Wikipedia:Reference desk/Archives/Science/2007 April 11

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

April 11

HIV tests -- false negative/positive statistics

There are now several HIV tests in use in the UK and USA.

Depending on the time since an infection incident (if any), there will be a gradual decrease in "false negatives" for each test, such that after about 6 months in the standard test, the odds of a false negative are considered quite small.

(Ie, a person who gets a negative result 6 months after an incident, is usually advised they probably did not become infected. But a person who gets a negative result 1 month after an incident is advised there is still considerable chance they are infected but the test returned a negative result due to insufficiency/lack of antibodies).

For a person not infected with HIV, there will likewise be a small chance of a false positive with each of the standard tests.

What I'm looking for is test data on this. For each of the 4-5 tests in common usage (UK pinprick, etc), and a single point event test, I'm looking for the following:

1. For a person who became infected at a given date, the odds of a false negative under that test at N days after infection, for a range of values of N (0 < N < 270 days). Especial emphasis placed on the shape of the graph for smaller N (< 90 days).
2. For an HIV negative person in good/reasonable health, the odds of a false positive.

Obviously the more authoritative the better. As an aside are the tests used in the UK and US identical? If not how are they called and distinguished (to avoid confusion).

Many thanks. FT2 ^{(Talk | email)} 00:05, 11 April 2007 (UTC)

There can be no absolute statistics, because the rates of false positives and false negatives depends not only on the test used, but also on the prevalence of the condition tested for in the population tested. An HIV test will have a lower rate of false positives and a higher rate of false negatives when used to test a high-risk population, and a higher rate of false positives and lower rate of false negatives when used to test a low-risk population. Our article at Type I and type II errors may or may not help to clarify this. If it doesn't, ask here again for more help. Some sample rates of false positives and false negatives can be found in HIV test and in the references for that article. - Nunh-huh 02:18, 11 April 2007 (UTC)

Donors in Donor Priority Lists

I was just curious if people who have donated a kidney are, at some point later in their lives, in need of a kidney transplant, are they moved up the list? —The preceding unsigned comment was added by 65.94.231.58 (talk) 01:15, 11 April 2007 (UTC).

I don't know, but if it's later in their life, they might even be bumped down. − Twas Now ( talk • contribs • e-mail ) 02:22, 11 April 2007 (UTC)

Determining priorities for transplanted organs is a very complex question of medical ethics. Different countries will have different criteria. In the United States, the matching of organ donors to recipients is handled by the United Network for Organ Sharing (UNOS); their criteria are in the various sections of Policy 3. I'm still reading—it's a hefty document. TenOfAllTrades(talk) 03:33, 11 April 2007 (UTC)

Ah, here it is: Section 3.5.11.6, Donation Status.

"A candidate will be assigned 4 points if he or she has donated for transplantation within the United States his or her vital organ or a segment of a vital organ (i.e., kidney, liver segment, lung segment, partial pancreas, small bowel segment)....Additionally, at the local level of organ distribution only, candidates assigned 4 points for donation status shall be given first priority for kidneys that are not shared mandatorily for 0 HLA mismatching, or for renal/non-renal organ allocation irrespective of the number of points assigned to the candidate relative to other candidates."

So yes, donating an organ bumps you up the kidney list in the United States. I suspect that most other jurisdictions have implemented similar protocols. TenOfAllTrades(talk) 03:46, 11 April 2007 (UTC)

So how much are 4 points worth? How many points does a typical recipient have? ^tucker/_rekcut 19:08, 11 April 2007 (UTC)

Cool! Thanks! I figured it made sense that way...

COHb

Will an anemic patient survive for a longer period (than anyone having the right level of Hb in blood) in an enclosed space with an increasing concentration of CO ? Thanks, -Pupunwiki 03:18, 11 April 2007 (UTC)

They would probably die sooner, as there would be even less good haemoglobin to carry oxygen, even if they absorbed CO slower than a healthy person, there would be less Hb left over. Please don't try it out. GB 07:12, 11 April 2007 (UTC)

Yeah - exactly. It's not that CO is poisonous in itself - it's that it ties up haemoglobin and prevents oxygen from being carried around. If you had less haemoglobin in the first place, you'd be much worse off. SteveBaker 16:53, 11 April 2007 (UTC)

Types of DNA

What is the most resilient type of DNA? I searched around and it appears to be mitochondrial, but I'm not sure. And aren't there really only two types of DNA (mitochondrial and nuclear)? Thank you. 192.216.82.178 03:33, 11 April 2007 (UTC)

I suppose you could also include viral, but those aren't really "types" of DNA per se. bibliomaniac15 05:37, 11 April 2007 (UTC)

You might consider A-DNA, B-DNA, and Z-DNA as different types. All depends on how you slice the cake. --David Iberri (talk) 05:44, 11 April 2007 (UTC)

Ancient DNA studies find that mitochondrial DNA survives longer in sub fossil bones than nuclear information - BUT this can be attributed to the fact that there are many more copies of mitochondrial DNA to start with. Which of nuclear/mitochondrial actually degrades faster, I'm not sure. I'd put my money on nuclear being more stable. Aaadddaaammm 07:40, 11 April 2007 (UTC)

I realise this isn't exactly on point to your question, but you might also enjoy reading Deinococcus radiodurans.

