Science desk
< November 16	<< Oct \| November \| Dec >>	November 18 >

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.

November 17

Please, help me decide whether to drink my tea.

I dispensed hot water into my plastic CamelBak water bottle so that the tea-bag can do a better job than it would in cold water. Now I remembered my mother telling me about pouring hot beverages in a plastic container causing cancer. However, no one else said this.

Does a hot beverage leech plastic chemicals into the drink, thereby causing one to develop cancer? If not, what ill effects could it cause?

Also, would anyone please post sources backing this up? Thanks. --98.190.13.3 (talk) 04:22, 17 November 2010 (UTC)[reply]

Our Reuse of water bottles article has some info. I don't know what kind of plastic a CamelBak is made from, so I don't know any specifics of what may be leached out, etc. DMacks (talk) 04:25, 17 November 2010 (UTC)[reply]

Some (but NOT all) plastics contain Bisphenol A which has been shown to be linked to a number of health issues. Hotter water tends to dissolve more Bisphenol A than cold water (this is a nearly universal property; hot water tends to dissolve more of anything than cold water). There are also a host of other chemicals which may (or may not) be linked to health issues depending on the specific type of plastic. Again, you would have to know the specific type of plastic in your water bottle before you could decide if you just killed yourself. --Jayron32 04:29, 17 November 2010 (UTC)[reply]

Your mother may have heard about microwaving plastic. The Mayo Clinic says microwave-safe containers are okay, but notes that the FDA warns that non-safe plastics that melt when you nuke 'em may potentially leak chemicals. So, as long as you don't want your tea really really hot, you should be fine. Clarityfiend (talk) 04:39, 17 November 2010 (UTC)[reply]

The CamelBak article says that their water bottles don't contain Bisphenol A (BPA). Red Act (talk) 04:44, 17 November 2010 (UTC)[reply]

Almost all of the chemicals which can leach out of plastic food containers are not toxic at all (chemists don't want to get sued!) but some of them can taste pretty bad. Even bisphenol A is only a serious threat to fetuses and possibly infants. Ginger Conspiracy (talk) 06:15, 17 November 2010 (UTC)[reply]

You need a teapot. The spherical ceramic kind are best. 92.28.250.11 (talk) 10:20, 17 November 2010 (UTC)[reply]

@Ginger: They're relatively nontoxic. Companies are willing to put up with a small (non-zero) number of lawsuits over their products, if the difference in cost between a product and its alternative (safer) product is GREATER than the money that could be saved by the lower number of lawsuits, companies would then choose to use the less safe, but cheaper, product since the outgoing cash on the lawsuits on it is still small enough to make it worth it. When actuarial science meets economics, that's what you get. The only reason that companies took Bisphenol A out of baby bottles wasn't because some new science indicated they should, it was that the likely cost of keeping it in the bottles grew exponentially such that the more safe, but more expensive, plastic became cheaper when factoring in the likely lawsuits. --Jayron32 16:08, 17 November 2010 (UTC)[reply]

chemistry

if the hydrogen gas is flammable and it burns with pop up sound oxygen is also necessary for burning the combination of these two [water] is used to extinguish fire —Preceding unsigned comment added by Bharat.293 (talk • contribs) 04:30, 17 November 2010 (UTC)[reply]

I am guessing that you are asking for an explanation of the relationship between hydrogen, oxygen, water, and burning. Lets see if I can give you the short-short version.

Elemental hydrogen and elemental oxygen are both relatively unstable. Both hydrogen and oxygen would be more stable if they were part of a chemical compound rather than as pure elements (nearly ALL elements, with the notable exceptions of the noble gases, are like this). The reason for this is that both elements have the wrong number of electrons for their most stable state. Hydrogen has too many electrons (it would rather have a positive charge than neutral) and oxygen has too few electrons (it would rather have a negative charge than neutral). When we chemically react hydrogen and oxygen, what happens is hydrogen and oxygen rearrange their electrons such that oxygen gets extra electrons and hydrogen gets to give up some of its electrons. This makes BOTH of them more stable. In chemistry, stability is related to potential energy; the more potential energy stored up in something, the less stable it is. When it releases that potential energy, usually as heat, the new substance that forms is more stable than the substances that formed it. In this case, the new substance is water, which is composed of atoms of hydrogen and oxygen, but in this case arranged such that BOTH have the number of electrons that makes them most stable. This is why water is not very chemically reactive; it already has its electrons in a relatively stable arrangement, so it will no longer burn. Burning is just the outward sign of the chemcial reaction between the fuel (in this case hydrogen) and oxygen.
To sum up, the process of burning involves a rearrangement of the electrons between hydrogen gas and oxygen gas to form a new substance (H₂O) which is more stable than the starting state. Because this new state is more stable, it isn't going to react further, so it will now put out flames rather than burn itself. --Jayron32 04:46, 17 November 2010 (UTC)[reply]

