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April 5

Types of copper

What are the types of copper? — Preceding unsigned comment added by Anto Christopher (talk • contribs) 07:51, 5 April 2012 (UTC)

Do you mean phases? Plasmic Physics (talk) 08:02, 5 April 2012 (UTC)

Our article copper refers to isotopes, alloys, oxidation states, ores, and many other topics that might be relevant depending on what you mean by "type". --Colapeninsula (talk) 09:16, 5 April 2012 (UTC)

See also police ranks of the United Kingdom.--Shantavira|^{feed me} 14:33, 5 April 2012 (UTC)

Well, there are those who cop a dollar and those who cop a feel, so that's two right there. :-) StuRat (talk) 17:20, 5 April 2012 (UTC)

An "expert-witness" in this morning's New York Times mentions 14 million alloys of copper, though I'm still not sure if he's speaking hyperbolically or literally. In any case, ASME and ASTM publish numerical and nomenclature standards for alloys of copper. There are catalogs full of this stuff. I only use two types of copper, "pure enough for electrical wiring" copper, which is some 99.9% copper, and "machine copper," McMaster calls it brass "Alloy 330" which is alloyed with tin, iron, nickel, and a bunch of other gunk, to make it hard enough to hold its shape when milled or worked. Nimur (talk) 15:36, 5 April 2012 (UTC)

The exact quote is

He said it would be nearly impossible to prove that Rubedo is a unique mixture of metallic components, adding, "It may be one of the 14 million alloys that people have cooked up over the decades."

14 million sounds about right for the total number of metallic alloys that have been cooked up and characterized over the past few centuries. --Carnildo (talk) 00:17, 6 April 2012 (UTC)

volume measurer

everyone knows the story of Archimedes' "Eureka! Eureka!" (I have it!) as he ran dripping naked through the streets of Athens, when Archimedes figured out how to determine for the King whether his "pure gold" crown was adulterated. Gold was the heaviest of the metals used, so if he put the supposedly pure Gold irregular crown in a full tub (as he just realized getting into one himself) the amount of water displaced would show the volume of the crown in terms of an exactly equivalent volume of water: the weight of displaced water is easy to measure for them, and if this was heavier (more volume, i.e. less dense crown at same weight) than that displaced by an amount of known pure gold WEIGHING the same as the crown (a simple pan balance lets you assemble such nuggets), in whatever shape or size -- then, since gold was the heaviest of the used metals, it must have been adulterated by something lighter, and the crown would be adulterated. The story continues that the crown was submerged, then pure gold weighing the same as the 'pure gold crown' was submerged, and when the former volume of water weighed more than the latter, the king's pilfering goldsmith was executed.

now my question today is really about how we measure volume today.

How do we measure the true volume of irregular objects? Is it by submersion? What about this idea: simply put it into an open container, close it, pump in x cc of air from outside, and see how much pressure goes up. Would this tell you how empty the (known volume) container was, i.e. how much volume was in it? (I think this would be true because if it's nearly full, all but 100 cc of empty space, the pressure would double. but if the 100 cc represented 1/10th of the available free space in there, the pressure would increase by far less. what do you guys think of this idea? (it depends on being able to pump an exact amount of air and to measure pressure accurately, and to have an enclosed container with known volume and no leaks. No water needed and it just takes a second! 188.6.92.6 (talk) 14:45, 5 April 2012 (UTC)

in practice what is the usual way of measuring the true volume of something today? submersion still or another technology? 188.6.92.6 (talk) 14:47, 5 April 2012 (UTC)

The displaced liquid method is today still a good, easy, and accurate way of measuring volume of an odd-shaped object. In industry, 3D scanning, either articlulated probe type, laser type, or camera type, is gaining ground. In 3D scanning, probing the surface is used to build up the shape in a computer, which can then calculate the volume.

Your air pressure method is not particularly good, because a) it is easier to measure liquid volume accurately than it is for pressure, b) you have to know the air volume pumped in accurately as well, as source of error, and c) the pressure of air varies in proportion to temperature, introducing yet another source of error. Raising the pressure of air increases its temperature, which will then slowly come back down, dropping the pressure, complicating the measurement still further, as you would have to wait until it stabilises.

More often than not though, the material in the object is known, so all you have to do is weight it. If not, many objects of complex shape can be visuallised as several simple shapes joined together, so you can easily calculate the total volume.

Ratbone124.178.37.185 (talk) 15:28, 5 April 2012 (UTC)

isn't the temperature of air in a room pretty homogenous? I would think you could just have a simple piston-pump say, containing the exact volume of the whole container (like a giant syringe) with the containing part of the syringe/piston open to the air to freely circulate until the device is used. Meanwhile the temperature is measured. The object is placed inside, the syringe/piston is plunged, and if the container is empty, the pressure doubles. The less empty it is, the more the pressure increases over double pressure. The piston doesn't need to go all the way down, if the container is nearly full you don't have to subject the object to great pressure... I would think far harder than accurately pumping air (easy peasy with a piston) maybe you are right that measuring pressure is hard. But isn't that also easy - I mean can't you just have a membrane on the side that flexes out with pressure, or a kind of barometer attached to the whole thing (filled with water if you want) that is calibrated to 0 at room pressure and temperature and just goes up like a normal weather barometer? It seems this is also a solved problem.... how hard can measuring pressure be? 188.6.92.6 (talk) 15:35, 5 April 2012 (UTC)

Trouble is, while you are perfoming your measurement, the room aircon cuts in or someone opens the door. So right away you've got a temperature change of 1 or 2 degrees, perhaps more. That alone will make your measurement eeror larger than even a sloppy measurement of displaced lequid volume. When you raise the prerssure of air, you inherently increase its temperature (gas laws), and the temperature will then slowly decay back to room temperature, lowwering the pressure. So, you would have to wait until it stabilises. Not a good method. Ratbone124.178.37.185 (talk) 15:42, 5 April 2012 (UTC)

First of all, I believe you. But could you help me understand the physics more deeply. I can imagine for example a vessel (the whole container) with slots that are all open, everything is circulating freely. Let's say we can only measure the air temperature to +/- 5 degrees. (huge variation). You can have temperature measurements at various locations inside and outside. Now after placing the object inside (it might not be room temperature!) you press a button, and within 1/10th of a second the slots blast closed, a cylinder of compressed air is fully released, and you measure the new pressure. You know that the cylinder had been at x temperature roughly (you can measure it) and you know that it will cool off during decompression, but can't you calculate for this effect? Could you walk me through an actual example. I have a hen I want to see the volume of that can withstand 3x air temperature without ill affects. I put it into the machine which has enough room for the average sized hen to displace 20% of the air. I press the button, the slots close, the tank inside decompresses so that the typical 80% of the remaining space will be at 3x pressure, and we measure the difference between 3 times normal pressure at the temperature (+/-5 degrees) and what we actually get, the slots open to neatralize pressure, the doors slide open, and the hen has been volumed. (like 'weighted'). Now could you walk me through the physics of where we go wrong here? (meanwhile, air is being pumped into the pressure container again for the next hen).

The box looks like this:

T

 __S__S__S___
S T        T S
S     HH     |  T
S    _HH_    S
S   |    |   S
|txx|____|___|  T

T
Thusly:
S   = slots normally open until door is closed and button is pressed
T   = temperature measurement device, of which there are several both inside and outside the box
xxx = cylinder pumped full of air (to the extent you would like for best results/best equation)
t   = a thermometer attached to the cylinder of air.
      over time if the box isn't used for long, cylinder,
      which would be hotter when you first pumped it full of air,
      would cool to room temperature.

      It's important to know when this happens,
      as after this point you will cool the box by releasing it
H   = hen.

now with the proper equations, couldn't this work quite well? Also, if you know how conductive the air container is you can calculate just based on the time it was last discharged and the temperature outside (i.e. the gradient) and the materials it contains, how slowly it cools off after being pumped full. If it's like a thermos it could retain the heat quite well, and you would get the same temperature of air out as you put in (instead of colder air after the warmer container dissipated it under extra pressure). This makes a lot of assumptions, of course, but could you walk me through why the equation fails to be meaningful? 188.6.92.6 (talk) 16:09, 5 April 2012 (UTC)

Archimedes may have used his principle of buoyancy to determine whether the golden crown was less dense than solid gold.

Not an answer to the question, just a comment on the story that introduced it. In the version of the story I've heard (in a podcast somewhere on this website), Archimedes did not measure the density by measuring the water displacement. Instead, he balanced the crown on a scale with a gold reference sample, and then immersed the apparatus in water.This is suggested by our article Archimedes#Archimedes' principle, see illustration reproduced here. --NorwegianBlue ^talk 00:03, 6 April 2012 (UTC)

Sounds like an excessively complicated way to frighten the life out of poor chickens, should they manage to survive the overpressure. Wickwack124.178.55.81 (talk) 05:48, 6 April 2012 (UTC)

Volume measurer name ?

Is there a standard device for measuring volume, and, if so, what is it's name and do we have an article on it ? StuRat (talk) 02:12, 6 April 2012 (UTC)

Lots of things measure volume. Graduated cylinders, volumetric flasks, syringes, beakers, pipettes. It all depends on for what purpose one is measuring volume. --Jayron32 04:49, 6 April 2012 (UTC)

I mean something automated. You drop the object in, and it spits out a number on it's display. Doesn't this exist ? StuRat (talk) 06:34, 6 April 2012 (UTC)

It does now, Stu! Silence denotes consent. You want to go into business with me? I have a killer business plan. 188.156.29.4 (talk) 07:30, 6 April 2012 (UTC)

How about this: a 3D, raster scanning, laser that gives an accurate approximation? Plasmic Physics (talk) 14:32, 6 April 2012 (UTC)

Like this? DMacks (talk) 15:11, 6 April 2012 (UTC)

Stu, these guys are trying to get us to abandon our business plan because an insanely expensive device technically already fills this space, with lasers and precision software. We can manufacture ours in China for four-fifty if we don't mind the compressed air tank exploding and the air ducts not closing hermetically after 200 cycles, but for twenty or thirty dollars we can cut this whole "3D raster scanning laser approximation thing" which no farmer would EVER use to actually volumize a Hen, if you'll patent this with me (I think we have 90 days since I spilled the beans here) we are going to strike it rich. They say build a better mouse-trap, but you know what's better than a surface-plotting, mouse-modelling, laser-guided mousetrap? A cheap one!! Are you in or out? 188.6.92.6 (talk) 16:24, 6 April 2012 (UTC)

Just send me a check each month, please. :-) I imagine the laser scanner would have a problem with hollow objects, while submerging in a liquid would work better, as long as there are holes to let the liquid in and air out. I wonder if water is the best option, though. It certainly is cheap, but it's tendency to soak into things like fabric might be a problem. It might also be important to rotate the object and vibrate it, to get all the air bubbles out. Then again, considering all these issues, perhaps using a gas instead of a liquid would be better. StuRat (talk) 18:18, 6 April 2012 (UTC)

I read your user page with your philosophy. ("I believe that the movement of wealth from the poor and middle-class to the rich is a serious problem, eventually destablizing a nation, and should be countered with a heavily progressive tax system (up to 90%)") I assume, since this is quite profitable, that you would like me to just send you 10% of the money, with 90% going to the government. Of course, the government is free to spend it on whatever the current president wants, a war we disagree with for example - but doesn't actually have to take any risk on my invention. Do I have this right? So let me take on all the risk, for the chance to give civil servants who didn't do anything for it control over how 90% of it is spent. Do I have your philosophy right? If so, I look forward to sending you 10% of your money soon... 134.255.105.197 (talk) 21:45, 6 April 2012 (UTC)

That 90% is for rich people, earning a million dollars a year or more. If you want to send me $100K a year and the gov the rest, I'm fine with that. BTW, I also believe in direct democracy, meaning we all get to vote on how the money is spent, as opposed to back room deals with corrupt politicians giving it to whoever donated the most to their campaign funds. StuRat (talk) 21:59, 6 April 2012 (UTC)

retrograde rotation of venus

Venus opposite rotation is remained mystery for duration of its discovering time till now ,and it has been refereed to some hypothetic events such as external object impact with planet which supposed it changed its rotational direction . HOw does this occure?--Akbarmohammadzade (talk) 14:54, 5 April 2012 (UTC) In fact Iwant to have any discussion ,in addition by let of Wiki site I ask you to see (http://www.gsjournal.net/Science-Journals/Research%20Papers/View/4029 )to know how i am thinking about the subject--Akbarmohammadzade (talk) 15:01, 5 April 2012 (UTC)

If you've already written a published paper on this, then why would you ask us about it ? StuRat (talk) 17:06, 5 April 2012 (UTC)

All the planets have a tilt to them, some more than others, and even the sun has over a 7 degree tilt. This last point was discussed recently here: Wikipedia:Reference_desk/Archives/Science/2012_March_12#Why_does_the_Sun_have_a_7.25_degree_tilt_.3F. As for the mechanisms, I suppose a moon-sized object could strike at a shallow angle, counter to the old rotation direction. However, I think it more likely it would strike at close to a right angle to the direction of rotation. Thus, Venus continued to rotate, rather than reversing direction, but the axis of this rotation was changed. This isn't necessarily an impact from a single object, either, there may have been many. This leads to a model of the early solar system with far more large objects in it than it does at present, since every surviving planet appears to have had it's rotation disturbed. StuRat (talk) 17:10, 5 April 2012 (UTC)

