Wikipedia:Reference desk/Archives/Science/2013 February 27

= February 27 =

practically, how many joules does it take for a helicopter to lift itself?
for a given weight, practically, how much power is required for a helicopter to ascend, say, 50 meters? 178.48.114.143 (talk) 00:11, 27 February 2013 (UTC)
 * It is not easy to answer this question without knowledge of other things like the helicopter's efficiency (how much of the energy goes to producing lift). However, we can make a rough estimate of how much work that needs to be done by it to rise 50 meters using the definition of work (power is different in this context; it's the rate at which this work is done, which would depend on how much time you give the helicopter to ascend).
 * The work is done against the force of gravity. If we can ignore the initial acceleration required to start the helicopter moving upwards, then the force exerted by the helicopter on the air is equal to its weight plus air resistance. If we can neglect the work done against air resistance, then the work is the work done against gravity only, $$W = -W_{gravity} = -(-mgh) = mgh = 50 m \cdot mg J$$, where m is the mass of the helicopter and g is the acceleration due to gravity.
 * Whether this reflects reality, I don't know, because this is only the work done in lifting the helicopter, and does not include the work done to overcome friction in components like the bearings, nor air resistance.--Jasper Deng (talk) 00:34, 27 February 2013 (UTC)
 * Specifically, that assumes that a helicopter is a 100% efficient machine - and it's VERY far from that! Energy efficiency in transportation says "The Sikorsky S-76C++ twin turbine helicopter gets about 1.65 mpg-US (143 L/100 km; 1.98 mpg-imp) at...260 km/h" - but when hovering, miles-per-gallon is zero! However, the helicopter probably uses about the same amount of power per hour at hover as at speed - so we can get another estimate by assuming that 143 L/100km at 260 kph - means that it consumes 143 liters every 20 minutes...or around 400 liters an hour. Gasoline says that gasoline contains 35 MJ/L - so the helicopter is consuming around 14,000 MJ per hour.
 * That's likely to be an over-estimate because there is no air-drag in hover. Also, when close to the ground, the helicopter uses less energy than when flying at high altitude because of "ground effect".  But we could probably say that the number is between 5,000 and 10,000 MJ per hour in low-altitude hover.  If it takes maybe 20 seconds to get to 50m altitude then (for the Sikorsky) the number is between 2.8MJ and 5.6MJ.  Interestingly, Jasper's calculations for a 5,000kg Sikorsky suggests 2.5MJ is required just to overcome gravity.  But a lot depends on the time it takes to get up to 50m...it's complicated!
 * SteveBaker (talk) 14:43, 27 February 2013 (UTC)


 * I think I answered this pretty practically in November 2012: "about a hundred horsepower" (or around 75,000 joules per second) for a small helicopter (referenced by a few reliable sources, particularly the Pilot's Handbook of Aeronautical Knowledge and Helicopter Flying Handbook from the FAA). The critical thing to keep in mind is that a helicopter pilot can directly control the aerodynamic efficiency of the helicopter - so the total engine power required to hover stably, or lift off, depends on how the pilot set the collective and other controls.  The collective sets the angle of attack for each blade, and determines how efficiently engine-power creates lift - the rest of the engine power is wasted as drag on the rotor-blade.  The pilot can throttle the engine - changing the total power delivered - independently.  (This is one of the main reasons helicopters are harder to fly than airplanes!  You want to go up, so you think "increase engine power!"  But that is frequently the wrong control change - you should often keep the engine power exactly the same and use collective to increase aerodynamic efficiency of each blade, causing the helicopter to rise).  More on this in chapters two and three of the Helicopter Flying Handbook; the correct pilot control action really depends on what type you're flying: e.g., "Some helicopters do not have correlators or governors and require coordination of all collective and throttle movements. When the collective is raised, the throttle must be increased; when the collective is lowered, the throttle must be decreased."  Nimur (talk) 19:14, 28 February 2013 (UTC)

Solar powered garden fountains - suitable latitudes?
I'm interested in getting a solar powered garden fountain, but am concerned about it's performance because I live in a rather high latitude (40+° N). Are there solar panel suitability/performance range maps available? I am imagining something like how the USDA maintains plant hardiness/suitability zone maps for North America. The Masked Booby (talk) 02:12, 27 February 2013 (UTC)


 * Some questions:


 * 1) Will the solar panels point straight up, or be tilted towards the south ? Will they all point in the same direction ?  (I assume they won't actually track the Sun.)


 * 2) How big are the solar panels, or, alternatively, what's their peak output ?


 * 3) I assume the fountain only needs to run during the day, but what hours, specifically ? (Trying to run the fountain near dusk might be difficult.)


 * 4) When will you shut the fountain down for winter, and start back up in spring ?


 * 5) Will the solar panels be in shade for any part of the day ?


 * 6) What's the weather like there ? Are there many cloudy or rainy days ?  And do you care if the fountain runs at those times ? StuRat (talk) 02:33, 27 February 2013 (UTC)


 * Would an article like this help? Vespine (talk) 02:43, 27 February 2013 (UTC)


 * The point here is about how much efficiency you need. It may be that the pump will work even with the panels pointed in the wrong direction, etc - or it may be that you have to have so many solar panels to make that work that way that you have to do something better.  But let's talk about how to get good efficiency.


