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In my apartment lawn, there are these semicircular, dark patches of grass. Does anyone know what they are? I'm convinced they are the work of aliens. Here are the pics. [1]. Thanks for any input 199.76.188.69 (talk) 01:14, 26 June 2008 (UTC)[reply]
Well, dude, if you already know--why ask us? --Trovatore (talk) 01:17, 26 June 2008 (UTC)[reply]
That's very interesting. Is there anything interesting at the physical centres? Sprinkler outlet or anything?
I feel like this one is the most likely explanation. A sprinkler that disproportionately sprayed water at a specific radius would produce these effects. EagleFalconn (talk) 04:00, 26 June 2008 (UTC)[reply]
Maybe your best bet is to wait 'til you see the lawn maintenance people, then run out there and ask. Or maybe the building manager. Do post the answer here though. Oh and when you say aliens, do you mean there are immigrants taking care of the grounds? :) Franamax (talk) 01:28, 26 June 2008 (UTC)[reply]
I've found the answer! No, they aren't from ET aliens nor illegal aliens. They are fairy rings. They're caused by fairies and elves dancing, though the scientific explanation is that they're caused by fungal growth.
(In seriousness, I'm quite surprised and amused by the various folklore about the rings)199.76.188.69 (talk) 01:49, 26 June 2008 (UTC)[reply]
Lawn mower wheelies? The nap (direction) of cutting can do effects like light and dark as you know. Julia Rossi (talk) 02:51, 26 June 2008 (UTC)[reply]
There may have been at one time a series of decorative walls in a semi-circle repeating pattern through the lawn. When the walls were removed, they landscaper planted grass seed that wasn't of the same variety as the rest of the lawn. So the different variety of grass is a different shade of green. Dismas|(talk) 03:36, 26 June 2008 (UTC)[reply]
The "darkness" of the grass is often proportional to the amount of nitrogen-bearing fertilizer applied to the lawn; a broadcast spreader slings fertilizer out within a circle of a certain diameter (say five feet for a hand-pushed spreader). An accident with such a spreader could easily leave a dark, over-fertilized area of grass.
Consider a controllable, variable DC voltage source some distance away from a load. The load requires a constant voltage and a varying current (consider a house with various numbers of lamps lit); the amount of current required is a fixed function of time, and may be considered to be known in advance if it helps. We can achieve this by varying the source voltage, but as the required current increases, the amount of power dissipated in the lines to and from the load increases quadratically. It seems that a way to reduce this waste would be to install reservoir capacitors near the load, so that when more current is required it can be drawn from them and when less current is required they can be replenished, with appropriate adjustments to the voltage source to make this happen. The RMS current from the source should thus be reduced even though the absolute average current (dictated by the load's requirements) is the same. However, the obvious approach of simply putting a capacitor in parallel with the load fails completely because the load by definition has constant voltage, so the capacitor has constant charge and 0 current. (In the perhaps more realistic case where the voltage is allowed to change slightly, the capacitor is merely inefficient, as it stores much more charge than it ever may dispense before being recharged.)
What is the correct approach, then? A variable capacitor, perhaps? Ideally we'd have some sort of (ideal) rechargeable battery that can store energy but has no correlation between amount of charge and potential difference (except that obviously no energy implies no voltage). Furthermore, how might this approach be extended to support an AC source — not a rectifier, but a true AC source and load where a capacitor in parallel would fail to store any energy permanently? --Tardis (talk) 02:42, 26 June 2008 (UTC)[reply]
Isn't it enough to have everything in parallel? If the sole need for changing current is turning items on and off, they'll get the mains voltage regardless when they're in parallel. You can accomodate the increasing voltage across the mains wire by using a higher voltage than necessary, and placing a variable resistor before it branches to the parallel circuits. You said the voltage needed is constant, so that sounds about right. But practically speaking, the actual example you gave (a house with appliances) can tolerate the variations in voltage that would result. Someguy1221 (talk) 04:06, 26 June 2008 (UTC)[reply]
It's not so much the precise control over the voltage that's the problem, but the non-linear wasted power in the lines. To take an extreme example in the vein of the house analogy, consider some small constant load and then a thousand hair dryers that are all used together for less than a minute per day. The average required power (thus average current) is not very large, but the RMS power (to which the waste is proportional) is huge because of the spikes. As the number of dryers increases, the useful power increases linearly, but the wasted power increases quadratically, so eventually almost all of the power is being wasted in the lines. Drawing the average current over the lines at all times would be preferable, if there were a way to store the "hair dryer energy" locally until it was needed. (Of course, the house is just a good example of a variable load; any sort of pulsed power arrangement would have this problem.) Does that make the question clearer? --Tardis (talk) 13:43, 26 June 2008 (UTC)[reply]
Active electronics or an electromechanical regulator at the load (like a variable capacitor or storage batteries and controller) ) would not be free of cost. You need to balance the cost of a novel regulator system at the load with the cost of having larger feeders or of placing a source of power closer to the load. A battery bank near the load or rotating equipment at the load could also regulate voltage. Batteries might be more economical than capacitors, unless the load cycle was quite short. If a huge load of short duration is to be placed on a power system, flicker must be limited. The customer who creates the flicker (like a factory which uses a huge of current once an hour) is made to pay for the system reinforcement, or may be told to build his own generating plant. A compromise which has been used for a customer who needs to do 30,000 amp short duration fault tests a few times a day is to use utility power to spin up a flywheel on a customer generator, which is then disconnected from the utility and serves to supply the fault current as it spins