Wikipedia:Reference desk/Archives/Science/2010 February 26

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February 26

Turning a switch backwards

I don't know how standard these types of switches are around the world, so I'll just preface this by saying that I'm in the US.

What happens if I turn the twist type switch, the one up near the light bulb, on my lamp backwards? Does the little knob (similar to a watch crown) just unscrew? Since all my lamps work, I don't want to risk breaking one by testing this out. And I'm certainly not going to take one apart. Thanks, Dismas|^(talk) 04:30, 26 February 2010 (UTC)

I doubt there is any standard for the construction of twisting switches. It could turn freely, doing nothing. It could unscrew. It could refuse to move until you use enough force to break it. I expect there are switches out there that do each. --Tango (talk) 04:33, 26 February 2010 (UTC)

As Tango said... there are many types of twist switches. For any switch, there are many types. When I recently replaced a pull-chain switch, I was surprised to find four completely different types of pull-chain switches to choose from. -- kainaw™ 05:17, 26 February 2010 (UTC)

I have accidentally turned a twist switch backwards. The knob unscrewed from the switch (and returned to normal when I twisted it forwards again). As the last person said, this may not be the same for all of them. --Anonymous, 11:52 UTC, February 26, 2010.

I've seen them that work both ways. I have a table lamp that unscrews and an old standard lamp which allows you to rotate the knob in either direction to turn the light on or off. SteveBaker (talk) 14:07, 26 February 2010 (UTC)

Lorentz transformations

Why do Lorentz transformations have to be linear? My teacher mentioned something about one-to-one correspondence (whatever that means), but could someone be more specific? Thanks. 173.179.59.66 (talk) 07:00, 26 February 2010 (UTC)

We have an article on One-to-one correspondence (actually under the more formal title of "Bijection" but the two terms mean the same thing, and the former redirects to the latter). --Jayron32 07:04, 26 February 2010 (UTC)

Alright, so why is one-to-one correspondence important, and why does linearity conserve it? —Preceding unsigned comment added by 173.179.59.66 (talk) 07:48, 26 February 2010 (UTC)

One-to-one correspondence is important because it is desirable for the Lorentz transforms to express some sort of group structure. Without one-to-one correspondence, there would exist transforms without a well-defined inverse transform. It is desirable to have a group structure because then the transforms are universal - they're the same (structurally) for every reference frame and can transform from any reference frame to any reference frame. A linear function is an "easy" way to obtain a one-to-one correspondence that has an inverse map of the same form. It may even be the only way (perhaps a proof could start with the inverse function theorem). 118.209.163.175 (talk) 08:07, 26 February 2010 (UTC)

Unfortunately, my knowledge of linear mapping is pretty non-existant. Is there a more physics-oriented explanation? —Preceding unsigned comment added by 173.179.59.66 (talk) 08:50, 26 February 2010 (UTC)

I think you should tell your physics teacher that you have no idea about what one-to-one correspondence means. I'm sure he will have a long chat with your math teacher. DVdm (talk) 08:55, 26 February 2010 (UTC)

There are plenty of one-to-one functions that are non-linear. For example, $y=x^{3}$. The Lorentz transformation has to be linear to ensure that light travels in straight lines (in a flat space-time) for all inertial observers. Gandalf61 (talk) 11:31, 26 February 2010 (UTC)

Ok, so how does non-linearity make light not travel in straight lines in other reference frames? —Preceding unsigned comment added by 173.179.59.66 (talk) 17:00, 26 February 2010 (UTC)

Snell law. Any inhomogeneity of the medium may lead to the bending of the path of the lightray. Dauto (talk) 19:02, 26 February 2010 (UTC)

The real reason the Lorentz transformations must be linear is that we require them to respect space-time homogeneity. As gandalf hinted above, the fact that light travels in straight lines is strong experimental evidence for space-time homogeneity. Dauto (talk) 16:49, 26 February 2010 (UTC)

Mistake in Textbook?

I believe I found a mistake in my physics textbook, and Introduction to Mechanics by Kleppner/Kolenkow. I wanted to confirm that my assessment is correct.