 * The efficiency of a panel as a function of the angle between the surface and the suns' rays varies a bit depending on the kind of panel - but straightforwardly, the number of photons you intercept depends on the area of the panels as seen from the direction of the sun...to put it another way - it's proportional to the area of the panel multiplied by the cosine of the angle between the direction the panel is pointing and the direction of the sun. Some panels have reflective glass or plastic that reflects the sun rather than absorbing it when the sun is coming from an extreme angle - which make it much more important to point the panels towards the sun.
 * But the sun moves across the sky throughout the day - and through the year - and it depends on the latitude of the place where you live.
 * So, echoing SutRat's answers:
 * Will the solar panels point straight up, or be tilted towards the south ? Will they all point in the same direction ?  (I assume they won't actually track the Sun.) -- You get solar panels built into other equipment that points straight up, or that you can fix at some specific angle, or that you can manually adjust, or that are driven mechanically to preserve maximum output through the day.  ::#* If you want to position the panel once and never change it - then you'll presumably want to aim it where it points towards the sun at midday in mid-spring/mid-autumn.  That'll produce less power at noon in summer and winter and less still in early morning and late evening...but it's a compromise.
 * If you're prepared to go out there and reposition the panel every few months to account for the changing position of the sun at zenith...then you'll get optimum power each day at noon - but worse in morning and evening.
 * If you don't mind popping outside to adjust your solar panels a few times a day to compensate for the motion of the sun across the sky - then you'll get better still output from your panels.
 * If you have a fancy motorized mount with a computer that knows where the sun will be minute by minute through the day - then you'll do even better.
 * How big are the solar panels, or, alternatively, what's their peak output ? -- This depend on how much power does the motor requires - but also on how many batteries you have (if any).
 * If you don't have any batteries in the system, then the panel has to be large enough to collect diffuse energy from the sky even when it's a cloudy day in mid-winter. That's a LOT of solar panels!
 * If you have have batteries that can accumulate power when the sun goes behind a cloud, then you don't need as many solar panels to continue to generate power when that happens.
 * If you have enough batteries to run the pump for 24 hours without discharging, then you don't need so many solar panels because the excess power that they generate at noon makes up for the deficiency at morning and evening.
 * If you had very, very large batteries, you could charge them in the summer and you wouldn't need such large panels to get your through the winter. Clearly that's unreasonable - but in many places you can feed the excess solar power that you generate in the summer back into the electricity grid and get a reduction in your electricity bill.  You can use that money to buy electricity in the winter...so (in effect) the entire electricity grid becomes a giant year-long-capacity battery!


 * I assumed no batteries and no operation in winter, when presumably ice will form. StuRat (talk) 16:21, 27 February 2013 (UTC)
 * I live in the north of England at a latitude of 53 degrees. My sister had a small, solar powered fountain in her garden. When the sun came out the fountain ran and when the sun went behind a cloud it stopped. Unless the panels are connected to batteries I suspect this would always be the case although they don't tend to mention it when they advertise them. Richerman ''   (talk) 10:35, 28 February 2013 (UTC)
 * At 41 degrees at the equinox and kindof mid day it can be stopped by your body (or was it hand?) from a few feet or meters away. Also clouds. This was at least an 8 year old model, though. Sagittarian Milky Way (talk) 12:35, 28 February 2013 (UTC)

Dark matter?
I don't pretend to understand the fine points, but for those here who do, what would be reasonable conclusins to draw from this article, which posits that there may be no such thing as dark matter? ←Baseball Bugs What's up, Doc? carrots→ 04:46, 27 February 2013 (UTC)
 * The long-and-short of it is this: There's a prediction about how the universe should behave given a) what we know about the laws of physics and b) how much matter we can reliably identify in the universe. The problem is that the universe does not obey the predictions in certain ways; and that means that either a) or b) is wrong: That is either the laws themselves are incorrect, and need tweaking or revising, or that there's some large amount of matter which is previously (and still) unaccounted for which is throwing off the observations.  B) is what is meant by Dark matter, which just means "matter which, if it existed, would allow us to match theory to observation without having to rewrite the laws of physics".  There's been lots of complex proposals as to what this dark matter is, but nothing has ever definitively been proven.  The article is about looking at the problem from the other perspective: that instead of trying to come up with some "Dark matter" which is unobserved (and maybe unobservable) to explain the problems, instead the article is proposing that tweaks need to be made to the theory instead.  -- Jayron  32  05:04, 27 February 2013 (UTC)
 * The problem with many articles like that is they select the bits that support MOND without touching on the parts that indicate a theory like that is wrong. What one should try and do in science is try and falsify a hypothesis not just calculate more of the thing you set the theory up to explain in the first place. See the criticisms section of the MOND article. It may be something like it will work but there's definite difficulties with the straightforward versions. Dmcq (talk) 11:10, 27 February 2013 (UTC)
 * That's a bit much. The criticisms section there goes on about the Pioneer anomaly being explained by MOND, whereas that is explained very well by the pressure of reflected heat radiation, so what's happened to the extra MOND should have contributed? Does one then just ignore that it predicted yet more deviation? Dmcq (talk) 11:16, 27 February 2013 (UTC)


