Wikipedia:Reference desk/Archives/Science/2006 October 26

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October 26

When They Test You For Genital Herpes, Do They Take A Blood Test?

Like, how do they do it? Danke.100110100 01:04, 26 October 2006 (UTC)

IIRC, generally it's a blood test. Samples of an open sore can be tested, but that (obviously) requires that one already have a suspicious open sore on your genitals. DMacks 05:08, 26 October 2006 (UTC)

I'm not sure but I think this user might have a rather poor understanding of the human circulation system and may think you need to take blood from the genital region. You don't, the blood will usually come from the arm as always Nil Einne 13:58, 26 October 2006 (UTC)

A blood test for HSV2 (genital herpes) does exist, and tests for the antibody (the thing you producde to fight it). Alternative methods include swabs of the genitals, and pcr amplification of the material found. The latter I have seen only in academic studies. --TeaDrinker 19:27, 26 October 2006 (UTC)

There are several ways to test for genital herpes infection. One method of testing is a viral culture from a smear or swab of a vesicle (blister). That test is not perfect; it will be positive in about 70% of active lesions. Direct fluorescent antibody testing of a smear of vesicle fluid is about 90% sensitive. Polymerase Chain Reaction (PCR) testing is very sensitive but is also very expensive and therefore not typicaly used for routine clinical diagnosis. Blood tests are useful in two situations in regards to genital herpes infection. First, in a person who is experiencing what appears to be their first herpes outbreak, blood tests obtained when the rash breaks out and about 2 weeks later will demonstrate an increase in antibodies to Herpes Simplex Virus (HSV), thus strongly supporting the assumption that the herpes virus casued the original outbreak. The second situation in which blood tests can be useful is to rule herpes out as the cause of a chronic recurring genital rash. If a serum antibody test is negative for herpes, that strongly suggests that herpes is not the cause.

Microscope Lenses

My teacher gave a very bizarre and un-scientific sounding explanation for how lenses work within optical microscopes. They were talking about reflective lenses, giving a diagram of a curved lens reflecting directly away from it, resulting in a larger image on the same plane as the original. After what I already know, and reading Microscope and Lenses, I can see that they use refractive lenses on common optical microscopes. My question is this: Is it possible to magnify things reflectively, and is it used anywhere? One example that I can think of is Reflector telescope (which seems like the reverse of the described process), but would actual microscopes use such things? The Uglymancer 01:23, 26 October 2006 (UTC)

As far as I know, no microscope uses reflection to increase magnification, but I could be wrong. I do know that binoculars use reflection in part of the optical path in order to have a long light path contained within a short physical housing. See the article on porro prism and the article on binoculars for more. 192.168.1.1 6:50, 24 Rocktober 2006 (PST)

Is it possible to magnify things reflectively, and is it used anywhere? -- youbetcha, see [1] Dallas67 04:39, 26 October 2006 (UTC)

The optics used in the steppers used to "print" the patterns on integrated circuits are mostly reflective (because the wavelengths used keep getting shorter and shorter, and there aren't good transmissive materials for these wavelengths). Steppers actually use reducing optics rather than magnifying optics, but they still prove the point. The really interesting stuff is coming up though; we're crossing the boundary into what used to be called "soft X-rays" but now are called (less scarely) "extreme ultraviolet"; these will need "grazing reflective optics" where the light is reflected at very shallow angles. I think you can find more about X-ray mirrors if you follow some links from our Chandra X-ray Observatory article or here.

Atlant 23:23, 27 October 2006 (UTC)

Help with relativity

I am trying to help my daughter with all of her homework and we are never going to be finished with all of it, so I need alittle help. One of her questions that we both can't figure out is,"You have learned about the Newton's concepts of motion, force and energy, and Einstein's theories of special and general relativity add a new twist as how the working s of nature are perceived. Comment on a feature of one of the theories that interests you. Use your imagination and come up with an idea how it might have real life application or even used in a science fiction story."Please help she really needs this grade. Every time you all have always hit the nail on the head with the questions. THANKS!!!!!!!!—Preceding unsigned comment added by 205.188.116.74 (talk • contribs)

You want us to tell your daughter what interests her and use her imagination? I'm not sure we can help with this part. Look at the articles on astronomy, generaly relativity, special relativity and newtonian physics. There are plenty of applications from astronomy to medicine. --Tbeatty 02:50, 26 October 2006 (UTC)