Atlant 12:28, 11 April 2007 (UTC)

there's DNA in chloroplasts, too.

G-C rich DNA (that is, DNA with a high concentration of (connecting) guanine and cytosine residues) is much more resistant to separation into two strands (helicization? is that a word?) by approximately three halves. ^tucker/_rekcut 19:05, 11 April 2007 (UTC)

Separation of dsDNA into two single strands is just denaturation. --David Iberri (talk) 19:51, 11 April 2007 (UTC)

It's difficult to determine how you measure 'resilience'. Ultimately, all DNA is made from the four basic natural nucleic acid bases which are all equally (more or less) susceptible to mutation or phosphodiester backbone damage. The difference would depend on the number of mutagenic factors the DNA is subjected to (such as sunlight, toxins and intracellular compounds such as reaction oxygen species) and the capacity of the DNA repair machinery to deal with it. If we're talking about DNA from a dead cell, then there won't be a damage repair mechanism, but there will be nucleases around in the environment that will just chew the DNA up, but I don't think it's really going to be affected by the source. G-C base-pairs have more hydrogen bonds holding them together (3) than A-T base-pairs (with 2), but that only really affects the stability of the DNA duplex. England Expects 23:29, 11 April 2007 (UTC)

Well, it was a question from a competition a while ago and I don't really know what exactly they're looking for or what the answer is, but thanks for everyone's help. 192.216.82.178 02:30, 12 April 2007 (UTC)

Raw meat

What does raw meat taste like? This is assuming that the sense of taste is of a human's. bibliomaniac15 05:37, 11 April 2007 (UTC)

You could try steak tartare, carpaccio or sashimi or many other dishes [1] and find out. Red meat, unsurprisingly, has a taste of blood. Bacon is pretty salty, I can't comment on chicken and fish varies in taste, but fresh raw tuna and salmon is very nice indeed. Rockpocket 06:15, 11 April 2007 (UTC)

It can taste pretty good, but the pleasure is tempered by the knowledge it might contain live parasites which would infest the consumer, as well as bacteria which could cause food poisoning. Edison 14:21, 11 April 2007 (UTC)

The center of a beefsteak cooked very rare or rare can be as low as 115ºF-125ºF and feel cool. ~ hydnjo talk  15:00, 11 April 2007 (UTC)

Oh man! If you havn't ever eaten a thick steak cooked 'very rare' - you havn't lived! Hmmm - look, it's lunchtime - go now! You'll want a heavy red wine to go with it. Forget about veggies and namby-pamby side-salads - release your inner carnivore! (You do have good health insurance - right?)  :-) SteveBaker 16:49, 11 April 2007 (UTC)

Technically, if the animals were raised in sanitary conditions, and the meat is cleanly prepared, there is no risk of food-borne illness, no matter how rare it is cooked. The trouble is identifying whether it has been "prepared cleanly," which is especially difficult given the long chain of processing and handling steps that foods undergo (at least, in "industrialized nations")... Nimur 18:20, 11 April 2007 (UTC)

Chernobyl

If I was to stand about 1 km away from Chernobyl Reactor 4 what radiation dose would I recieve and how long would it take me go get radiation posioning/sickness and the die, and repeat the same question except 1-100 metres outside the Zone of Alienation, Cheers -- 210.55.145.223 05:41, 11 April 2007 (UTC)

As I understand, the radiation level at Chernobyl is quite uneven. Since it mainly comes from dust particles which fell in the days following the explosion in the reactor building, those areas where that dust has accumulated have the most radiation. That depends on the winds during those days and the terrain on the ground. For example, there isn't much radiation in the center of roads because the radioactive dust was washed off the roads and into the gutters by subsequent rains. The gutters, however, contain a high amount of radioactive dust. StuRat 14:59, 11 April 2007 (UTC)

There've been several documentaries produced and quite a few magazine articles. I think one that I saw was titled Living Under the Cloud: Chernobyl Today [2] but the Bulletin of the Atomic Scientists has had several articles across the years.