Example 7.3 in Chapter 7 goes as follows:

Consider a particle rotating in a verticle plane as shown in the sketch. [The sketch consists of a three-dimensional Cartesian coordinate system with a circle representing the particle's plane of rotation. This plane is perpendicular to the xy plane, with the ω vector making a 45° angle with the x- and y- axes. The position vector of the particle, r, is said to be at an angle θ from the z-axis, with θ being in the clockwise direction]

First we shall calculate v directly from the relationship v = dr/dt. To find r, note that z = r cosθ, x = -r sinθ/√2, and y = r sinθ/√2. Hence,

$\mathbf {r} =r({-\sin \theta \over {\sqrt {2}}}\mathbf {i} +{\sin \theta \over {\sqrt {2}}}\mathbf {j} +\cos \theta \mathbf {k} ).$

Differentiating, we have, since r = constant,

$\mathbf {v} =\omega r({-\cos \theta \over {\sqrt {2}}}\mathbf {i} +{\cos \theta \over {\sqrt {2}}}\mathbf {j} -\sin \theta \mathbf {k} ).$

Next, we shall find the velocity from v = ω × r. Assuming that ω can be resolved into components,

${\boldsymbol {\omega }}={\omega \over {\sqrt {2}}}\mathbf {i} +{\omega \over {\sqrt {2}}}\mathbf {j} ,$

we have

${\boldsymbol {\omega }}\times \mathbf {r} =\omega r({-\cos \theta \over {\sqrt {2}}}\mathbf {i} +{\cos \theta \over {\sqrt {2}}}\mathbf {j} -\sin \theta \mathbf {k} )$

as expected.

However, unless I've made a mistake in evaluating the cross product, I get the negative of their answer, so there does appear to be a discrepancy! Is this a mistake on my part, or because they defined positive θ to be in the clockwise direction (and dθ/dt would be one, which was tacitly assumed in this example but should be -1 due to how they defined θ)? —Preceding unsigned comment added by 173.179.59.66 (talk) 07:47, 26 February 2010 (UTC)

Yes, there is a sign mistake. I don't know if it is in the expression for $\mathbf {r} \,$, $\mathbf {v} \,$, or ${\boldsymbol {\omega }}\,$. Could be any one of them. I would have to see the picture in order to tell which one is wrong. Note that ${\frac {d\theta }{dt}}=\omega \,$. Dauto (talk) 16:20, 26 February 2010 (UTC)

difference between distance and displacement

What is difference between distance and displacement —Preceding unsigned comment added by NADEEM BHAT (talk • contribs) 08:22, 26 February 2010 (UTC)

Distance is the absolute value of displacement. When you go from a place with coordinate x=4 to a place with coordinate x=1, your displacement is -3 and you covered a distance 3. See our articles Distance and Displacement. - DVdm (talk) 08:47, 26 February 2010 (UTC)

I would say distance is the norm of displacement. If you are working in one dimension, then that's the same thing as absolute value, but with higher dimensions displacement is a vector. --Tango (talk) 18:01, 26 February 2010 (UTC)

Countering momentum

Suppose you are playing football (association football or soccer if you will) and you receive pretty strong pass from player exactly in front of you. If you take a shot without stopping the ball (back towards the player that sent the pass), the speed/strength of the shot feels substantially more powerful then if ball was resting. Obviously, the added strength comes from kinetic energy other player invested, but what is the exact mechanism behind this? To me, logical thing is that you would spend part of kinetic energy you invest on actually countering other player's kinetic energy, thus stopping the ball before sending it back, resulting in weaker shot overall, but opposite is true. Can you shed some light on this phenomena? --124.148.229.154 (talk) 08:57, 26 February 2010 (UTC)

Try Deflection (physics) and Collision --Digrpat (talk) 13:37, 26 February 2010 (UTC)

Some of the kinetic energy of the incoming ball gets turned into elastic energy as the ball is compressed/dented/squashed as it hits your foot. As the ball departs your toe, heading off in the opposite direction, the ball uncompresses - giving back that stored elastic energy as kinetic energy in the opposite direction. The ball is essentially acting like a spring. But because every action has an equal and opposite reaction, your foot feels that much higher impact. That's why balls have to be made of inflatable, rubbery kinds of things. SteveBaker (talk) 14:05, 26 February 2010 (UTC)

Your mistake was to assume that part of the invested energy is used to counteract the already present kinetic energy. That doesn't happen. In fact, the two add up. Dauto (talk) 16:03, 26 February 2010 (UTC)