 * MOND isn't promoted as a full theory of gravity by anyone, not even its creator (I heard him lecture). Basically, it fits one system, that of galactic rotation, perfectly, where dark matter explanations are rather inadequate. It does not do well in explaining larger-scale dark matter phenomena. Basically, the founder of the theory said this: because the current theory that is used to explain this phenomenon (galactic rotation) is pretty bad at it, and because MOND is really good at it and can put up decent fitting for some larger-scale stuff, then MOND is worth researching more.
 * That is, it is an incomplete theory by any standard and hardly offers a replacement for the dark matter explanation. That said, both dark matter and MOND still lack a mechanism – a particle or something that can drive it. Dark matter has some ideas for that particle, while MOND has none. SamuelRiv (talk) 18:02, 27 February 2013 (UTC)
 * How are dark matter explanations inadequate for galactic rotation? It's hardly surprising that MOND explains rotation curves accurately; they were designed specifically with that purpose in mind.  In the Bullet Cluster, dark matter passed right through a galaxy and is significantly offset from the galaxy's core.  That's very hard to explain with MOND, and completely consistent with dark matter.
 * One other thing that hasn't been mentioned is that one can directly measure, from the cosmic microwave background and distant supernovae, the density of baryonic matter (protons + neutrons) AND the total density of matter in the universe. The baryonic density can also be measured by looking at galaxies and summing up their masses.  All these measurements indicate baryons only comprise 17% of total mass.  So it's not the case, as Jayron implied, that dark matter allows us to avoid rewriting the laws of physics.  The laws of physics have to be rewritten no matter what, because if dark matter exists, it cannot be made of any known particle.  That would require a fundamental rewrite of the Standard Model likely much more drastic than MOND.  --140.180.255.158 (talk) 21:43, 27 February 2013 (UTC)
 * I think MOND is more drastic. The Standard Model already has dark matter in the form of neutrinos, and it's not much of a leap to suppose there are other particles like neutrinos but with a higher mass and even weaker coupling to ordinary matter. It's also not hard to add those particles to the Standard Model. But I think no one has found a theoretical extension of the Standard Model (or general relativity) that implies anything like MOND. -- BenRG (talk) 18:09, 28 February 2013 (UTC)

American and Russian Tank Designs
Which tank is better in your opinion, the cheap Russian tank which can be prodeced in large numbers , or the expensive American and German tanks of fewer numbers - if we assume that military budget is equal for both sides- or in other words which is more important the numbers or quality ? — Preceding unsigned comment added by Tank Designer (talk • contribs) 07:46, 27 February 2013 (UTC)
 * It isn't a straightforward question but I think it is generally agreed that in the case of the German tanks the best was the enemy of the good. I have a suspicion that some of the high up German technocrats actually wanted Germany to lose the war. I think you have a bit of a misapprehension though. The main Russian and American tanks were good designs and better than the earlier German tanks. Dmcq (talk) 10:16, 27 February 2013 (UTC)

Well thank you for this information, but I am asking about the tanks in present time (M1 Abrams , T90 and Leopard 2A6 ,) it is known that T-90 is lighter and carry lower amount of ammunition -22 rounds- , and M1 Abrams can carry 42 rounds , so what case is better : larger number of T-90s or better quality of M1 Abrams-also Leopard 2A6-? It is known that they are similar in armour and firepower, but the difference is in safety and comfort of tanks. — Preceding unsigned comment added by Tank Designer (talk • contribs) 11:29, 27 February 2013 (UTC)


 * Your recent questions, including this one, have asked for opinions for which whole books can be written, which may, but most probably not, provide a definitive answer. This Reference Desk is intened to help people with specific questions about science subjects. The Reference Desk is capable of coming up with very good answers and assistance on questions on specific science topics. It is not intended to provide opinions on intangibles and broad subjects. Though some may offer opinions, you are likely to form a much better opinion if you do your own research. If you have a deep interest in tanks, why not visit your State Library? You can probably get books on inter-library loan from a military college in your country, or even visit the college library yourself.  What you need is not restricted information.
 * There is not likely to be one "best" tank - what sort of tank is best for any country depends on a myriad of variables, a lot of which will be intangible. For example, what sort of tank is "best" depends on what sort of battles will occur - small, large, against tanks or against other systems.  And what sort of battles, or applications for the tanks depends on military planning.  For instance, in the military planning of my country, (Australia), it has been anticipated that military operations may involve long penetration deep into other country's jungles, where the ability to tow trucks out of bogs & rivers, rapidly construct bridges, and be largely field repairable is more important than accurately firing dirty great shells.  So tanks were purchased that look very different to that of other countries that wanted tanks for a rapid shock and awe penetration (firing shells) across open terain or deserts.  The cost of a tank is not at all closely related to its complexity, capability, or sophistication. For example, factory labour cost in Russia and China is well below that of the USA.  Depending on Govt policy, whole of life cost may be more important, or less important, than ex-factory cost.  Another factor is that the purchase of expensive military hardware is always a political decision - will it provide jobs in a senator's or minister's home State?  Will it provide technology experience transferable to other industries? - always an important reason.  One strongly supects that, on purely objective operational grounds, countries such as Australia, and perhaps even the USA, would like to purchase Russian equipment.  But that is never going to happen for obvious political and "face" reasons - on both sides.  These factors are always at least just as important, and usually more important, than straight technical merits of military hardware design.  This is why Australia builds its own submarines - they aren't as good as Russian subs, they aren't as good as USA subs, they aren't as good as other subs - we know that.  But it means $billions injected into local industry, and not $billions injected into some other country.  Economic strength is important, wins wars, and prevents wars.
 * Finally, just like the strength of a patent is unknown until tested in court, what tank or any other sort of military hardware is "best" cannot really be known until it is used in a major war, and we found out who won, and we have the oportunity to post mortem research and figure out why they won.  I'm sure that if you were to ask 5 American generals and 5 Russian Generals in a off-the-record free discussion which is the best tank, you'll get 10 different answers with 10 sets of different reasons.
 * Wickwack 58.170.157.73 (talk) 13:15, 27 February 2013 (UTC)
 * I agree with all of what Wickwack has pointed out, but I'll also note that the premise of the question seems flawed. The T-90 and the M1 Abrams aren't that different in unit cost (perhaps a factor of 2 or 3), but more importantly, the T-90 isn't "produced in large numbers" relative to the Abrams; there are roughly 4 Abrams for every T-90.  Generally, it looks like you might be interested in reading on "quality vs quantity" topics with respect to the military.  Certainly there are some good case studies like the air war in Vietnam. — Lomn 13:38, 27 February 2013 (UTC)