Atlant 16:53, 11 April 2007 (UTC)

Decomposition of plastics

I happened to read that cockroaches can digest plastics,can't we study that mechanism and decompose plastics?202.88.225.217 07:59, 11 April 2007 (UTC)

• Where did you read that? Sounds unlikely to me. - Mgm|^(talk) 11:05, 11 April 2007 (UTC)

Bacteria can to a certain extent.[3]

Plastic is a term widely applied to many thousands of different types of materials. Most are petroleum-based polymers. Even among the "commercial" plastics, there are a huge variety of grades, types, and chemical composition. Nimur 18:24, 11 April 2007 (UTC)

Insects can chew plastic, so that they can penetrate a plastic bag to get a good feed off the contents. I have seen this with weevles. Cockroaches have bacteria in their digestive tracts, and termites are supposed to be a specialised cockroach with heavy jaws that can chew and digest wood. In actual fact it is the bacteria breaking up the lignin and cellulose in the wood inside the digestive tract. GB 06:55, 12 April 2007 (UTC)

Electromagnetic radiation versus "colo[u]r"

We see the color green at a certain wavelength range, same with orange, & blue, and they are sequential, for example, as a case in point, ROYGBIV. Do all other organisms with color vision see color in the same sequence, per se? A response on my talk page would be greatly appreicated, or to let me know there's a response would still be appreciated, thanks!100110100 12:22, 11 April 2007 (UTC)

[replying here, don't be selfish hiding the answers in your talk page :p]

We don't actually have sequential colour vision. We only sample 3 wavelengths and it's not colour until our brain interprets them as colours. Thus, colours are quite meaningless to other creatures who may only sample 1 wavelength or perceive hard to comprehend colours (for human) with their tetrachromacy. --antilived^{T | C | G} 12:27, 11 April 2007 (UTC)

Am I right in thinking that we have no way of determining that we all see red as the same colour? ie Your red might be what I would call blue, but there's no direct comparison because I can't see what you see, we call the colours the same thing, even if they're different, we have the same conditioning as to which colors are calming, which are vibrant etc. Or is there evidence that shows we all see the same colors? I would find it hard to think of an experiment to prove it. Capuchin 12:50, 11 April 2007 (UTC)

I would argue this is a bit meaningless since colour is just what the brain perceives. I.E. you can't say that your blue is the same as my blue since it's all just in the brain. Nil Einne 14:34, 11 April 2007 (UTC)

No, it's actually pretty easy to show - I believe we all see the same blue and green, but there are two slightly different reds. You just measure the response to stimuli of the three different kinds of cones in the eye ... WilyD 15:03, 11 April 2007 (UTC)

My 16 year old son is 'weak-green' colour blind - he didn't know it until a few weeks ago - his green receptor is sensitive more to yellowish-green light than to pure green - he is seeing a different part of the spectrum than 'normal' people do - but he didn't know that for 16 years. So it's abundantly clear that there isn't any way we'd know if our perception of 'red' is the same as everyone elses - and in a sense, it doesn't matter.

Furthermore - the idea that the sequence: Red/Orange/Yellow/Green/Cyan/Blue is somehow all that we see is flat out wrong (incidentally, there is no 'indigo/violet' - that's a myth, the visible spectrum ends at blue). Our perception of the spectrum of 'pure' colours (single frequencies of light) does indeed go from red through all of those colours to blue - but there are other colours like 'magenta' and 'white' that aren't anywhere on that spectrum. Take white light from the sun, split it through a prism and examine the rainbow...there is no magenta anywhere to be found. That's because magenta isn't a pure colour - it's a mixture of two pure colours (red+blue).

So consider a colour like yellow - is that a mixture or a pure colour? Well - there are actually two utterly different colours that our eyes see as "yellow"! They have the same name because we can't tell them apart. There is the pure colour (such as you'd see in a rainbow or from a sodium streetlamp) and there is the yellow that you see on your computer monitor which is a mixture of red and green - just as magenta is red and blue. The second kind of yellow isn't a part of the 'rainbow' (just as magenta isn't). Our eyes simply can't tell the difference between 'pure' yellow and the red/green mixture that we also call 'yellow'. But if you take the yellow light from a sodium lamp and put it through a prism to make a spectrum - you pretty much get just yellow light out. But if you take the yellow light from your computer monitor and stick that through a prism instead - you get just red and green light out of the prism - no yellow at all.

In case you think this is a fine distinction - a colour halfway between red and green versus a colour that's a mixture of red and green, consider this: Magenta is red+blue - but the colour halfway between red and blue is green. So the "real" difference between yellow and yellow (so to speak) is comparable to the difference between magenta and green!!! It's hard to imagine two more different colours! So the "real" difference between two seemingly identical yellows would be HUGE to some kind of hypothetical creature that could distinguish them.

What kind of creature might that be? Some other animals (notably the humble goldfish and certain freshwater shrimp) have much better colour perception than we do - others (like bees) see colours outside of our 'normal' range (bees can see ultraviolet light). For goldfish, 'pure' yellow and 'red/green-mixture' yellow would look as different as green and magenta look to us. This is impossible to fully grasp...but it's very clear that it's true.