Indeed. I suspect our OP is confusing "kinetic energy" and "momentum" - the two are not the same thing. Kinetic energy is mass times velocity squared. Momentum is mass times velocity. In a collision like this, both momentum and energy have to be conserved. This matters because momentum is a signed quantity - when the ball is coming towards you, its momentum is the opposite of what it is when it's moving away. For momentum to be conserved, your foot has to exchange some momentum with the ball. But kinetic energy is a scalar quantity - so it's perfectly OK that the energy from your foot going forwards adds to the energy of the ball coming towards you when it turns around and heads off across the pitch. The elasticity of the ball allows it to briefly store that kinetic energy as elastic energy while the ball slows down and transfers momentum to your foot. SteveBaker (talk) 21:19, 26 February 2010 (UTC)

fuel oil

what is hsd,fo,lshs? what are thire advantages and dissadvantages? is this all are commersially available fuels like petrol,diseal etc. —Preceding unsigned comment added by 117.204.85.94 (talk) 10:24, 26 February 2010 (UTC)

They could be many things. Could you give some of the context that they are being used in? --Phil Holmes (talk) 10:07, 26 February 2010 (UTC)

Seeing as you were quite vague I didn't have much to go on, but this is the science reference desk so that gives us a hint. Running a good search here for "LSHS" "FO" gave me a lot of results for two types of hydrocarbon in oil. Regards, --—Cyclonenim | Chat  10:27, 26 February 2010 (UTC)

• HSD is "High Speed Diesel" (which we don't have an article about!)
• FO is "Fuel oil" (also called Furnace Oil)
• LSHS is "Low Sulphur Heavy Stock" (which is another kind of fuel oil - and something else we don't have an article about).

This web page: "suppliers.jimtrade.com/91/90371/56222.htm" (which I can't link directly to because it's on the Wikipedia blacklist for some reason) compares LSHS to FO. This article is about High Speed Diesel. SteveBaker (talk) 13:55, 26 February 2010 (UTC)

Animal tastebuds

Hi

1. What animal's tastebuds are similar to a human being's? 2. What type of animal's tastebuds are more complex or has a better sense of taste then a human being's

Thanks, NirocFX

41.193.16.234 (talk) 11:14, 26 February 2010 (UTC)

I believe that taste is a matter of physiology. Cats for instance have little use for sweetness, so they are not going to go crazy for ice cream or honey if presented with it. So there's not really a more 'complex' set of tastes -- organisms have evolved faculties in response to what's available in their environment. Vranak (talk) 17:35, 26 February 2010 (UTC)

I believe you might be confusing, as most do, the gustatory (taste) and olfactory (smell) senses. Taste consists of merely five basic sensors, while olfaction provides a complete spectrum of virtually limitless combinations of aromas and odors. So while the evolutionary benefit of taste is a more protective one (in that bitter/sour tastes may warn for dangerous alkaline/acidic agents, and sweet tastes are generally harmless (except, of course, for ethylene glycol), and salty is tied to a drive for Na⁺ intake), olfaction is a sort of everything sense, in that odors can represent dangerous predators, tasty prey, cute potential mating partners, etc. So while taste is probably similar across the board, I would say that the vast majority of mammals possess a greater sense of olfaction than do humans. If I'm off track, and you really meant to focus on gustation...I don't think there can be anything more complex than the 5 taste sensations we currently know about. DRosenbach ^{(Talk | Contribs)} 18:44, 26 February 2010 (UTC)

The comment on ice cream is likely not correct. I strongly suspect that quite a few cats quite like ice cream provided it isn't too cold, as they do other dairy products like cream, milk, yoghurt and cheese. As an example of OR which gave me a cold hand (particularly since I couldn't find the thing for a while), my cat licked a small amount of ice cream off my hand earlier quite happily. This doesn't mean it's a good idea to regularly feed ice cream (or other dairy products) to cats BTW, they are lactose intolerant when adults as with most mammals. Nil Einne (talk) 01:58, 2 March 2010 (UTC)

The human sense of smell is actually vastly underrated. Partly because much of what we attribute to "taste" is in fact "smell" - but also because we don't use that sense very much. There was an experiment done about a year or two ago to see if humans could follow a scent trail such as a dog can. While it's clear that we're not up there with the bloodhounds, even completely untrained, unpracticed, blindfolded humans did pretty well at following a trail. SteveBaker (talk) 21:10, 26 February 2010 (UTC)

Thanks Vranak, DRosenbach and SteveBaker

I was hoping you guys might tell me about an animal that has more taste sensors than a milipede has legs and expected our human butts to be kicked by animals as usual when it comes to stuff like this.

