased) access. I know I can use remote desktop to connect to the campus computer, but it doesn't seem to be working at the moment. Thanks, 90.203.22.59 (talk) 20:40, 6 December 2012 (UTC)
 * Most educational institutions have some sort of proxy service through which you can access google scholar and from there fulltext of any journal your university has a subscription to. Assuming the institution being referred to is a university, I'd start with the library homepage and then look for something labeled 'databases'. Alternatively Google Scholar may have a direct institutional login, I've never explored that option myself. (+)H3N-Protein\Chemist-CO2(-) 21:11, 6 December 2012 (UTC)


 * You should check with the e-Resources librarians at your institution. They should know what they are or aren't paying for, whether you can use your institutional login at home, or what. --Mr.98 (talk) 21:36, 6 December 2012 (UTC)


 * At some institutions, you can sign into the library webpage and then access those resources through that page. Mattmatt1987 (talk)


 * I agree with the above posters - check the library web site or call the library for details. If it can't be done through their site, then it is also possible that your university offers a VPN client. This will basically make your home computer connect to the university's network over the internet, and act as if it is a computer directly on that network. You can then visit the journal's page the same way as on campus, because you will be accessing it from a university IP address. 209.131.76.183 (talk) 15:55, 7 December 2012 (UTC)
 * Don't be surprised if this does not work consistently. The IP addresses for VPN connections are often clustered in a specific subnet; this allows content providers to price electronic resources with versus without VPN connections allowed, the latter being less expensive (because they consistently result in fewer concurrent connections).  -- Scray (talk) 00:22, 8 December 2012 (UTC)

power with a highly damped, frictionful system
Walking is highly damped as following the removal of force, velocity ceases quickly. The power consumption due to friction is F*v, where F is the frictional force. Hence when the animal is not accelerating, power is constant and dependent on velocity. But sometimes, the animal has a net acceleration, and force applied is more than just friction. What I can measure from the video feed is net acceleration. How do I calculate power consumption in that case?

How would I make a good estimate of the coefficient of friction (kinetic) from experimental data of behavior? 128.143.100.192 (talk) 20:58, 6 December 2012 (UTC)


 * I doubt that friction has much if anything to do with it. Could it be friction between the feet and the gound surface?  No, as when in contact the feet do not move laterally.  Could it be friction within the joints of the legs?  Maybe, and this could be measured in a suitable jig with the subject given a spinal block aneasthetic.  But I expect such friction will account for only a very small part of the energy consumed in walking.  Friction-wise, walking up stairs must be much the same as walking on a horizontal surface, but it sure makes you breathe harder.  The answer probably lies in the chemo-thermodynamics of muscle contraction.  Work done on the air due to the air's viscosity must be very small.
 * Why do you think that walking is highly damped (presumably you meant damped by friction)? If you are walking, and decide to stop, you stop instantly without conscious effort.  But this does not mean there is no unconscious muscle action.  The whole time you are not in bed, the automatic regions of the brain constantly evaluates position sensors in the limbs, the ear canals, and the visual scene and continually sends motor signals to the muscles to keep you stable and upright.  This same automatic system means that when you stop walking, while inertia would tend to keep you going forward, the right muscles automatically apply force to overcome this inertia.
 * Wickwack 124.182.154.215 (talk) 01:47, 7 December 2012 (UTC)


 * It's definitely friction from the ground. Consider biking or ice skating. 71.207.151.227 (talk) 03:09, 8 December 2012 (UTC)
 * Ice skating is totally different in terms of muscle movement. Biking is heavily affected by tyre deflection and wind resistance.  At walking speeds, windage has a lot less influence.  However, as I said, how can you have friction with the ground when in walking the feet do not slide along the ground.  Can you explain your assertion?  Wickwack 121.221.91.19 (talk) 07:23, 8 December 2012 (UTC)
 * It's impossible to walk on a frictionless surface. I think external friction >> internal friction-- note that ones ankles start to get sore (at least for beginners) in ice skating because of the increased role of internal friction to stop movement which the body is not used to. 128.143.1.222 (talk) 17:07, 8 December 2012 (UTC)
 * Yes, it is impossible, because your feet slip - you either fall over or you skate. However, that has no relavence to the OP's question.  I'll say this one more time, (for anyone who has completed high school science, it should be obvious):  In walking, your feet do NOT slip.  If there is zero slip, there is zero energy loss (or power consumption) due to friction with the gound.  So, you cannot have external friction greater than internal friction, unless "external friction" includes the power loss in displacing the surrounding air.  Wickwack 124.182.187.188 (talk) 11:06, 9 December 2012 (UTC)
 * Hurting ankles does not imply friction. They hurt because skating involves unusual muscle action, leading to stress in unusual places. Also perhaps shock loading due to beginers hitting the surface hard.  Wickwack 124.182.187.188 (talk) 11:10, 9 December 2012 (UTC)