The best analogy I have is with sound. The 'yellow' we see coming from a TV or a computer screen is like a chord formed of two notes on the piano - say 'A' and 'C' struck together. The 'pure yellow' we see coming from a sodium lamp looks every bit as yellow as from a computer screen - but it's like playing a single 'B' on the piano. Our ears can easily tell the difference between a musical instrument playing A+C and one playing just a B - but our eyes cannot do the same trick - for us, red+green looks identical to pure yellow. The same is true for 'cyan' - the colour that could be between green and blue - or that could be a mixture of green and blue. For a goldfish...not so there are more utterly different colours than we can see.

Of all the things I've ever learned from the study of science - this is the one that blows my mind the most. But then there is that whole bizarre tetrachromacy thing - some humans (specifically one lady who works in a wool shop in England who was identified as a tetrachromat from a genetic prediction of her family history of colourblindness) have two different kinds of colour receptors (in the green region) where the rest of humanity has just one - these very rare people (who, as it turns out must be female and who must be very rare) can evidently see those two 'different' yellows that I've been going on about (although probably not the two different cyan's). The lady in question explains that she's always wondered why so many people are so poor at matching the colour of yarns.

I'd give a lot to be able see what that lady sees - even if just for just an hour! SteveBaker 16:37, 11 April 2007 (UTC)

Just to hammer SteveBaker's point home a bit further, the reason we can distinguish individual musical notes from a chord that averages to the same frequency is that the organ of the cochlea actually contains thousands of individual receptors that are each tuned to be receptive to only a very narrow band of frequencies. This extravagance is possible because we only have two "point receptors" (two ears) for sound whereas we have millions of point receptors for light (all those rods and cones in our retina), so each light receptor in the retina has to be designed a bit more economically.

With regard to seeing in other ways: I can't give you tetrachromacy, but it's interesting to note that people who have had the lenses removed from their eyes (as a result of cataract surgery) often gain ultraviolet vision; witness Claude Monet in the real world and the fellow in K-PAX in the fictional world. Ultraviolet vision would be interesting, but it would be even handier if our eyes came with removable UV-only filters.

Atlant 17:02, 11 April 2007 (UTC)

The business with cataract removal is a very different thing. What that does is remove the UV filter so that our blue receptors can pick up light further into the ultraviolet than before. There are still the same old three receptors at work. So the result of this operation can only appear to the person who is seeing it as more blue light than usual. The world has more blue in it - but it's not different-looking blue. Certain things such as the patterns of UV-reflective scales that flowers use to attract bees become visible as blue streaks and spots that 'normal' vision doesn't see as being any different from the rest of the petals. They definitely don't percieve "new" colours. We could (and indeed routinely do) give ourselves that kind of vision (albeit in the infra-red) artificially with night vision goggles and infrared cameras. Some domestic video cameras see into the infrared a little - and we rarely even notice that the images are not 'natural'. Night vision goggles (in effect) take IR light and move it into the green part of the spectrum - suddenly you can 'see heat' - that's no different from the cataract removal moving UV light into the blue part of the spectrum. It's an interesting thing to experience - but it's hardly life-changing!

The tetrachromat on the other hand percieves things in ways we never can - no matter what technology or trickery we uses. They see things in ways that are tough even to describe. They see colours that we call 'yellow' that are as different as magenta and green are to you an me but which don't look like any other colour that we have a name for. My son (who as I explained recently discovered he is colour-blind) does not understand how it is possible for there to be 'more colours' than he can see - specifically - that I can easily tell whether the LED on the front of his Wii is shining green (meaning it's turned on) or yellow (meaning it's on standby). My normal eyesight is as miraculous and impossible-seeming to him as a tetrachromat's vision is to me.

(The Wii's power LED is the way we finally discovered his problem - I was complaining that he kept leaving it turned on - so the drive motor is spinning and getting worn out instead of putting it on standby. "But how can you tell?"..."Well, look at the LED - it's green now - and I turn it off and it goes yellow."..."No it didn't! It just went a little bit dimmer you can hardly see any difference."..."Yes it did - watch again!"..."No - it didn't change."...."Er, let's do a Wikipedia search for colorublindness tests"...time passes...we both stare at pictures of numbers made up of red, green and orange dots...my kid fails to see some of the numbers that stand out as clear as day to me..."OMG - you're colourblind! Fortunately, in the mildest and least bothersome way.") But now I really feel bad about this - there is no possible cure. I honestly think he'd have been better off not knowing. But we have learned that sticking a piece of red-tinted plastic over the Wii's power LED lets him tell whether it's on standby or not. The LED is evidently a red/green mixture "yellow" and not a "pure yellow" or else that trick wouldn't have worked. It's tempting to wonder whether his 'blindness' to certain colours lets him see the difference between the two kinds of yellow differently than I do...differently is "better" if you put the right spin on it! I suspect it does - and when he get a little less sensitive to discussing the matter - I'd like to see if in fact his vision is better than mine in some interesting ways - it might cheer him up considerably to discover that but I don't want to set him up for another failure just in case it's not! SteveBaker 18:33, 11 April 2007 (UTC)