It is not a peer-reviewed journal, so I suspect that the OP might not a specialist in the subject. As far as "external impact events", I don't understand why these exotic theories are needed. The explanation that makes the most sense to me is that its rotation is retrograde due to its thick atmosphere. Although the exact evolution to its current state is up for debate, with an atmosphere 92 times denser than Earth's at the surface, Venus should have had more than enough time to spin down from a conventional orbit due to tidal effects. After it had reached a near-zero rotation, it's not inconceivable that drag from the atmosphere could set it spinning in either direction, and since it seems that the dominant winds in the atmosphere are retrograde, it should be of no surprise that its rotation is retrograde as well. -RunningOnBrains^(talk) 06:19, 6 April 2012 (UTC)

Isn't the retrograde motion of the atmosphere due to the retrograde motion of the planet, not the other way around ? What do you think causes the motion of the atmosphere ? And I see how a planet would eventually stop revolving, down until it became tidally locked, but I see no way for it to start spinning again without an impact. StuRat (talk) 06:39, 6 April 2012 (UTC)

I don't see why you would imagine that Venus's atmosphere would make a significant difference. Internal redistributions of angular momentum do nothing to affect the total angular momentum of the planet. A planet needs to interact with an external objects (e.g. other planets or the sun) in other to accomplish a net change in angular momentum. Besides which, the Earth is covered with a fluid (the oceans) significantly more massive than Venus's atmosphere. Dragons flight (talk) 19:00, 6 April 2012 (UTC)

Yes, but the oceans are contained do basins, and so rotate with the earth and don't have a general direction of flow, while the atmosphere has a net retrograde motion in comparison to the surface. The earth's atmosphere is the number one reason for changes in the length of day. However, I see your point about external torques, my false assumption was that the jet stream and Hadley cell structure would be similar for venus in the past, but this is simply not true, because these structures only form on a rotating planet. Still, the atmosphere could have been responsible for slowing its rotation, then it would take a much less significant impact event to turn the planet retrograde.-RunningOnBrains^(talk) 21:48, 7 April 2012 (UTC)

Escaping from a black hole

It's standard to point out that even light cannot escape from a black hole. Likewise, if a cannonball is fired very fast outward toward the event horizon, the gravity will decelerate it to zero and then negative velocity. But a cannonball has no countervailing force accelerating it to offset the gravity. What if, instead of a cannonball, we have a rocket that is continuously firing its thrusters? Could a time profile of generated thrust be found that would let the rocket escape? Or would the required thrust rise to infinity as the event horizon is reached? Duoduoduo (talk) 15:36, 5 April 2012 (UTC)

The escape velocity at any point within the event horizon of a black hole is at least the speed of light. No object with mass cannot be accelerated to (much less beyond) the speed of light, and so no rocket operating within the bounds of known physics can escape. It would fall more slowly, but still it would fall inexorably. As I understand it, though, your cannonball example is fallacious. Within the event horizon, gravity is sufficiently strong that you cannot move away from the singularity, even temporarily. So there's no launching a cannonball "up but not quite far enough"; it can only fall. — Lomn 15:54, 5 April 2012 (UTC)

the scape velocity for outer distance from event horizon is same as first star ,the sun radius is 1/200 AU , the escape velocity from its surface is 617 Km/s .if it came to be for example any black hole ,the necessary velocity will be equal .Akbar mohammadzade IRAN — Preceding unsigned comment added by 78.38.28.3 (talk) 16:11, 5 April 2012 (UTC)

This is not true. What will remain the same is the escape velocity from the Sun's former radius. Escape velocity is dependent on the radius from the center of gravity. So the escape velocity of a black-hole sun would be greater than the speed of light within its event horizon. -RunningOnBrains^(talk) 06:46, 6 April 2012 (UTC)

The article escape velocity says: It is the speed needed to "break free" from a gravitational field without further propulsion. But my question involves further propulsion.

By analogy with escape from Earth: An unthrusted cannonball, propelled only by its initial explosion to just less than the escape velocity, would not quite make it out; but if you add some more thrust continuously, couldn't you make it out at a slow speed? Duoduoduo (talk) 16:11, 5 April 2012 (UTC)

No. Within the event horizon, gravity is constantly drawing you in at a rate greater than the speed of light. Your hypothetical rocket, therefore, would have to have, at some point, a velocity also greater than the speed of light in order to escape. Per known physics, this requires infinite energy and so cannot be done. Alternately, consider light: it is always traveling at the speed of light (the black hole does not slow it down), and yet is unable to escape. How, then, can a rocket that can never reach the speed of light ever escape? Rather, the geometry of spacetime is sufficiently altered that no escape is ever possible. The images at Black_hole#Event_horizon illustrate this. This is also my point above about the problem of the cannonball analogy: no paths within a black hole's event horizon exist that move closer to (or even remain the same distance from) the event horizon; all possible futures move strictly towards the singularity. — Lomn 16:23, 5 April 2012 (UTC)

Thanks, Lomn -- very clear and helpful. Duoduoduo (talk) 16:52, 5 April 2012 (UTC)

It's not very accurate. Gravity doesn't draw you in at a rate. It accelerates you. As you said, ordinarily you don't need to accelerate to the escape velocity to escape from something, so the argument that "you would have to accelerate to the speed of light to escape, therefore you can't escape" makes no sense and people shouldn't use it. However, it is true that you can't escape from a black hole by any means. -- BenRG (talk) 16:58, 5 April 2012 (UTC)

My understanding is that accelerating (in any direction) while within an event horizon actually causes an object to fall faster. --Tardis (talk) 14:03, 6 April 2012 (UTC)

Inside the event horizon, "inward" and "outward" are no longer spatial directions, but they are timelike, i.e. like future and past. That's what the Schwarzschild metric is saying. Icek (talk) 20:07, 5 April 2012 (UTC)

I can see where the OP's confusion comes from. One thing that is often glossed over in a casual discussion of escape velocity is that it is only the velocity at which you will escape the planet's gravity. It is not a velocity you must achieve to escape the planet's gravity. Indeed, one could envision an imaginary spacecraft with near-infinite fuel that could escape Earth's gravity at walking speed; all they would have to do is adjust their thrust slowly downward as they moved away.

Another thing that is glossed over is that escape velocity is dependent on the distance from the center of gravity of the object you are escaping from (see the equation at the top of the escape velocity page), but the number that is most often cited is the escape velocity at the surface. The further you get from, say, Earth, the lower your escape velocity will be.

In regards to your original point though, you can't think about these cases classically, because black holes are, by definition, non-classical objects. I'm not really knowledgeable on the in-depth mathematical details of black holes, so I presume Icek knows what he's talking about, but I know enough to know that even in a thought exercise where your spacecraft somehow wasn't annihilated by the ridiculous tidal forces that the concepts of acceleration, velocity, direction and distance will become non-intuitive inside the event horizon (even though, technically, physics can't say anything about that which lies past the event horizon).-RunningOnBrains^(talk) 06:46, 6 April 2012 (UTC)

Body fat monitor

How do body fat monitors (example) work? How accurate are they? --SupernovaExplosion ^Talk 15:42, 5 April 2012 (UTC)

I don't know much about this, only enough to tell that the relevant article is Bioelectrical impedance analysis. --NorwegianBlue ^talk 23:38, 5 April 2012 (UTC)

Thanks for the link! --SupernovaExplosion ^Talk 04:19, 6 April 2012 (UTC)

Caramel Catastrophe!

I had some delicious leftover caramel that I'd made--I put it in a plastic tupperware container (one I've regularly used in a microwave) and put it in the fridge.

Later, when I was microwaving it to drip onto ice-cream, the caramel melted through the bottom of the tupperware and burned the heck out of my hand!

I didn't leave it in for an excessive amount of time, and my friend says it has something to do with the 'heat capacity' of sugar---can any of you science-types shed some light on what that may or may not mean? Does that mean that sugar can hold a very high amount of heat without becoming a sugar vapor or some such? (please note, I'm not referring to the brand: Tupperware--I'm actively promoting the genericide of that brand)66.30.10.71 (talk) 15:51, 5 April 2012 (UTC)

a large heat capacity means that it can hold a lot of heat energy without it's temperature increasing (feeling hot). OR, If it's hot it can lose a lot of energy (and maybe burn more stuff) before it's temperature reduces. So heat capacity is not directly responsible for melting through the bottom. Sugar does not normally vaporize. It melts and then begins to char to form caramel. If you keep heating it it will go all the way to charcoal. Most foods will never heat up enough to melt the plastic as once they reach the boiling point of water, any more energy supplied to them from the microwave will just be carried away by the steam that is formed. In the case of your caramel, no such luck. the water formed by charring reaction was too little to carry away the heat in the steam. All that energy instead raised the temperature of the caramel enough to melt the plastic. Staticd (talk) 16:26, 5 April 2012 (UTC)

I don't know, don't you make caramel in a very hot pan before the sugar 'melts' - obviously too hot for tupperware (you wouldn't put sugar in a tupperware container and put it in a microwave and heat it until it became caramel. so maybe getting it to melting temperature is equally hot, even though it was 'already caramel'. just a theory. 188.6.92.6 (talk) 16:39, 5 April 2012 (UTC)

It's never a good idea to microwave plastic containers. Long before they actually melt, they leach toxic chemicals into the food. I suggest glass or ceramic for your microwaving needs. StuRat (talk) 17:00, 5 April 2012 (UTC)

What about the containers in which prepared ("ready") meals or dishes are sold explicitly to be microwaved within them (some of which I save and re-use), and the plastic kitchenware specifically sold for repeated use in microwaves (which I also use)? I agree, of course, that any plastic container not clearly designated for microwave use should be avoided. {The poster formerly known as 87.81.230.195} 90.197.66.205 (talk) 17:37, 5 April 2012 (UTC)

They might be better than foam take-home containers, which seems to melt immediately, but still I bet pretty much any plastic they sell as "microwaveable" is likely to melt and/or give off liquid chemicals and fumes before glass and ceramics designed for the microwave. One worrying sign is if tomato sauce heated in a plastic bowl turns the bowl red, and the color won't come out. This show a level of interaction between food and container which should not be allowed. StuRat (talk) 19:31, 5 April 2012 (UTC)

I'm not sure, but if the caramelization involves enough charring, the substance would become electrically conductive and absorb a lot of energy from the microwaves. (There's a somewhat related demonstration where a beer bottle is microwaves on its own - nothing happens - then a torch flame is applied to it for a moment, and remicrowaved ... and the glass melts) Wnt (talk) 18:57, 5 April 2012 (UTC)

The caramel could simply be heating very unevenly (it's viscous, so a local high-energy spot might not redistribute very well), leading to "that one spot" getting hot enough to melt through. I would assume that there was enough caramel total and that it is polar or ionic enough to absorb sufficient microwave energy, but if not (or it can't conduct the heat and MW energy away effectively), wherever the energy concentration is greatest just keeps hitting the plastic itself. DMacks (talk) 21:46, 5 April 2012 (UTC)

AFAICT, the microwave energy will be absorbed by the rotational energy levels of the -OH groups in caramel, same as water. Need'nt be conductive. From there equipartition theorem calls for it's redistribution. Staticd (talk) 11:37, 6 April 2012 (UTC)

My point is that the redistribution is slow...microwaving a frozen block of meat gives some areas that get hot enough nearly to start to cook while others are still frosty. It's unlike a mug of water, where the molecules are free to move around and allow physical redistribution of the "hotter" ones so the whole thing winds up heating more evenly. I assume that's why "defrost" is a long time of cycling of a lower energy level instead of the 100% "boil water" mode. DMacks (talk) 11:59, 6 April 2012 (UTC)

Agreed. I've found that just about everything turns out better when microwaved longer on a lower setting. Pizza, for example, doesn't turn out burnt at the edges and frozen in the center (although it's still soggy, so I prefer a convection oven). StuRat (talk) 18:13, 6 April 2012 (UTC)

When you add sugar (or salt) to water it elevates the boiling point. This can be above the melting point of the plastic which can be as low as 105°C. Oil also boils at high temperatures, and so microwave containers have warnings about these kinds of foods. Graeme Bartlett (talk) 22:56, 6 April 2012 (UTC)

And thermoplastics (soft plastics) don't so much have a melting point as a wide range of temperatures over which they slowly change from solid to tacky to gel-like to runny. Anything beyond completely solid is problematic if you intend to eat the contents, as toxic chemicals will leach out at much higher rates then. StuRat (talk) 23:31, 6 April 2012 (UTC)

Instantaneous communication at a distance

Suppose I have a stick of fixed length. I assume it is absolutely incompressible. I press one end of the stick, and the other end moves (and so does an incompressible object that the far end is touching). Have I communicated instantaneously at a distance? How does this relate to the relativistic notion that (is this right?) information travels no faster than light speed? Duoduoduo (talk) 16:52, 5 April 2012 (UTC)

Yes, that would violate the laws of physics. Thus, a perfectly incompressible solid is not possible. StuRat (talk) 16:57, 5 April 2012 (UTC)

This is a common question on the reference desk. See WP:Reference desk/Archives/Science/2012 January 16#Information transfer Faster Than Light - DIY and WP:Reference desk/Archives/Science/2008 January 10#Communicating Faster than Light Using a Rotating Rod for starters. -- BenRG (talk) 17:16, 5 April 2012 (UTC)

Perfect rigidity is excluded by relativity. In an object composed of matter, the atoms are held together by electrostatic forces. When every position along the object is not accelerating, the atoms are found with an equilibrium seperation, similiar to a spring. When a position along the object is accelerating, the seperation between atoms at that point compresses to a minimum relative to their acceleration. As with any process, it takes time to be compressed (however short), before the minimum is reached. Since all the atoms inside the object are free to move and compress, the whole object can compress as a cumalative effect.