down. Capacitors at the customer site could also be used to supply spikes of energy. In the early days of central station DC power in the 19th century, the mains could be quite large, to prevent excess voltage drop, and the generating stations might be located every couple of miles. Sensing conductors (non-load carrying) could be placed at various points in the close to the load to inform the generating plant of the user voltage, to supplement line drop compensation. Regulation of AC systems was later accomplished by placing AC substations closer together than the generating stations, which supplied higher transmission voltage. The substations transformed it down to distribution voltage and were a point of voltage regulation. AC capacitor banks along the line furnished additional regulation. Simply placing pole mounted or pad mounted transformers at closer spacings improves regulation, without tap changers, which are usually found only in substations. In summary, I would hesitate to pay to reinforce an entire distribution system either for delicate load which requires a constant voltage, or for demanding loads which impose huge infrequent brief loads. Another approach would be to run a dedicated line from a dedicated transformer to the high surge load, or to run a transmission line to it an give it its own transformer, with rectifiers if it is a DC load. Or a motor generator could be used to give it adequate voltage. It sounds like engineering questions which utilities and industrial customers face frequently. The questions are how bad is the load factor, and what is the cheapest way to prevent excess flicker. Edison (talk) 12:28, 27 June 2008 (UTC)[reply]
While showering I use bar soap to clean my cavities. Can germs survive long on soap bars? Thanks. Imagine Reason (talk) 03:51, 26 June 2008 (UTC)[reply]
Soap is really like a surfactant that aids is removing germs from the skin. I'm not sure of how long, but bacteria and fungus can certainly live/grow on soap or in detergent. Wisdom89(T / C) 03:54, 26 June 2008 (UTC)[reply]
Yes...the purpose of soaps and detergents are to remove undesired material from whatever you're washing, in this case yourself. They are not designed to actually destroy what they are washing away. If you're really concerned, you can go for antibacterial soap, but outside of disinfecting your hands before performing surgery, there is no evidence to my knowledge of an actual benefit to using it; Specifically with respect to a study on hand soap, I recall the quote (something like this, anyway), "It's getting the bacteria off your hands that's important, not killing them as they rinse down the drain." Although I'll admit I've thrown out a bottle of soap after it sat unused for years and took on a funny color...Someguy1221 (talk) 04:00, 26 June 2008 (UTC)[reply]
Soap is in fact primarily composed of a surfactant. What happens on your skin is that bacteria and dirt and such get stuck on the oil on your skin, or in the case of your hair in the sebum. This oil is non-water soluble, making it very difficult to get rid of a lot of this stuff. Soap decreases the surface tension between the water and the oil making the oil (and hence all the stuff in it) water soluble so it is possible to wash away. So you really don't have anything to worry about with the soap getting dirty. If you're really that concerned about it, a quick rinse (5 seconds) of the bar of soap at the end of your shower would be sufficient to eliminate anything stuck to the surface since the surfactant most commonly used is very water soluble. EagleFalconn (talk) 04:08, 26 June 2008 (UTC)[reply]
Has someone been listening to Jay Mohr's "The Magical Power of Soap"? It's hilarious.... [2] - Nunh-huh 04:19, 26 June 2008 (UTC)[reply]
One of my favorite Pluggers cartoon panels showed a plugger coming in from working hard outside, maybe on his car; the caption was "A plugger has to wash the soap after he washes his hands." If you're concerned, take that advice and just clean off the soap a bit after you shower. ;-)
Antibacterial soap that contains antibiotics (bactericides, really) is a bad idea. It just helps breed superbugs. As for the OP, just wash off the surface of the soap in the stream of water before you start using it to clean your cavities, and you'll be fine. Also, harsh soaps are meant for ordinary skin, not mucous membranes. Be gentle with your mucous membranes. --arkuat(talk) 07:37, 27 June 2008 (UTC)[reply]
What I consider interesting is that the largest stars have far larger quantities of fuel but their large size also increases the rate of reaction such that the lifespans of such stars become way shorter. Are there any methods of formulae to calculate what is the theoretical "optimum size" for stars to live the longest (with active fusion?). I assume if you start out with too little fuel (though with enough critical mass to start a reaction), your lifespan will also be cut short. John Riemann Soong (talk) 04:26, 26 June 2008 (UTC)[reply]
No, see our Red Dwarf -"The lower the mass of a red dwarf, the longer the lifespan." and the cited reference.John Z (talk) 04:48, 26 June 2008 (UTC)[reply]
Though I think the original poster likely has a point. If you take a star just ever-so-barely this side of a non-fusing brown dwarf, it probably fuses for a shorter period than a star perhaps 10% larger. The reference you mention (at red dwarf the star, not Red Dwarf the BBC programme) isn't specific enough regarding dwarf stars to address this issue either way. — Lomn 04:53, 26 June 2008 (UTC)[reply]
See main sequence, it has the answer. Basically, if the star has enough mass to ignite the hydrogen in its core, it would normally stay in this "steady state" for a very long time; the less massive is the star the longer, very possibly longer than the lifetime of the universe so far. So, the smaller the better. Lifetime-versus-mass table is here. Cheers, --Dr Dima (talk) 05:43, 26 June 2008 (UTC)[reply]
Actually, I'll follow up with more definite data to my previous answer -- this is not a trend that is indefinitely sustainable. Brown dwarf notes that those stars (less massive than red dwarfs) can cease fusion on a scale of millions of years. There is some point larger than "minimum size for fusion" at which "length of time fusing" maximizes. — Lomn 12:59, 26 June 2008 (UTC)[reply]
Brown dwarfs are a special case: they're fusing deuterium rather than hydrogen, so they've got less usable fuel to begin with. "Star" is generally defined as any body capable of sustained fusion of hydrogen. --Carnildo (talk) 23:28, 26 June 2008 (UTC)[reply]
I was trying something i found in an old science book but since I'm only in high school i only know that much of chemistry so perhaps someone could explain it better.