 * These questions don't have black-and-white answers. : For example, suppose "team A" in a conflict has a vehicle which has a "worth" in battle that is twice that of the vehicle that "team B" manufactures at half the price. Which is best?


 * If you have to ship your army halfway around the world (as the USA did for wars in Iraq and Afghanistan) - then it's clearly a bad idea to have to ship twice as many vehicles, provide them with fuel, feed and provide sleeping quarters for their crews, maintain them...etc. So for "long distance" wars, team A's approach with fewer, high quality vehicles is clearly a good idea.
 * For a war that's close to home, a simpler design that needs less technical sophistication and training from the crew, probably less maintenance too - is clearly a good thing, so maybe team B's approach is better. Short supply lines make it easy to get the extra fuel, food, etc to the battle front.
 * You also have to consider that tanks have to do other things than fighting other tanks. If you need to have forces spread over a long border with a hostile neighbor to protect against incursions from light-weight forces - then you might just need a large number of cheap vehicles just to have them spread along the entire length of the border...so again, team B have the best approach.
 * There are other less tangiable considerations too - cultural matters: Team A will have half the number of people out there in harm's way than team B. If team B's people place a lower value on human lives than team A - then they may not care much about that...but team A might well believe that having half as many people killed or injured in battle more than justifies the more costly vehicles.
 * High-tech vehicles require more training for crew and maintenance people than lower-tech vehicles. That's justified if your army is staffed by career professionals who stay in service for decades.  But if you rely on conscripts or part-time soldiers who come into the military, get brief training and only work out a short stint before returning to daily life, then team B's K.I.S.S approach is clearly better.  Issues as fundamental as how good your nations' educational system is has an impact on what kind of tanks you make!


 * I'm sure I've missed many arguments here - but the point is that there are a huge number of trade-offs. Simple calculations about ammunition capacity and armor thickness are an overly-simplistic way to look at this problem.  The success or otherwise of team A & B's approaches depends on much, MUCH more than that!  To couch this in purely military terms: In modern warfare, you need to consider "strategy" - not just "tactics".
 * SteveBaker (talk) 13:43, 27 February 2013 (UTC)


 * You have made very good points, and you have remined me of another important factor: In anticipation of a future time of major war, where a rapid acquisition of large numbers of tanks may need to occur, military planning may involve the conversion of truck and car factories to large scale tank production.  Any tanks made under such circumstances will be a design, retrieved from the files, that is compatible with existing or easily adapted factory tooling, standard available engines, compatible with assembly line floor widths and crane load capacities and the like.  The rationale may be to settle on a tank design that is not the best possible, whatever that means, but one that can be made in a hurry.  A good tank you have in quantity now is better than a perfect tank you won't get until the war is long over.  Wickwack 121.215.21.148 (talk) 14:38, 27 February 2013 (UTC)


 * The current Russian tanks seem pretty good to me! The tanks have developed in an arms race and it isn't over yet. I'd guess in the future people will only have a few of the heavy tanks and go more for lots of drone tanks which don't have to bother with armour quite so much - but who am I to guess? Perhaps they'll make them even more impregnable if they don't have to worry about frail humans inside.. But as the contributors before said - best really depends on what's cost effective for the circumstances. Dmcq (talk) 13:46, 27 February 2013 (UTC)


 * In the 1991 Gulf War, the "quality over quantity" argument went in strongly favour of "quality" - the Iraqis lost a total of 4,000 of 4,230 tanks deployed, the Coalition lost 4 of 3,360 tanks deployed. Of course many of these were lost to aircraft; however 180 British tanks destroyed 300 Iraqi tanks without loss (BTW, it's not only the Americans and Germans who know how to make tanks). To qualify that, many of the Iraqi tanks were of very old designs, and as Dmcq says above, the latest Russian tanks are firmly in the "quality" bracket too. Alansplodge (talk) 15:03, 27 February 2013 (UTC)


 * On the other hand, if you can make them cheap enough, and no human lives are involved, why bother with armor at all? There was a time in the first war against Iraq where the cost of the missiles the US were using to shoot down Iraqi aircraft cost twice the price of their targets.  When there are humans in the conflict, you have to concern yourself with the ethics of under-protecting the crew of your vehicle - but in some future "drone war", the economics of the situation start to matter.  SteveBaker (talk) 15:13, 27 February 2013 (UTC)
 * That's not quality vs quantity but quality vs hopelessly outdated tanks, some dating back to WWII. Their best tank was the monkey model T-72, a downgraded version of a simplified tank firing half charges. The M1 Abrams, Challenger II or Leopard II is leagues ahead of the T-55. It's simply not a justified comparision Steve.