 * From our power (physics) article:

$$P(t) = \mathbf{F}\cdot \mathbf{v}$$


 * And we also know that:

$$\mathbf{F} = ma$$


 * So, it should be possible to determine power usage from mass, velocity, and acceleration. StuRat (talk) 05:23, 7 December 2012 (UTC)
 * Not true. Consider a creature or a car moving at constant speed i.e., zero acceleration.  Do you really believe the energy expended (energy is power x time) is zero?  For a simple mass in a vacuum it will be zero, but real creatures and cars are not like that.  Wickwack 121.221.90.227 (talk) 10:29, 7 December 2012 (UTC)
 * Unless this is a homework problem and the OP is being asked to consider a spherical animal in vacuum, in which case you would compute $$P=m\mathbf{a}\cdot\mathbf{v}$$ for instantaneous power, but since a and v are presumably in the same direction you have $$P=mav$$, and to approximate this value over a finite amount of time you can just take $$\bar{a}=\frac{\Delta{v}}{\Delta{t}}$$ and $$\bar{v}=\frac{\Delta{x}}{\Delta{t}}$$. 24.92.74.238 (talk) 02:50, 8 December 2012 (UTC)
 * This is for trajectories of animals that can only walk forward, v and a can't always be in the same direction, or the animal would accelerate to infinite velocity. Interestingly enough, when I plot acceleration against velocity, I get a straight line upper bound in my plot where acceleration is positively correlated with velocity. This is interesting, because I would have expected a straight line downwards or at least a parabola where acceleration starts to reverse at high velocities. I will upload a plot later.
 * Currently the way I am doing things, the animals appear to exert close to zero power since the average acceleration is close to zero (when considering both deceleration and acceleration) and I can't figure out how to estimate the coefficient of friction, given behavioral data. 71.207.151.227 (talk) 03:09, 8 December 2012 (UTC)
 * To see why you P = mav guys have not understood it, please try the following experiment: Stand up, and wait for your breathing to return to your rest rate.  Then, swing your arms together back and forth like a pendulum, about 30 degrees or so each way, about two swings per second. This is quite comfortable and easy for healthy people.  Keep it up for a minute or more.  Mechanically, neglecting joint friction and tissue viscous loss, there is no power consumption over a full swing cycle, just as for any pendulum.  Notice how your breathing rate significantly increased?  That's because the exercise is consuming oxygen, which means you are expending energy.  Walking comprises such oscillatory muscle movements which consume energy above that required to actually move you along.  An analogy is a piston heat engine.  Not all the heat energy is converted to mechanical effort, and there is energy consumption, not due to friction, while ever the engine is running without any mechanical load.  Walking is not a thermochemically efficient process.  Energy is consumed in contracting muscles even if there is no mechanical load.  Wickwack 121.215.62.189 (talk) 04:43, 8 December 2012 (UTC)
 * To answer the OP's apparent need, he could put a test human or test animal (whatever he wants) on a powered treadmill and feed the human or animal with air via a mask as is sometimes done in sports research. Measure the oxygen consumption at various walking speeds.  The energy consumption over a period of time, and thus the power consumed in watts is easily inferred by standard chemical calculations for the mass of oxygen consumed.  This has of course been done before - I expect a search will reveal data for humans and perhaps other mammals.  It will be of the form  P = Krest + Kmove x Speed + Kmove x Speed2.  The OP could then interpolate values for the animal he is interested in, on the basis of body mass.  Wickwack 121.215.62.189 (talk) 04:43, 8 December 2012 (UTC)