We've tried something similar to this at school using a Hoffman voltameter but this more primitive setup worked just as well. Opposing poles of a 9 volt battery are attached to 2 graphite pencils partially submerged in the water/salt solution. The result should be the separation of the hydrogen and the oxygen no? that worked fine. Something like H2O -> H2 + O. (forget the sodium here)
Then i tried just putting the wire directly in. I didnt do this for long because i suspected it might produce chlorine. Anyway NaCl is ionic right so the ions should seperate in water? So its something like this perhaps NaCl + H20 -> Na + CL ?? . Im not sure what to do there. Anyway my question is was chlorine produced?
Also there were white solids floating in the water.
This could be salt but that means chlorine wasn't produced.
The inverse works that same way being they were seperate and it wasnt salt but something else.
So what was the solid?
Salt dissolves in water to give Na+ and Cl-, there is also a small amount of autoionisation of water itself to give H+ and OH- ions.
At the negative electrode hydrogen will be produced, (even if sodium were made it would react immediately with the water to give the same product hydrogen)
At the positive electron two reactions can occur:
2OH- turns into H2O + 0.5 O2 +2e-
2Cl- turns into Cl2 +2e-
The first reaction you already know, from the electrolysis of water, the second makes chlorine, and occurs in the presence of Cl- ions.
In fact the chlorine can often react with the water to give bleach
ANSWER If you run this cell - you'll get some chlorine give off (green gas), and some dissolves (making the solution bleachy). Small amounts aren't harmuful, large amounts are.87.102.86.73 (talk) 12:18, 26 June 2008 (UTC)[reply]
I was thinking of this reaction the other day and I was trying to think of some common substance that would provide the ions needed to charge the water but not get in the way of oxidizing the water to produce oxygen and I thought either sodium bicarbonate (baking soda) or acetic acid (vinegar) would work. Anyone tried this?72.219.143.150 (talk) 19:57, 26 June 2008 (UTC)[reply]
bicarb is a good choice, as is NaOH if you can get it (pearl caustic soda) - used for clearing drains - be careful - can burn you.. both will work - and you only need a small amount - enough to increase the ionic conductivity of the water.
acetic acid - not so sure - acetate should be relatively stable to oxidation - but that could depend on what your electrode is made of.87.102.86.73 (talk) 23:08, 26 June 2008 (UTC)[reply]
Note aqueous electrode oxidation may produce peroxides eg hydrogen peroxide, - as well as oxygen.87.102.86.73 (talk) 11:06, 27 June 2008 (UTC)[reply]
Is this picture for real? Why doesn't Arnold take just a LITTLE bit of care of himself, like a few minutes? Or is it just not possible given his age? If I don't want to look like that when I'm old, what can I do?
Firstly he is 60 - if you look anything like as toned as he does (even in that photo) at 60 you will be more than happy with yourself. Secondly - a single photograph designed specifically to show-off someone's flaws is always going to look much worse than if you were to see him in the flesh. The photographer will have had maybe 50 shots of him in that little segment and will have deliberately picked the one that made Arnie look worst. Finally the photo to the left is from a competition-event so it's a ridiculous comparison to make. In short Arnie looks great for 60, and I expect the reason is that he has kept himself physically fit for a long time in his life, he keeps his mind and body active (judging by the work he's undertaken in his life) and that helps too. I wouldn't be amazed if the shot was photoshopped a bit but then so will the competition shot. 194.221.133.226 (talk) 11:17, 26 June 2008 (UTC)[reply]
Not to mention he was posing in the first one in a way designed to show off the muscles, whereas in the second I wouldn't be surprised if the photographer did everything he could to catch Arnie in a non-posed moment designed to show off the flab. Confusing Manifestation(Say hi!) 00:06, 27 June 2008 (UTC)[reply]
I asked this question above but didn't get the answer I was looking for. I'd like the answer in terms of number of roads, not total distance. —Preceding unsigned comment added by 82.120.107.22 (talk) 11:02, 26 June 2008 (UTC)[reply]
With all due respect, phrasing the question as "What distance..." probably wasn't the best way to get the answer you need. If you ask for a distance, that's what we'll give you. The new form of the question is rather vague. Are you only counting distinct roads, or do you count a road every time you walk on it? Is the shopping mall a "road"? The school building? A sidewalk through the middle of the park? Do you count a road when you merely cross it (after all, you're "on" it for 10+ metres)? For me personally, I would bet that 95% of my walking is not along a road. -- Coneslayer (talk) 11:21, 26 June 2008 (UTC)[reply]
What if you pace back and forth on the same road your entire life? Does that count as 1? I wonder how many responses are necessary to point out that "roads" is not a unit of measurement. This question is no different than asking "How many hedgehogs does a the average person walk?" -- kainaw™ 12:34, 26 June 2008 (UTC)[reply]
I'm not sure this response is from the OP since the IP adresses are different. Anyway, significant thought has gone into some of the original answers and I suggest you read them and use some ingenuity to apply these estimates to the actual solution you are looking for, whatever that may be. Jdrewitt (talk) 07:24, 28 June 2008 (UTC)[reply]
Since you haven't apparently clarified the original question, I'd have to answer very few since many children don't walk on roads most of the time, or at least what we commonly consider roads. As I mentioned in my response to the original question, most children live in the developing world (in particular a large number live in India and Africa and to a lesser extent, China) Nil Einne (talk) 06:48, 29 June 2008 (UTC)[reply]