 * A better comparision would be the M1 Abrams against the T-80. You can buy three T-80s for the cost of an M1.--178.167.236.242 (talk) 00:01, 3 March 2013 (UTC)
 * Apocryphal story about the Russians in WWII; an American advisor sees them chaining three tanks front to back before crossing a frozen river and asks why. The Russian commander answers "So that in case one tank breaks through the ice and sinks, the other two can haul it back up again." the American asks, "Won't the crew be already drowned?" The Russian answers "Oh, we can always get a new crew." Gzuckier (talk) 17:21, 27 February 2013 (UTC)
 * Note that most Russian tanks are equipped to drive underwater, e.g. for crossing deep rivers. 24.23.196.85 (talk) 00:49, 28 February 2013 (UTC)
 * Recommended reading on the subject: . Gzuckier (talk) 17:21, 27 February 2013 (UTC)

Thank you every one for this valuable information and for your patience too.

Why is sleep associated with "scary"?
After reading the Night terror and Sleep paralysis articles, I was wondering, why is sleep almost always associated with the frightening, even unconsciously? Why aren't sleep problems more often associated with, say, feelings of bliss or, instead of an "intruder in the room", maybe being in a celestial location? What is it that the body is trying to tell us that it needs to make such drastic and terrifying emotions?

Bonus question: These scary sleep problems have names, but are there names for ultra-realistic dreams that sadden you to the point where you wake up drenched in tears (or anything similar—just not scary)? – Kerαu noςco pia ◁ gala xies 09:50, 27 February 2013 (UTC)


 * Where do you get the 'almost always' from? Have a look at the percentage occurrences mentioned in the articles. Dmcq (talk) 10:04, 27 February 2013 (UTC)
 * A lot of people find being awake a lot more scary.--Shantavira|feed me 10:28, 27 February 2013 (UTC)


 * Our brain has been shaped by the hunderds of millions of years long arms race where animals try to eat other animals and try to avoid being eaten themselves. Count Iblis (talk) 12:23, 27 February 2013 (UTC)


 * You're more vulnerable while asleep, as the Count says. A couple of counterargument quotes:
 * "Sleep... the most beautiful experience in life... except drink!" -- W.C. Fields
 * "Sleep is wonderful. You're alive... and unconscious!" -- Rita Rudner
 * ←Baseball Bugs What's up, Doc? carrots→ 12:56, 27 February 2013 (UTC)


 * And another quote...
 * "There is not any book
 * Or face of dearest look
 * That I would not turn from now
 * To go into the unknown..."
 * Edward Thomas, "Lights Out" (1914) Alansplodge (talk) 14:45, 27 February 2013 (UTC)


 * One possible reason is that sleep requires darkness, and, being diurnal, people can't see well in the dark, and thus fear what might be lurking out there (hostile people or animals). StuRat (talk) 16:13, 27 February 2013 (UTC)


 * Note that ~3/4 of dreams are unpleasant . However sleep paralysis is a different thing altogether; the emotional/frightening component is kind of logical, considering the predicament of being more or less conscious and yet incapable of voluntary movement. Hard to imagine how that could NOT be considered scary; but it's just a software timing failure in the old meat-computer, the reticular activation system not properly turning off the muscle inhibition circuit before it turns the sensory awareness circuit on. Similarly, there is no reason why true night terrors, not associated with any bad dream, could not be just the result of a misfire of the brain's terror circuit in the absence of a valid stimulus. (Seems like sleep is a time of frequent brain malfunctions). Similar to deja vu; no reason to think of that as anything more than a misfire of the "hey I recognize this" circuit without valid stimulus, no need to invoke past lives etc. On another note, those "I'm falling!" jerk-awake dreams (somtimes accompanied by the belief "Hey, I levitated in my sleep!") might have something to do with our ancestors sleeping in trees for a zillion years, and the evolutionary benefit of being hypersensitive to a fall and snapping immediately awake. Gzuckier (talk) 17:30, 27 February 2013 (UTC)


 * Is that 3/4 of all dreams, or 3/4 or remembered dreams ? (Presumably traumatic dreams will be easier to recall.)  StuRat (talk) 18:41, 27 February 2013 (UTC)

Stagnation pressure
Does stagnation pressure only exist at stagnation points? Clover345 (talk) 13:40, 27 February 2013 (UTC)


 * According to the first line in that article, yes. StuRat (talk) 16:02, 27 February 2013 (UTC)
 * but the article also says its the static pressure + dynamic pressure. Both pressures exist everywhere in a fluid doesn't it?  Doesn't this contradict that first line? Clover345 (talk) 18:55, 27 February 2013 (UTC)
 * First is a theoretical model view for calculation. In reality Dynamic pressure is added (or subtracted if negative). --Kharon (talk) 20:40, 27 February 2013 (UTC)


 * Total pressure exists everywhere, and stagnation pressure only exists at stagnation points, but total pressure and stagnation pressure are equal (except in the vicinity of shock waves.) Seeing total pressure and stagnation pressure are equal, many authors who write about incompressible flows, and low speed flows, don't complicate things by talking about total pressure. Those authors only write about stagnation pressure and they say it exists everywhere. Dolphin  ( t ) 07:23, 28 February 2013 (UTC)

Space Travel Time Dilation Chart
I remember reading a book in the library about the history of space travel & the possible future of space travel & I remember that on one page that there was a chart showing the velocity of a space craft, if using current technology & the amount of time dilation for each step. For example if the space craft was travelling at 0.1c (or 10% light speed) than there will be this much time dilation or if travelling at 40% then there will be this much time dilation. And the chart showed you all the way up to I think 99.9... something %. I can't remember the title of the book so I couldn't find it again in the library so does anyone know where I can find a similar chart from somewhere else. Scotius (talk) 14:52, 27 February 2013 (UTC)