Good job you didn't punish your son before running some tests. dr.ef.tymac 18:03, 12 April 2007 (UTC)

Railroads

What is the advantage of a train on railroads versus one or more trucks on a asphalt road? Does a train use energy more efficiently and if so why? What about the energetic costs of the rails themsellves? 84.160.202.9 13:29, 11 April 2007 (UTC)

The benefit of a railroad is lower rolling resistance due to the steel wheels on a steel rail deforming much less than rubber tires on an asphalt road. anonymous6494 14:11, 11 April 2007 (UTC)

Consider also the air resistance; train cars follw more closely than is possible for trucks. Consider also the allowable loading of a railroad freight car versus a truck. But then consider that a train cannon split into 100 separate delivery vehicles to different destinations not adjacent to freight tracks without special apparatus to lift a piggyback container onto a truck. Edison 14:19, 11 April 2007 (UTC)

Another big factor is that trains don't have to stop nearly as often as trucks, so lose less energy to braking. StuRat 14:46, 11 April 2007 (UTC)

I presume that diesel-electric train engines are more efficient then trucks too. Railways can also be electrified of course altho this is less common for freight. BTW, the train and rail transport articles mentions the advantages briefly Nil Einne 14:55, 11 April 2007 (UTC)

It only takes one or two railroad engineers to drive a 100-car train - versus 100 truck or bus drivers to do the same thing on roads. (Did you know that there are fewer locomotive drivers in the USA than airline pilots - and the railroad guys are better paid than the pilots! That amazes me.) Trains don't get stuck in traffic and the railroad company can decide what speed limits to apply so they can always drive at the most fuel-efficient speed. Someone already mentioned the lower rolling resistance and air resistance. You can put much more weight into a train car than the law allows for trucks so you have less empty space for denser loads. Trains are also a lot safer - so lower insurance costs. SteveBaker 15:45, 11 April 2007 (UTC)

Where can i assign for hire? ;) 84.160.202.9 18:01, 11 April 2007 (UTC)

Another efficiency benefit is that a locomotive engine is a HUGE almost industrial sized engine compared to the much smaller truck engine. Large engines in general are more efficient than small engines. -Czmtzc 15:53, 11 April 2007 (UTC)

"Almost industrial sized"? What's your definition of "industrial sized"? A typical diesel-electric locomotive (e.g. the GE Dash 9-44CW) generates over 3 megawatts of electricity, enough to power a medium-sized town. —Steve Summit (talk) 02:35, 15 April 2007 (UTC)

What about the costs of the rails themselves (energetic as well as financial) compared to roads?

And wouldn't it be even more economic to encapsule the rails in some tunnels (just like pneumatic tubes) to further reduce air resistance? 84.160.202.9 18:01, 11 April 2007 (UTC)

I think you can find statistics for cost-per-mile of a railroad. I belive cost per mile of an interstate highway is ~$50 million US. (That's a lot of money!) As far as a pneumatic tube, I doubt that the huge construction cost would even begin to counterbalance the minimal gain in fuel-efficiency. It might not provide ANY fuel efficiency gain; in that case, there would be zero benefit. Nimur 18:33, 11 April 2007 (UTC)

Railroad costs in 1908 and 1995, around $250000 US per mile.

[4] Interstate highway costs varying between $1 million and $1-billion per mile (yikes!).

Of course you should also take in to account the total commercial value returned by each mile; this is not easy to estimate, but you could presume that interstate commerce is more facilitated by highways than railroads in 2007. Most of my day-to-day stuff arrives on a truck, not on a train; but I know things like lumber and construction supplies arrive in my town via rail. Nimur 18:37, 11 April 2007 (UTC)

This is true only of goods that don't have far to go from their point of production. If you live very far (more than a few hundred miles) from a major port or point of trade for goods, everything you get on a truck likely spent some time on the rail before that. This is especially true for anything shipped to the US from overseas, cargo containers spend more time on the rail than they do on the truck, prior to being unloaded at the point of sale. --66.195.232.121 20:27, 12 April 2007 (UTC)

Air resistance isn't really a problem for trains - only the front of the locomotive and the back of the last car offer any resistance - so the total air resistance of a mile-long train is going to be pretty much comparable to a single truck on the road. It's truly negligable. Also, air resistance goes up as the cube of the speed you travel - and most trains go pretty slowly. The really high speed TGV's and Bullet trains start to feel problems and the front ends of the locomotives are streamlined - but on big freight trains they don't bother with even the most basic rounding of the front ends. The railroad companies are very concerned with fuel economy - if there was a cost-effective way to improve it, they would.