More Importantly, Children not left in my care are taught to use Pavements (sidewalks to those of you somewhat illiterate) in the apparently none-developing (?) world. However, i'd say for the ones I encourage... about 1 road, then a car, of course technically they don't reach adulthood, so perhaps they don't count. Then of course, for those children that survive me, we have to take into account Adulthood, and how it is defined by the particular culture. In some ancient, but still existing cultures a 14 year old is considered an adult, while in Korea, children don't exist, they've all been replaced by MMORPG characters. Then in order to average it out you also have to factor in all children who ever lived and all who ever will. Ok, i'll just quickly add that up on my NP-complete machine... well i'll be damned Tango was right, precisely 42. - Jimmi Hugh (talk) 22:27, 2 July 2008 (UTC)[reply]
You might want to re-evaluate your inquiry. --Proficient (talk) 11:09, 30 June 2008 (UTC)[reply]
Applications of the theory of General Relativity[edit]
So, it got around to that time of the week again, where, precipicated by the reading of an article [3] about the continued accumulation of evidence for a fractal (non-homogenous) distribution of matter on cosmological scales, I was given cause to resume a habitual musing, to wit: What has GR ever done for me, anyway? Raising the question in various irc channels was, to say the least, unproductive, so I decided to bring up the matter here, where I expect a more reasoned, interesting, and informative response. To be as precise as possible, I will reformulate the question and append a precautionary note, so as to preclude the most obvious inanity from replies. Which, if any, technological advances have directly required the acceptance, or at least conception, of General Relativity Theory?
1. GPS does not require GR for its fundamental theoretical operation, and those correctional factors which it does employ are not predicted exclusively by GR, nor would have been in the remotest part difficult to ascertain empirically. —Preceding unsigned comment added by 153.1.253.80 (talk) 12:39, 26 June 2008 (UTC)[reply]
Well, the fact is that GR doesn't have a lot of human-scale benefits is not terribly surprising, given the scales it operates on. Waving away GPS is a bit cheap, IMO—other theories may incorporate the correctional factors but none of them have passed the many, many tests of all aspects of GR. (Which is not to say that GR is complete or final; it's likely it's not. There's that whole incompatibility with the standard model problem. But it does indicate that GR is probably substantially correct, and certainly a more reliable theory than anything else out there at the moment.) GPS is the only technology out there at the moment that incorporates GR. Saying that other theories "could" account for the same factors is not how science works—the question is about falsification, not verification.
But asking "What has GR done for me lately?" is sort of like asking "What has Jupiter done for me lately?" It's out there, it's big, it's useful for astronomers—but it doesn't affect things on a human scale much at all. --74.223.170.249 (talk) 13:01, 26 June 2008 (UTC)[reply]
You have, perhaps inadvertantly, hit upon the main kernel of frustration that mandates my original question: it is exactly this (supposed, and vociferously defended) nonapplicability of GR to the if-not-yet-then-quickly-becoming infamous "classical realm" which is the root of its objectability. Its cosy incubation within the comsological regieme -- where experimentation is all but off-limits, and observable data can be accounted for by multiple, conceptually-incompatable, theories -- enables it to dawdle merrily in the penumbra of falsifiability, metamorphosising ever more convincingly into an academic white elephant that devours great swarths of cognitive resource for diminishing returns, with the impunity accorded to its (de facto) religious sacredness. Were this vertible lightning-rod-for-inspiration not planted firmly in the fertile imaginative ground of our blossoming physicists, who knows what other paths of least potential credulity might be traversed by the bolts of intuition to which they are prone. Perhaps one might stumble upon a theory of cosmological evolution which entails consequences germane to that sphere in which we live, and have the liberty of active investigation through the controlled variation of parameters. Good lord, that might almost be considered progress... —Preceding unsigned comment added by 153.1.253.80 (talk) 14:10, 26 June 2008 (UTC)[reply]
Please review the guidelines at the top of this page, particularly regarding diatribes, debates, and soapboxen. -- Coneslayer (talk) 14:18, 26 June 2008 (UTC)[reply]
the beauty of physical soapboxes, as stood-upon in public streets, was that the uninterested had the liberty of simply walking past. please feel free and welcome to act analogously. —Preceding unsigned comment added by 153.1.253.80 (talk) 14:53, 26 June 2008 (UTC)[reply]
Woa! That's a lot of metaphors!! --Tango (talk) 14:22, 26 June 2008 (UTC)[reply]
Actually, Jupiter is a great and constant friend to us by keeping most of the riff raff out of our neighborhood. --Sean 14:29, 26 June 2008 (UTC)[reply]
If you're going to reject GPS as an answer, you'll probably reject everything else on similar grounds. You can always just measure the systematic error in your results and correct for it without knowing the cause. It's much nicer to know the cause, though! Other than GPS, I expect the effects of GR are limited to aerospace - I expect it had to be taken into account when planning communications with probes near the sun. --Tango (talk) 14:22, 26 June 2008 (UTC)[reply]