Since this question requires imagination, we can't exactly help without practically completing it for you. Try the articles on Classical mechanics, Special relativity and General relativity if you need some reading material. -- Consumed Crustacean (talk) 02:53, 26 October 2006 (UTC)

Because we're all a bunch of guys that don't have any imagination.X [Mac Davis] (SUPERDESK|Help me improve) 07:10, 26 October 2006 (UTC)

Nah, we have active (some would say over-active) imaginations. But even when we're pondering physics, I doubt we're imagining the same things as someone who's had the same level of science and worldly experience and been in the same science class as your daughter. DMacks 08:49, 26 October 2006 (UTC)

Or her teacher. Pffst, stupid assignment. —Bromskloss 10:01, 26 October 2006 (UTC)

I wouldn't say it's a stupid question. It's perhaps not really a physics question but it is IMHO a good question. It requires one to actually think and understand the concepts rather then just parroting something which is good since too many students can only parrot. No one said your answer has to be the same as the teachers or anyone else's. Provided you demonstrate an actual understanding of the concepts and an ability to think, I would assume you would be given your marks. This isn't a right or wrong answer question... BTW we do have imaginations. I'm thinking how old is your daugther? Over the age of consent I hope? I'm imagining what she looks like...... Er you probably don't want to hear the rest. (Okay I made this up but I'm sure there are some wikipedians thinking it). Nil Einne 13:49, 26 October 2006 (UTC)

BTW I got this from the reference desk a while back "it would be really kewl if your parents went on a trip to a nearby blackhole for 2 weeks then you will be 20 years older than your parents when they return --RedStaR 20:55, 27 September 2006 (UTC)". While it's talking about black holes, in actual fact if your parents were travelling at or close to the speed of the light they could easily end up younger then you as time would be travelling much slower for them (see Time dilation). This obviously has implications for SF... Nil Einne 13:48, 26 October 2006 (UTC)

How would helping high school children with homework, help them when they have to write their exam all on their own? It's the same with study groups - you don't write exams in groups. Also, if the answer requires imagination, the only way to develop imagination is 'try it yourself' and not ask for help. As Einstein added the fourth dimension into Newton's classical laws of physics, perhaps your daughter should do some research on time travel or time paradoxes - right into the realm of science fiction where imagination has no boundary. Sandman30s 12:02, 26 October 2006 (UTC)

Hmm, time travel, black holes as portals thru time and space, parallel dimensions...I'm sure something interesting is out there. StuRat 00:11, 27 October 2006 (UTC)

chemical reactions

how could i know what chemical products can be produced in a reaction simply by knowing the reactants (and apart from memorizing them)? for example, for the thermal decomposition of hydrated copper (ii) sulphate crystals...is sulphuric acid formed and copper oxide and hydrogen gas? how can i tell, simply by looking at it?

One important thing to do is to write down a reaction for whatever you propose and see if it can be balanced. If you can't make it balance, it's probably not something that happens:) After that, it comes down to proposing lots of likely things. That means knowing the general kinds of reactions that happen, not memorizing a lot of specifics. For example, you perhaps know that many acids are compounds that can release H+ in solution, and you can probably even recognize many such compounds by their name or formula without explicitly memorizing every acidic compound explicitly. For the situation here, here's a hint: many hydrates can undergo a common reaction by virtue of their being hydrates. DMacks 08:41, 26 October 2006 (UTC)

Well, apart from some simple and relatively well-known cases, as in classic inorganic chemistry, you can't actually tell exactly how stuff is going to react. That's what chemists know and what they're needed for. But the knowledge itself doesn't follow any simple rule. It's a mix of stuff learned by heart (and generalizing it) simple but approximate rules and intuition, more or less. It's basically a large part of what chemistry is all about, so I can't really suggest more than just "learn chemistry". --BluePlatypus 14:08, 26 October 2006 (UTC)

In the example you mention, a small amount of chemical experience will suggest that the water in the crystals is not very strongly held there, whereas the other ionic bonds in the copper sulphate are much stronger. So heating hydrated copper(II) sulphate (at a moderate temperature, anyway) will cause there to be a thermal decomposition reaction, merely producing steam and a white solid known as anhydrous copper sulphate. If hydrogen was going to be given off, it would require there to be a redox reaction, and simply heating a solid isn't likely to do this. I'm afraid that experience with experiments will enable you to predict the outcome of unfamiliar reactions much more easily. --G N Frykman 22:04, 26 October 2006 (UTC)

Geography

I have some topics on which i am not getting any information. i have searched a lot but not getting what i need, so i need ur help for this.