Thanks, NircoFX

41.193.16.234 (talk) 12:09, 27 February 2010 (UTC)

I seriously doubt that any animal would or could enjoy such a wide variety of foodstuffs as we humans do. Vranak (talk) 17:44, 27 February 2010 (UTC)

When you refer to "more taste sensors," I'm unsure if you speak of type or token -- see type-token distinction. Is there really an advantage of possessing an overwhelming quantity of taste buds...and I mean more so than a human possesses? I don't think that would really help. DRosenbach ^{(Talk | Contribs)} 04:38, 1 March 2010 (UTC)

NE Snowstorm

In this, it appears that this weather system looks like a hurricane cloud. So according to this map, the eye of the storm is over Hempstead LI. Does that mean the right now, Hempstead sees clear skies? --Reticuli88 (talk) 15:29, 26 February 2010 (UTC)

Yes.Accdude92 (talk to me!) 16:00, 26 February 2010 (UTC)

Not really. Extratropical cyclones don't have eyes. Dauto (talk) 16:30, 26 February 2010 (UTC)

Many weather forecasters have been calling this storm a "Snowicane". In Halifax wind speeds were comparible to that of Juan and the storm itself has a size and shape similar to Hurricane Gloria. ~AH1^(TCU) 23:59, 27 February 2010 (UTC)

Using mouthwash and brushing teeth

Is it worth using mouthwash in addition to brushing your teeth? Is also using mouthwash likely to lead to any significantly less need for dental work over the years? Thanks 78.146.242.196 (talk) 16:27, 26 February 2010 (UTC)

Yes, have you read our mouthwash article?--Shantavira|^{feed me} 16:47, 26 February 2010 (UTC)

Have you read the article? Because if you had, your answer would be "No". 89.242.83.202 (talk) 18:55, 26 February 2010 (UTC)

It depends. Really you have to ask yourself, does mouthwash make your mouth feel cleaner? Or do you find it harsh? It certainly has its uses, and its limitations. Vranak (talk) 17:32, 26 February 2010 (UTC)

Mouthwash certainly provides a statistically significant benefit, but whether or not that translates to a clinical benefit can be debated. To begin, one must first discriminate between various mouthwashes -- those that are antiseptics (such as povidone iodine, chlorhexidine, Listerine, Crest Pro-Health), those that are breath fresheners (such as Scope) and those that are meant to provide other benefits, such as fluoride rinses (such as Act). So assuming we are discussing antiseptics, mouthwashes will not kill all bacteria because there is no way to get the mouthwashes of enough antiseptic concentration to remain in contact with oral pathogens for a long enough time -- even 2% chlorhexidine requires 10 minutes to kill P. gingivalis, a serious periodontal pathogen. And periodontal bacteria are generally found in the gingival sulcus/periodontal pocket and mouthwashes cannot get into these areas. And even if they are forced under the gumline with a Waterpik or similar apparatus, gingival crevicular fluid flow rates completely renew the pocket fluid 40 times an hour, giving any drug placed under the gumline about a 1 minute half-life. That being said, 'dilution is the solution to pollution' -- and removing bacteria from oral surfaces (enamel and gums and other soft tissue mucosa) certainly aids in preventing translocation of bacteria from, say, the tongue or the tonsils back onto teeth after brushing (as no one brushed tonsils and no one brushes the underside of the tongue or far back enough on the tongue to reach all the bacteria). So yes -- proper use of good antiseptic mouthwashes certainly contribute to dental health. DRosenbach ^{(Talk | Contribs)} 19:00, 26 February 2010 (UTC)

Not wanting to spread disinformation, so shoot me down immediately if it's all scare stories - but don't newspapers come out with a link between mouthwashes and oral cancers once every two years or so? Vimescarrot (talk) 21:46, 26 February 2010 (UTC)

See Listerine#Safety. Vranak (talk) 21:56, 26 February 2010 (UTC)

Roughly what percentage of alcohol is there in, e.g., Listerine?--Frumpo (talk) 12:07, 27 February 2010 (UTC)