The distinction I intended to extend by discounting GPS as having required GR was that between enabling and facilitating theory as regards technological advance. Refinements in theoretical prediction of the involved variables will generally assist in the implimentation, precision, effectiveness or applicability of a particular invention. However, this facilitation is not on par with the enablement exhibited when a theoretical breakthrough makes concievable technology which, prior to its arrival, was speculative, if even imaginable. In this sense, I was asking whether GR has lead to any qualitative, rather than simply quantitative progression in our ability to harness the physical universe. —Preceding unsigned comment added by 153.1.253.80 (talk) 15:01, 26 June 2008 (UTC)[reply]
Perhaps you are underestimating the effect of GR on GPS. It's not just a matter of the results being a little imprecise, the error is very large. According to [4], the readings would drift by about 10km a day. GPS would be useless if it wasn't corrected for relativistic effects. I think trying to distinguish between qualitative and quantitative effects is a bad idea - Newtonian physics is a decent approximation of GR on every day scales, and it a rough approximation even at quite large scales. Most results of GR can be interpreted as simply corrections to Newtonian physics. The only things that can't are likely to be related to black holes (which are too far away to affect us) or gravity waves (which aren't even measurable with current technology, although we're getting close). --Tango (talk) 15:41, 26 June 2008 (UTC)[reply]
Out of interest, what other theories do predict the correctional factors for GPS? 91.143.188.103 (talk) 18:37, 26 June 2008 (UTC)[reply]
None that I'm aware of. GR also predicts the precession of planetary orbits (with applications to astrology, I guess ;-). Gravitational lensing allows us to see things otherwise invisible, including extrasolar planets. And of course there is a huge difference between technological application and falsifiability. --Stephan Schulz (talk) 23:21, 26 June 2008 (UTC)[reply]
I find it beautiful that a theory created decades ago now find a place in the cars of many a person who distrusts scientists. There are few moments that rival the confirmation of a long-predicted scientific result. Many a time the original scientists aren't there to observe it. Imagine Reason (talk) 04:29, 27 June 2008 (UTC)[reply]
Oh, and by the way, gravitational lens. Or don't you like looking at pretty distorted pictures of distant galaxies you'd never be able to see with just unassisted human technology alone? --arkuat(talk) 07:33, 27 June 2008 (UTC)[reply]
You seem to be conflating the idea that because GR doesn't relate to human technologies it is somehow outside the range of experiment/experience. This is not true. GR has satisfied many experiments—over and over again it has managed to accord with very precise predictions and succeed where no other competing theory has be able to. You can test GR many different ways, without too much trouble (unlike String Theory). Most of those ways don't translate into human technologies. But that has no relation to its ontological status. That doesn't mean it's "correct" in an ultimate sense—it has to be "incomplete" on some level, because it is not compatible with quantum mechanics. But so far nothing else has come close to displacing it or being as experimentally rigorous. The other advantage of GR is that it is conceptually quite simple—once you re-define what "gravity" is then the rest of it falls into place rather easily, which is also to its credit (it is not ad hoc or based on purely mathematical tricks). --140.247.10.12 (talk) 10:52, 27 June 2008 (UTC)[reply]
How come the bulb in a projector is so expensive (perhaps a few hundred dollars)? Are they hard to manufacture? Do they have some particularly expensive material in them? Thanks. —Preceding unsigned comment added by 140.247.241.132 (talk) 14:15, 26 June 2008 (UTC)[reply]
Relatively few of any given type of projector lamp are produced so you don't get huge economies of scale in their production. But probably more importantly, like so many things in life, projector lamps are expensive simply because they can be and you'll still pay for them. Capitalism takes away at least as often as it gives. Meanwhile, you may enjoy our article about metal halide lamps.
The article on negative feedback amplifiers didn't seem to explain why they increase stability and linearity. Can anyone explain this to me? Thanks. —Preceding unsigned comment added by 140.247.241.132 (talk • contribs) 14:19, 26 June 2008
(note: please sign your posts.) I'll try, but I'll admit I'm no expert. When the signal goes through the amplifier's "output chain" (output tubes/transistors, wiring, _maybe_ output panel, part of speaker cable, speaker, etc.) the first time, all the components are going to have non-linear effects on the signal. If feedback were not used, these non-linear effects would add up and cause distortion (possibly audible and unpleasant). Feedback "feeds back" or adds a bit of the inverse ("negative") of the signal/distortion and this removes a bit of the non-linearity. It can be enough to get an amplifier with a good low THD (0.01%) from imperfect non-linear components, which are cheaper than completely linear ones. Flayked (talk) 22:28, 27 June 2008 (UTC)[reply]
If you look at the formula for the gain of the amplifier you will find that it depends on the feed back amount and the gain of the underlying non-ideal amplifier. But when this last gain is high, it has very little effect on what comes out. This means that the n egative feedback amplifier is very linear, and has very low distortion. To look at the instability, you will need to understand why an amplifier would be unstable, that is oscillate. If the output is added back to the input then the output can head off to infinity, with an unstable result. But if you subtract output from input, that will cancel any tendency to add, and make oscillation unlikely. In reality there will be time delays on going through the amplifier, but even so the input will be settled down by the feedback and oscillation is harder. Graeme Bartlett (talk) 22:46, 27 June 2008 (UTC)[reply]
Why should an amplifier without any feed back be unstable? I would venture that this is impossible!