 * Our article on Time dilation contains such a chart, I believe. Zzubnik (talk) 15:58, 27 February 2013 (UTC)


 * That contains a graph (which, unfortunately, is linear, so doesn't show much detail beyond .95 c). For a chart, see Lorentz_factor. StuRat (talk) 16:08, 27 February 2013 (UTC)

Also Is there one that shows the speed as well, as in if the space craft is travelling this many Kms or miles than its travelling at this percent of light speed & the amount of time dilation for that is this. Scotius (talk) 16:15, 1 March 2013 (UTC)

Calculating Shaft Power
I had some marine engine data which mentioned the ship's maximum speed at maximum engine power at the maximum rpm. e.g. 13 knots at 9000kW at 120 rpm. Is it possible to calculate shaft power with this data? If not, then what other data is required for such calculation? —  Hamza  [ talk ]  16:25, 27 February 2013 (UTC)


 * I think you do need more info. You could either go from engine power to shaft power if you knew the efficiency of the transmission of power from the engine to the shaft (you could just guess at this efficiency) or you could look at the speed of the ship, along with the size, mass, hull design, prop efficiency, wind, water salinity/density, etc., and try to determine the shaft power that way.  This later method has far more variables, and is likely to be less accurate. StuRat (talk) 16:33, 27 February 2013 (UTC)


 * 1) No 2) Displacement (ship) 4) Fluid mechanics are extremely complex and ship construction is never sure their new designs work as intended like reaching planned max speed. Engine results are far more exact to estimate just from design as are Transmissions. Therefor shaft power is considered given aka can be estimated exact enough just by giving engine power. --Kharon (talk) 20:30, 27 February 2013 (UTC)

The writers of the IQ tests are the smartest persons on earth?!
Hi,

let suppose that IQ measures the human intelligence;

That means that to challenge another personin the test whoever writes those tests should be smarter, isn't it?Exx8 (talk) 17:42, 27 February 2013 (UTC)


 * No, since writing a question when you already know the answer is far simpler. For example, you can easily come up with a sequence of numbers that follows some pattern too complex for most people to guess.  Try this one: 429,-75,-100,-36,-99,384,-75,-100,96,125,300,-36,-19,96,-51,-100,300,125,-100,-64,-75,-99,224,-100,-96,341,300,-100,-64,-75,-99,224,-100,-19,300,261,-75,44.  What's the next number in the sequence ? StuRat (talk) 17:48, 27 February 2013 (UTC)

To find the answer, add 100 to each number, take the square root, then look it up in a table of letters, where 0 = space, and 1-26 = a-z:

x  x+100  sqrt(x+100) letter - --- --  429    529          23  w  -75     25           5  e -100      0           0 -36    64           8  h  -99      1           1  a  384    484          22  v  -75     25           5  e -100      0           0 96   196          14  n  125    225          15  o  300    400          20  t  -36     64           8  h  -19     81           9  i   96    196          14  n  -51     49           7  g -100      0           0 300   400          20  t  125    225          15  o -100      0           0 -64    36           6  f   -75     25           5  e  -99      1           1  a  224    324          18  r -100      0           0 -96     4           2  b  341    441          21  u  300    400          20  t -100      0           0 -64    36           6  f  -75     25           5  e  -99      1           1  a  224    324          18  r -100      0           0 -19    81           9  i  300    400          20  t  261    361          19  s  -75     25           5  e   44    144          12  l

Lookup table 0     a 1 b 2 e 5 f 6 g 7 h 8 i 9 l 12 n 14 o 15 r 18 s 19 t 20 u 21 v 22 w 23

Now can you determine the next number in the sequence ? StuRat (talk) 18:36, 27 February 2013 (UTC)


 * I can't answer that question. You must be smarter than I. Gzuckier (talk) 18:00, 27 February 2013 (UTC)


 * Or maybe StuRat just found it in a book somewhere and copied it out. Wickwack 124.182.57.146 (talk) 01:06, 28 February 2013 (UTC)


 * I think you're missing StuRat's point. Anyone can come up with a simple encryption of a common phrase and demand that the person being tested decrypt it.  But is that a test of intelligence or a test of code-breaking ability?  In this case, it's clearly the latter.  One could very easily write a computer program to generate such tests (pick a random phrase from WikiQuotes - convert it to numbers, then string together a random sequence of arithmetic operators and constants to change that sequence and voila! Instant StuRat-quality IQ test).  The computer program to do that could be exceedingly dumb.  So if you regard decryption puzzles as IQ tests - then certainly you don't need to be intelligent to make them.  However, I very much doubt that intelligence figures much in your ability to solve them.  The harder problem is to come up with problems that require lateral thinking and which test many different aspects of intelligence.  Also, simply generating a puzzle like the one above is not sufficient to form an IQ test question - you also have to ensure that the answer is reasonably unique - and certainly that it's the most likely answer.  Mathematically, there are an infinite number of arithmetic sequences that would have generates StuRat's number sequence - who is to say that one is "better" than another?  To take a trivial case, if I asked you to name an animal whose name begins with the letter 'A' - and I claimed that the correct answer was "Aardvark" (because I'm no very smart and that's the only animal I could think of that began with 'A'). You, being smarter than me, might think: "Well, it could be Antelope or Amoeba or Aardvark or Anteater." - now the real puzzle for you is not thinking of an animal that starts with an "A" but thinking "Which of those answers is the one that Steve would have been thinking of?".  This is no longer an intelligence test!  So how do we know that StuRat's sequence can only be completed in this manner?  In fact, you DO have to be comparably smart to the smartest person who might be taking your test in order to be certain that there is no other, simpler, answer to the puzzle.  That means solving your own puzzle in ways other than the one you thought of when you wrote it...which requires at least as much intelligence than the person doing the test.  The only thing that makes it easier for the test-writer than the test-taker is that they have more time and access to things like computers.  SteveBaker (talk) 14:36, 28 February 2013 (UTC)