Bharat, are you remarking on the oddity that the combustion of hydrogen and oxygen produces water, yet water is what we commonly put fires out with? —Steve Summit (talk) 06:14, 17 November 2010 (UTC)[reply]

That has to be it. So the answer is "Yes, that's right" I'd say. Ginger Conspiracy (talk) 06:17, 17 November 2010 (UTC)[reply]

Water can be used to put out SOME fires. This is because it typically acts either to reduce the 'heat' of the combustion reaction. Other forms of (non-water) extinguisher target other parts of the fuel triangle, like for example a C02 extinquisher works by denying the fire access to oxygen.. Sfan00 IMG (talk) 20:40, 17 November 2010 (UTC)[reply]

We have a fire triangle article. DMacks (talk) 21:01, 17 November 2010 (UTC)[reply]

Although the water itself does not put out the flame, the instant reaction of hydrogen and oxygen produces a flame that immediately extinguishes itself. Water does not put out all fires, as a grease fire will continue burning as its heat fuel source is removed only briefly, and water can often start electrical fires though even hot water will cool a red-hot iron nail as liquid water cannot be hotter than 100°C under normal circumstances. ~A H 1^(TCU) 03:23, 18 November 2010 (UTC)[reply]

It's really quite simple: wood will burn, but the ash that's left over will not burn again. Hydrogen and oxygen burn, and the ash is water. Sand is the ash of silicon and oxygen. Since fire needs oxygen, if you cover a fire, it goes out. Of course, the fire may ignite the thing you cover it with, and then you just have a new uncovered fire. If the cover won't burn, then the fire does go out. You can thus put out many fires by covering them with ashes, sand, or water. --Tardis (talk) 15:42, 18 November 2010 (UTC)[reply]

Catfish

I just watched an episdode of River Monsters, and the host would take very large catfish, such as the Piraíba or Jaú, out of water. They made strange grunting and moaning sounds, which he said were because of air moving over the gills, rather than water. This set me wondering: can they breathe in air? Thanks, --The High Fin Sperm Whale 06:20, 17 November 2010 (UTC)[reply]

Some can for a while, see airbreathing catfish. Sean.hoyland - talk 06:40, 17 November 2010 (UTC)[reply]

Yes, I am familiar with airbreathing catfish. My question was whether or not a large one, such as those I mentioned earlier can. --The High Fin Sperm Whale 18:42, 17 November 2010 (UTC)[reply]

Yet another mistake in the solutions?

Read Question 4 (b) (iii) (inelastic) here: http://www.tqa.tas.gov.au/4DCGI/_WWW_doc/006039/RND01/PH866_paper02.pdf to which the solutions are here (scroll down to the second last page): http://www.tqa.tas.gov.au/4DCGI/_WWW_doc/006121/RND01/PH866_report_02.pdf . Where did he get 10.6 eV from?? If he got it by subtracting the gap between n=2 and n=3 (which would be 1.9 eV) from the 12.5 eV that the electrons originally had, that's wrong, isn't it? Since the hydrogen atom is in the ground state, there are no electrons to excite in n=2, correct? 220.253.217.130 (talk) 08:42, 17 November 2010 (UTC)[reply]

My guess is the person writing the answer key should misread the n=1 level for the n=2. Assuming the collision is involving the ground state hydrogen atom, you should be able to use the energy bled off of the colliding electron in an inelastic collision to do the n=1 -> n=2 transition and the n=1 -> n=3 transition. The n=1 -> n=4 transition won't happen because you would need excess energy in the colliding electron. So the only two answers should be 0.4 eV (1->3) and 2.3 eV (1->2). --Jayron32 15:59, 17 November 2010 (UTC)[reply]