I once worked on a railroad locomotive simulator (like a flight simulator for airline pilots) - and the big win from buying this simulator was that they could train their engineers to drive the train efficiently. On very long coal trains in the northern USA, there are some fairly counterintuitive things you have to do to drive efficiently. On a series of hills for example, it's tempting (from experience with driving a car) to reduce the throttle when starting down a hill and increasing it when you go up the next hill. However, when all of the weight is half a mile behind you, as you crest the top of a hill, the bulk of the train is still not down in the preceeding valley - so you need maximum power - even as the locomotive starts down the hill because most of the train is still being pulled up. Then as you head up the next hill, you may actually need to stomp on the brakes because most of the train is still pushing down the previous hill and might speed you up to dangerous speeds if you use your instincts from driving a car!

Another thing I learned is to avoid slowing down too gently as you pull to a halt at a station because all of couplings between the cars would end up in tension. When you pulled away again, the engine would have to accellerate all of the cars at once up to speed - and it doesn't really have the power to do that! Instead, you have to stop fairly abruptly so that all of the couplings are compressed together. Then, as you pull away, you only have to get one car at a time moving as the train un-compresses. It's a really complicated learning experience. SteveBaker 18:49, 11 April 2007 (UTC)

(I added paragraph breaks there.)

Just to complicate things further, if it's a passenger train you do want maintain tension in the couplings when you stop. Because people are more impatient than freight, passenger trains have a higher power-to-weight ratio, and can start the whole train at once... and that keeps the ride smoother. --Anonymous, April 11, 2007, 22:10 (UTC).

"ikky" tight curve #1

"ikky" tight curve #2

Yeah - that's undoubtedly true. My simulator was for gigantic coal trains. For short passenger trains (with relatively very light wagons), probably all of the tricks I described are bogus. But the fact remains - driving trains isn't as simple as you'd think. There are all sorts of other bizarre things you have to remember - pulling long trains around tight curves without derailing is another ikky problem. SteveBaker 00:47, 12 April 2007 (UTC)

For examples of how "ikky" it can get, take a look at the full-size versions of the thumbnails at the right. These are the Spiral Tunnels in British Columbia and the Tehachapi Loop in California. For some reason we don't have a proper picture of the Horseshoe Curve in Pennsylvania. (And these three are hardly unique; they're just the first ones I could think of off the top of my head. See Spiral (railway) for many more) —Steve Summit (talk) 01:31, 12 April 2007 (UTC)

Wow! That's all one train?! Holy cow! The problem of going around tight curves with a long train is that it's like pulling on one end of a piece of string. If you put the string into a 'U' shape and pull on one end, you end up straightening out the string...pulling the cars off the rails at the bottom of the 'U'. All that stops that from happening is the lateral force exerted by the rails on the flanges of the wheels. But if the pulling force is too strong then because the couplings are higher up than the rails there is a net force trying to overturn the rail cars. You have to pull them rather gently around corners to avoid that. But if the curve is a 360 degree loop - and also uphill, this must all get rather nasty. on The longest trains there will be locomotives pushing from behind as well as the ones pulling from the front - which makes things even more complex. SteveBaker 14:28, 12 April 2007 (UTC)

Well, they're not always a full 360 degrees, anyway. I think the two Spiral Tunnels are about 270 degrees each, and the Tehachapi Loop looks to be about 315.

I'm not sure I believe our Spiral (railway) article where it implies that these loops and spirals are 360 degrees by definition. For the loops I've looked at, they're usually more like 270 degrees, and the elevation they lose/gain is, it seems to me, the secondary purpose for their existence. The major purpose is to provide the effect of a switchback, but without having to actually "switch back", and in tight quarters (such as of course are often found in the mountainous regions where you need switchbacks in the first place, which is why so many spirals end up being in tunnels also).

File:BigHillandSpiralTunnels2.jpg

old and new inclines of the CPR line along the "Big Hill", before and after the addition of the Spiral Tunnels

For example, if you look at this map of the Spiral Tunnels in BC, and this diagram showing among other things the way the grade looked in profile before and after the tunnels were constructed, you can see that the tunnels allow the line to reverse, and go partway back up the river valley it's descending, at a wide, flatter spot where the line can continue to lose altitude at the same time. (See also this nice model.) The tunnels themselves accomplish elevation gains of 50 and 55 feet, but the "reversed" section of line between them provides another 170 vertical feet. —Steve Summit (talk) 17:01, 14 April 2007 (UTC)

Thanks to all, this improved my understanding. 84.160.202.9 20:08, 11 April 2007 (UTC)

Density and volume

I have two containers, one that is smaller and filled with liquid like cane syrup that will fit inside the other larger container. Can I determine the density and volume of the syrup by getting the weight of the empty containers and then the weight of the small container filled with syrup and subtracting the weight of the smaller container to get the weight of the syrup and then put the small container with the syrup into the larger container and fill it with water until the smaller container just begins to float and then weigh the larger container and subtract the weight of the containers and the syrup to get the weight of how much water was added and then divide the weight of the syrup by the weight of the water to get the density of the syrup and its volume? 71.100.6.150 17:55, 11 April 2007 (UTC)

I think what you said is valid: see Displacement (fluid). It sounds like you're making it complicated, though. Here is what I would do.