If you have a long pole, and pull very fast on it, it will stretch momentarily before returning to its normal length. If you push the pole, it will shorten. Even the motion of atoms relative to each other, is constrained by the speed of light; the distant end of the stick will only respond to a push on the near end at a rate slower than the speed of light.

This is the behaviour of matter: no matter how rigid something is, nothing is pefectly rigid, and every thing can compress and stretch. Plasmic Physics (talk) 23:10, 5 April 2012 (UTC)

Thanks, Plasmic Physics. You say "Perfect rigidity is excluded by relativity". I thought it was excluded by the physics of the small, not by relativity. Is it not possible to conceptualize an (empirically non-existent) physics of the small involving perfect rigidity? If so, It seemed to me that this kind of instantaneous communication at a distance would be possible, and would not violate relativity because nothing physical is moving at the speed of light. Is it really relativity that precludes perfect rigidity? Duoduoduo (talk) 20:27, 7 April 2012 (UTC)

Follow-up

These two questions may be identical, but I had actually been thinking of something similar for a while -- say a hypothetical "see-saw" was constructed that spanned several light years through deep space. The object is a single piece and rotates along a hinge (like a see-saw). When one end is raised, the other is lowered. Assuming the far end is raised with great speed, when would those at the near end begin to perceive a lowering? My initial feeling is that, since the atoms and molecules of the "see-saw" cannot be accelerated beyond the speed of light, the other end would not be affected until such a time as the displacement of the atoms "rippled" through to the other side. Probably an objective observer (viewing the entire object from the side) would see a slow warp in the object until the "ripple" reached the terminal point and "straightened out" the object. Is this an accurate description? Evanh2008 (talk) (contribs) 02:23, 8 April 2012 (UTC)

Sure, but it wouldn't be any less odd or strange to the observer than watching a flexible bar bend and then straighten out. Just on a really huge scale. --Jayron32 02:29, 8 April 2012 (UTC)

Have they ever tried multi-stage balloons?

They launched a 1.35 million m³ balloon before, for the world altitude record (~50 km). Why not tie a barely inflated small balloon to it, so that by the time the carrier balloon bursts (or stops rising?), it's payload has expanded enough to be buoyant? Sagittarian Milky Way (talk) 18:10, 5 April 2012 (UTC)

The expansion won't change the buoyancy. It is displacing a greater volume of air, but that air is reduced in density by the same amount (the pressure inside and outside the balloon need to be equal). The buoyancy is determined by the number of molecules of lifting gas and the difference in molecular weight between the lifting gas and air. The number of molecules of gas in a given volume at a given temperature and pressure will be the same, regardless of what the gas is, so one molecule of helium will always displace one molecule of air. --Tango (talk) 18:50, 5 April 2012 (UTC)

(ec)I don't understand. Why have a second balloon rather than allowing the carrier balloon to vent lift gas so it doesn't burst? Wnt (talk) 18:50, 5 April 2012 (UTC)

If you vent your lifting gas, you won't have the lift any more. Balloons don't usually have much more lift than is needed to counter their weight, so there isn't much you can vent before you start falling again (and you might as well have just not had that bit to start with). --Tango (talk) 22:17, 5 April 2012 (UTC)

how about this one guys?

why not just shoot a giant artillery round up that is itself a piece of artilley, timed so that when it reaches its apex, right before it starts falling again, it shoots its payload? Oh, and guess what the payload is... right, another smaller round of artillery and so on, right down to a miniature little gun that fires a little pobe into space? All the heavy artillery can parachute down for another fun go just the same. Why is this better than just one giant piece of artillery firing straight into space? Because whatever it fires doesn't have to undergo such a huge explosion, just enough to make a nice arc, and the next one likewise, and so on and on. See http://en.wikipedia.org/wiki/Schwerer_Gustav - it weighs 1300 tonnes - let's trim it down to 700 tonnes without a lot of track and equipment and guiding stuff, etc, and fires a 7-ton shell 23 miles. I suppose at a 45 degree angle. If instead it shot straight up, surely it could reach 10 miles straight up, no? Then we repeat: the 7-ton (7000 kg) projectile is itself a piece of artilly that, at the same ratio, can fire a 70 kg piece another 10 miles up in the air. We are at 20 miles. The 70 kg piece itself is able to fire 0.7 kg i.e. 700 grams, which ought to be enough to launch a 7-gram projectile another 10 miles (especially due to much lower air resistance), getting close to reaching low-earth orbit http://en.wikipedia.org/wiki/Low_Earth_orbit . Admittedly we need to trim bit more to really reach 10 stages of matruschka artillery, i.e. the 100 miles that's required for low-earth orbit, but I think we can actually do better than being able to launch only a projnectile that is 1/100th of the piece that is firing it, since all we need is an explosive chamber (no wory about reloading it on the spot), a mechanism to keep it upright right before firing (for example it could deploy its chute and fire up through a whole in the center whenever it is hoizontal (parallel to the ground) according to simple level device that doesn't weigh much. Sure, it would lose a bit of altitude after deploying chute, but you don't have to worry about timing and trajectory and all that stuff...very simple. Basically like those handheld artillery pipe things from vietnam era warcraft (don't remember what they'e called). Just a pipe you put explosive in and a shell, aim and ignite. So, what's wrong with this way of getting to space repeatedly, always retrieving the larger and larger pieces of 'artillery' as they fall with a gps tracker? 79.122.54.224 (talk) 18:57, 5 April 2012 (UTC)

Sounds like a multi-stage rocket, only less efficient and more complex. I always thought a rocket launched from a balloon or balloons would be the best combo. StuRat (talk) 19:26, 5 April 2012 (UTC)

A multistage rocket would be much gentler on the payload. You've specified that the payload is a space probe, and delicate electronic devices like a space probe tend to not fare so well when subjected to explosive forces. Red Act (talk) 19:53, 5 April 2012 (UTC)

See Gerald Bull, the HARP Project, and the supergun. All the parts are out there somewhere, dunno about the drawings though. I think the missile was designed to fire while still in the gun barrel, but not positive on that. Franamax (talk) 19:33, 6 April 2012 (UTC)

how about THIS one guys

I don't get multistage rockets, if you're not jettisoning a LARGE amount of your weight at each stage (as with my artillery) then isn't the following easiest: fill a giant balloon in ana aerodynamic shape with jet fuel. Put a small hole on the bottom with the ignition, the pressure from the balloon and gravity keeps the ignition fed and the balloon (being rubbery) contracts by itself. So you go from a huge balloon filled with jet fuel to a tiny one. Why waste all this metal on 'stages' and so on? It would be better if we could suspend the belloon, of course, so it doesn't have to stay rigid but that's tough. I guess you could have a long pole that you hang the aerodynamic balloon on, that siphons from the top and has ignition at the bottom (past the bottom of the balloon, that way the rocket-shaped balloon is suspended the whole time, but this adds an unnecessary pole to the whole thing that weighs exta. illustration 1:

Diagrams of proposed space launch vehicles

ILLUSTRATION 1 (NOT IMPORTANT) P P = PAYLOAD H H (for hook) = TOP OF POLE, SUSPENDING IT ALL B+B , B = BALLOON, + = SIPHON | = POLE, HERE UNDER COMPRESSION, NOT TENSION X = FIRE BB|BB BB|BB BB|BB BB|BB BB|BB BB|BB BB|BB BB|BB BB|BB BB|BB BB|BB | | | | X XXXX XXXXXXX basically, why have multiple stages instead of just jet fuel and an elastic balloon filled with it and in the shape of a rocket? Another option (bette I think) for aerodynamics is if you can find some very light metal that can withstand continuous jet-fuel burning, then you put the ignition a good distance ABOVE the aerodynmaic balloon, so that it's igniting straight down but doesn't reach the balloon of jet fuel, which feeds up through the a tiny line that is way off to the side out of the way of the flames, like this [^] /[ ] <--- [payload] <<-- FAR LEFT: FUEL LINE, HELD OUT AWAY FROM IGNITION FLAME fuel--> / [_] line / [x] <--- ignition point and can be diected by payload for correct guidance / x|x / xx|xx <-- flames licking the metal (x = flame) / | / | <- far left - fuel line / | <--- metal to suspend the aerodynamic balloon, now under tension, not compression / | | | | | | + <-- good insulation on the line so it doesn't heat up too much fom the above flames | + | + |__________+ | + | + | (F) \ (F) \ (FFF) \ (FFF) \ (FFF) \ (FFF) <--- balloon with jet fuel (F) in it \ (FFF) could just be canvas with rubber bands around it \ {FFF} \{FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} {_F_} it could also have two fuel lines for balance, thusly: [^] /[ ]\ / [_] \ / [x] \ / x|x \ / xx|xx \ / | \ / | \ / | \ / | \ |-+-+-+-+--|-+-+-+-+--| \ | / \ + / \ + / \ + / \ + / \ + / \ + / \ (F) / \(F)/ (FFF) (FFF) (FFF) (FFF) (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} {FFF} (FFF) {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} {FFF} (FFF) {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} {FFF} (FFF) {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} (FFF) {FFF} {FFF} {_F_}

Hang it on a long pole that siphons from the top (due to the pressure from the balloon) and near the top of pole put the ignition. Now you go all the way to space with a shrinking balloon that weighs less and less. 79.122.54.224 (talk) 21:32, 5 April 2012 (UTC)

I haven't read your whole idea because your premise is wrong. Multistage rockets *do* jettison a large portion of their mass. Take the Saturn V rocket that took man to the moon, for instance. The first stage (excluding fuel) was 2,300 tonnes. The rest of the rocket (including the fuel for the other 2 stages) was 621 tonnes. By jettisoning the first stage and using the much smaller second stage engine, they were able to lose about 80% of their mass. --Tango (talk) 22:17, 5 April 2012 (UTC)

The idea of keeping the fuel in a "balloon" is not that much different from keeping it in the casing of a rocket, except that a rocket will actually work flying up through the air. And when the balloon shrinks, what you're really saying is, keep the extra bits of outer shell even though much less fuel is being held by it; whereas the multistage rocket dispenses with the bits of extra shell when the fuel they enclose is burned.

It's more fun to think of ways of having really powerful fuel, like red oxygen, variants on inverse sodium hydride and other such bizarrities. Wnt (talk) 23:11, 5 April 2012 (UTC)

All this stuff about altitudes is beside the point. Space is an arbitrary altitude, and getting there briefly isn't very useful (but for silly people with too much money and astronomers with too little). What's useful is orbit, which allows you to stay up for a long time. Orbit is so much more than "getting up really high"; even if you build a 100km tall tower, when you stepped off it you'd drop like a stone. To be in orbit you need speed; absolutely huge speeds, in fact. The lowest elliptical orbital speed that article quotes is 6.5 km/s, which is 14500 mph. That's a massive speed, and to get to it you need to expend a massive amount of energy - that's why astronauts strap themselves to gigantic exploding bomb things and blast violently into orbit on a huge column of flames. Air launched systems (be it from balloons or aircraft like Pegasus) have some plusses from range safety and bad-weather avoidance, and some modest gain from avoiding some of the atmosphere, but you don't get to orbit without that massive speed, and you won't get a balloon going at 14500 mph. -- Finlay McWalterჷTalk 23:36, 5 April 2012 (UTC)

It's understandably hard to get your head around the forces involved, but if you did you'd see why the balloon idea is just silly. Have you ever had a water balloon explode in your hand because you tried to throw it too hard? Now imagine that you're not throwing something, but applying almost 10,000,000 pounds (4,500,000 kg) of force to it (forgive my abuse of units, fellow scientists). There is no "elastic material" in existence that could survive such a journey.

Your entire thought experiment is unnecessary, however. You seem to assume that we could somehow save the amount of fuel needed to get into space by cutting down on the weight of materials used to build the rocket. Using the space shuttle as an example, the fuel needed to reach orbit weighs 20 times as much as the object it is lifting! So you see, our efforts would be much better spent trying to reduce the weight of the fuel, as Wnt points out above. -RunningOnBrains^(talk) 07:13, 6 April 2012 (UTC)

Also, a balloon is going to be far less aerodynamic; it'll tend to bulge out rather than be long and thin. Rockets have fins etc for stabilisation in the atmosphere; without a rigid structure these aerodynamic features would be less useful (imagine a floppy airplane). --Colapeninsula (talk) 09:18, 6 April 2012 (UTC)

= be-all end-all

I have the be-all end-all of all fuel-containing solutions!!!!

This eliminates 100% of the enclosing container.

This is what you do.