 * That's why I made the sequence so long, so it would be unlikely there would be any simpler solution than the one I gave. StuRat (talk) 06:44, 1 March 2013 (UTC)


 * I think there are some problems of this nature - it's known that the most intelligent people sometimes have trouble with intelligence tests because they can think of perfectly valid reasons to pick a answer other than the one that the person who framed the question didn't think of.
 * For example, I might ask what the next letter in this series is: A,H,I,M,O,T (it's "U") - but who is to say that if you converted each letter into a number 1,8,9,13,15,19 that someone couldn't come up with an equation that would produce something 21 instead of 20 as the next in that series? Maybe converting the letters into morse code produces a pattern who's obvious next letter is "R". Maybe the first letters of the names of the first six days of the week in the Kx'a language of southern africa are A,H,I,M,O and F?
 * Once these kinds of alternative answers come up, this super-smart person now has to decide which of the possible answers is the most likely to have been in the mind of the (clearly less intelligent) question writer. Avoiding those kinds of accidental alternative answers is not a trivial matter - and it most certainly it doesn't help if you already know which answer you think is "most correct". SteveBaker (talk) 18:09, 27 February 2013 (UTC)


 * It's easy to develop questions that are more difficult to answer than they are to conceive and verify (there are tons of examples in mathematics). Also, almost all (maybe all) aptitude tests have some sort of time limitation (even if only practical... the researcher has other things to do with their life). There's also Steve's point, that sometimes you can think yourself out of an answer, or know so much more that the obvious answer seems just too obvious. Shadowjams (talk) 18:16, 27 February 2013 (UTC)


 * Clever sequence, Steve, but a cinch with google: t, u, v, w, x, y. Not the point of the question exactly, but try this one (if I've got it right): class, rate, world, estate, element, sense, .. IBE (talk) 18:22, 27 February 2013 (UTC)


 * Heaven! 80.254.147.84 (talk) 00:07, 28 February 2013 (UTC)


 * Same problem - too many answers ("chord", "sea", "-day adventist", "generation", "son")...now I have to wonder which of those YOU were thinking of and pick that one. This is a poor IQ test question.  So, again, the person setting the problem has to be able to think of *all* of the possible answers - and that requires more intelligence than finding a single correct answer. SteveBaker (talk) 20:17, 28 February 2013 (UTC)
 * Seventh-day adventist - not bad at all. But 80.254 was right, and the correct answer was "whichever one is the one that most springs to mind when you get the sequence, for a typical person." Ok, don't start on me ;) IBE (talk) 19:19, 2 March 2013 (UTC)
 * I have got a feeling that as soon as I post this others will come in with support. '[Smart] is the wrong term. In tests I have always come well up in the upper percentiles – but I am not what I consider smart. I've come across countless people, like those depicted a TV soaps as hill-billies and trailer-trash and self- made millionaires that still have trouble reading and writing -but who are  really  smart...  but they are  just not good at IQ tests.  Think Henry Ford and his court case about him being ignorant pacifist  If you teach a kid how to do the tests they just become good at doing the tests. I don't think their intelligence comes to match mine after all that effort (as a tutor) because  – to use a phrase – Old age and cunning will always triumph over youth and skill so I still have the advantage. Our only  'equality' is now that we can also amaze but doing things like reciting  pi to twenty decimal places. But tell you folk out there.. 3 decimal places does for almost every thing including designing jet engines. The ability for remembering the signs and cosigns  for 10-30-40-50-60 degrees extra is just something we happen to be good at. But what good is that in trying to get over to our accountant, lawyer etc. that their bill is too high (“we have looked in to your consernse Mr Aspro but our billing is in line with blah-blah-blah”).   In  other word 'intelligence'  is is a much broader attribute than (say) a child psychologist can objectively measure with the simple analytical tools at his/hers disposal. If the IQ examiners are  smart at anything, then it is  to get paid well  for what their doing - which is not a lot.--Aspro (talk) 19:25, 27 February 2013 (UTC)


 * There is always a question of what, exactly, an IQ test is testing. However, test results are reasonably stable (if you're in the 90th percentile for one IQ test, the odds are high that you'll be in the 90th percentile for all the other IQ tests).  So we can still answer the OP's question.  Do you need someone who scores highly on IQ tests to write IQ test questions? SteveBaker (talk) 20:23, 28 February 2013 (UTC)

Writing tests like this one is more difficult than a standard IQ test. Count Iblis (talk) 21:42, 27 February 2013 (UTC)
 * A fun test written by someone who doesn't know the difference between your and you're, plus it turns into a chain letter at the end. – Kerαu noςco pia ◁ gala xies 23:10, 27 February 2013 (UTC)