Lambert's cosine law

In Lambert's cosine law it describes how if the sun were a perfect black body radiator it would appear to us as a circle the same brightness all over, yet it darkens towards the rim. The full moon on the other hand should darken towards the rim, but it seems almost as bright at the rim as at the centre. There is an explanation about the sun at Limb darkening, but why does the full moon seem brighter at the edge than one might expect thanks? Dmcq (talk) 10:10, 17 November 2010 (UTC)[reply]

Is the moon really brighter at the edge? I've never noticed that. Perhaps it's obvious, but the moon is not a black body radiator, it's a reflector. But I'm not sure if that makes a difference. It probably depends on how directional the reflection from moon rocks is. In a black body the light is emitted evenly in all directions, but in a reflector it may have a preferred direction. Ariel. (talk) 10:30, 17 November 2010 (UTC)[reply]

It should also bear stating that a blackbody is an unrealistic ideal, something like an ideal gas or perfect vacuum. We can do mathematical calculations which show how a blackbody should behave, and some real objects aproximate these calculations, but no real object acts like a blackbody. --Jayron32 15:30, 17 November 2010 (UTC)[reply]

The Moon's maria on the Earth-facing side make it brighter towards the center in total but also brighter on most of its rim. Ginger Conspiracy (talk) 17:49, 17 November 2010 (UTC)[reply]

I got my wording a bit confused, the lambert law should apply to the moon, just the article mentioned the sun. I don't think the maria quite explain the effect, they don't say why the edge shouldn't be fairly dark. I was wondering perhaps the moon isn't as dark as it should be towards the edge because it has lots of craters and what we see at the edge are the reflections from the sides of the craters, does that sound like it might be possible? Dmcq (talk) 10:48, 18 November 2010 (UTC)[reply]

The article isn't talking about the edge of the moon as viewed from here, but rather as viewed from the sun: the edge of the lit part of the moon. Of course, those are the same when it is full; the article goes on to say that the lack of darkening indicates that the moon isn't Lambertian. If it were, we would expect its limb to be bright (which it is), but more terminator darkening than we see. --Tardis (talk) 17:44, 19 November 2010 (UTC)[reply]

You're mistaken. The article is talking about the edge of the moon as viewed from Earth.--Srleffler (talk) 18:58, 21 November 2010 (UTC)[reply]

Please help me clear up this hogwash about adjusting braces at the SAME orthodontist.

Hi, when I saw that getting braces in India was only going to be as low as $225 to as high as $1800 USD, I then had a desire to travel to India to get the deeply-discounted orthodontic work.

Then someone told me that I'd have to fly back to the same orthodontist to get them readjusted every so often. I thought that was a bunch of hooey, because get THIS logic:

TRUE OR FALSE: A driver who buys his Ford in Texas and moves to Maine, must drive his car all the way back to that same Ford dealer in Texas to realign his axles (and perform other substantial maintenance.)

See the fallacy here? He only has to drive it to the nearest Ford dealer from his house in Maine. Or if an independent mechanic is closer, he can simply drive it to them and they can do all the maintenance needed just so long as they have the right equipment.

Now the orthodontists' equipment is state-of-the-art back Stateside, so they'll undoubtedly have all they'll need to readjust Indian braces.

So since the logic of having to travel back to the same orthodontist to readjust braces falls under the same level of fallacy as the car maintenance analogy, I didn't buy what they said.

(Also, think of military families whose family must move with the active duty serviceperson, and sometimes on short notice. If their child got braces in Fort Hood, and they move to the base in Landstuhl, their child would hate to fly all the way back to Fort Hood for a mere readjustment. Some kind of accommodation would need to be made. That would be to just get it done at a local orthodontist.)

So can someone clear this up? I still hope to get braces at the cheapest possible place, but get them readjusted wherever I happen to be. Thanks. --70.179.178.5 (talk) 10:33, 17 November 2010 (UTC)[reply]

I'm confused as to what sort of answer you expect since apparently the last one wasn't good enough. Wikipedia:Reference desk/Archives/Science/2010 September 5#Why can't I just get a local orthodontist to readjust braces installed in India?. Ultimately even though as I said last time, it seems unlikely that you won't be able to find some other orthodontist to deal with your braces, no one here can guarentee your local orthodontist will be able to handle your braces. Your best bet if it concerns you is to ask your local orthodontist yourself. Nil Einne (talk) 10:45, 17 November 2010 (UTC)[reply]