You FREEZE the rocket fuel in a long rocket-shaped cone, and melt off the bottom as you use it!!! You can space the distance between the burning unit and the bottom of the frozen rocket fuel such that just enough heat goes up to keep melting it. Granted, you still need loads of rocket fuel, but you now have ONLY a payload at the top of the chunk of frozen rocket fuel and a drip container and burning unit at a variable distance at the bottom:

 P      P = payload
FFF
FFF
FFF
FFF     F = frozen rocket fuel
FFF
FFF
FFF
\D/     D = drip container, dripping down to burning unit
 |
 |      | = variable length somewhat insulated pneumatic aparatus for keeping the burning 
 |           just far enough to melt what you want, with a tube inside to drip from D to xx
/xx\
xxxx    x = blastoff fire
xxxx

For a mini rocket, how can you get ANY more streamlined than having NOTHING containing the rocket fuel? Depending on how fast you get into orbit, you can keep the whole thing in a cryogenic container until just the right moment, then blast off and hopefully get into orbit before it all melts. Does this math work, guys? — Preceding unsigned comment added by 188.6.92.6 (talk) 09:34, 6 April 2012 (UTC)

Your problem isn't maths, it is engineering. I can't see how that design could possibly work - what's holding the engine onto the frozen fuel? You are also mistaken in thinking that the container is a significant issue. Let's take the Saturn V first stage as an example. Unfueled, it weighed 131 tonnes. The fuel tank (empty) weighed only 11 tonnes. The tank is only a fairly small amount of the weight. Conventional, multi-stage rockets are very well designed and are very efficient - you're not likely to improve on them without a lot of training and experience (you would be better off trying to invent whole new ways of getting into orbit - so much work has gone into rockets that they are pretty much as good as they'll get). --Tango (talk) 12:17, 6 April 2012 (UTC)

I only read thru your second sentence, will be back later to finish. "what's holding the engine onto the frozen fuel?" - the engine is pushing up on the frozen fuel! even if it were not the frozen fuel would rest on it of its own pendulous weight, but the thrust makes this even greater than 1g (a lot greater than one g). so the same thing holds onto it as a server (waiter/waittress) who swings a plate from below to weave past something). more later. 188.6.92.6 (talk) 14:03, 6 April 2012 (UTC)

Tango. I've read the rest of your comment. First of all, I welcome any suggestions as to 'new ways' to get into orbit other than rocketry. Secondly, let us say that the fuel tank weighed only 11 tonnes. Now let me ask you another question: how much do you think those 11 tonnes cost? How about designing them?

How much is a safely and properly designed conventional rocket in MONEY TERMS, as opposed to a device that is meant to just push some frozen fuel up into the air by burning it off, just like an inverse candle: what could be simpler than a candle?

I am talking, amateur rocketry for example. If all you have to do is freeze the rocket fuel (or maybe even gasoline!!) in a special shape, place it onto this aparatus, and have it lift off without having to jettison anything or have any moving parts or do anything other than control how fast the fuel melts into its recepticle - which can be done with a simple feedback loop (it can be purely mechanical if you want) moving the rocket exhaust out whenever the recepticle is more than half full and retracting it again if it is less than half empty? One thing nobody has done, however, is done the calculations: does frozen fuel stay frozen long enough to make it to space? Rentry is seriously hot - how about getting to space to begin with?

This would be like a home kit you buy for very cheap, plus your fuel costs, a form to freeze your gas into and a trajectory, put it outside with your payload on top and blastoff into low earth orbit. I can't imagine anything simpler than this, if the physics allow it. 188.6.92.6 (talk) 16:17, 6 April 2012 (UTC)

If the fuel isn't securely attached it will just fall off. Rockets don't fly smoothly through the air, there is a lot of turbulance. You have also neglected to consider the oxidiser. There is no oxygen is space, so you have to take it with you. Rockets generally have two fuel tanks, one containing the fuel itself and one containing liquid oxygen. They have to be mixed in just the right ratio just before being fed into the engine or the rocket doesn't work. --Tango (talk) 16:24, 6 April 2012 (UTC)

Well we can't take frozen 02. i'll have to think about this... do you want to take up my other bright idea with me if stu doesn't? 188.6.92.6 (talk) 16:47, 6 April 2012 (UTC)

Freezing rocket fuel sounds like a poor way of creating a solid fuel rocket. Now I suppose if you can have caseless ammunition you ought to be able to design some kind of rocket made entirely out of fuel. Modeling different rates of burning, creating fuel with good tensile strength, controlling the rate at which it burns so that it somehow remains firmly pressed against a nozzle (or otherwise forms its own?) ... Some kid at Caltech should come up with such a thing sometime for a class project. Somehow I doubt it would replace sanely made rockets though. Wnt (talk) 04:31, 7 April 2012 (UTC)

I'm sorry, but all solid rocket fuel is frozen. If you mold it first, then it was liquid before it was frozen. Your point doesn't change my proposal at all. I am "that caltec kid." 134.255.3.231 (talk) 09:00, 7 April 2012 (UTC)

Perhaps you should read combustion chamber. In some toy rockets and fireworks, the casing is made of paper, or there is no casing at all. In powerful rockets, the operational chamber pressure is large (because it is a rocket engine). Chamber walls are necessary for structural integrity. High pressure hot gas impinging on explosive fuel or on load-bearing structure can cause structural damage, even if only a tiny gas leak impinges on a tiny portion of the structural material. As was discovered during the investigation into STS 51-L, a very small amount of exposure can cause total structural failure. You are suggesting to expose the entirety of the structural, load-bearing portion of the rocket to the internal environment of the combustion chamber. This will cause dangerous explosions. If you are indeed building small rockets, consider taking ample safety precautions. Even small toy rockets can contain large amounts of energy. Before you ever fire up your rocket, use water to test structural integrity at high pressures - well above your design pressure - because when your design fails, and the water rapidly depressurizes, it does not expand volumetrically, unlike hot gas. Remember that at high temperatures and pressures, and in the presence of oxidizer, solid steel can be more flammable than gasoline. Rocket science is difficult because the inside of a rocket motor is a very unusual environment. Many of your intuitions about material strengths, time-constants, and general behaviors of material are dangerously invalid in this chemical and physical environment. It is for this reason that rocket engineering relies so heavily on rigorous theoretical analysis and very careful experimental validation of subcomponents. Nimur (talk) 16:37, 7 April 2012 (UTC)

April 6

Dandruff and eyes

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Virtual work

Here's something I don't understand about virtual work.

By definition, $\delta W={\vec {F}}\cdot \delta {\vec {r}}$. Normally, we write ${\vec {F}}\cdot \delta {\vec {r}}={\dot {\vec {p}}}\cdot \delta {\vec {r}}={\frac {\mathrm {d} ({\vec {p}}\cdot \delta {\vec {r}})}{\mathrm {d} t}}-{\vec {p}}\cdot \delta {\dot {\vec {r}}}$ and solve from there.

But couldn't we just say that $\delta W=\delta T=\sum _{k}{\frac {\partial T}{\partial q_{k}}}\delta q_{k},\ \mathrm {where} \ q_{k}=\mathrm {k^{th}} \ \mathrm {degree} \ \mathrm {of} \ \mathrm {freedom}$? It doesn't give the right answers, but physically I don't see why this shouldn't work. 65.92.5.132 (talk) 02:19, 6 April 2012 (UTC)

Hello, if T is kinetic energy then surely ${\frac {\partial T}{\partial q_{k}}}$ is zero? For example, a point particle has $T={\frac {1}{2}}m{\dot {q}}^{2}$ which isn't a function of $q_{k}$.

Perhaps we need:

$\delta W=\delta T=\sum _{k}{\frac {\partial T}{\partial {\dot {q_{k}}}}}\delta {\dot {q_{k}}}$ and then connect this to $\delta {\vec {r}}$ ? Sorry I get confused with generatised coords. 94.72.209.210 (talk) 13:07, 6 April 2012 (UTC)

There are situations where T can depend on $q$ as well as ${\dot {q}}$. For example, if a point mass is attached to a rigid rod which is pivoted at the origin and rotating in the xy plane w/ angular speed $\omega$ (the point mass is allowed to slide up and down the rod), then ${\vec {r}}=q\cos {\omega t}{\hat {x}}+q\sin {\omega t}{\hat {y}}$ (q is the distance from the point mass to the origin) and so $T={\frac {1}{2}}m({\dot {q}}^{2}+\omega ^{2}q^{2})$. And besides, $\delta {\dot {q}}_{k}=0$ by the definition of a virtual displacement. 65.92.5.132 (talk) 15:09, 6 April 2012 (UTC)

You aren't defining your terms, but it looks like you're using standard notation, where T is kinetic energy. If so, you forgot U (potential energy). Even when working in generalized coordinates, you must still use total energy. And, as pointed out above, kinetic and potential energy can both vary as functions of q, dq/dt, or any other generalized coordinate. Nimur (talk) 00:41, 7 April 2012 (UTC)

Sorry for not stating it, but I'm defining $\delta W$ to be the total work done, including work done by any conservative force. Also, why would dependence on ${\dot {q}}$ matter? Isn't $\delta {\dot {q}}=0$? 65.92.5.132 (talk) 02:34, 7 April 2012 (UTC)

Dreams: maker versus watcher

I am often struck by the disconnection between what things in dreams appear as and what they are interpreted as. In a dream, I can know that what I see as a tiger is 'really' a lion. Sometimes I've thought that the concept was real and its graphic rendition was fouled up, but this time it appears otherwise...

A dream I had last night involved the story of a little girl living out on the streets who was taken in by a caring family. After she told a social worker how much she loved it there and hoped she'd be adopted, her would-be parents took her up to an even nicer bedroom to stay. There's a sudden heavy thump on the ceiling that makes the hanging light in the room sway, and they tell her that's "their daughter Grace". Then they hurriedly lock the door leaving the girl inside, and as Grace comes down into the room she screams and runs over to a tiny window which she manages to get open and slide out through. As a fast-tempo musical theme from The Nutcracker (I think) starts up I see Grace, who is a giant snake with glowing eyes that follows the girl out the window and chases her around the manor's rooftop.

Now what was odd, though, is that all the while I'm watching this giant snake slither out the window and around the dormer windows this little girl was so desperately skirting, (eventually devouring a Garfield-like "pet cat" the girl had with her the whole time after she slammed a far door on it) I never once believed that what I saw as a giant snake was anything other than an older girl. I was still thinking, this is the cannibal freak daughter that was locked up in their attic chasing her. But the set-up - the thump on the ceiling, even the family adopting street urchins so that when they disappear nobody notices so that their "daughter" can have its preferred human meat - it all points to the person creating the dream knowing it was a giant snake the whole time.

Is there any term for this? Who makes the dream? Who watches the dream? Where are these people located in the brain? What mechanism notices incongruities between what we see and what we think we see, and how is it deactivated?

I suppose what perplexes me the most is, if one part of me had the ability to cobble together such an elaborate plot, why is a different and seemingly unconnected part able to watch it? Why isn't there just one place in the brain capable of conceptualizing the idea of "snake" and "little girl"? Wnt (talk) 13:24, 6 April 2012 (UTC)

(...crickets...) I don't think you'll get any refs for this one :). Dreams are so subjective, and you can easily appreciate how hard this would be to study scientifically. Anecdotally, I think I know what you mean, but I don't think it implies a "dream maker" vs. "dream watcher" dichotomy. Rather, I think it points toward the differences between what you "see" in a dream v.s what you "know" in a dream. Example: I've had dreams about people I know where (to borrow acting terms) the "part" of person X is "played" by person Y. Often, person Y is indeed a famous actor/actress, maybe because my brain is more inundated with images of the famous person, so their face fills in, even when the dream is about someone I know personally. This, to me, sounds similar to what you describe. You "knew" it was a cannibal human, but it was "played" by a giant snake. Does that make any sense? SemanticMantis (talk) 22:00, 6 April 2012 (UTC)

It sounds like we're observing very close to the same thing - which means there's something reproducible to it. But it sounds like in the case you describe the known version was indeed more fundamental than the seen version, which is interesting. I can't believe there isn't data on it though, at least, psychological anecdotal data, given how many shrinks tried to infer Freudian things from dreams - surely some must have taken a general interest in how they work, and come up with some such observations. (Actually, as I was fairly consciously aware while having it, this dream was rooted in a certain Commons thread where for a time I played the part of the social worker, but the less said there the better) Wnt (talk) 04:24, 7 April 2012 (UTC)

Just to pick up on SemanticMantis's point, the meaning of your dream will be apparent to you if you think hard enough about it. Dreams, as they are currently perceived by the psychological community, are a way of making sense of memories, and as memories are derived from your own experience, nobody else can tell you what a dream means. So the maker of the dream, the watcher of the dream, are all you. --TammyMoet (talk) 08:11, 7 April 2012 (UTC)

The idea that YOU are one "consciousness" experiencing it self and reality in real time is an illusion. Reality and perception are CREATED in the brain by many "modular" components which play disparate roles. I'm having a hard time finding references in Wikipedia, possibly split brain and Dissociative disorder could be taken as references, maybe even Alien hand syndrome. There is a term I've come across called "reality testing" which I can't find in wiki but would be relevant. Reality testing is something your brain does continuously in parallel and subconsciously. While you are asleep, your reality testing "process" is greatly diminished which is why when you are dreaming, very unusual scenarios might not seem out of the ordinary at the time. Vespine (talk) 00:34, 10 April 2012 (UTC)

Middle East climate

Why is the Middle East so dry even though it directly borders parts of Europe and Asia that aren't? --204.184.214.187 (talk) 14:43, 6 April 2012 (UTC)

Not all of the Middle East is very dry; Israel, Lebanon and parts of Jordan and Syria have a more moderate climate. Also, the Middle East is not a unique dry part, but part of a much larger dry area spanning from northwest Africa to parts of west India. At Köppen climate classification, you can view a world climate map illustrating this. What exactly the cause of the drought is, may be complicated and is probably related to wind patterns, but I don't know much about that. -- Lindert (talk) 15:19, 6 April 2012 (UTC)

In a nutshell, it seems you don't get much rain around the Persian Gulf and the Red Sea because the atmospheric circulation known as the Hadley cell carries water away from the Middle East, and the mountains around the Iranian plateau (mostly, I think, the Zagros Mountains) trap what rain does blow in. This paper has more details, though it's rather dry and technical. Smurrayinchester 15:51, 6 April 2012 (UTC)

It's part of a dry subtropical lattitude band that stretches from the Sahara to the Gobi and also includes the US Southwest and adjoining northen part of Mexico. It's southern hemisphere counterpart includes the Namib and Kalahari in Africa, the Atacama in South America and the Central dessert in Australia. Roger (talk) 20:42, 7 April 2012 (UTC)

The answer is the Hadley cell, but not the reason given above. The reason the subtropical latitudes are so dry is that air rises in thunderstorms near the equator, it then must sink at some point. These desert regions are areas where there is a net subsidence, or sinking of the air, which in general suppresses the rising motions that contribute to rain. This is a very general, averaged effect, so local effects such as the Indian monsoon cause areas at similar latitudes to be quite moist.-RunningOnBrains^(talk) 21:57, 7 April 2012 (UTC)

Vitamin B12 absorption

I've read that vitamin B12 absorption from low dose supplements (less than about 50 micrograms) is limited to a maximum of about 1.5 micrograms, presumably because the limiting factor is the availability of enzymes needed for absorption.