From the Listerine article: Ethanol, which is toxic to bacteria at concentrations of only 7%, is present in concentrations of 21.6% in the flavored product and 26.9% in the original gold Listerine Antiseptic. Vranak (talk) 17:43, 27 February 2010 (UTC)

I printed out the cited article (McCullough + Farah, 2008) from the Listerine article decrying the use of alcohol-based mouthwashes for their contribution to oral cancer and showed it to my professor at Columbia a few months back. She laughed and we went through it together, noting the many problems with the McCullough review (and likely the reason that no one cares about it, other than kooks who either don't understand how to read and interpret literature or those who believe that amalgam and fluoride also cause cancer, autism, Alzheimer's and ADHD):

1. There are very few papers published showing these results. This paper, from the Australian Dental Society, is sort of a stand-alone and if it was so groundbreaking, would have been accepted for publication is a more prestigious (perhaps international) dental journal.
2. Randomized clinical trials are few, if existent. Most are retrospectives, and conflicting results are demonstrated. Some findings show statistically insignificant results, while others merely establish a greater risk for tobacco-induced neoplasms in conjunction with alcohol-based rinses.
3. Other "great" results showing "direct links" are from in vitro studies.
4. The focus study in this review (Guha, et al., 2007) is a virtual joke -- "These authors did not know the type of mouthwash being used nor its alcohol content, however the most common mouthwash in the countries from which the data were drawn, possibly with up to 80 per cent of the market share, contained levels of alcohol over 20%." How compelling can one's rejection of the null hypothesis be when one is using inconclusive, self-reported data?

When one reads the McCullough review with some perceiving eyes, the "results," if you can even call them that, become a lot less meaningful. DRosenbach ^{(Talk | Contribs)} 05:00, 1 March 2010 (UTC)

As an aside, chlorhexidine based mouthwashes shouldn't be used with standard toothpaste; they interfere with each other. You're meant to leave a while (30min, IIRC) between using each of them. CS Miller (talk) 20:06, 2 March 2010 (UTC)

Sand grains on earth

The site http://www.wolframalpha.com/ gives the number of sand grains on earth as (1x10^20 to 1x10^24). I want to know how such a number is obtained. What would be the procedure and the "size" of error involved. Thanks.--119.155.13.92 (talk) 16:33, 26 February 2010 (UTC)

Well, one thing I see right off the bat is that they give a range that is 4 orders of magnitude. That would be like estimating that there are somewhere between 60 million and 600 billion people on earth, so their procedure results in a substantial error size... Googlemeister (talk) 16:39, 26 February 2010 (UTC)

You can get an estimate by working out the total area covered by sand (A), the average depth of the sand (D), the average volume of a sand grain (V) and the average packing efficiency of sand (E). The number of sand grains is then A*D*E/V. There is going to be a fairly large error on each of those factors so the final value has a very high error (+/- 2 orders of magnitude in the figures you give). --Tango (talk) 18:06, 26 February 2010 (UTC)

Googling gives quite a lot of information on how you might calculate this[1][2][3][4]. Basically how many grains in a unit volume, times total volume of sand. --Normansmithy (talk) 18:07, 26 February 2010 (UTC)

How long do preserved animals "keep"?

Shortly before I took high school biology, my teacher purchased a collection of several preserved animals for dissection. I was talking with my teacher earlier today and learned that a crawfish and a foetal pig from this collection are still in one piece. Since they've been stored in a cupboard without temperature control, are they likely still to be good for dissection purposes? I can't remember for sure, but I'm guessing that the biology class was seven years ago. Nyttend (talk) 16:59, 26 February 2010 (UTC)

As long as the specimens are covered by the fixing solution (often formalin and some other goodies), they should be fine still. The fixing solution prevents bacterial and fungal growth, either of which might destroy the specimens. Also, once the tissues are fixed, they don't get more fixed with time so there shouldn't be any change in the tissue over the years. The original specimens used to draw Grant's Atlas of Anatomy (first ed. is 1943) are still on display (and in good condition) at the University of Toronto's Anatomy department. -- Flyguy649 ^talk 18:12, 26 February 2010 (UTC)

Circular logic? How was Avogadro's number originally determined?