Cool, N.E., that IP user must've been on a similar mental plane as me. Seems like we have a lot in common; I'll have to study his editing history now. --70.179.178.5 (talk) 11:13, 17 November 2010 (UTC)[reply]

You do indeed have a lot in common as it seems you both use the same ISP and appear to live in the same area in or near Manhattan. Nil Einne (talk) 14:07, 17 November 2010 (UTC)[reply]

One thing they don't share is time. The old IP address made edits through mid-September. Then, they stopped. Once the old IP address stopped making edits, this IP address began making very similar edits (similar topics, similar grammar, similar misspellings...). -- kainaw ™ 15:29, 17 November 2010 (UTC)[reply]

... but they do share a blog at "bigyesbomb"! Dbfirs 01:45, 18 November 2010 (UTC)^{[citation needed]}[reply]

(ec)As much as possible you need to stick with one person when adjusting, this is because you make a plan for the adjustments, when to do what, which tooth, etc. If you go to a different person each time you'll get a mix. The solution to this is very good records, and presumably the military does that, but I doubt a regular orthodontist is going to carefully read and follow the documented plan. You'll be lucky to even get a written detailed plan. That said, you can probably get them installed in India or wherever, then stick with the same new local person for all the future adjustments. But you better hope that he has tools or equipment to service the exact type of braces you are getting, I believe there is a wide variety of types, and a variety of ways of attaching them. Ariel. (talk) 10:49, 17 November 2010 (UTC)[reply]

"Someone told you"... Did you ever ask an orthodontist if they would be willing to manage the periodic readjustments of braces installed elsewhere? There could be many reasons why an orthodontist would prefer to have the braces be his or her own handiwork, but people move all the time and they don't generally have this problem. I suspect that you won't get an answer that satisfies you here on the RefDesk and would be better suited simply asking the orthodontist that you hope to be followed by. --- Medical geneticist (talk) 16:45, 17 November 2010 (UTC)[reply]

Periodontal resident here -- orthodontics is a field in which comprehensive care can and often does exceed the length of a treating resident's residency -- for example, if the average ortho case takes 18 months to complete, any case began within 18 months of graduating needs to be completed by another resident. That being said, ortho residencies are highly regulated and organized by their administrators and professors, and the case will likely go though a thorough hand-off review. But outside, in the real world, I can certainly see an orthodontist refusing to pick up a case. Excepting death of the original practitioner (and perhaps even not excluding it) there has to be a really good reason to switch mid-therapy, because, as you can probably figure out, there are multiple ways to accomplish most any goal in dentistry, and mixing a case up between clinicians is like having two contractors building your house -- you might have to have good luck to make sure pipe A is able to meet up with pipe B when they're done. There are many orthodontic systems, many possible treatment planning options, many types of appliances, many different springs, hooks, wires, brackets and they can probably be used in different ways depending on the level of training, experience and personal preference. And this is assuming that the Indian orthodontist practices on a level congruent with American standards. Eastern European/Russian root canal therapy is a really strange thing to look at from a United States-trained dentist's perspective, and so is Central/South American restorative work (crown and bridge). Foreign dentistry may be inexpensive because there's no Manhattan premium, but it may also be inexpensive because it's shoddy. DRosenbach ^{(Talk | Contribs)} 19:40, 17 November 2010 (UTC)[reply]

Thank you for responding, Dr. Rosenbach. Not having straight teeth could give me dim job prospects because in this kind of economy, employers are looking for any slightest excuse to throw out an applicant, when they have so many resumes/applications for so few job openings, that they may even throw me out just because my teeth aren't straight enough. I would have to walk on so many eggshells just to have the slightest glimmer of hope for a job offer, that having to pay so much just to make it in the world defeats the whole deal. If there was a charity/schooling orthodontist office anywhere close to 66502, please let me know where the nearest location is and I'll consider that. Otherwise, this daunting catch-22 is one more reason why I must move overseas for good. --70.179.178.5 (talk) 08:40, 18 November 2010 (UTC)[reply]

Unfortunately, it appears as though the closest reliable discount for orthodontics might be UMKC in 64108, just over 2 hours away. DRosenbach ^{(Talk | Contribs)} 05:27, 21 November 2010 (UTC)[reply]

All cars of the same model (and production run) are (or should be) identical, so any garage with suitably experienced staff is usually happy to carry out your repairs and adjustments. Only identical twins have identical teeth (and even then, not always), so there is an element of creative art in the work of an orthodontist. It's rather like asking an interior designer to make small adjustments to your bodged DIY decorating, or an architect to improve the look of your self-built home. They might do it for you, but they will not necessarily be delighted to be asked. Dbfirs 01:17, 18 November 2010 (UTC)[reply]

Why does the sex drive far exceed what is biologically needed for reproduction?