How long does it take after taking one supplement before the body is able to absorp 1.5 micrograms from the next supplement? Count Iblis (talk) 15:58, 6 April 2012 (UTC)

Uniqueness of the attractor for Earth's climate

Earth's climate tendencies can be viewed as an attractor within which the actual climate evolves. For example, in the above discussion of the Middle East's climate, the Hadley cell carries water away from the Middle East, giving it a mostly dry climate; the Jet Stream over the US follows a time path that, while not precisely repeating itself annually, stays within certain limits; etc.

But is it known whether the observed global climate attractor is unique? In other words, if we held constant the locations of the continents and oceans and the orbit of the Earth, but started from initial conditions of say zero wind velocity everywhere (or started with new initial conditions after some massive external disruption), would we eventually return to the attractor that we are familiar with? Or are there multiple attractors, each with a different basin of attraction in the space of initial conditions? Duoduoduo (talk) 17:38, 6 April 2012 (UTC)

It partly depends on the timescale you look at. There is, I believe, a pretty strong case that Snowball Earth is also an attractor -- if the entire Earth were covered with ice, it would reflect so much sunlight that it would never warm enough to melt the ice. It is widely believed that the Earth has been in such a state several times, but that it eventually escaped because over the course of a few hundred million years so much CO2 accumulated in the atmosphere that it eventually produced enough global warming to melt the ice. That account isn't universally accepted, though. Looie496 (talk) 17:46, 6 April 2012 (UTC)

One big effect seems to be the motion of the continents. When they move they change ocean currents, and, to a lesser extent, air currents, causing massive changes in the climate. So, you would have to stop plate tectonics, among other things, to get a constant climate. The "snowball Earth", for example, might be caused by plate motions. So, I would argue there would be a single equilibrium point if none of the inputs changed, but, since they do, we end up with multiple equilibrium points, depending on the current inputs to the system. StuRat (talk) 17:58, 6 April 2012 (UTC)

My gut feeling is that the Ice Ages approximate such an attractor (I know the orbit is supposed to be involved, but have you looked at the comparison? It's not very convincing looking). In any case, the transition back and forth from Ice Age to not is so extreme, I think it would have a good chance to kick us out of any smaller attractors if they exist. Of course, climate has changed on that time scale in many places, e.g. North Africa which in Carthaginian times was better suited for horses than camels. Wnt (talk) 20:32, 6 April 2012 (UTC)

As Looie said, it depends on the timescale you are looking at. We will never reach a true attractor in climate (even if one might exist locally), because the solar and geophysical inputs are constantly changing: on short scales we have seasons, on intermediate scales we have the Milankovitch cycles, on longer scales we have drifting continents, and on even longer scales we have ever-increasing solar luminosity, and decreasing tides and increasing length of day due to the tidal acceleration of the earth-moon system. This is not to mention the occasional high-impact events such as supervolcanoes and impact events, and other bizarre occurrences (Messinian salinity crisis, megafloods) which can affect the earth for thousands of years, or human activities such as deforestation and anthropogenic emissions causing photochemical smog, ozone depletion, and global warming.-RunningOnBrains^(talk) 20:56, 7 April 2012 (UTC)

Explaining light

Some years ago I received from an aquaintance by e-mail a curious physics explanation. I need some help to provide a serious point-by-point answer. The following is the exact text as I received it, with no attempt to correct typos e.g. Cerb to Cerne , paricles to particles, degenartion to degeneration.

(text starts)That collider at Cerb, Switzerland is another non-starter. They are using something ( electrons/magnetism throigh a material conductor) to find out what conditions were like when there was nothing. It's a dead end. All you ever get when dividing matter into ever smaller paricles is another surface. No one has yet considered that there may be a dimension where light can not go less than 186k/sec., and that this dimension ( the big bang) ocurred when an anomaly, or time warp, retarded light and as it descended from super light, time came with it and the release of supercharged energy created heat, which when it expanded created matter. Desension of superlight from hyperspace couldn't dispell itself fast enough resulting in a dense coagulation, or degenartion of energy, into more solid form.

I've never understood why if the speed of light is a constant, its speed can become squared; as in E= Mc2. But if the speed of light can be squared ( in fact) there should be a universe, or dimension, where this is the standard. (text ends) 84.209.89.214 (talk) 23:52, 6 April 2012 (UTC)

Technobabble like this example can sound quite serious, but it is pure gibberish. A point-by-point explanation would be useless, because this text excerpt is absolutely meaningless. It name-drops several interesting topics in physics, and it misspells CERN, among other errors; but it doesn't actually say anything. Nimur (talk) 00:44, 7 April 2012 (UTC)

Perhaps you should reply in technobabble too ? "The light-force emanations vibrate into crystalline astral projection planes in non-isotropic Euclidean parallel multi-verses, thus ensuring the isotope strings of an invariant cosmology." StuRat (talk) 00:54, 7 April 2012 (UTC)

One fairly big problem is that there was not "nothing" at the Big Bang, there was an incredible amount of energy contained in an infinitely small space. The idea of the Large Hadron Collider is not to create "nothing from something", but to create as large an amount of energy in as small a space as possible. The rest is basically gibberish. Speed squared is not a speed, any more than distance squared is a distance (for instance, 5 inches squared is not 25 inches, but 25 square inches, which is a measurement of area). Smurrayinchester 09:05, 7 April 2012 (UTC)

just one easy to contradict point. Let's say a typical rat is 5-6 inches. If a person is 5-6 feet, that means that the length of a person in terms of rats is 12 * ratlength. But if 12 * ratlength is meaningful, then there must be a rat the size of a person. We just have to find it and put it on television. (this is the same argument used about the fact that you can 'square the speed of light' in e=mc^2, exactly the same as multiplying the length of a rat by 12. it doesn't have any bearing on the speed of light!) 188.156.106.2 (talk) 08:39, 7 April 2012 (UTC)

There are rats the size of people (infants, at least). See nutria. StuRat (talk) 16:43, 7 April 2012 (UTC)

Nutrias are nothing! Check out these guys. Evanh2008 (talk) (contribs) 21:23, 7 April 2012 (UTC)

Yep. Tecnobabble, and not even good technobabble. In addition, heat could not have existed before matter since heat is a property of matter. Evanh2008 (talk) (contribs) 09:03, 7 April 2012 (UTC)

Escape velocity

Is the escape velocity of the Earth's gravitational field in m.p.h. is equal to the length of the circumference in miles? 84.209.89.214 (talk) 23:53, 6 April 2012 (UTC)

No. Why would it be? That the numerical values, expressed in different units, are similar is merely a coincidence. Dragons flight (talk) 00:01, 7 April 2012 (UTC)

Earth's circumference in cubits is 8.8×10⁷ while Earth's escape velocity is 8.8×10⁷ cubits per hour. Lord! it's a miracle!

For any spherical body, the ratio between its circumference and its escape velocity is given by the following expression: ${\frac {{\sqrt {2}}\pi r^{\frac {3}{2}}}{\sqrt {GM}}}$

Subbing in Earth's dimensions gives us 3575 seconds, which is coincidently very close to the 3600 seconds in each hour. Thus it's pure coincidence that an object travelling at escape velocity around Earth would take approximately one hour. Anonymous.translator (talk) 02:16, 7 April 2012 (UTC)

As a side note, "Eight Eights" (88888888) is probably the shortest mnemonic possible for memorizing both Earth's equatorial circumference and Earth's escape velocity. For someone living in ancient Egypt at least.Anonymous.translator (talk) 02:24, 7 April 2012 (UTC)

Escape Velocity = the speed at which a prisoner is pardoned after making a generous contribution to a retiring US President, or governor of a US state. StuRat (talk) 02:36, 7 April 2012 (UTC)

April 7

How much extreme weather damage can the world economy take?

The international reinsurance company Munich Re says that the amount of damage from extreme weather tripled from 1980 to 2010, and they expect that trend to continue.[1] Assuming that remains true, (1) how long until the damage from extreme weather causes the world economy to collapse? And, (2) assuming global wind power growth continues along its current trend, will it prevent such a collapse in time? 71.215.74.243 (talk) 02:52, 7 April 2012 (UTC)

At some point neither private companies not governments will be able to insure people living in high risk areas, like below sea level in a place regularly hit by hurricanes. This will result in people no longer being able to get mortgages to build there, and will thus limit damages from future storms, restoring the balance. StuRat (talk) 03:09, 7 April 2012 (UTC)

So do you think the world economy can absorb an infinite amount of storm damage? 71.215.74.243 (talk) 11:07, 7 April 2012 (UTC)

No, but an infinite amount of storm damage isn't possible, so that's pointless to worry about. You seem to be making a common extrapolation error. Whenever anything is increasing, all you have to do is assume it will forever increase at the same rate to conclude that it will eventually become infinite. The assumption that anything which is currently increasing must therefore increase forever is fundamentally flawed. StuRat (talk) 17:10, 7 April 2012 (UTC)

I agree. Let me rephrase my question: If the amount of economic damage from extreme weather continued to tripple every 30 years, do you think the world economy would still remain functioning indefinitely? Or are you saying that the damage must eventually level off before it causes economic collapse? 71.215.74.243 (talk) 22:20, 7 April 2012 (UTC)

Yes, it will level off or even reduce. For example, now that global warming is fairly well established, people will eventually stop building on eroding coastlines. StuRat (talk) 22:33, 7 April 2012 (UTC)

The premise is false. Data from impartial sources [2] show that there's no such increasing trend.

Also the global wind power cumulative capacity can't grow forever either, since the total amount of available wind power on Earth is limited. Anonymous.translator (talk) 03:18, 7 April 2012 (UTC)

You don't think that graph you linked to shows a trend? Wind power can't grow forever but there is enough for dozens of times expected human needs even assuming everyone started using US per-capita power. 71.215.74.243 (talk) 03:31, 7 April 2012 (UTC)

Looking at the dotted line, no, I don't see a clear increase. For most years the damage is roughly $10 billion while in the occasional "disastrous" years the damages spike up.Anonymous.translator (talk) 03:47, 7 April 2012 (UTC)

Do you think the average of the dotted line in, for instance, the first, middle, and most recent third of the graph comprises a trend? 71.215.74.243 (talk) 11:07, 7 April 2012 (UTC)

I'm pretty sure that as a publicly traded company whose core business is insurance, Munich Re has a legally mandated duty to be honest about the past claims they have honored and the future claims they expect. As a global company, I would expect they actually have a better picture of climate related losses than data that solely comes from the US. That said, I seem to recall that Munich Re also blames most of the trend on increased human development in vulnerable areas, and relatively little of it on actual changes in weather conditions. Dragons flight (talk) 05:11, 7 April 2012 (UTC)

One thing I left unstated is that most of the damage is due to people building in unsafe areas. Once we stop that, the problem is largely solved. StuRat (talk) 04:27, 7 April 2012 (UTC)

Yes. People need to remember such things as the fact that flood plains are for floods. HiLo48 (talk) 05:31, 7 April 2012 (UTC)

Indeed, Munich Re used to say that sort of thing, but not in the past several years. Their 2010 report linked above, for example, says, “In Germany, extreme precipitation resulting in floods is becoming increasingly common. This affects not only people living on rivers: there are more and more cases of heavy rain and flash floods. Anyone may be affected.” 71.215.74.243 (talk) 11:07, 7 April 2012 (UTC)

That's a rather meaningless statement, rather like saying that anybody can be struck by lightning, so you might as well play golf on top of a hill in a thunderstorm. While anyone can be affected by some type of bad weather, by no means is the risk equal in all locations, far from it. StuRat (talk) 16:16, 7 April 2012 (UTC)

One line on that graph is normalized for inflation but not for population growth or overall real estate value. Wnt (talk) 15:44, 7 April 2012 (UTC)

Not much, and the weather doesn't have to get a lot more extreme overall. According to a recent BBC Horizon documentary, we're now seeing significantly more fluctuations in the weather, while the average has only shifted slightly due to global warming. The latter is driving the former. A prediction of climate models is that cyclones can hit places where they have sitorically never occurred, e.g. Dubai seems to be at risk of being washed into the ocean by a category 5 hurricane.

A small perturbation is enough to let the World economy crash, because the economy has been mismanaged anyway. We were about to hit a global depression, simply due to mismanagement in 2008. So, it's a bit like flying a plane that could theoretically fly to the thunderstorm, but with incompetent pilots flying it, it's already at risk of crashing due to pilot error when the weather is fine. Obviously, you then don't want to fly a plane with these pilots through thunderstorm.