1. Obtain the mass of the Syrup
   1. (First measure the weight of the unfilled container (see tare weight)
   2. Then obtain the measure of the filled container, and subtract to find the mass of only the syrup.
2. Obtain the volume of the syrup. All you need is the volume of the original container; you can use a graduated cylinder to help measure this. In fact, you can use the graduated cylinder as the tare-weight, so you really only have one container.

Hope this helps, Nimur 18:30, 11 April 2007 (UTC)

I think what you are trying to say is

(1) Two containers of unknown volume

(2) One container is smaller than the other

(3) The small container can fit into the larger container

(4) You have a perfectly accurate (weight) scale

(5) You have access to unlimited volume of pure water

(6) You have an unknown volume of Syrup

You want to measure the density of the Syrup (relative to the density of pure water). Can it be done?

Answer:

I believe so

(1) Using the scale you can measure the volume of pure water because 1 litre of pure water is 1 kilogram.

(2) You can find out the mass of the Syrup using the scaler

However I have no idea how you can find the volume of the Syrup.

202.168.50.40 23:48, 11 April 2007 (UTC)

You only need one container. Weigh it (call this weight 'E' for Empty), fill it with pure water up to the very top - weigh it again, subtract E to get the weight of the water. Because we know the density of water (1kg per Liter) we can cross out 'kg' and write 'L' and we know the volume of the container. Dump out the water and refill with syrup (right up to the top) - since the volume of water is the same as the volume of syrup, you now know the volume of syrup. Now weigh again, subtract E to get the weight of the syrup - then you know both the weight and volume of syrup - which makes calculating it's density one press of the divide key away! SteveBaker 00:23, 12 April 2007 (UTC)

You made a mistake. We have a fixed (but unknown) volume of Syrup, so we cannot fill up the container with Syrup. We still need another way of finding the actual volume of the Syrup. 202.168.50.40 00:53, 12 April 2007 (UTC)

We already know what volume the container can hold because we can measure how heavy it is when filled with water, and water has a density of 1 g/cm^3. --Bowlhover 02:19, 12 April 2007 (UTC)

Knowing the volume of the container does not tell you the volume of the Syrup because you cannot fill up the container with Syrup. 202.168.50.40 03:37, 12 April 2007 (UTC)

There's not enough syrup, not even for a very small container? Then do it the other way round: pour some syrup into the container, mark the syrup level, refill the container with water up until the mark, and weigh the water. That'll tell you how much syrup you had. --Bowlhover 04:10, 12 April 2007 (UTC)

Excuse me - but the question specifically states that the small container is "is smaller and filled with liquid like cane syrup" ...filled...not half filled or something. But if it were only partly filled then you'd need to mark the level of the syrup and fill with water to that level in order to figure out the volume - it's not clear whether marking the containers is allowed though. SteveBaker 14:20, 12 April 2007 (UTC)

The direct answer to the original question is "No", the procedure you (71.100.6.150) describe will not do the job. The second measurement, where you put the small container into the larger one, does not measure the volume or density of the syrup. To see this, just consider a case where the larger container is an oil tanker; you'd need tons of water to fill it until your little syrup container floated, and measuring how many tons would tell you nothing about the volume of syrup. --mglg_(talk) 01:06, 12 April 2007 (UTC)

low-UV compact fluorescent floodlights?

Does anyone know where I might find some flood-style compact fluorescent lamps with low-UV output so they can be used in a museum and not damage exhibits? Thanks! --Allen 18:34, 11 April 2007 (UTC)

I would check your local hardware store; but here's Froogle's selection. Some of them contain detailed specs such as UV levels. Nimur 18:40, 11 April 2007 (UTC)

The article Illuminating Alternatives: Research in Museum Lighting might shine some light on the subject. (Boo.) Also, this link has links to lighting research and standards, and they also happen to sell museum lighting fixtures as well. --Elkman ^(Elkspeak) 19:07, 11 April 2007 (UTC)

You might also care to look into solid-state (LED) white lighting. There is infinitesimal UV emission from them. -- mattb @ 2007-04-11T22:47Z

Thanks for the responses! Nimur, I couldn't find the UV level information you're talking about, but perhaps I was looking at the wrong individual products. --Allen 02:17, 12 April 2007 (UTC)

Mangos

The article has a link to an external video on how to cut a mango but no where can I find an article on how to extract the seed from the shell and grow a mango from the seed. Any help would be appreciated. Nebraska bob 19:04, 11 April 2007 (UTC)

Dear Bob, i see that just like me you're putting your final hope onto global warming. Until this really happens, you'll have to move from Nebraska to some far away tropical country to grow mangos. Aside from that, for what ive seen, nearly all articles on any plants lack any information on how (humidity, temperature, climate) they could be grown. This is a point worth improving. 84.160.202.9 20:07, 11 April 2007 (UTC)