Just wanted to know.--X sprainpraxisL (talk) 10:56, 17 November 2010 (UTC)[reply]

Define "needed".

For males, at least, and in species that are not 100% monogamous, it's evolutionarily advantageous to have as many offspring as possible. It's just another example of how rank selfishness, much as we'd like to define it as a social ill, is a biological imperative. —Steve Summit (talk) 12:29, 17 November 2010 (UTC)[reply]

(e/c)This is rather open ended. What species are you talking about, or just in general? Are you referring to humans? If so, whose sex drive...? Let's assume you're referring to an 'average' human, though I guess you could generalise and substitute in other species. In a historical and evolutionary sense, the simple answer to your question is that it doesn't. Let's say in basic terms the sex drive spurs the activity that results in reproduction. For the sex drive to "exceed what is biologically needed for reproduction" that would mean the reproduction rate would result in more children born than needed to replace their parents and an ongoing population boom. Historically this is not the case - populations in general remain fairly stable, unless or until something upsets the equilibrium. Now it is true that nature generally does produce more offspring than needed for replacement, and evolution will favour population growth if it can (greater spread of genes, etc), as well as needing to 'hedge its bets' for the bad times; indeed the excess individuals are the fodder for natural selection, but in normal conditions these excess offspring are removed from the population through conflict, starvation, etc and the overall population numbers remain relatively constant. When something does upset the natural balance we can see population booms like the recent one in human populations. As I said at the start, sex drives vary greatly both between and within species, but a generalisation would be that the average sex drive of a species is that which has been favoured by natural selection to maximise that species' long term survival - different species use different strategies (fewer offspring and greater parental care, more offspring and less parental care...) to try to achieve that best balance between producing enough offspring for the species to survive and prosper, and expending too much energy on reproduction. That's all pretty general; really you could do an entire thesis on this. --jjron (talk) 12:39, 17 November 2010 (UTC)[reply]

A relatively higher sex drive will tend to be selected for as long as it isn't detracting from survival or child rearing. Ginger Conspiracy (talk) 17:53, 17 November 2010 (UTC)[reply]

For humans who engage in media and other forms of entertainment, suggestive shows, films and commercials likely help promote the drive. DRosenbach ^{(Talk | Contribs)} 19:30, 17 November 2010 (UTC)[reply]

Unlikely to be the case. People have been randy long before there was television. In fact, my understanding is that various studies have shown that people's sex drives decrease as a function of how much television they watch. I don't have references handy, though. --Mr.98 (talk) 01:21, 18 November 2010 (UTC)[reply]

Keep in mind that before the invention of modern medicine, a significant fraction of human babies died in childbirth. thx1138 (talk) 19:33, 17 November 2010 (UTC)[reply]

None of that answered his question. You are talking about how many babies are born, but he asked why humans want to have sex far more often then is needed to get pregnant. The answer is twofold: Humans are the only species with Hidden estrus, so the male does not know when to have sex, so the males will try as often as possible. The the second reason is that sex reinforces pair bonding, so the male is motivated to stay with the female and help raise the babies. In an animal instinct can do this, but in humans a greater incentive is used. Note that contrary to impressions, and sitcoms, married men have sex far more often than single men. Ariel. (talk) 20:36, 17 November 2010 (UTC)[reply]

See libido. Also this article from LiveScience. ~A H 1^(TCU) 03:19, 18 November 2010 (UTC)[reply]

Only? Are you sure? The article you linked to doesn't think so. It links to a ref on the occurance in vervet monkeys which mentions the occurance in other primates. It doesn't really mention any others per se from what I saw although it does mention others with prolonged sexual receptivity although that's more complicated then concealed ovulation per se (for example it mentions orangutans where it says it's often due to forced encounters). I believe some dolphins may also show concealed ovulation/hidden estrus although I couldn't find a decent ref. Nil Einne (talk) 18:06, 18 November 2010 (UTC)[reply]

What kind of moss covered bug is this?