And your specific question would be .... ??? JackofOz 10:53, 26 October 2006 (UTC)

i have some topics on which i am not getting any information. they havee asked to choose any one realms of the earth.i have taken earth. then they have asked to find

• the diversity of enviroment that exists in the world which supports life.
• ollect stories,legends,festivals,poetry,sayings,rituals,associated to water
• write a report on how mans interaction with nature has resulted in different hazardous situation such as:- deforestation and soil erosion causing floods and droughts water scarcity in rural and urban areas.

if u would just tell me from where i can get this information and the sites i would really be gratefull to you. thx.

This is a very large research project. Looks like dozens of hours of work, and 20-40 pages. I would take time to invest in research techniques, such as using Google, or libraries. Good luck! --Zeizmic 11:44, 26 October 2006 (UTC)

Or Wikipedia of course: biodiversity, water, deforestation, soil erosion, flood and drought might be good starting points (haven't looked at them myself - that's your job). DirkvdM 14:27, 26 October 2006 (UTC)

Derivation and Use of Clausius-Clapeyron relation

Hi everyone. From the article Clausius-Clapeyron relation, I'm confusing about its derivation. It says that

Along the coexistence curve, we also have dμI = dμII.  

And after that we use the $d\mu =-sdT+vdP$ relation to give the final formula. My question is that we use two points in the coexistence (phase-equilibrium) curve to derive the formula. Those two points describe the same situations where there are two phase appeared in equilibrium in the system. Hence, the Clausius-Clapeyron equation should be used only for describing a change between two points along the phase-equilibrium curve.

But commonly we use it to refer about latent heat for describing phase transition. We try to describe two situations between a point at equilibrium (two phase) and a point outside the equilibrium curve (only one phase). So why is the equation valid in this case? I know I'm missing some important concept here. Thanks everyone for your attention. -- 131.111.164.228 11:13, 26 October 2006 (UTC)

It's derived at equillibrium between the two phases and describes the relation between the latent heat of the phase change and the associated changes in temperature, pressure and volume. The latent heat of the phase change is what it's all about. If you want to study the change between states that aren't in equillibrium, then you set up a thermodynamic cycle. --BluePlatypus 14:00, 26 October 2006 (UTC)

As far as I know, the Clausius-Clapeyron equation gives an approximation to the slope of the coexistence curve--that's it. ("Equillibrium" is a tricky word because thermodynamic equillibrium is a separate concept from phase equillibrium.) It says nothing about states of single phase, nor does it even describe the coexistence curve properly--firstly, it's a linear equation (which most coexistence curves are not); secondly, it only describes the transition between two out of three possible phases. In other words, I am questioning the validity of your last paragraph. Ckerr

It is not an approximation at all. It is exact. Nor is it a linear equation. It's a PDE. Now, if you chose to integrate that as if it wasn't, which some people certainly do, then it becomes an approximation. But it's not one present in the equation itself. --BluePlatypus 21:00, 26 October 2006 (UTC)

The points I and II are actually the same point on a PT diagram; they are different points on a VT, or PS, or VS diagrams. You do not take two different points on a melting curve on a PT diagram, you just take one point (which gives you the temperature at which your element would melt under given pressure, provided the pressure does not change in the process). Now you would like to know what is the melting curve slope at that point. Here the Clausius-Clapeyron equation becomes useful. You see, that single point in a PT diagram becomes an infinite number of points when you go to the VT diagram. Indeed, under given P and T, when you are on the melting curve, you can vary the fraction of phase I in the mixture continuously between 0 and 100%. This changes V (as the specific volumes of the two phases differ, generally speaking), without changing either P or T or mju (but changing S). The Clausius-Clapeyron equation is then derived from the fact that mju stays constant. The two points used in the derivation are at the same P and T; simply, one is 100% phase I, 0% phase II; and the second one is 0% phase I, 100% phase II. The slope is thus defined at a point (PT) proper, not at two close points. Hope this helps. Dr_Dima

Thank you all very much -- 131.111.164.110 09:24, 30 October 2006 (UTC)