What I forsee happening within a few decades, is droughts and floodings causing India and China to have to import huge amounts of rice and grain for a few years in a row. They have enough financial reserves to do so, but that would drive food prices up by so much that other countries will close their markets to prevent all their rice and grain from from being exported to India and China. That will then lead to collapse of free trade agreements and ultimately to the collapse of the World economy. Count Iblis (talk) 17:42, 7 April 2012 (UTC)

In that case, what they need is system to collect flood water and store it until the next drought. StuRat (talk) 18:25, 7 April 2012 (UTC)

So simple, yet so difficult! How should this be accomplished on the massive space and time scales necessary to help? Shall we start a lobby to subsidize rain barrels? SemanticMantis (talk) 19:21, 7 April 2012 (UTC)

In the case of underground aquifers, it may be as simple as redirecting rivers to refill them rather than draining to the sea. StuRat (talk) 19:47, 7 April 2012 (UTC)

Going back to the original question, I don't see what wind power has to do with any of this. Additionally, the correlation between severe weather events and global warming is unknown; indeed, we should see a reduction in Atlantic hurricanes in the next decade or two due to a downturn in the West African rainfall cycle. -RunningOnBrains^(talk) 20:33, 7 April 2012 (UTC)

I've read many times that it's not simply the number of Atlantic hurricanes that matters; supposedly global warming is predicted to increase their average intensity -- fewer less intense ones combined with more very intense ones = more damage. Is that not right?

Also, the problem is not solved once we stop building in unsafe areas (although obviously that is important too). An amazing percentage of the world's population has already built in unsafe areas -- there are lots of hugely populated coastal cities (and for good reason since coastal cities can have ports).

I think the questioner's point about wind power was simply that, to the extent that wind power supplants CO₂-releasing power generation, we can slow down or stop the growth of CO₂ levels in the atmosphere. S/he is asking whether present trends in the growth of wind power usage are fast enough to do that before we reach a tipping point. I'm pessimistic about it, but it's not all-or-nothing -- increased use of wind power might delay or minimize the extreme-weather effects of CO₂ even if not stopping them entirely. Duoduoduo (talk) 21:06, 7 April 2012 (UTC)

It could be true: certainly increasing the sea-surface temperatures in the absence of all external factors will increase the amount of latent heat available for the hurricanes to tap into, but I've seen studies that suggest that wind shear will increase dramatically over the tropical Atlantic with a warming planet, which would lead to less intense hurricanes. We simply don't have enough information to know for sure. One thing that is known, however, is that increasing sea levels will make residents increasingly vulnerable to the hurricanes that do occur. (I'm not talking Al Gore's nonsense about a 30m increase in sea levels: even a conservative estimate of 20 centimetres (7.9 in) in the next 100 years could have devastating long-term impact.-RunningOnBrains^(talk) 21:22, 7 April 2012 (UTC)

As far as wind power costs, even if the world managed to stop emitting fossil fuels in the next 20 years (an impossible goal, as China is clearly not going to try to stop burning coal any time soon), CO2 in the atmosphere is far from its steady state. The residence time of CO2 in the atmosphere is hundreds of years, so it would likely take thousands of years (without active intervention) to return atmospheric CO2 to it's current levels, never mind pre-industrial levels.-RunningOnBrains^(talk) 21:30, 7 April 2012 (UTC)

We not only need to stop building in unsafe areas, but move away from them. Ports can remain (and be hardened against foul weather), but the rest of the city should be moved. This will happen slowly, as storms destroy buildings in unsafe areas and the foolish people who built there are unable to get insurance and mortgages to rebuild there again.

Also, there's a natural life of a building, after which it would be rebuilt (except for a few historic sites). When the buildings in unsafe areas reach that age, nobody will be willing to risk their money to rebuild there. StuRat (talk) 21:19, 7 April 2012 (UTC)

The idea of moving away from the unsafe areas is interesting -- the Maldives, I've read, are shopping around in Australia for land they can eventually move their entire country to; and I read very recently that one of the South Pacific island nations is also shopping around for land. But these are tiny populations. It would take an extremely long time, with an extreme amount of social disruption and a massive amount of highly expensive infrastructure investment, to move everyone in say Mumbai farther inland. And then transportation costs to the port would be higher than now. So it's not going to happen to any significant extent, unless gradually rising sea levels concentrate people's minds to organize it. Duoduoduo (talk) 21:37, 7 April 2012 (UTC)

There's no need to move the entire city at once. Start with the coastal area, then gradually move inland. This could all happen over a century. For a case where it is happening rapidly, look at Christchurch, New Zealand, which has the misfortune of being on an active fault which destroyed the downtown area recently. StuRat (talk) 21:41, 7 April 2012 (UTC)

Male sexual activity and cognitive performance

How close have any studies come to establishing or disestablishing an effect of sexual activity on cognitive performance in men? Have there at least been correlational studies that controlled for the confounding fetal-androgen effect, or lab studies using the same combination of hormones that are released in orgasm? NeonMerlin 10:18, 7 April 2012 (UTC)

What kind of cognitive performance? PMID 19105078 is a recent review, and PMID 16490297 is an example study controlling for fetal androgen exposure. PMID 19250266 includes the study of prolactin, which is the only hormone conclusively shown to be released during male orgasm. 71.215.74.243 (talk) 11:38, 7 April 2012 (UTC)

Physics LC circuit

The natural frequency of LC circuit is 1 kHZ. How do we reduce it to 0,5 kHZ, using the equation: natural frequency = 1/(2 π sqrt{LC}) I think duplicating the T (2 π sqrt{LC}) is not the correct answer? Thank you in advance!--Atacamadesert12 (talk) 17:40, 7 April 2012 (UTC) — Preceding unsigned comment added by Atacamadesert12 (talk • contribs) 17:39, 7 April 2012 (UTC)

if you want the frequency to be halved then 1/(2 π sqrt{LC}) needs to be halved, i.e. LC needs to be four times as large. Dmcq (talk) 19:41, 7 April 2012 (UTC)

Or you could increase the value of 2π -RunningOnBrains^(talk) 20:21, 7 April 2012 (UTC)

Double it, to be precise, so π ≈ 6.28318530717958647652528677. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 20:26, 7 April 2012 (UTC)

Electronics on board Apollo 11 spacecraft

What were the specifications of the Apollo 11 spacecrafts' electronics? By this, I mean the motherboard, memory, processor, etc. Also, how differently would the spacecraft have been designed if it were designed today with modern electronics?--99.179.20.157 (talk) 18:40, 7 April 2012 (UTC)

Apollo Guidance Computer is a good start. Almost everything would be different if implemented today; not least you wouldn't have a room full of ladies hand-knitting the program. -- Finlay McWalterჷTalk 18:43, 7 April 2012 (UTC)

It was the memory core that got hand-knitted, not the program.--Aspro (talk) 19:02, 7 April 2012 (UTC)

Maybe your right Core rope memory--Aspro (talk) 19:10, 7 April 2012 (UTC)

Great link, thanks! SemanticMantis (talk) 19:22, 7 April 2012 (UTC)

(ec)I'm not at all sure that the motherboard concept even existed at the time. Please clarify whether you are interested only in the on-board computers or all of the electronics in general. Most "electronics" don't contain motherboards, processors or memory, those components are specific to digital computers. Roger (talk) 18:49, 7 April 2012 (UTC)

A good resource for finding your way around more recent spaceflight computer systems is Category:Radiation-hardened microprocessors, which lists a bunch of devices that are, or have been, used in spacecraft. -- Finlay McWalterჷTalk 18:55, 7 April 2012 (UTC)

You should also remember that the onboard computer systems were never intended to be general purpose computers; they were calculators and autopilot controllers designed specifically for the single purpose of automating certain timing operations for the manned flights to the moon. These computers have more in common with a cockpit instrument than with modern general purpose personal computers. For example, there is no "screen" on the infamous Apollo Guidance Computer; only a few cryptic numerical status indicators and some warning lights. However, if you compare this to, say, a RADAR altimeter instrument in terrestrial aircraft of that era, you see that AGC actually provides much more information and control, including automatic fly by wire control of some spacecraft and propulsion systems. Nimur (talk) 19:48, 7 April 2012 (UTC)

Using tachyons to blast something out of a black hole

Would it be possible to "rescue" people or objects from beyond the event horizon of a black hole by hitting them with a super-high energy beam of tachyons at just the right angle? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 20:37, 7 April 2012 (UTC)

I take you mean hypothetically. Well, hypothetically one might have better luck using a transporter. How do you think these questions up?--Aspro (talk) 20:46, 7 April 2012 (UTC)

Off-topic discussion
The following discussion has been closed. Please do not modify it.
AAAAAAAAAAAARGH!!!!! Whoop whoop pull up Bitching Betty | Averted crashes 20:55, 7 April 2012 (UTC) So...is that the sound of you trying the transporter idea? -RunningOnBrains(talk) 21:13, 7 April 2012 (UTC) Maybe the transporter reassembled his molecules with those of a fly The Fly (1986 film)? Are hybrid flies allowed to hold a WP account?--Aspro (talk) 21:25, 7 April 2012 (UTC) BE SERIOUS!!!!Whoop whoop pull up Bitching Betty | Averted crashes 21:28, 7 April 2012 (UTC) Verily - but you first. --Aspro (talk) 21:36, 7 April 2012 (UTC) Some chill pills for Whoop whoop.

I couldn't even begin to approach the highly theoretical math involved, but my understanding is that tachyons can't interact with normal matter, as this would create paradoxes. -RunningOnBrains^(talk) 21:33, 7 April 2012 (UTC)

The serious answer is "no, because tachyons do not exist as far as we know, and even if they do exist, we can't know anything about them, and even if we did know all about them, we couldn't manipulate them, and even if we could manipulate them, we couldn't use them to manipulate ordinary matter." 71.215.74.243 (talk) 22:42, 7 April 2012 (UTC)

Can you pull something out of a black hole? E.g., what if it was a microscopic black hole, or maybe a one-pound black hole -- could you stick part of a sturdy object partway in it and then pull it out? Duoduoduo (talk) 22:56, 7 April 2012 (UTC)

No. As summarized in the previous black hole thread, once within the event horizon, conventional "spacewise" directions become "timewise" -- all future paths move to the singularity; all paths back out require you to move backwards in time. Additionally, it only confuses matters to think about "an object" that stretches into a black hole. That whatever-it-is is in fact a large number of independent subatomic particles bound together by various forces, but those forces are overcome by the black hole once the particles pass within. — Lomn 23:07, 7 April 2012 (UTC)

How about creating a wormhole inside the event horizon? Plasmic Physics (talk) 23:58, 7 April 2012 (UTC)

My understanding is that that would just create a second entry and a second event horizon for the black hole. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 11:35, 8 April 2012 (UTC)

Wormholes fall into the same category as tachyons - you can formulate them mathematically, but there is no evidence that they actually exist or can exist in the real universe and if they do there are all sorts of unanswered questions about how they would actually work. --Tango (talk) 14:40, 8 April 2012 (UTC)

It shouldn't if you face towards the future when you create the worm hole. Plasmic Physics (talk) 13:42, 8 April 2012 (UTC)

You mean like Jodie Foster flying though a worm hole in Contact (film) or was that just another cinematographic illusion?--Aspro (talk) 14:53, 8 April 2012 (UTC)

New theories are constantly being developed as to how to stablise a wormhole, including using the negative pressure exerted by the Casimir effect. Plasmic Physics (talk) 03:06, 9 April 2012 (UTC)

Radicals and VSEPR

What geometries does VSEPR theory predict for radicals? For instance, does the methyl radical have a trigonal planar geometry or a trigonal pyramidal geometry? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 21:26, 7 April 2012 (UTC)

Lone electrons are treated just like non-bonding electron pairs. The methyl radical is triagonal pyramidal. Plasmic Physics (talk) 22:55, 7 April 2012 (UTC)

Thank you. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 02:17, 8 April 2012 (UTC)

You can "treat it" however you like in a handwaving method that is solely an empirical analysis, but if you do so, you'll get the wrong answer:) Methyl radical is sp² hybridized—trigonal–planar geometry with respect to the three C–H bonds and the lone electron in a p orbital. The general trend (setting aside effects of electronegativity and resonance of the substituents) is that a non-bonded electron is p not sp^x; one rationalization is that doing so allows a greater filling of s-like atomic orbitals (as usual, better to have vacancy in the higher-energy p levels). DMacks (talk) 14:32, 8 April 2012 (UTC)

Modelling software predicts a tetrahedral, trigonal pyramidal structure for methyl. The bond angles are greater than in methane, but less than 120 degrees. One s and two p AOs of carbon are degenerate, mixing with the hydrogen s AOs. One p AO of carbon is not degenerate, and is the (non-bonding) HOMO. To achieve a stable electrostatic equilibrium position, the valence MOs are distributed in a tetragonal fashion. For methane all the p AOs are degenerate with one s AO to form four degenerate MOs with the hydrogen AOs. Plasmic Physics (talk) 02:15, 9 April 2012 (UTC)

April 8

Illumination in lumen additive or parallel?