As to linearity, any negative fb tends to compare the input with the output thus improving linearity, reducing distortion/noise etc. —Preceding unsigned comment added by 79.76.191.71 (talk) 23:58, 27 June 2008 (UTC)[reply]
Calculating the horizontal force exerted by a 60 ton vertical press on an aluminum extrusion[edit]
we have an aluminum extrusion and we want to be able to test the 4 welds on it, so we want to stick it in our 60 ton press and make sure that it will not break under a force of 36,000 psi.
knowns:
round extrusion
OD=7.00"
ID=3.18"
thickness=1.00"
60 ton press
do not want to exceed max force on ID = 36,000 psi
We plan on making cone shaped hardened tooling for each end of the press, also we will be making 2 half mooned hardened sleeves that will fit inside the aluminum extrusion to apply equal pressure across the whole inside diameter. Both ends of the sleeve will have a coned shaped groove to fit the tooling of the press. As the press is activated the press will drive the 2 half moons apart applying a force to the inside diameter of the aluminum extrusion to test the welds.
My question is how can I calculate the force on the inside of this aluminum extrusion? I'm guessing it will have some relationship to the downward force of the hydrualic press —Preceding unsigned comment added by Gbalamut (talk • contribs) 14:25, 26 June 2008 (UTC)[reply]
It will be related to the downward force of the press. You need to know the shape/size of the internal cavity. Then, you can assume the aluminum has a constant pressure across the surface of the internal cavity. Knowing that pressure, you can consider it to be hydrolic - surface size and pressure of the press compared to surface size and unknown pressure of the internal cavity. That will give you a good estimate. -- kainaw™ 14:48, 26 June 2008 (UTC)[reply]
QUERY From your description it sounds like you are using the press to get a force outwards from the inside of a (circular cross sectioned?) extrusion.. One problem I see here is that if you use 'half moon' inserts then the force will not be equal all around the edge - especially if the aluminium deforms before breaking - did that make sense?
An alternative might be using hydraulic pressure directly to apply this inner force which has the advantage of being equal all over - in this case you'll need to be able to form a pressure proof join between the hydraulics and extrusion - maybe the hydraulic method won't produce enough pressure.
ANSWER (possible) Assuming your conical inserts have angle A (A is half the angle at the cone tip), then the force outwards at the contact of the conical insert and half moon should be F/ tan(A) where F is the total hydraulic press force, then divide by the surface area of the contact between half moons and extrution to get the pressure.
Did that make sense - it's possible that I've misread what you are doing..87.102.86.73 (talk) 15:06, 26 June 2008 (UTC)[reply]
One thing I don't get is
OD=7.00"
ID=3.18"
thickness=1.00"
Surely OD=ID+2xthickness , at least approximately ??87.102.86.73 (talk) 15:08, 26 June 2008 (UTC)[reply]
The cone is acting as a Wedge (mechanical device) - the force is split between the two sides - for a more evenly distributed force use sections ie using a 6 segment splitter will result in force on outside of each splitter of F/6 tan(A)
The knowns above are the specifications on the aluminum extrusion. I agree that more segments in a splitter would be a better design, but for the moment I am just assuming that the pressure will be equal. cause my biggest concern is that the 4 welds hold.Gbalamut (talk) 19:49, 26 June 2008 (UTC)[reply]
Given that the 2 piece splitter gives most of its force along one axis (in opposed directions) I'd suggest trying the splitter in different orientations..
Did you get enough info? (blank space means yes..)87.102.86.73 (talk) 19:53, 26 June 2008 (UTC)[reply]
The angle on my conical surface is 30 degrees, max force on the inside of the extrusion is 36,000 psi, and the area is (PI X 3.18 X 1) so I believe what you are saying is.