Expert In Opacity Needed

Hey. The article Opacity (optics) is in need of a rewrite from an expert. I'm not one. However, if anyone has any assistance that they can render, it would be appreciated! Scalene^{•UserPage•Talk•Contributions•Biography•} 11:34, 26 October 2006 (UTC)

• You could always post a message on Wikipedia talk:WikiProject Physics. They may be able to help you. – ClockworkSoul 13:51, 26 October 2006 (UTC)

I rewrote it, although I'm not claiming it's complete (or that I'm an expert). Feel free to remove the help tags if I've done a good job with it. I also proposed a merge, if anyone's interested in helping with that. --Tardis 15:33, 26 October 2006 (UTC)

Added a paragraph. Some important things are still missing (self-absorption, relation to line profiles, absorption mechanisms, etc...). Which level are we aiming at: high school? undergraduates? experts in adjacent fields? --Dr_Dima

All levels. A Wikipedia article should start out with an intro that those with the least background can understand, then build up to the most complex theory, at the end. StuRat 23:50, 26 October 2006 (UTC)

Eiinstein's theories

I am trying to solve a problem but I am stumped, please help. How can you add a new twist as how the workings of nature are perceived when using Einstein's theories? and how might it have real life applications? Thanks!!!

—The preceding unsigned comment was added by 205.188.116.74 (talk) at 00:58, 27 October 2006.

Your question is vague - Albert Einstein worked on many theories throughout his life, which are you referring to? However, in any case, you should do your own homework (see the top of this page). — QuantumEleven 13:07, 26 October 2006 (UTC)

See the same question above. The user is simply repeating the question with less info thinking he or she can con us in to helping. However our answer is the same Nil Einne 13:41, 26 October 2006 (UTC)

A good try though ! If this is a different user trying to get the homework done from us, see Wikipedia:Reference_desk/Science#Help_with_relativity -- Wikicheng 13:46, 26 October 2006 (UTC

I am doing my own homework, and I am not pertending to be someone else in order to get you to help. All I was asking is to help out, but as usual you have one or two of you trying to be smart about a question. I will not ask another question from you and the donations that come from the Miller Foundation will also be ceased. Thanks anyway.

I sincerely hope that until you grow up you will not be allowed to handle any significant amount of money. Cryonic Mammoth 16:52, 26 October 2006 (UTC)

This question I actually feel negatively towards. It evokes some anger response in me. That's scary. Please try before you ask here. This is a very easy question, and we would answer it, but I know I wouldn't feel like I answered it much at all without at least a page. X [Mac Davis] (SUPERDESK|Help me improve) 04:28, 27 October 2006 (UTC)

If you don't mind my asking, to exactly which Miller Foundation are you referring, and over how much of their funding do you have direct control? There's the Miller Foundation, the Brittaney Miller Foundation, the Rush-Miller Foundation... [2][3] Try to be more explicit in your vague, unsubstantiated threats. Black Carrot 21:06, 27 October 2006 (UTC)

research project

i have to find info on sampling and randomisation but i cannot find anything on representitive sample any search engine i use comes back with no information

Please sign your posts and don't add a space as the first character because it screws up text. I suggest you use a different search engine. Google finds quite a bit of stuff with representitive sample. Of course, it helps a LOT if you spell it right and search for representative sample instead but Google will usually make you aware when you spell a word wrong (and as I said you get useful stuff however you spell it). Alternatively use a spell checker. BTW, sampling on Google also give a lot of useful links as does randomisation and the alternative (predominantly US) spelling randomization. So really I don't know what search engines you tried but you should add google to your list. I also found useful wikipedia articles with these searches and using the wikipedia search would probably find the same article... Nil Einne 14:15, 26 October 2006 (UTC)

Randomisation? DirkvdM 14:30, 26 October 2006 (UTC)

Well actually Sampling (statistics) is probably what the above reader needs, but I was trying to encourage him/her to actually search Nil Einne 14:46, 26 October 2006 (UTC)

Focus and focal point

Quick question for the physics people: Is the focal point of a parabolic mirror located at the same point as the geometric focus of the parabola? Thanks, Zagalejo 14:18, 26 October 2006 (UTC)

Yes. Virogtheconq 14:39, 26 October 2006 (UTC)