If various LED light emitters is fitted within the same "bulb". Will their light intensity add up as in total_lumen = lumen_1 + lumen_n + lumen_n+1.. Or will a light source made up of many parallel light emitters never be equal to a single (incandescent) light source of x lumen? That is parallel photons will not add up into less photons with the same energy but larger amplitude? because phase synchronization will not happen? Electron9 (talk) 00:58, 8 April 2012 (UTC)

I believe it's additive. However, our perception is not. That is, twice as many lumens doesn't seem "twice as bright". When lighting a room, it's helpful to have lights in each corner, to avoid any dark corners (this is especially true with dark walls, like wood paneling, where little light reflects into the corners). StuRat (talk) 01:15, 8 April 2012 (UTC)

Right. Photons do not join together or combine into fewer photons of different amplitude or frequency. 71.215.74.243 (talk) 01:23, 8 April 2012 (UTC)

Thus the whole idea about replacing incandescent light bulbs with multiple emitters is fundamentally flawed because it just won't be the same ? Electron9 (talk) 01:49, 8 April 2012 (UTC)

No, that's not at all what's being said. For the same total number of lumens, and same quality of light, the room will generally be perceived as being better lit if it has multiple, lower-lumen lights in multiple locations, instead of one bright light in one location. But if you wanted the more poorly lit look of a single light for some reason, you could just put the multiple lights in essentially the same location. Red Act (talk) 03:29, 8 April 2012 (UTC)

Agreed. Also, having the light sources spread out will be perceived as more light. StuRat (talk) 03:41, 8 April 2012 (UTC)

Won't it be much light but no real intensity anywhere? Electron9 (talk) 14:37, 8 April 2012 (UTC)

A room lit uniformly (as if the entire ceiling glowed, or the ceiling an walls glowed) might not be perceived as "better lit" since the lighting effect would be very "flat," and shapes of solid objects would not be as well defined. It might seem quite blah and boring. Edison (talk) 17:34, 8 April 2012 (UTC)

Indeed; this is one of the efficiency advantages of lengthy fluorescent bulbs (which I can't stand at 50 Hz, for what it's worth.) 71.215.74.243 (talk) 23:14, 8 April 2012 (UTC)

Strange zoo bird

Any idea what this bird is? I've gone through all the birds on its zoo's list and none seem to be it. 71.215.74.243 (talk) 01:05, 8 April 2012 (UTC)

I think it may be the Chestnut-breasted Malkoha - which is on the list. AndyTheGrump (talk) 01:23, 8 April 2012 (UTC)

More on it here - with another video. Surprisingly, it is one of the Cuculidae - the Cuckoo family. Common in Southeast Asia. AndyTheGrump (talk) 01:32, 8 April 2012 (UTC)

No doubt! Thanks, I don't know how I missed it from the accurate description linked from the list. 71.215.74.243 (talk) 01:36, 8 April 2012 (UTC)

Cool bird! "Unlike many cuckoos, it builds its nest and raises its own young." Marking as resolved. SemanticMantis (talk) 01:57, 8 April 2012 (UTC)

Resolved

Octane rating of jet fuel

What is the octane rating of jet fuel? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 02:30, 8 April 2012 (UTC)

Jet fuel is essentially kerosene (C8 to C16 molecules), not gasoline (C4 to C12 molecules). Octane rating is irrelevant to jet fuel, since predetonation is not a concern in a gas turbine. However, if it was measured it would be considered to be very high, since octane rating is a measure of fuel's resistance to predetonation, not its volatility. Acroterion (talk) 02:38, 8 April 2012 (UTC)

Thanx! Whoop whoop pull up ^{Bitching Betty | Averted crashes} 11:30, 8 April 2012 (UTC)

Do different chicken breeds have different tastes?

I'm not talking about their eggs, whose taste apparently depend not on their shell color, but the chicken feed. 66.108.223.179 (talk) 02:32, 8 April 2012 (UTC)

If so, it must not be very noticeable, or I'd expect various brands to claim they have better tasting breeds. StuRat (talk) 03:35, 8 April 2012 (UTC)

This document suggests that the slower growth rates of traditional breeds gives a better flavour. Alansplodge (talk) 01:37, 9 April 2012 (UTC)

mercury spectrum

what elements may have a spectrum of wavelengths very similar to the spectrum of mercury — Preceding unsigned comment added by 197.255.118.206 (talk) 06:43, 8 April 2012 (UTC)

"Similar"? You can eyeball the visible spectra at http://www.umop.net/spctelem.htm Hg is in the lower left. 71.215.74.243 (talk) 10:16, 8 April 2012 (UTC)

sodium spectrum

what are the wavelengths of the red, yellow, yellowish-green, green, greenish-bue and violet colours of sodium . — Preceding unsigned comment added by 197.255.118.206 (talk) 08:15, 8 April 2012 (UTC)

This smells like homework to me, but if you can figure it out from http://physics.nist.gov/PhysRefData/Handbook/Tables/sodiumtable2.htm that's probably okay. 71.215.74.243 (talk) 10:20, 8 April 2012 (UTC)

physics

what is the difference between spectroscopy and spectrometry — Preceding unsigned comment added by 197.255.118.206 (talk) 08:31, 8 April 2012 (UTC)

Spectroscopy is the science of determining characteristics from a spectrum. Spectrometry is the measurement of spectra. In many cases they can be used interchangeably without too much ambiguity, but it's better to be precise. 71.215.74.243 (talk) 10:06, 8 April 2012 (UTC)

Specialists in the field often have heated debates. The difference I learned from them in a bar long ago is mainly that half of them take offense at one term and half at the other:) Okay, the main difference I hear when they're sober is that spectroscopy deals specifically with absorption/emission of electromagnetic radiation over a range of frequencies whereas spectrometry especially refers to "things over a range" other than EM effects but may also be used as a parent field for the other. DMacks (talk) 14:28, 8 April 2012 (UTC)

I think your definitions are better than mine, because there's no such thing as acoustic spectroscopy. 71.215.74.243 (talk) 20:02, 8 April 2012 (UTC)

Could Kinect ever come with a force-feedback body suit?

I get to burn some extra calories with the XBox360's Kinect, but there's always room for improvement.

If a force-feedback body suit could make me feel (somewhat) what I see on the screen, there is potential to burn away even more of our bodily excesses. If a zombie pushes me, I would feel an extra urge to push him away, for example.

However, what technological hurdles would its developers need to overcome in order to make a viable force feedback body suit work as intended? --Tergigress (talk) 10:29, 8 April 2012 (UTC)

See Haptic technology for starters. SemanticMantis (talk) 12:29, 8 April 2012 (UTC)

There aren't really any hurdles except expense. It's already possible to get stationary bikes with force feedback. Handgrips or gloves with force feedback could be built, but they would cost a lot, especially gloves. Looie496 (talk) 15:57, 8 April 2012 (UTC)

Why can't the Kinect recognize fingers and head movements?

I can't look up or down; only straight ahead. There are many RPG elements (yet to come?) that'll require you to nod or shake your head.

Moreover, you can imagine many games that would allow/require the use of your individual fingers. A "Guitar Hero"-style game for Xbox360s would finally teach the player how to play a real guitar without even needing a guitar-like accessory. However, the Kinect only recognizes arm movements, and not much more.

Would the Kinect only need a software patch update? Or would changes have to be made to its hardware that it would essentially take a "Kinect 2.0" to pick up the finer details of our bodily movements? --Tergigress (talk) 10:29, 8 April 2012 (UTC)

My understanding is that Kinect "sees you" as outlines of forms — as a silhouette — with some depth mapped on to that (it makes an infrared map like this and then creates the form map like this). It's extremely hard (and unreliable) to see things like fine head and hand movements with just silhouette. It would take significant changes to the way the Kinect worked to do these sorts of things. I wouldn't expect it as just a software patch. (And I would note that even well-informed air guitar is not really instructional for how to play a real guitar.) --Mr.98 (talk) 12:53, 8 April 2012 (UTC)

Its Z-resolution seems to be a bit better than those pictures suggest - check out the various things at http://openkinect.org/wiki/Gallery - the first facial animation shows (in its bottom left) the raw Kinect spacial data stream. That's pretty good, but its resolution is probably too poor to properly distinguish fingers, especially at a reasonable distance, for most angles of the hand. Given an order-of-magnitude improvement in resolution (which probably isn't unreasonable to expect in say 5 or 10 years) you could locate fingers for a guitar game pretty accurately. -- Finlay McWalterჷTalk 13:27, 8 April 2012 (UTC)

Try a google with 'kinect finger' and yoiu can see some videos of people experimenting with this. I guess you'd need a bit of intelligence about how fingers work as well as just detecting them to get it working really well but I don't see any great problem. Dmcq (talk) 13:42, 8 April 2012 (UTC)

which reaction formula is right?

For the reaction between aluminium and NaOH, I'm interested in knowing which of the two reaction formulae is correct:

• the one listed here, under the "reaction of aluminium with bases" section: http://www.webelements.com/aluminium/chemistry.html 2Al(s) + 2NaOH(aq) + 6H₂O → 2Na⁺(aq) + 2[Al(OH)₄]^- + 3H₂(g)
• or the one given in the Wikipedia article, Sodium hydroxide#Reactions. 2 Al + 2 NaOH + 2 H₂O → 2 NaAlO₂ + 3 H₂

They give different products and disagree in the initial amount of H₂O. Katherine.J.W. (talk) 13:52, 8 April 2012 (UTC)

The section here, Sodium_aluminate#Reaction_of_aluminium_metal_and_alkali, suggests they may both be right. --Tango (talk) 14:14, 8 April 2012 (UTC)

(edit conflict) Looks like [Al(OH)₄^–] is just a hydrated form of [AlO₂^–]. As Tango's link says, there are lots of different forms possible with various numbers of "oxide" and "hydroxide" ligands depending on how much water is present (which makes sense, per Katherine.J.W's good observation that "the initial amount of H_{2O" is a key difference). DMacks (talk) 14:22, 8 April 2012 (UTC)}

Mystery bird

• http://dl.dropbox.com/u/50919224/mystery%20bird%20close%20up.jpg

I took a picture of this bird that was wading and fishing at Oyama Lake last July. The complete image is 15mp and actually has a fish jumping on the other side of the frame. I hope to upload it to wikipedia after I identify the bird. The fish is most likely a trout.--Canoe1967 (talk) 16:26, 8 April 2012 (UTC)

It is one of the Heron species. AndyTheGrump (talk) 16:30, 8 April 2012 (UTC)

Probably the Great Blue Heron. Mikenorton (talk) 16:31, 8 April 2012 (UTC)

Yup - If the 'Oyama Lake' referred to is the one in Canada, that makes sense. In Europe, one would expect to find the very similar Grey Heron, which is what I initially thought it was. AndyTheGrump (talk) 16:39, 8 April 2012 (UTC)

Thank you both. I should have mentioned which planet the lake was on. I am glad I didn't label it as a kingfisher before upload.--Canoe1967 (talk) 16:41, 8 April 2012 (UTC)

Er, yes - I don't think Kingfishers get that big. Actually, I may have a decent picture of a Grey Heron myself - or if not, I might see if I can take one to download (fairly common on the banks of the Thames above London, and in surrounding parklands etc). It would be nice to get a picture of one in flight - very distinctive. AndyTheGrump (talk) 16:47, 8 April 2012 (UTC)

I had the camera tilted approximately 40 degrees. I can't see a way to crop it decently to include the heron and fish with normal rotation. I will upload as is and get proposals and votes on the best way to fix it.--Canoe1967 (talk) 17:11, 8 April 2012 (UTC)

I managed to upload, but it seems the file is too large for thumbnails. I need to look into that. File:Heron and small trout.png --Canoe1967 (talk) 18:09, 8 April 2012 (UTC)

Changed to jpg upload. File:Heron and small trout.jpg. Where would I seek photgraphic opinions on how to fix the rotation, or is it fine as is?--Canoe1967 (talk) 18:59, 8 April 2012 (UTC)

 Done I asked for opinions in the graphics lab help area.--Canoe1967 (talk) 20:15, 8 April 2012 (UTC)

calcium metaborate

I'm trying to write a balanced chemical equation for the reaction of calcium oxide melted with boric acid to yield CaB₂O₄. Have I worked this out correctly? CaO + 2B(OH)₃ = CaB₂O₄ + 3H₂O. 86.7.42.12 (talk) 16:47, 8 April 2012 (UTC)

I hope we aren't doing your homework, but the oxygen count looks off.--Canoe1967 (talk) 17:15, 8 April 2012 (UTC)

Hmmm, I count seven each side? 86.7.42.12 (talk) 17:19, 8 April 2012 (UTC)

Yes, I corrected my above post. It has been a while since I have done those. I had problems with factoring trinomials even more.--Canoe1967 (talk) 17:26, 8 April 2012 (UTC)

Okay. And no, you're not doing my homework (I wish I were still that young!), especially as I think I have obtained the correct chemical equation anyway, just want to check for mistakes, like whether the charges are balanced or not. It's been a while since I last worked with chemistry too. 86.7.42.12 (talk) 17:34, 8 April 2012 (UTC)

If I remember correctly, you just need the correct yield compounds and the element counts need to have the same totals on each side, so it does look okay in that respect.--Canoe1967 (talk) 18:04, 8 April 2012 (UTC)

None of the redox numbers change because this is not a redox reaction(which is what you're most likely thinking of when you asked about balancing the charges), so all you need to do is balance each side of the equation, which you've already done. So yes, this is correct as is. 112.215.36.180 (talk) 06:59, 9 April 2012 (UTC)

Why do cows eat grass?

The cows at a nearby farm seem to prefer almost anything over grass. If I bring my organic waste there, like banana skins, apple skins, even rotten vegetables, they come running to me from a great distance, eat all of it, hang around for a while (presumably hoping for more food), and then slowly retreat and start to eat their regular diet of grass.