F press = Tan 30 X 36000 psi X 9.990
doesn't seem to make sense? what about friction? doesn't that need to be taken into account. I would assume that the force from the press and the force exerted out vertically would be pretty close in value?? got any suggestions on websites were I could refresh my memory on all these formulas? its been about 15 years and i am quit rusty. —Preceding unsigned comment added by 147.154.235.53 (talk) 20:14, 27 June 2008 (UTC)[reply]
You need to look at Wedge (mechanical device) for an explanation - curiously friction does come into it in the static case - but stiction could cause some 'noise' in the force/distance moved by ram graph
Your surface area should be perimeter length x length of extrustion , see Cylinder (geometry) - 2pi x r x h the area of the tube walls.87.102.86.73 (talk) 09:33, 28 June 2008 (UTC)[reply]
I've just done my morning stroll to get some coffee and once again was serenaded by a mockingbird that lives nearby. Or perhaps he only works here and commutes from somewhere else, just like me. But that's not important right now. What has me curious is why does a mockingbird sing all those different songs? I can find plenty of sites that tell me how many songs might be in an average repertoire, and that some birds have imitated car alarms and dogs barking, and things like that, but there has to be some purpose to all that parroting (so to speak), other than making LarryMac smile. Our Northern mockingbird article says "The Northern Mockingbird's mimicry is likely to serve as a tool for increasing the size of its repertoire and thus its ability to attract females" which seems to beg the question. It learns songs so that it knows more songs, and that makes it a good catch? Any ornithologists out there? --LarryMac | Talk 14:51, 26 June 2008 (UTC)[reply]
What I always heard was that knowing a large number of songs meant that the bird had a productive habitat (capable of supporting many species). I suppose it could also indicate that the bird had "been around a while", and hence had good genes for survival. (At an old apartment, the mockingbird liked to mimic "Garbage Truck Backing Up". I translated this into English as: "Hey, ladies! My habitat supports a garbage truck! Have you ever seen how much those things eat? This is a great spot!") -- Coneslayer (talk) 15:12, 26 June 2008 (UTC)[reply]
Could it also be possible that the birds use mimicry in order to locate other members of their local flock? I read somewhere[citation needed] that Budgerigars do this - copying other birds of their own/other species and local sounds in order that they might be identified as members of a particular 'clan' within a much larger gathering (the idea being that birds from a similar area will have similar learned vocalizations), making it easier for friendly birds to find each other. --Kurt Shaped Box (talk) 21:57, 26 June 2008 (UTC)[reply]
An individual bird may be conditioned to respond to the sound in a Pavlovian sense, but I don't think any bird would be able to understand, abstractly, that "garbage truck sound → garbage truck → food". − Twas Now ( talk • contribs • e-mail ) 00:57, 27 June 2008 (UTC)[reply]
You're right, it does beg the question. The same question might be asked of peacocks and birds of paradise: Why have such elaborate feathering, which both hinders the bird's movement and attracts predators with bright colouring. Basically, females choose the best male singer. Their offspring, then, are hardwired for the same: the male offspring will be a good singer, and the female offspring will be attracted to good singers. Look up sexual selection for more. − Twas Now ( talk • contribs • e-mail ) 00:57, 27 June 2008 (UTC)[reply]
IANAO, but I had always heard that mockingbirds sang other birds' songs so as to claim a large amount of territory: the thrushes think that there's already a thrush eating the food over there, so they don't visit, and similarly for the robins, the jays, etc. The mockingbird then gets its pick from all the different kinds of food and nesting areas that the other species would otherwise divide. --Tardis (talk) 13:17, 27 June 2008 (UTC)[reply]
I am wondering how the floods occurred. June 2008 Midwest floods it states that the levees were breached. Why were the Levees breached in the first place? Weren't they strong enough? Considering US has the latest civil engineering technology could they have prevented the flooding? My other question is that when the streets get flooded, there is a storm sewer every block or so, so why did the storm sewers did not work in this occasion? --64.228.196.97 (talk) 16:32, 26 June 2008 (UTC)[reply]
Levees fail to hold back rising waters for two reasons: Either the water is too high and "overtops" the structure, or a weakness in the soil allows seepage to collapse the structure from underneath. Found this in Wired article: http://www.wired.com/science/discoveries/news/2005/09/68746 --64.228.196.97 (talk) 16:42, 26 June 2008 (UTC)[reply]
A cynic might suggest that we're too busy spending our borrowed money to blow shit up to have any left over to properly maintain and improve our national infrastructure.
There is no practicality to building levees tall enough to withstand flooding that has never occurred before. There's a kind of a viscious cycle going on near big rivers like the mississippi: They build levees to keep the floods out, but since this makes the river more confined, the waters rise higher, so that when they do fail the flooding is worse. I don't know too much about hydrometeorological engineering, but I would think that the cost of maintaining 4000 miles of 50-foot tall levees during a 500-year flood would require about, say 1000 engineers (on each side), each equipped with a construction team to fix any weak spots they may find. It's just not practical.