It's not a proof, but it may help to remember that each point on a parabola is as far from the focus as from the directrix; it's therefore reasonable that the image of the focus in a parabolic mirror would be the directrix (many different images, in fact, from each point on the mirror). Since all the shortest-distance (perpendicular) lines from the directrix are parallel, light emitted from the focus is redirected into a parallel beam (the definition of the focal point). (It makes no real difference whether it's truly parabolic or is a circular paraboloid, except that in the former case the focus is a line, not a point.) --Tardis 15:06, 26 October 2006 (UTC)

Cool, thanks all. Zagalejo 23:41, 26 October 2006 (UTC)

Blind Dreams

What do people dream about if they have been blind all their lives? Is it purely sounds and textures? Paul venter 14:29, 26 October 2006 (UTC)

Good one. If the visual cortex is intact, I suppose they will have visual aspects to their dreams. But if it is never used (for lack of input), it might degenerate. Don't know, though. Of course, this would also depend on the cause of the blindness - I just assumed it was in the eyes, but if the visual cortex were deficient from birth they could never imagine colour, I suppose. DirkvdM 14:35, 26 October 2006 (UTC)

[4] might be useful and [5] both of which I found from [6] (most alternatives also work). Short answer - usually yes if they're not congenitially blind, probably not if they are. Of course, although I haven't read the 2004 paper, I'm somewhat skeptical whether you could establish whether a congenitially blind person has visual dreams. Without any knowledge of what vision is like, how will they know if they are seeing something? Food for thought. Nil Einne 14:53, 26 October 2006 (UTC)

If they "knew" what shapes things were without touching them, that would be a way to know they were seeing them. There was an interesting sci-fi book where almost everyone was blind. One girl could see, so knew about things before she felt them, and everyone else thought she must be psychic. StuRat 23:43, 26 October 2006 (UTC)

One could check for activity in the visual cortex. That doesn't necessarily mean that they see the way we do, but of course the same goes for anyone else. I only assume that other people see things roughly the way I do (in a literal sense, I mean). That when I see the colour red, they will have a similar sensation. But of course I can never really know that. DirkvdM 09:38, 27 October 2006 (UTC)

You know, by a common definition of "psychic" (aka extrasensory perception), she would be. She has a sense they don't have, don't understand, and can't really even imagine. I've heard people claim that claivoyance is the "sixth sense", one with a corresponding sensory organ, which we just haven't tracked down yet. Of course, it'd be more than the sixth, even among the traditional senses, but the point is the same. Black Carrot 20:51, 27 October 2006 (UTC)

And, as a matter of fact, you can know that. Not yet, sure, but once we've actually got some clue how the brain works (cross your fingers), it should be fairly straightforward to demonstrate whether or not we process basic visual information the same. For instance, the retina has been thoroughly studied, and at least one stage (the photoreceptors themselves) has a fairly conclusive answer: there's quite a bit of similarity in the distributions of red, green, and blue photoreceptors from person to person, but some variation, which may or may not be accounted for by the brain itself. Black Carrot 20:55, 27 October 2006 (UTC)

One thing to keep in mind that not everybody who is defined as "blind" has absolutely no vision at all. Many do have some vision. --Robert Merkel 09:37, 28 October 2006 (UTC)

That's "legally blind", which is a rather silly legal term. They should be called "low vision" to distinguish between them and the truly blind. StuRat 16:09, 28 October 2006 (UTC)

See no evil ... ? DirkvdM 08:13, 29 October 2006 (UTC)

Vitrectomy aftermath

I assume the vitreous humor regrows after a Vitrectomy, but does anyone know how long it takes? My guess is the fluid would come back within a day or two, but the fibrous network may take some time after that - does it require leaving some of the humor behind as a seed, or will it all come back naturally? Virogtheconq 14:47, 26 October 2006 (UTC)

The vitreous does not regenerate spontaneously. To maintain the proper shape of the eye, chemically stable, optically clear, sterile viscous fluid is infused in its place. —Preceding unsigned comment added by Jodahy (talk • contribs)

The article also says that a gas is used and diffuses out gradually, meaning that eventually the humor returns. Obviously that won't happen spontaneously, but I'm interested in the timescale on which it happens. Virogtheconq 13:26, 27 October 2006 (UTC)

s and d orbitals

What does the s and d areas in the peroidic table have to do with orbital notation? 216.253.128.29 16:10, 26 October 2006 (UTC)nicholassayshi

• This picture might help you out a little bit.

