Wikipedia:Reference desk/Archives/Science/2007 December 15

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December 15

Best armor-penetrating caliber cartirdge?

I understand the .45 has too low velocity, whereas the 9 mm has not enough power... so what's the ideal pistol bullet against standard body armor? —Preceding unsigned comment added by 83.5.250.239 (talk) 01:14, 15 December 2007 (UTC)

See Body_armor#Performance_standards. As a general rule, most decent soft body armor is effective against pistol rounds. Friday (talk) 04:13, 15 December 2007 (UTC)

So-called armor-piercing handgun rounds usually rely on construction, like the teflon bullet and Black Talon, ammunition although their abilities are opften overstated. Rmhermen (talk) 17:40, 17 December 2007 (UTC)

Black Talon is not armor-piercing ammunition, this is an unfortunate fabrication perpetuated by the media. It is essentially normal hollowpoint pistol ammunition, the only difference is that its jacket is scored in such a manner as to create small sharp points when it peels back during impact. It is stopped by all normal pistol-resistant body armor. Black Talon is still sold, both vintage and new, and is not illegal. It was merely renamed Ranger SXT by Winchester to avoid the negative publicity (the joke was SXT stands for Same Xact Thing). As to the original question, for penetrating armor, velocity and diameter are king. Cartridges which combine high velocity and low diameter apply all their kinetic energy to a very small area of armor, allowing them to penetrate. Some cartridges augment this penetration by using hard metal cores, such as steel or tungsten instead of lead. Standard 5.56x45 will penetrate most police body armor, but some military armor will resist 5.56 and 7.62x39 or even 7.62x51. 5.7x28 is a new armor piercing cartridge by FNH, as is 4.6x30-something by H&K. —Preceding unsigned comment added by 71.111.215.3 (talk) 05:08, 18 December 2007 (UTC)

"Troubleshooting" PCR protocol

If a person is "troubleshooting" a methylation specific PCR protocol with pre-defined primers, what sort of things are they going to be able to vary? I'm told it will be by trial an error. Are there online tools to advise regarding the lengths of time for each stage in each PCR cycle etc? --Seans Potato Business 02:07, 15 December 2007 (UTC)

You can vary annealing temperature, length of annealing or extension time per cycle, magnesium concentration, Mg:dNTP ratio, Mg:salt ratio. You could try hot-start PCR, touch-down PCR or two step PCR. You can, literally, spend weeks playing with PCR variables. There are loads of online tools that will give you every variable known to man that you can use to calculate cycle length, here is one, however the length of time depends very much on which enzyme you are using. Most commercial enzymes synthesize DNA at a rate of around 2kb/min, so if your amplicon is less than that, 1min extension will probably be sufficient with 30 sec of melting and annealing. Rockpocket 02:25, 15 December 2007 (UTC)

It appears that, for methylation specific PCR, there are a few extra things you could play with too, during the bi-sulphite modification step. Rockpocket 02:30, 15 December 2007 (UTC)

Whats special about frequency?

A question thats been bothering me for some time is: why is it not possible to change the frequency of an electrical or optical signal with out going through some non-linear process (like a mixer)? (excluding the Doppler effect which requires relative motion between source and receiver). After all, we can change the wavelength and/or the velocity quite easily. Whats special about frequency?--79.76.208.92 (talk) 03:14, 15 December 2007 (UTC)

How do you mean "changing wavelength quite easily"? Wavelength and frequency are inexorably tied together... Arakunem^Talk 03:28, 15 December 2007 (UTC)

V=f*λ in a certain medium. Change the medium, you change the velocity and the wavelength, but you cant change the frequency! —Preceding unsigned comment added by TreeSmiler (talk • contribs) 03:36, 15 December 2007 (UTC)