 * I strongly disagree, I got all of them correct - but I can't get every answer in a standard IQ test right - so it's definitely not harder than an IQ test. The problem with the questions he puts forward are that the answers are HORRIBLY dependent on small details of language use.  He asks how many birthdays an average man has - and claims that the answer is "1".  Wiktionary says:
 * Birthday:
 * The anniversary of the day on which someone is born.
 * The anniversary of the day on which something is created.
 * The date on which someone is born or something is created, more commonly called birthdate or date of birth.
 * So the guy who set the question somehow assumes that the last of these three definitions is the one that's "correct" and hopes that we'll assume that he meant the first one. Why?  It makes no sense.  This question has two, equally valid answers - neither is "wrong" or "right".  So it's not a good test of intelligence.  In order to get the answer "correct", I had to see both answers (easy!) and assume that he's trying to trick us (as he does throughout the test) - so now I have to think "Which of the two equally valid answers is the one that this idiot thinks that I won't think of?"   Since I'm clearly smarter than he is (because I've realized that he's wrong and because I happen to know the correct definition of the word "birthday") it basically leaves me having to think like an idiot in order to pass the test.  In summary, this question divides the world into four groups of increasing intelligence:
 * Those who assume the first definition of "birthday" is the only valid one will answer "5+".
 * Those who assume the last definition is valid and incorrectly assume that the first definition is incorrect will answer "1".
 * Those who know that both definitions are valid and don't realize that we're being deliberately tricked. Those people would produce the "wrong" answer because they assumed that an honest questioner would have used the word "birthdate" if he wanted the answer "1".  They may also assume that the use of the words "average man" clearly indicates that the lifespan of the person is pertinent to the question, which would not be the case if the answer was "1" - so they choose the most common definition for the word and answer "5+".
 * Those who know that both definitions are valid and manage to spot that the author is deliberately trying to trick people in group (1) because he's clearly in group (2). So therefore we should pick "1" as the answer.
 * So for people of gradually increasing intelligence, the expected responses are 1, 5+, 1, 5+. This question is therefore not a valid test of intelligence...it's a piece of junk.
 * SteveBaker (talk) 15:01, 28 February 2013 (UTC)


 * Being able to write an IQ test does not mean that the author is really smart - he could be really dumb.  Two factors: 1) He could have spent his whole career figuring it out, but you and I tried to do the test in half an hour;  2) A specialist in any trade, be it wood working, electronics, dentistry, or devising IQ tests, can, and should be able to, outperform a lay person or non-specialist.  I make my own furniture and can join boards, as in making table tops, so that it is almost impossible to see the join - it has always impressed my wife, and never fails to impress other women.  It often doesn't impress men, though, because they, like me, learnt the simple secret to it in high school woodwork class.  Wickwack 124.182.57.146 (talk) 01:18, 28 February 2013 (UTC)

Engineering and medicine
Could one claim that engineering is physics applied to real world situations and that medicine is biology applied to real world situations? If this is true, what's applied chemistry? Clover345 (talk) 18:52, 27 February 2013 (UTC)


 * Hummm! Medicine is a 'discipline' often avoid of any proven science/effectiveness. With engineering/physics, its clear if it  works or it doesn’t work.--Aspro (talk) 19:30, 27 February 2013 (UTC)


 * All of the advances of medicine of the last century have been forms of applied science, and have been justified by the methods of science, and have been consilient with biology. One of the main differences between medicine and engineering is the importance of the personal interaction with between patient and doctor. This is not science (though it can be studied scientifically). There is a perfect controlled experiment of history. Homeopathy was a very popular form of medicine in the 19th century. It was completely unable to adopt the methods of science and its theoretical underpinnings are completely incompatible with biology (despite desperate fantasies). Homeopathy really is medicine without the science. alteripse (talk) 19:36, 27 February 2013 (UTC)
 * It was the practice of medicine I was was referring too. The labs may have used chemical science in the manufacture of drugs etc., but the 'evidence based medicine' that you would you receive from your GP is only a few decades old. There are still a lot of whishy-washy areas that it has not reached, and that are non proven and very doubtful. (google it) So the practice of medicine is still still very much a discipline  and not a science  - it just draws on the benefit gained from  scientific research, which is a different thing entirely..--Aspro (talk) 22:27, 28 February 2013 (UTC)
 * Aspro you are not using words with enough precision to even try to figure out what you are saying, let alone argue meaningfully. alteripse (talk) 01:48, 1 March 2013 (UTC)
 * The practice of medicine is a discipline. . If you can't figure out what I'm saying  saying then why bother to comment? There are many examples of 20th century treatments like insulin coma therapy and lobotomy that are now embarrassments.There was never any scientific justification.--Aspro (talk) 22:25, 1 March 2013 (UTC)
 * A body or discipline or profession relies on applied science to the extent that its theories and explanations remain consistent and consilient with other established science, that other science is used for advancement and research, and that the same methods that advance knowledge in other sciences are used to assess theories and treatments. Not because a treatment that seems plausible is tried, becomes favored for a while, and is then discarded as evidence mounts that benefits do not offset costs. alteripse (talk) 02:33, 2 March 2013 (UTC)


 * Engineering includes virtual world application (like CAD) and theory (like Patents)as well. --Kharon (talk) 20:11, 27 February 2013 (UTC)


 * What about vets? Clover345 (talk) 21:33, 27 February 2013 (UTC)


 * One form of applied chemistry might be materials science. StuRat (talk) 00:40, 28 February 2013 (UTC)

Is there a request for references here? This is the reference desk, not a discussion forum. μηδείς (talk) 01:02, 28 February 2013 (UTC)


 * Um - I only have one reference for this...and well.... :-) SteveBaker (talk) 05:12, 28 February 2013 (UTC)