Actually I was thinking of a Green house until Global warming gets here but the picture of tropical breezes and barefoot girls and the hamock I had to return to the store 'cause the stand was not included. (The store aut to be puttin' a sign on their display sayin' you have to buy the stand too or own a couple a trees...) Anyway since I commited to a Green house does anybody know how to grow fruits from seed or is it just the same as growin' wheat for bread and corn for ethanol? Nebraska Bob 00:59, 12 April 2007 (UTC)

Mango seeds are easy to germinate [5]. Growing them up to tree size is a different matter. --mglg_(talk) 01:10, 12 April 2007 (UTC)

Catalytic Strategies of Catalase

I am having difficulty finding information about the catalytic strategy of catalase. I know a catalytic strategy would probably lower the activation energy, but I can't find out how catalase lowers the activation energy, or what it lowers the activation energy of.

After I am done with my research, I plan to edit the catalase page, its not grossly inadequate, but it leaves out details.

Thanks-- cyanide_sunshine

There is some discussion on the mechanism of catalase action in these sources (in varying levels of detail). [6] [7] [8] Rockpocket 21:10, 11 April 2007 (UTC)

human ageing

what is the best abailable 'advice' for staying young and healthy so far? scientific stuff...not un proven claims.

and do you think it will ever be posible to rverse ageing? —The preceding unsigned comment was added by 75.68.247.175 (talk) 23:03, 11 April 2007 (UTC).

No smoke, no booze, no drugs, eat a balanced diet and don't mess around with guns, explosives, fire or trains? --Kurt Shaped Box 23:26, 11 April 2007 (UTC)

thnx! but i also meant things like...whether if genetic engineering can someday REVERSE ageing..

Stem cell research. Nebraska Bob 01:06, 12 April 2007 (UTC)

It won't reverse aging, but you might find Calorie restriction interesting. --Allen 02:20, 12 April 2007 (UTC)

The "balanced diet" advice is often given but essentially doesn't say anything - what is "balanced"? Icek 02:42, 12 April 2007 (UTC)

Also avoid suntans and especially sunburns, as they rapidly age the skin. StuRat 03:04, 12 April 2007 (UTC)

You might try looking here. Longevity-Czmtzc 13:05, 12 April 2007 (UTC)

XBOX 360

Does anyone know when Microsoft will produce the XBox 360 that uses the 65 nano cpu? —The preceding unsigned comment was added by 205.157.110.11 (talk) 23:11, 11 April 2007 (UTC).

An entire CPU in 65 nm? Astounding! (Our article says "mid-2007"). -- mattb @ 2007-04-12T00:33Z

What does it matter? It will have zero impact on your gameplay experience. Even if the CPUs went 65nm, the graphics card would still require the same monster fans to cool. --72.202.150.92 05:03, 12 April 2007 (UTC)

I heard no news recently but as the above mentions the last date given was 'summer 2007' - so at least we know not before that date.87.102.84.170 16:11, 13 April 2007 (UTC)

Sudden Alertness

It seems a interesting unknown to me, when usually after I eat a big lunch I take a nap. After awaking from my nap , my hearts beats very fast for a couple seconds, feeling like adrenaline throughout my body. It makes me very alert and gives me this sudden awareness of getting me up. I’ve always experienced this and I thought maybe it was a regular human evolution within the body? Thanks for your information if you could explain and if you also experience it.

CorinneQ 23:51, 11 April 2007 (UTC)Corinne Quinones

Waking up in an unusual manner could be a sign of a sleep disorder. Talk to your doctor about it; we don't do medical advice here. --Anonymous, April 12, 00:11 (UTC).

Hm, thank you very much for your adivce. I just thought it may be a normal thing that the body does to wake up a person to alert their body. Thank you again.

CorinneQ 00:14, 12 April 2007 (UTC)Corinne Quinones

When this happens to me its usually because I left my dessert out on the table for after I wake up but when I wake up its gone. Nebraska Bob 01:12, 12 April 2007 (UTC)

Maybe the bedouins stole it.  :) JackofOz 01:16, 12 April 2007 (UTC)

There aren't that many Bedouins between Canada and Mexico so I don't think it was them. (Besides after seeing the movie Flight of the phoenix with my girl friend last week its a pretty good bet that I would not be here right now if it had been one of them.) Nebraska Bob 02:54, 12 April 2007 (UTC)

?? when did dessert come into play on this?

well anyways u might be having problems with your fight or flight mechinism. perhaps something starttles you in your sleep like a loud noise or something?? Maverick423 14:05, 12 April 2007 (UTC)

Thank you very much Maverick, your right about being startled about something, thank you for your help, it makes much more sense to me.

CorinneQ 20:57, 12 April 2007 (UTC)Corinne Quinones