Frequency is tied to the energy of a photon, so you're asking how to increase/decrease the energy of a certain photon without absorbing it and re-emitting a new one. I don't know of a physical process that can do that. I'm not sure it's something "special" about frequency though...each parameter is modifiable by a system that interacts with that parameter. Maybe frequency is more intrinsic to the photon itself than to its motion through space, so it's harder to act on it? DMacks (talk) 06:14, 15 December 2007 (UTC)

Not only photons. What about electromagnetic waves? why is frequency invariant there? Does this have to do with energy as well?--TreeSmiler (talk) 12:04, 16 December 2007 (UTC)

A factory at the end of a road is building cars and sending them out on the road with a fixed frequency. As long as all cars are driving at a constand speed, the distance between them (i.e., the wavelength) will be constant as well. If they encounter rough terrain, they might have to slow down (making the wavelength shorter). The frequency with which they pass a roadside observer will not change, however, regardless of the terrain. Think of what would happen if the observer saw a higher frequency than that of the factory. There would be more cars leaving the area between the factory and the observer than entering it. You can't have that (apart from mabye a short while) because soon that area would be empty of cars. Similarily, the crests and troughs of a wave (be it an electromagnetic wave, sound wave, …) don't just disappear or pop up out of nothing. —Bromskloss (talk) 10:57, 15 December 2007 (UTC)

Good point. But if you were to take a fixed number of cars after they left the factory, and sped them all up at the same time, you standing at the end of the road would see an increased frequency (for a short time).The wavelength (distance between cars) would remain the same. The converse is also true.--TreeSmiler (talk) 12:04, 16 December 2007 (UTC)

Frequency is energy is the best statement of this. Because you can't change the speed of a photon, (it MUST go at the speed of light) - the only way to change it's energy is to change it's mass - which isn't an easy thing to do. However, there is a way - which is to set yourself in motion relative to the thing that emitted the light. If you move away from the emitter, the light will be red-shifted and if you move towards it, the light will be blue-shifted - the doppler effect. However, that's pretty inconvenient! I guess you could drag a black hole or something suitably massive nearby - that would bend space and thereby cause a doppler shift...sadly, that's even more inconvenient. SteveBaker (talk) 13:46, 15 December 2007 (UTC)

If by "changing the frequency", you mean producing a time-compressed or time-dilated version of an arbitrary signal, it's not hard to see why it's difficult. Consider a linear blackbox that outputs a time-compressed version of its input, the delay between an input impulse and the corresponding response gets progressively shorter. Eventually the delay will be negative — the system is non-causal! Now consider a linear blackbox that outputs a time-dilated version of its input, the delay between input and output get sprogressively larger. If the system is to exhibit this property on a sustained basis indefinitely, it will need infinite information buffering capacity.

--72.78.102.231 (talk) 15:11, 15 December 2007 (UTC)

Good explanation. Thanks. It only needs to be done for a finite time and I have a big buffer. How can it be done?--TreeSmiler (talk) 12:04, 16 December 2007 (UTC)

I don't know what constraints you have, but can't you just sample the input at one rate, store the samples somewhere, and play back the samples at a different rate? --64.236.170.228 (talk) 13:56, 17 December 2007 (UTC)

Yes you certainly can! But I was hoping for a method that doesn't involve sampling in the conventional sense. So how can you slow down a wave that is already propagating at velocity 'v' in in certain medium?--TreeSmiler (talk) 02:26, 19 December 2007 (UTC)

Energy efficient halogens

hello,

i currently live in a gorgeous, new build appartment. Like most new builds, it has low ceilings, so unlike having one big 'normal' light they have about 9 little halgen spot light things. whilst they're no doubt very trendy, they also burn out really quickly and are SO expensive (£5 for 2). They're also no doubt not very good for the future of the planet. Any one know of any engery efficient alternatives? there don't seem to be any in my supermarket and i'm not really sure i know what the techincal term is to search on the internet. Any ideas? thanks! 81.110.23.77 (talk) 15:22, 15 December 2007 (UTC)