Purpose of flat tunnel roof
What's the purpose behind the flat internal "roof" in tunnels? Since Sasago Tunnel did not collapse after these roof panel fell out, I can only assume they are not load bearing. Dncsky (talk) 22:27, 6 December 2012 (UTC)


 * My initial guess, and I know nothing about this, is that the flat roof simply conforms to the flat tops of vehicles. Bus stop (talk) 22:32, 6 December 2012 (UTC)
 * But doesn't the roof restrict vehicle height? Dncsky (talk) 22:36, 6 December 2012 (UTC)
 * Yes, I see your point. I was just about to revert my initial post. Bus stop (talk) 22:38, 6 December 2012 (UTC)


 * Also, is this a Japanese only thing? I've never seen a tunnel with an internal roof like that, though admittedly I haven't been to a lot of places and haven't seen a lot of tunnels. Dncsky (talk) 22:38, 6 December 2012 (UTC)


 * This article (which may only be accessible if you visit it from Google) says the space above the panels is for ventilation. A major concern in long road tunnels is where a fire in one place blocks traffic, and the smoke travels back down the stationary jam behind the blockage, choking more people. Some tunnels have ducts, some have impellers that are suspended in nacelles, and some use a concrete ceiling plane.  A concrete barrier means that fires and explosions (e.g. from tanker lorries)  are less likely to penetrate into this ventilation space.  They can also run utilities (lighting, phone, CCTV) through there, and again they're better protected from fire (naturally they could also run these through fire resistant conduits).  The Sasago incident resembles the Big Dig ceiling collapse. -- Finlay McWalterჷTalk 23:00, 6 December 2012 (UTC)
 * Thanks. I've seen ducted fans in long tunnels, but never the roofs. Guess I'll keep an eye out next time I travel. Dncsky (talk) 23:10, 6 December 2012 (UTC)


 * This page of info about the ventilation system in the Gotthard Road Tunnel shows a very similar ceiling construction to the Sasago tunnel, and explains some how this giant tunnel's ventilation is done. -- Finlay McWalterჷTalk 01:25, 7 December 2012 (UTC)


 * From the diagrams, it also looks like the side panels either contain lights or reflect lights (probably painted white, versus grey concrete). StuRat (talk) 05:15, 7 December 2012 (UTC)


 * I've been through some mountain tunnels in America, and they all seem to be this box-shape design. So I would expect they are made the same way as the one in question. ←Baseball Bugs What's up, Doc? carrots→ 10:10, 7 December 2012 (UTC)

Africans and headlice
At my day program, we were having a discussion on hygiene, and the job coach, who is African-American, mentioned that Black people do not get headlice. The word for "louse" is present in every language in the world, appearing on the Swadesh list, the Dolgopolsky list and the Leipzig-Jakarta list, so presumably it is present in Sub-Saharan African languages too. Is it really true that African-Americans don't get headlice? Given the linguistic universality of the term, I find it hard to believe. If it is true, then how did all those Bantu languages come upon their words for louse? Enzingiyi (talk) 22:33, 6 December 2012 (UTC)


 * Well, there are other lice but head lice, so a language could have a word for them, without headlice existing. HiLo48 (talk) 22:38, 6 December 2012 (UTC)


 * I also seriously doubt that there's a lexeme for them in every language in existence, though I could see it being generally ubiquitous. Snow (talk) 23:11, 6 December 2012 (UTC)


 * In a study in 1985, only 0.3% of African-American children were infested with head lice, in contrast with 10.4% of the non-African-American children in the study. However, according to the same web page, there's an African variety of head lice, which is better adapted to being able to hold on to kinky hair than the European variety of head lice is.  Red Act (talk) 23:05, 6 December 2012 (UTC)
 * Here are two more studies sharing the conclusion that children of African descent are less likely get headlice.Dncsky (talk) 23:17, 6 December 2012 (UTC)

Interesting to know why your colleague thinks do-rags originated with black men in prison. μηδείς (talk) 01:41, 7 December 2012 (UTC)