You could just say build the levees taller, and make them out of concrete so they can't leak, but this causes yet another problem: If you build a levee taller, this decreases the frequency of major floods, but makes the potential for more catastrophic flooding when the levees finally do get overtopped (or the soil fails underneath). Besides, the reason there's all this fertile land for growing crops out there is because of floods like this (see Floodplain#Ecology). I hope this helps.-RunningOnBrains 17:59, 26 June 2008 (UTC)[reply]
Exactly. Current thinking is to build sacrificial flood-zones into the flood protection system. Towns and cities, and infrastructure like power plants and water works get protection, and the flood control scheme deliberately sends the water to low-value areas like farmland (and the farmer gets compensation for the loss of his crop). Its pragmatic - expensive flood control measures are used to protect only expensive things. This is a change from the "man conquers nature" mind-set that has prevailed (not just in the US, in many places). In a lot of ways this is an analogous shift to the changes in forest-fire management strategy, where thinking is shifting from total suppression to managed burns. Unfortunately the media (certainly not just in the US) has a "something must be done" mindset, and (faced with pictures of sad looking cows in flooded fields) it takes a strong-willed politician to say "we shouldn't be spending billions to keep some fields from getting wet". -- Finlay McWalter | Talk 20:20, 26 June 2008 (UTC)[reply]
As to the storm sewer, that's dependent on the sewer having somewhere to drain. That's great when it rains and the water drops down to river level. It's useless when river level is above the drains. — Lomn 18:24, 26 June 2008 (UTC)[reply]
In low-lying areas where there is not any convenient downhill location to shunt water to, storm sewers are often designed as dry well systems. This works fine for a few inches of rain, but it has only a finite capacity that will be totally overwhelmed during a riverine flood. Dragons flight (talk) 21:17, 26 June 2008 (UTC)[reply]
Depending on the extent of the fire, they may also be removed in the fire elevator. Wheelchair uses tend to choose low or ground floors for their residences; however, they may not have many choices in workplaces. I don't know of a fire warden who has been pre-selected for the job, but fire fighters are certainly trained for such carries. It is thus a wise wheelchair user who keeps his/her body weight as low as possible. ៛ Bielle (talk) 17:29, 26 June 2008 (UTC)[reply]
Some stairwells in Ireland have toboggan-like slides which are intended to allow anyone assist a disabled person down the stairs. They look outrageously dangerous, but I suppose it is better than the alternative PlasticupT/C 18:18, 26 June 2008 (UTC)[reply]
They use an escape chair/evacuation chair - a specially designed lightweight chair designed to manouvere up and down stairs and around tight corners. Exxolon (talk) 19:06, 26 June 2008 (UTC)[reply]
It would be interesting to see images of said toboggan like slides. --Proficient (talk) 11:11, 30 June 2008 (UTC)[reply]
My (seven-storey) building has 2 units per floor reserved for people with wheelchairs. When the fire alarm rings they just stay put. I think the fire department is supposed to help them out if necessary. —Preceding unsigned comment added by 64.231.11.47 (talk) 00:58, 27 June 2008 (UTC)[reply]
This is referred to as shelter in place, another term for which Wikipedia doesn't yet have an article. [6]
Although not specifically designed for the disabled, the Barbican Estate in London has (or had) helter-skelter style escape chutes. Temporary chutes are also available.--Shantavira|feed me 07:26, 27 June 2008 (UTC)[reply]
Dr ansong (talk) 18:45, 26 June 2008 (UTC)why would a blood type o+ agglutinate with A and B. and also water?--Dr ansong (talk) 18:45, 26 June 2008 (UTC)[reply]
Having just watched The Godfather III on DVD again tonight, I find myself pondering one of the more famous scenes in the movie - i.e. where one of the villains of the piece is killed by having his own spectacles jammed into his throat, severing his carotid artery. Now, one of the 'making of' bits on the bonus discs (I gots me the new trilogy box set) has Francis Ford Coppola commenting that he has no idea if it's actually possible to kill someone with a pair of glasses (sounds like one of those things he gets asked by fans on a regular basis). So, I'd like to know if it would actually be possible, as seen on screen...
Looking at my own specs, I'd guess that I could probably give someone a few bloody gashes with them (were I so inclined), if I held them end-on in my fist and got a few good downward stabs in - but I doubt I'd be able to stick them deep enough into the flesh to cause a fatal wound. I think that they'd just bend/break. Thinking about it, you could probably fashion a shiv from a broken lens and the frame - but that's not what was done in the movie, so let's forget about that for now. --Kurt Shaped Box (talk) 21:41, 26 June 2008 (UTC)[reply]
I haven't seen the film, so I can't say whether they way they do it is possible. As for breaking the lenses and using the shards as a weapon - it's worth noting quite a lot of glasses are made out of plastic these days for safety reasons, so you would need to make sure your glasses are actually glass before relying on them for your killing needs! --Tango (talk) 22:03, 26 June 2008 (UTC)[reply]
But of course the movie is set in the early 80s. I suspect that glasses were more deadly back then. I mean, we even ate full-fat yoghurt back then, and that both ways up and against the wind! --Stephan Schulz (talk) 22:34, 26 June 2008 (UTC)[reply]
Yes, I remember whittling my first personal computer from a tree branch, back when the internet was stll being used to play volleyball. Those were good days... Franamax (talk) 22:51, 26 June 2008 (UTC)[reply]
Some plastics can have very sharp edges when broken too. Not sure about the particular plastic used in spectacle lenses (I think mine are polycarbonate) but I once saw a man stabbed in the gut with a shard from a plastic beer glass during a bar right. I believe that someone was once murdered after having his throat torn open with the handle from a plastic fork too. --Kurt Shaped Box (talk) 23:17, 26 June 2008 (UTC)[reply]
Are we talking lenses or frames here? Maybe it was not the specs but the technique of the hand that did – looking at the shiv article, there's one made of two tongue depressors so maybe not too far off in concept. Julia Rossi (talk) 13:35, 27 June 2008 (UTC)[reply]
The scene in the movie involves the victim's glasses being pulled off, folded and jammed end-on into his throat with a downward stabbing motion. Nothing to do with shivs or broken lenses at all. Just the frames with the lenses in them. --Kurt Shaped Box (talk) 15:25, 27 June 2008 (UTC)[reply]
Based on the very close proximity of the artery to the esophagus, and there being no obstructions, that leads me to think it is very possible as long as the glasses had an edge that could cut through a little of that tissue. Mac Davis (talk) 18:11, 27 June 2008 (UTC)[reply]