Sorry, but I don't know much about alternatives, but the ones you have aren't supposed to burn out all that quickly. If you will accept a tip, never touch them at all. Put on a pair of those disposable latex gloves before you open the pack, and handle them as little as possible even so. They have to be cleanroom clean. --Milkbreath (talk) 16:03, 15 December 2007 (UTC)

See Halogen lamp for further information on this kind of lamp (you could have found Milkbreath's tip there). As an alternative with greater efficiency I suggest fluorescent lamps. Icek (talk) 16:22, 15 December 2007 (UTC)

You can get led based MR16's [1], [2]. They last longer and are more efficient. Led's have seen remakable improvements lately in efficiency and warm white colors, but halogens still have a better quality of light (IMO). Halogen MR16's put out about 15 lm/W, warm white leds put out about 45 lm/W. The newest white led's are putting out over 100 lm/W. --Duk 17:57, 15 December 2007 (UTC)

LED "bulbs" for general lighting are very exciting and promising for the future, but they currently have the disadvanges of being very expensive, and it tending to be hard to find bulbs that are bright enough. What is currently available at good brightness levels and cost are compact flourescent bulbs. Until recently, it wasn't possible to find compact flourescent spotlights that are small enough to fit into standard fixures (or at least, the spotlight fixtures I have in my kitchen), but that's changed now. They still aren't available yet at stores around where I live anyway, but they are available online.[3] Compact fluorescent bulbs cost a little more than incandescent or halogen bulbs initially, but they more than make up for it with a long lifespan, and more importantly, a large savings in electricity usage. MrRedact (talk) 05:47, 16 December 2007 (UTC)

Fluorescent MR16. --Duk 06:06, 16 December 2007 (UTC)

preventation of Bow legs?

My father has bent legs, mine were ok , until recently, though, they have started to bend. Iam in my late 20s and please tell me, how can I prevent further bending? —Preceding unsigned comment added by 131.220.46.26 (talk) 16:14, 15 December 2007 (UTC)

We are not qualified to offer treatment advice for a medical condition. You should seek medical advice from a professional. -- kainaw™ 16:31, 15 December 2007 (UTC)

I'm aware of the dangers from non-qualified medical advice, including dangers of lawsuite. But, on the other hand, I, personally, didn't see a "medical professional" worth his title the last ten years. I can really feel with people who try to get medical facts on their personal condition from others than the officially so called professionals. 84.160.244.197 (talk) 22:21, 15 December 2007 (UTC)

Nowadays it's relatively easy to search in the literature yourself, using e. g. PubMed. Icek (talk) 01:43, 16 December 2007 (UTC)

Bow legs are generally caused by a pathological condition or they are genetically based. In the former your family doctor can offer an opinion or a referral to a specialist. In the second case there is precious little you can do, except stoically put up with the 'you've got legs like your dad' remarks. Check it out with your doctor he'll be glad of an unusual case among all the coughs and snotty noses at this time of the year. Richard Avery (talk) 10:06, 16 December 2007 (UTC)

It is not correct that "there is precious little you can do". Bow legs can be corrected surgically. Bow legs also predispose for osteoarthritis of the knee, so this is not just a matter of putting up with remarks. Check it out with your doctor. --NorwegianBlue ^talk 15:44, 16 December 2007 (UTC)

I happen to fully agree with you, 84.160, but keep in mind that "professional" just means that you get paid for it :) --Taraborn (talk) 11:03, 16 December 2007 (UTC)

Umm... From the introduction of our article Profession: "Professions are usually regulated by professional bodies that may set examinations of competence, act as a licensing authority for practitioners, and enforce adherence to an ethical code of practice." --NorwegianBlue ^talk 15:50, 16 December 2007 (UTC)

wanna pee . . . but can't . . .

I thought I had already posted this query but I can't find it. So, here goes again.