I was walking around Norfolk, Virginia this weekend and I saw this piece of moss walking along a concrete pillar. I got a few photographs and videos of it. I feel like I've seen the bug before (without the moss). Such an interesting defense mechanism, I someone out there can help identify it. The insect does look familiar.

Here is a photo of the moss. On the right side you can see what I think are mandibles.

Here is a photograph of the creature upside-down.

Here Is a (handheld, sorry) video of it walking around.

Here Is a youtube video of it righting itself.

I went out again, and upon closer inspection, I this is the same bug, without the moss covering. The mandibles look the same, the size is similar, the legs look the same (from the upside-down shot), and the movement was similar.

Keegstr (talk) 15:06, 17 November 2010 (UTC)[reply]

It may not be moss at all, but may instead be Mimicry. Some bugs like planthoppers and walking sticks have amazingly complex ways to mimic their environment. I do not recognize this exact bug you have found, but I would assume that the stuff you are calling moss is actually an integral part of the bug itself. --Jayron32 15:23, 17 November 2010 (UTC)[reply]

It's a brown lacewing larva.[1] Very helpful in the garden for keeping aphids under control. Ginger Conspiracy (talk) 18:13, 17 November 2010 (UTC)[reply]

Thanks for the ID. Any idea what the deal with the moss is? Is it defense / camouflage? Keegstr (talk) 19:03, 17 November 2010 (UTC)[reply]

Sometimes known as 'trash bugs' apparently [2] and your assumption seem to be correct [3]. Mikenorton (talk) 10:52, 18 November 2010 (UTC)[reply]

Reaction products of opianic acid with strong acid

Is this image likely to be right? Reaction with strong hydrochloric or hydriodic acid. Possibly isomeric phenols are formed?
--Wickey-nl (talk) 17:25, 17 November 2010 (UTC)[reply]

Looks OK to me. You shouldn't get any isomerization whith hydrochloric or hydriodic acid, simply the hydrolysis of the two ester groups. I wouldn't like to guarantee that they get hydrolysed in the order that shown, but that's a fairly minor point. Physchim62 (talk) 17:58, 17 November 2010 (UTC)[reply]

nit: s/ester/ether/ DMacks (talk) 18:04, 17 November 2010 (UTC)[reply]

(ec)It's a plausible reaction, probably S_N2-like. Methyl ethers are stable to just about everything except strong acid. My protecting-groups table for "phenols, methyl ether" says the only suitable deprotection conditions are aqueous pH<1 and AlCl₃(but only slowly unless warmed). I don't see an easy mechanism for isomerization. DMacks (talk) 17:59, 17 November 2010 (UTC)[reply]

FWIW, there is a known aqeous-acid-catalyzed isomerization of a different part of the structure in opianic acid and related compounds: the 1,2 acid-aldehyde portion can form cyclic ester-hemiacetal. (see doi:10.1007/BF00568016 and refs therein). DMacks (talk) 18:14, 17 November 2010 (UTC)[reply]

Cold-weather urination

Why does being in the cold make people urinate more? The core body temperature where the kidneys are must be either the same or nearly the same as in warm surroundings. 92.15.28.182 (talk) 18:09, 17 November 2010 (UTC)[reply]

I'm not sure it does, in general. Maybe because people drink more hot diuretic coffee? Ginger Conspiracy (talk) 18:14, 17 November 2010 (UTC)[reply]

People generally sweat less in cold weather but I don't think that's the whole explanation. A quick search for 'urinate more cold weather' comes up with some results, the most promosing looks to be [4]. That links to [5] for rats. From that same search, it seems in humans there's also greater urgency [6], in other words it may not simply be that you have more urine but that you feel a greater urge to urinate. Nil Einne (talk) 19:07, 17 November 2010 (UTC)[reply]

The phenomenon is called "cold-induced diuresis". Exposure to cold temperatures causes your body to respond with vasoconstriction in the extremities, which helps your body retain heat. The vasoconstriction causes blood pressure to become elevated, to which the kidneys respond by removing fluid from the blood stream in order to lower the blood pressure. The kidneys pass that removed fluid through the ureters into your bladder, which makes you need to urinate.[7] Red Act (talk) 19:37, 17 November 2010 (UTC)[reply]

The phenomenon is apparently sometimes also called "cold diuresis", as that's what the polyuria and hypothermia articles refers to it as. Cold diuresis is unfortunately a rather useless redirect. Cold-induced diuresis is part of the cause of immersion diuresis, as is explained a little bit in that article. Red Act (talk) 20:00, 17 November 2010 (UTC)[reply]