So, why don't we feed cows with organic waste instead of grass? Count Iblis (talk) 17:50, 8 April 2012 (UTC)

Why do cows eat grass? Because they evolved in grasslands. AS for them eating other things, they may well enjoy it. They may well even get some benefit from it - in small quantities. I doubt that a diet consisting solely of 'banana skins, apple skins, and rotten vegetables' would be good for them. AndyTheGrump (talk) 18:01, 8 April 2012 (UTC)

In the same way that horses are crazy for polo mints, but you'd not want to feed a horse solely on polo mints. 46.208.224.194 (talk) 18:03, 8 April 2012 (UTC)

Not even a polo pony? —Tamfang (talk) 01:15, 9 April 2012 (UTC)

Cows are ruminant animals. Every part of their digestive tract - from the shape of their teeth to the chemistry of their multichambered stomach - has evolved to support eating fibrous plant matter, like grass. However, other plant matter, like fruit, grains, and oats, are acceptable in some quantity. Here's a good overview of cattle feed for small-scale beef farmers, Feeds and Feeding for Junior Beef Cattle Projects, from the extension program of the Animal Science department at Texas A&M - you'll hardly find a more expertly qualified organization than that to explain cattle nutrition! In addition to grass and roughage, cattle need minerals, vitamins, and nutrients that are normally provided by oats and alfalfa, and other products; apples and fruits are acceptable supplemental sources, too, but tend to be too expensive for large-scale feeding. Nimur (talk) 18:23, 8 April 2012 (UTC)

We don't feed cows with grass either, rather, with corn, according to The Omnivore's Dilemma. →Στc. 20:07, 8 April 2012 (UTC)

Pasture fed cattle obviously eat other stuff too. Grass is sucessful because it tolerates grazing well + the cows/other grazers eliminate the grass's potential competition from other sapling plants that could grow up to shade out the grass. SkyMachine ⁽⁺⁺⁾ 23:55, 8 April 2012 (UTC)

Here in the UK, (and a lot of other countries) we got into a lot of bother by feeding cows on bits of other dead cows. Alansplodge (talk) 01:25, 9 April 2012 (UTC)

Cows are probably similar to primitive man, who had to subsist mainly on low-calorie veggies and grains. When they got something better, like fruit or meat, they were probably quite grateful, too. StuRat (talk) 01:20, 9 April 2012 (UTC)

As Nimur says above, the insides of cows are designed to eat grass and similar stuff and quite different from ours. Alansplodge (talk) 01:27, 9 April 2012 (UTC)

The similarity isn't in the actual diets, but that the majority of the diet of both cows and primitive humans was one thing, while both craved "treats" to supplement their meager diets. StuRat (talk) 02:10, 9 April 2012 (UTC)

Something I read long ago, so needs verification. I have heard that too much of very sweet or starchy foods can make cows and pigs ill by causing rapid fermentation in their guts leading to acidification and gas buildup. Staticd (talk) 06:00, 9 April 2012 (UTC)

Bayes' theorem in medical diagnostics

I'm trying to get a grasp on how Bayes' theorem can be applied to updating prior probabilities in medical diagnostics. In this context, Bayes' theorem could be written:

$P(SomeDisease|PositiveTest)={\frac {P(SomeDisease)\,P(PositiveTest|SomeDisease)}{P(PositiveTest)}}.\,$

My understanding is that $P(PositiveTest|SomeDisease)$ is the diagnostic sensitivity of the test, and $P(SomeDisease)$ is the doctor's prior probability of the patient having the disease in question. I'm having problems, however, in interpreting $P(PositiveTest)$. Which probability should be entered here? The probability that any random person in the community that the doctor serves has a positive test, whether they have the disease or not? This doesn't sound right, as the doctor must have taken other information, such as the symptoms, age and sex of the patient into consideration, when assigning a prior probability. So, what probability, exactly, should be entered here? I am aware of the article Likelihood ratios in diagnostic testing, this question specifically concerns the use of Bayes' theorem. Thanks! --95.34.141.48 (talk) 20:17, 8 April 2012 (UTC)

$P(SomeDisease)$ is the probability that someone in a particular group has the disease. $P(PositiveTest)$ is the probability that someone in that same group (regardless of their disease status) tests positive. Both would be based on observed frequencies within that group. What group? It could be all the patients tested during the development phase of the test (good because it gives a large sample, making for a more accurate probability), or it could be all the patients that the doctor has ever given the test to (good if test results vary by geography etc, but bad if it results in poor accuracy of probabilities due to small sample size), or it could be all the patients of particular age, sex, and symptoms that this doctor, or doctors in general, have ever given the test to. But everything in the formula has to refer to the same one of those groups. Choosing a group depends on how good the measured probabilities are as estimates of the true underlying probability, which depends on sample size, and on how much difference it is thought to make if you limit yourself to sub-groups with particular characteristics. Duoduoduo (talk) 20:54, 8 April 2012 (UTC)

Thanks! That was really helpful. Would the following statement be both a correct interpretation of your answer, and correct (albeit pedantical) mathematical notation?

$P(SomeDisease|PositiveTest,X)={\frac {P(SomeDisease|X)\,P(PositiveTest|SomeDisease,X)}{P(PositiveTest|X)}}.\,$

Where $X$ is the information that the patient belongs to a subgroup within the doctor's practice, that the doctor would test for the disease, for whatever reason. If the doctor is not doing a clinical study, the doctor's suspicion of the patient having the disease, or the patient's fear of having the disease, could be such reasons. --95.34.141.48 (talk) 05:52, 9 April 2012 (UTC)

Group 14 electronegativities

Why doesn't this group show any EN trend? Could someone give a detailed answer?--R8R Gtrs (talk) 20:32, 8 April 2012 (UTC)

A few questions from a science fiction author: Lethality of small amounts of high explosives and theoretical weapons systems

I'm currently working on a book for an RPG game developer that's in their "wish list" category, I dislike pure "pie in the sky" science fiction: I want my creation to be at least plausible.

One idea that has always intrigued me is the idea of a "utility launcher" weapon, based on the dizzying array of shotgun shells and grenades manufactured, especially the myriad special shells (flechette, HE, smoke) proposed for the Jackhammer automatic shotgun. My idea is that given the tactical adaptability needed of a 22nd century soldier (engaging rioting civilians, lightly armed insurgents, powered armor equipped enemy heavy infantry, light vehicles, etc) a common solution is a large-bore (perhaps 30-40mm) low-velocity pistol or rifle railgun. In this publisher's settings power cells have put energy levels capable of fueling a railgun in common use. The standard D-sized battery carries 180,000 KwSec of energy, as an example.

My question is threefold: First of all how practical would such a weapon system be? Would there be special challenges faced? Would they make conventional weaponry obsolete or would they be used as a side-arm in addition to a main battle rifle firing a round more like what we're used to seeing (for example a 5-8mm railgun main battle rifle)? Secondly: How much damage would a 30-40mm sphere of high explosive do? Would it be lethal enough to justify its use? What might it's kill/wound radius look like? For this I think we can assume advances in chemistry would produce an explosive 10-30% more powerful than hexogen Third: What sorts of rounds would a common soldier be issued? Obviously HEAT and HESH are likely, perhaps smoke and distraction device. Non-lethal stingball and CS rounds are other obvious possibilities, but one complaint I have with sci/fi is that there are often many advanced weapons but writers rarely look at theoretical grenades/explosives, except maybe plasma. I see no reason why they wouldn't advance along with the weapon technology. HominidMachinae (talk) 23:46, 8 April 2012 (UTC)

Are you thinking of something like the Milkor MGL? Alansplodge (talk) 01:04, 9 April 2012 (UTC)

In all honesty the fictional underpinnings of my idea come from the Scorpion pistol in Mass Effect 3 and the Pancor Jackhammer (which I consider fictional since fewer than 20 were ever made). I envision something like the sawn-off Vietnam Era variant of the M79_grenade_launcher or the XM29 HominidMachinae (talk) 02:08, 9 April 2012 (UTC)

The US military is already looking at explosives 28% more powerful than hexogen, so it wouldn't be unreasonable to assume that something 100% or even 200% more powerful will become available by the 22th century.

I really like your grenade idea. A bunch of army guys had similar ideas back in the 90's and that's how the OICW project started. 20 years later two countries are fielding (US and South Korea) computerized air burst grenades. My speculation is that these computerized grenades will gradually gain new capabilities as technology evolve and eventually become "smart grenades" that can automatically identify threats, change trajectory in mid-flight, and perhaps even switch between HE and HEAT right before impact. Just to throw some wild ideas around: sub-munitions, spherical/cylindrical grenades that can roll toward targets after landing, grenades that can turn into mines. You might also want to take a look at what other sci-fi authors came up with w.r.t. grenades:[3]Anonymous.translator (talk) 02:17, 9 April 2012 (UTC)

I assume that all rounds will have a dual-use proximity switch based upon capacitance: just like the touch screen of a modern smartphone they'll know if what they stuck to is humanoid (and thus use an instant fuze) or inanimate (and thus use a proximity fuze based on capacitance), in essence they work like a very lethal Theremin. As to what other sci-fi authors have done, usually they stick to modern explosives when offered a choice of grenade, otherwise they go to using a plasma grenade. Even settings in the far future such as Mass Effect or Halo use either fragmentation grenades much like those used since the 1800s or "Plasma" as the default "sci/fi sounding" grenade type. I hope for a more realistic interpretation. HominidMachinae (talk) 05:17, 9 April 2012 (UTC)

April 9

low blood pressure and i. v. heroin use

can low blood pressure be caused from heroin abuse? what can the outcome be? — Preceding unsigned comment added by 174.19.186.92 (talk) 02:20, 9 April 2012 (UTC)

I wouldn't expect it as a direct result, but perhaps it could be an indirect result, if while shooting up the addict damages a blood vessel and bleeds out. Death could result. This is most likely when mainlining into major arteries. StuRat (talk) 03:50, 9 April 2012 (UTC)

Perfectly elastic collisions for equal masses

From the article on Collisions:

"Collisions play an important role in cue sports. Because the collisions between billiard balls are nearly elastic, and the balls roll on a surface that produces low rolling friction, their behavior is often used to illustrate Newton's laws of motion. After a low-friction collision of a moving ball with a stationary one of equal mass, the angle between the directions of the two balls is 90 degrees. This is an important fact that professional billiard players take into account.[1] Consider an elastic collision in 2 dimensions of any 2 masses m1 and m2, with respective initial velocities u1 in the x-direction, and u2 = 0, and final velocities V1 and V2. Conservation of momentum: m1u1 = m1V1+ m2V2. Conservation of energy for elastic collision: (1/2)m1|u1|2 = (1/2)m1|V1|2 + (1/2)m2|V2|2 Now consider the case m1 = m2, we then obtain u1=V1+V2 and |u1|2 = |V1|2+|V2|2 Using the dot product, |u1|2 = u1•u1 = |V1|2+|V2|2+2V1•V2 So V1•V2 = 0, so they are perpendicular."

Although I see no problem with the math, I've been having a hard time trying to believe the result that the final velocities are perpendicular. Can anyone provide evidence for this statement (A video, if possible)? Also, if correct, what mechanism is directly responsible for the change in direction of the initial velocity? If the final velocities are indeed perpendicular, that would imply a force acting on a different direction than that of the moving ball's trajectory. However, the only forces I can see are the respective normals of each ball, which, as far as I know, are perpendicular to the contacting surfaces, that is, parallel to the initial velocity of the moving ball. Is there something I am missing?186.28.49.19 (talk) 02:57, 9 April 2012 (UTC)

The cueball need not hit the target ball straight-on (square with the direction of travel)--they are not point particles. Here's a handwaving explanation for if not colliding straight-on: if the cue makes a glancing blow on the target, you have to decompose the initial velocity as one vector normal to the contact and one orthogonal to it. The contact-normal component results in energy transfer to the target in the direction of that vector. The orthogonal component is not affected by the collision, so the cue keeps moving in that direction. DMacks (talk) 03:23, 9 April 2012 (UTC)

In the real world, there is an additional complication in that when the collision occurs, the cue ball will usually have some spin, which may include spin imparted by the cue tip (in any direction) and/or a previous bounce from a cushion (in a horizontal direction), and usually (sometimes solely) a component caused by its roll along the baize: after the collision, this spin (which is not greatly affected by the near-frictionless contact between the balls) operating through friction with the baize, will contribute to the subsequent direction of the cue ball, changing it from the Newtonian ideal. If the ball is spinning and/or travelling with any appreciable speed, it "skids" on the baize, and only when it slows down sufficiently in either sense does it fully interact with the baize, so the ball may well travel in a curve, or change direction abruptly after a brief interval, following the collision.

Rather than videos, may I suggest that you go to the miniclip.com website and try playing the 8 Ball Pool QFP game (in its solo version via the "Play Instead" button rather than the multiplayer mode reached via the "Play" button). This gives, for an online version, quite a good simulation of most of the parameters of a billiards-type game. (Confession time – I'm an addict.)

Pertinent to your particular query, you will find that a hard cue shot played from any position against a ball on the centre spot (a situation that is automatically set up at some re-racks, and always if one contrives to leave the last ball of a frame in the rack area when potting the penultimate ball) so as to pot the latter into a side pocket will always send the (skidding) cue ball at right angles, parallel to the length of the table, whereas if you play the cue ball more gently, its rolling friction-induced spin will be able to cause it to deviate from this ideal direction. You will also see that the helpful post-strike direction indicator (absent if you play in "Expert" mode) always indicates the "ideal" (spinless) post-collision directions of cue and object balls to lie at right angles.

Should you be so minded, you may through repeated playing be able to develop both a reasoned and an intuitive grasp of the balls' behaviours in various situations of speeds and spins. {The poster formerly known as 87.81.230.195} 90.197.66.4 (talk) 05:42, 9 April 2012 (UTC)

To answer your question directly it seems there is indeed something you are missing. Any spin ("English") applied to the Que ball will be imparted onto the object ball in some proportion. HominidMachinae (talk) 06:04, 9 April 2012 (UTC)