Reposted question removed. TenOfAllTrades(talk) 19:41, 15 December 2007 (UTC)

We are not qualified to make a medical diagnosis of your problem. See a professional doctor for a diagnosis. -- kainaw™ 17:06, 15 December 2007 (UTC)

Your question was removed from the Miscellaneous desk because we are not allowed to diagnose medical conditions. Please see the instructions at the top of this page – and see a doctor.--Shantavira|^{feed me} 17:56, 15 December 2007 (UTC)

If any medical problem or symptom causes you concern you are encouraged to consult your doctor rather than Wikipedia Reference Desk volunteers who are not allowed to answer such questions. Edison (talk) 02:39, 16 December 2007 (UTC)

Here we can't answer your questions, but there are many resources in the Internet to gather information from (including forums, such as www.wrongdiagnosis.com). --Taraborn (talk) 11:05, 16 December 2007 (UTC)

Termite ranges

The page on termite doesn't specify how widespread they are. I'm wondering how prevalent they are in the US. --BrokenSphere^{Msg me} 18:11, 15 December 2007 (UTC)

Here's a link to start,[4] a bit basic but gives a general indication. Richard Avery (talk) 18:23, 15 December 2007 (UTC)

DNA sequencing

hello i want to know if i had a bacterial culture and i sent it for DNA and Plasmid sequencing the report will show the 2 results and if not how i would know the difference between the 2 (Chromosomal DNA and Plasmid) —Preceding unsigned comment added by 217.53.194.136 (talk) 21:18, 15 December 2007 (UTC)

Homework hint: Plasmid#Plasmid DNA extraction. --JWSchmidt (talk) 03:06, 16 December 2007 (UTC)

The pain of childbirth

Didn't women fear childbirth in the times prior to modern pain relief techniques? I mean, was it a matter of concern while deciding whether to bear children or not? --Taraborn (talk) 21:48, 15 December 2007 (UTC)

I think it would vary from woman to woman. I'd say there would have been many who feared the pain, but just as many who thought of it as the natural way of things. My mother (now aged 82) is in the latter category. She had 4 kids, and has often said she'd much rather go through childbirth than have a tooth cavity filled. They saw it as a matter of lying back, doing what had to be done and getting it over with (that's after having lain back 9 months earlier and ... doing what had to be done and getting it over with). It also varied from culture to culture about whether to express the feelings of pain. In some cultures, the women shriek and scream, in others they remain as quiet as they can. -- JackofOz (talk) 22:07, 15 December 2007 (UTC)

As for "deciding" whether to bear children or not, it's probably worth noting that eras predating modern pain relief also predated modern birth control. — Lomn 22:56, 15 December 2007 (UTC)

Just a side note: According to the latest issue (15th December) of New Scientist, Egyptian doctors prescribed crocodile dung applied as a pessary as a contraceptive more than 3000 years ago. The article suggests that it might have been effective, due to the spermicidal effect of its acidity. It might have been effective for other reasons as well... --NorwegianBlue ^talk 15:11, 16 December 2007 (UTC)

It wasn't just the pain of childbirth that women feared, but the threat of losing their lives. Quoting the Wiki article on Maternal death, it states:

The death rate for women giving birth plummeted in the 20th century. At the beginning of the century, maternal death rates were around their historical level of nearly 1 in 100 for live births. The number today in the United States is 1 in 10,000, a decline by a factor of 100. The decline in maternal deaths has been due largely to improved asepsis, use of caesarean section, fluid management and blood transfusion, and better prenatal care. -- Saukkomies 00:31, 16 December 2007 (UTC)

"in sorrow thou shalt bring forth children" seems to just about cover it. DuncanHill (talk) 00:41, 16 December 2007 (UTC)

Kinda like when they said "all flesh is grass"? That's actually pretty much true for terrestrial life when you consider the various food webs... --Kurt Shaped Box (talk) 01:36, 16 December 2007 (UTC)

Thanks to all. --Taraborn (talk) 11:00, 16 December 2007 (UTC)