The orbital names are s,p,d,f- in that order. On the periodic table, each atom is written with its electron configuration, showing which electron orbitals are filled.

For more info, look at the article on Electron configuration. It is explained there in more detail. Laurən^whisper 16:25, 26 October 2006 (UTC)

• A little more specific info, spdf designate the different shapes of the orbitals, also. Laurən^whisper 19:15, 26 October 2006 (UTC)

I also am trying to understand elctron configuration in orbitals. How does that image present electrons relatve to the nucleus? Are those the varied directions the electrons will appear from that orbital (with the nucleus being in the center). or is that a visual representation of one electron? More simply put, I am confused about the visual representaion of electron(s) and/or nucleus in those images of the orbitals. 69.150.209.15 19:22, 26 October 2006 (UTC)

Those are representations of the 3D region where an electron in a given orbital is likely to be found around a centrally-located nucleus. We can't actually locate a specific electron at a specific position, and electrons really are free to be located anywhere at all, even miles away. But we can talk about, over time, the where they "usually" are, a "probability density". So an electron in an s orbital is generally found in a spherically-symmetric region around a nucleus, whereas an electron in a p orbital is generally found in a dumbell-shaped region along a certain axis through the nucleus. DMacks 19:40, 26 October 2006 (UTC)

Basically, that happy little diagram where the electrons moved around the nucleus like planets around the sun was wrong. It's just a really simple way of showing it. And for the rest, DMacks pretty much has it covered. --AstoVidatu 21:55, 26 October 2006 (UTC)

I seem to remember from college chemistry that s,p,d and f stand for "secondary, primary, diffuse, and fine," perhaps from the appearance of spectral lines. So what is g?Edison 00:01, 27 October 2006 (UTC)

"g" is the letter after "f":) No, seriously...see Electron shell#Subshells. DMacks 01:42, 27 October 2006 (UTC)

Warm-bloodedness in birds and mammals.

Did warm-bloodedness evolve in a common ancestor of birds and mammals (was the most recent common ancestor of birds and mammals warm-blooded), or did it evolve independantly in the two groups? Would it be possible to determine this by comparing the mechanisms birds and mammals use to maintain thermal homeostasis? grendel|khan 17:07, 26 October 2006 (UTC)

Good question. There are many cases of convergent and parallel evolution, but our articles don't list this one. Cryonic Mammoth 17:14, 26 October 2006 (UTC)

Good question indeed. Looking up "endothermy evolution birds mammals" in google gives plenty of good hits. There seems to be quite a bit of argument going on about the details of warm-bloodedness evolution, but most seem to agree that the primary thermoregulation mechanisms are already present in reptiles. It's just the metabolism rate in reptiles that's not high enough. Dementios

Hmm... I'm not sure about birds and mammals, but homeothermy of a degree is found in fish (particualrly some kinds of Tuna and Lamniform sharks (see Salmon shark)). This is believed to have arisen independentally. --TeaDrinker 19:58, 26 October 2006 (UTC)

I think the current concensus is that they evolved it seperately (at least that is what we are being taught in uni). It is unknown, though seems to be actively argued, whether dinosaurs were warm blooded. However, I think mammals diverged from reptiles just after the amphibians, so there is a big gap there. --liquidGhoul 22:25, 26 October 2006 (UTC)

I was taught that they were not. That a lot of the similarities between birds and mammals (like the heart for example) are covergent evolution. Common ancester between birds and mammals go as far back as the reptiles, so i'd think not. With fish, it's defintely convergent evolution. Things like Tuna and sharks are slightly endothermic for completely different reasons. Where as things like whales are endothermic because they're not fish at all =). --`/aksha 01:50, 27 October 2006 (UTC)

Colour of semen

I have noticed my semen is a little less opaque and yellower than that of others. Does this mean something about sperm count, or tell anything else about my health? I have not yet tried to get my wife pregnant but we are considering that before the end of the year. I've never pointed that out and I strongly doubt she noticed it. Please let me know if this is a bad thing because I'll need to bring it up.