Frog dissection

Is there a common species of frog that would usually be dissected in a lab class to measure the contraction of its gastrocnemius muscle? 149.169.142.37 (talk) 22:05, 17 November 2010 (UTC)[reply]

Rana pipiens. --- Medical geneticist (talk) 01:25, 18 November 2010 (UTC)[reply]

When I was at Uni, we used the Cane Toad (Bufo marinus) in a lot of pracs. --jjron (talk) 12:31, 18 November 2010 (UTC)[reply]

Direction of apparent weight

Is apparent weight a directionless quantity? Or is it a vector equal to the normal force in both magnitude and direction (for instance, in an accelerating lift, in which case the direction would be up). -- 220.253.217.130 (talk) 22:21, 17 November 2010 (UTC)[reply]

There may be more than one way to define "apparent weight" but I would describe it as a force vector equal to the normal reaction with the lift floor, but in the opposite direction. Dbfirs 22:59, 17 November 2010 (UTC)[reply]

So in general, apparent weight is equal to the vector difference Fnet – Fg, but in the opposite direction (including for arbitrary direction for Fnet)? 220.253.217.130 (talk) 23:20, 17 November 2010 (UTC)[reply]

Or equivalently, apparent weight is equal to Fg – Fnet ? 220.253.217.130 (talk) 23:22, 17 November 2010 (UTC)[reply]

Well I would define real weight as a downwards force of magnitude mg (I assume that this is your Fg). If the lift is accelerating upwards with acceleration a, the normal reaction will be mg + ma upwards (assuming that the person or object is not moving relative to the lift floor), so by my definition, the apparent weight will mg + ma downwards. I'm not sure what your Fnet denotes. I think the safest way to deal with the situation is to consider only real forces and apply Newton's second law (Resultant force = mass times acceleration). Dbfirs 23:40, 17 November 2010 (UTC)[reply]

Fnet is the net force, that is the vector sum of the force due to gravity and the normal force, and is equal to ma, where a is the acceleration of the lift and m is the mass of the person. 220.253.217.130 (talk) 23:53, 17 November 2010 (UTC)[reply]

Sorry, yes, I should have realised that you meant the same as my "resultant force". (It's after midnight here.) Dbfirs 00:12, 18 November 2010 (UTC)[reply]

So does that make Fg – Fnet a correct formula for the apparent weight? 115.178.29.142 (talk) 00:32, 18 November 2010 (UTC)[reply]

Only when the lift is accelerating downwards (and a ≤ g), or if you allow Fnet to be negative when the lift is accelerating upwards. The safest method to avoid confusion is always to draw a diagram showing the directions of real forces, then apply Newton's second law. Dbfirs 01:03, 18 November 2010 (UTC)[reply]

Glycerol

How stable is glycerol stored in a clear glass bottle? I see that it's hygroscopic, but would that just make it gradually dilute itself over time, rather than reacting with the water? Also, not much water is going to get in anyway. Does it decompose into anything given time and a small amount of normal air, in the top of te bottle? Does is decompose into anything in sunlight? I'm just wondering, because it seems such a bland thing, and yet I know it's used in various reactions, including making explosives. Would a bottle of the stuff still be just glycerin after a few years? 86.163.213.68 (talk) 23:45, 17 November 2010 (UTC)[reply]

Glycerol is pretty stable, especially if the bottle is kept closed. It can oxidize to compounds such as glyceraldehyde and glyceric acid, but this is very slow in the absence of a catalyst. Physchim62 (talk) 23:53, 17 November 2010 (UTC)[reply]

In my personal experience, glycerol is stable when stored at room temperature and out of direct sunlight for years. Proctor and Gamble have verified that room-temperature storage doesn't appreciably degrade glycerin over a the length of a two-year monitoring and testing program: [8]. I don't have (and was unable to find) specific information about the photostability of glycerol exposed to sunlight over extended periods, though I wouldn't be surprised if the sunlight affected the plastic bottle it's shipped in more than the glycerol itself. TenOfAllTrades(talk) 01:25, 18 November 2010 (UTC)[reply]

Thanks, these are both good answers. 86.163.213.68 (talk) 15:42, 18 November 2010 (UTC)[reply]