First, see a doctor for medical advice. Second, semen takes on a yellow tint as you age. It also turns yellow if you haven't had an orgasm in a long time. It also turns yellowish with a high sperm count. It also turns yellow in the presence of many common infections. So, yellow semen is just a symptom. You need to see a doctor to figure out what the cause of the symptom is. --Kainaw ^(talk) 19:02, 26 October 2006 (UTC)

Check your lighting, it easily appears yellow sometimes if it is in a "warm" color light. X [Mac Davis] (SUPERDESK|Help me improve) 04:51, 27 October 2006 (UTC)

I can't help but ask... what was your method of comparison? Rockpocket 06:42, 28 October 2006 (UTC)

Plane Crash

Which seats in a big airliner do you think are safer depending on the different types of crashes? It does happen quite often that a plane crashes and it is announced that "189 people died, and 3 survived" or sentences of that sort they never specify where the lucky ones were sitting. I was thinking more towards the tail of the plane, dont know why, it seems stronger somewhat but then that last Kentucky one only the co-pilote got dragged out of the burning plane by the fire brigade if I remember correctly. Any thoughts or statistics? Thank you. Keria 19:55, 26 October 2006 (UTC)

IIRC, you want to sit either near the tail, since that part usually absorbs the least amount of shock on impact, or you want to be around the wings, since that's the strongest part of the airframe. --BluePlatypus 20:53, 26 October 2006 (UTC)

Where is the fuel tank usually located? Might want to avoid sitting on top of that. --Kurt Shaped Box 23:02, 26 October 2006 (UTC)

I have heard, albeit not from a source that would know directly, that most deaths in plane crashes are from smoke inhalation and fire. Thus the best places to sit might be near the exits. Tuckerekcut 23:07, 26 October 2006 (UTC)

I have heard that the safest is by the wings, I believe from documentaries on National Geographic etc. It is both the strongest part of the plane as well as right by the exits. --Wirbelwindヴィルヴェルヴィント (talk) 23:30, 26 October 2006 (UTC)

Since you asked for statistics, I'd like to point out that since the odds of you dying in an airplane crash are lower than 1 in 5000 - over your entire lifetime, and your odds of dying on any given trip are roughly 1 in 6 million, your best bet in choosing a seat is to look at something like SeatGuru to choose the most comfortable seat -- particularly since your odds of being uncomfortable on any given flight approach 1 in 1. --ByeByeBaby 00:01, 27 October 2006 (UTC)

FYI the fuel tanks in most commercial airliners are in the wings. Larger aircraft such as the Boeing 747 also have tanks in the lower half of the fuselage. 192.168.1.1 5:21PM, 26 Rocktober 2006 (PST)

One look at controlled impact demonstration should change your mind about sitting above the wings. Xcomradex 01:41, 27 October 2006 (UTC)

Heh, it seems to be a total mess anyway, no matter where you sit. Hmm, all the passengers seem to be asleep, and the little one has apparently turned around, planting their face in the seat. :-) —Bromskloss 12:27, 27 October 2006 (UTC)

ByeByeBaby: These are odd statistics you are quoting there. Your two numbers only fit together if the average person does 6 mio / 5,000 = 1200 flight in his entire live. Do people really fly that much? Simon A. 13:17, 29 October 2006 (UTC)

No matter where you sit, the inside of the plane is likely to be a mess after any impact, since it's not designed to handle the same load conditions the structure is. I don't think it really matters where you'll be sitting since everything's going to get tossed around anyways - in that case, it'd probably be better to be seated close to the emergency exits, so avoid the previously-mentioned asphyxiation scenario. Virogtheconq 01:55, 27 October 2006 (UTC)

When we start falling I'll go lock myself in the toilets at the back. Thank you. Keria 08:55, 27 October 2006 (UTC)

There was a BBC documentary on this recently and the final conclusion someone drew was "We don't really know" because every plane crash is different. So those shots Xcomradex linked to are no indication at all. But two considerations were that near the wings the structure of the plane is extra reinforced and that it helps to sit near an exit. Especially that last bit seemed to be important. Also, bend forward before the plane crashes, so you won't slam into seat before you quite as hard, and try to immobilise your limbs so they won't go flapping about (your muscles can't possibly couteract the forces excerted on your body by a plane crash). Maybe wrap your arms aroun your legs. Oh, and if you land in water, inflate the life jacket at the moment you leave the plane, not before because it will hinder your movements in the melee of people scrambling over each other after the crash - the main reason to sit near an exit. DirkvdM 10:06, 27 October 2006 (UTC)

I don't have it handy, but a report I read from the FAA some time back said that sitting near an exit didn't guarantee you'd be able to use it. During even a mild crash, the fuselage telescopes slightly, resulting in compression forces which made it impossible to open the exits. One test showed that they had to tow the aircraft body forward for some distance to fully relieve the stresses and enable the exits to open. Additionally, in a severe crash, the fuselage will rupture in many places, creating new exits. 192.168.1.1 8:46, 27 Rocktober 2006 (PST)

On the other hand, given that there's a roughly equal probability of a "new exit" opening somewhere in the fuselage, then it's still somewhat safer to sit near the exits, since at least in the case you're not seated near one of the fissures there's still a chance of escape. Virogtheconq 01:36, 28 October 2006 (UTC)

The Science magazine?

Could anyone tell me how can I get the issues of the Science magazine, please? I live in Alexandria in northern Egypt.

--62.139.172.44 21:24, 26 October 2006 (UTC)

Go here for information on ordering an individual article/issue, and here to get information on subscribing. --AstoVidatu 21:59, 26 October 2006 (UTC)

Personal print subscriptions at $139 but digital subscriptions are only $99 and may be a much better deal oveseas to reduce shipping costs, delays, and losses. Most American and European universities and hospitals pay for free online access to Science at [[7]] for all students and faculty. If you belong to a school or similar institution with a library, ask the librarian if they have access. Another possibility would be to search online for American library associations that send duplicate copies of scientific and medical journals overseas to poorer institutions, although you may not get complete sets and the issues may be months or a year old. alteripse 23:38, 26 October 2006 (UTC)

Well, if you go to a library, you might just as well read the paper issue there. :) That can be any major public library. And surely, Alexandria must have a good library. DirkvdM 10:11, 27 October 2006 (UTC)

Basic Physics: Force Question

A car and driver at m=3,000lb total moves at exactly v=60mph along a level, straight highway heading west to Vegas for five minutes. Is it true that during that time, the car has no west-bound force (f=m*a), because its acceleration is zero? Seems strange if that's true. --12.104.14.109 22:39, 26 October 2006 (UTC)

Yes it's true that the total force is zero. However there will still be forces acting on it. Friction will act eastwards so as to retard (slow down) the motion. Therefore the engine will have to provide a force to overcome this. These forces are equal and opposite, so the add up to zero. Theresa Knott | Taste the Korn 22:51, 26 October 2006 (UTC)

How can the total force be zero? The car is travelling westward at 60 mph. If the engine's power output exactly matches the force of friction, than the car won't go anywhere. The engine has to overcome friction, and move the car. (Maybe I'm missing something obvious here--if that's the case, please tell me!) --Bowlhover 04:11, 27 October 2006 (UTC)

Oh yes, once the car is moving, it only needs to overcome friction to maintain its speed. It only needs to accelerate at the very beginning of the journey. --Bowlhover 04:16, 27 October 2006 (UTC)

In physics terms, net force --> net acceleration through Newton's second law, F=m*a. If the car is moving at constant velocity, there is no acceleration. If there is no acceleration, there is no total (or net) force. --M1ss1ontomars2k4 ^{(T | C | @)} 04:24, 27 October 2006 (UTC)

You are missing Newton's first law of motion. If it is travelling at 60 mph, with no friction, it will continue to travel at 60 mph forever till another force acts upon it. Thus, to maintain speed, engines have to put enough force to cancel out friction (including the air). --Wirbelwindヴィルヴェルヴィント (talk) 05:24, 27 October 2006 (UTC)

You are probably getting confused with kinetic energy, which is very much dependend on the mass and velocity. --Wirbelwindヴィルヴェルヴィント (talk) 23:34, 26 October 2006 (UTC)

Could be a trick question involving the earth's rotation. A toilet in a 3,000 lb RV still flushes counterclockwise when it's moving at 60 mph. It is a Coriolis force that causes the rotation. --Tbeatty 03:04, 27 October 2006 (UTC)

Huh? What does this have to do with Earth's rotation? Also, toilets definitely do not flush counterclockwise because of the Coriolis effect. See http://www.snopes.com/science/coriolis.htm. Even dropping a feather into the toilet will affect the water's velocity thousands of times more strongly than Earth's rotation can. --Bowlhover 04:11, 27 October 2006 (UTC)