I looked at Avogadro constant and the link at footnote 7 about Loschmidt doesn't work. The methods described in the body of the article all seem to be rather circular. The Coulometry method uses Faraday's constant, which is the charge of a mole of electrons. You can't know Faraday's constant then before you even know how many a mole of something is. The electron mass method which is next in the article uses the molar mass of the electron, once again it seems circular. And then the X-ray crystal density method uses the molar volume. How can you know the volume of a mole of things before you know how many a mole is? So where is the description of an experimental procedure to determine the Avogadro number that doesn't require knowing how much a mole of something is beforehand? 20.137.18.50 (talk) 17:19, 26 February 2010 (UTC)

I've always wondered this myself... Nyttend (talk) 17:34, 26 February 2010 (UTC)

Didn't you read the article you linked? It explains how to directly measure the Faraday's constant before you know how much a mole is. Let us know if you need more explanation about how it worked. Dauto (talk) 18:09, 26 February 2010 (UTC)

I followed the link to Faraday constant which said F was first determined by weighing the amount of silver deposited in an electrochemical reaction in which a measured current was passed for a measured time, and using Faraday's law of electrolysis. But what is current measured in? Coulombs per second. What's a Coulomb? The charge of a mole of electrons. Sorry, still I don't see the part where F gets determined without knowing anything about moles. 20.137.18.50 (talk) 18:38, 26 February 2010 (UTC)

Yes, you are right. This method only works if you have an independent measurement of the charge of the electron which they had. See the millikan droplet experiment. Dauto (talk) 18:51, 26 February 2010 (UTC)

Thanks20.137.18.50 (talk) 19:10, 26 February 2010 (UTC)

A coulomb is only about $10^{-5}$ moles of electrons. --Tardis (talk) 23:38, 26 February 2010 (UTC)

According to Scientific American "The first person to estimate the actual number of particles in a given amount of a substance was Josef Loschmidt, an Austrian high school teacher who later became a professor at the University of Vienna. In 1865 Loschmidt used kinetic molecular theory to estimate the number of particles in one cubic centimeter of gas at standard conditions." Avogadro didn't actually calculate his number; it wasn't calculated till 1909.[5] --Normansmithy (talk) 18:11, 26 February 2010 (UTC)

Sorry, that's exactly what it says in the Wikipedia article. Which reiterates Dauto's point. --Normansmithy (talk) 18:12, 26 February 2010 (UTC)

Geochemical/Mineralogical Equilibrium

Can this combination exist in equilibrium? Why/why not?

Calcite+Dolomite+Enstatite+Wollastonite+Diopside

I believe I have decided yes, but I'm not quite sure how to back my answer. —Preceding unsigned comment added by 131.123.82.201 (talk) 17:37, 26 February 2010 (UTC)

OK, dragging my mind back a few years (actually that's decades now, 'shudders'), to when I did metamorphic petrology and specifically the calc-silicates, I would say that in the case of a metamorphosed limestone with dolomite, there is likely not enough quartz to react with all the carbonates, so that calcite and dolomite will remain (note that they are only in equilibrium if there's no quartz available). That leaves wollastonite, the product of the reaction calcite + quartz and diopside from the reaction of quartz with dolomite. Not so sure about where the enstatite is coming from though, normally a high temperature and pressure mineral. Mikenorton (talk) 19:18, 26 February 2010 (UTC)

Could we also determine that if the combination of phases coexist at some given temperature, would this also be true if the temperature were to decrease by a few degrees? —Preceding unsigned comment added by 131.123.82.201 (talk) 23:28, 26 February 2010 (UTC)

Becoming an engineer in the UK with a Msc.

If you have a degree in a natural science, can you become an engineer in the UK with a Master of sciences? ProteanEd (talk) 18:47, 26 February 2010 (UTC)

This document explains the requirements for the various levels of professional accreditation for engineers in the UK. To become a Chartered Engineer, you need a Bachelors and Masters, both in Engineering or another recognised subject. A general science degree isn't sufficient, apparently. --Tango (talk) 21:04, 26 February 2010 (UTC)

If you read the document carefully, you'll see that it is not a requirement to have either a Bachelors or a Masters degree. "An accredited Bachelors degree with honours in engineering or technology, plus either an appropriate Masters degree accredited by a professional engineering institution, or appropriate further learning to Masters level" and "Applicants who do not have exemplifying qualifications may demonstrate the required knowledge and understanding in other ways, but must clearly demonstrate they have achieved the same level of knowledge and understanding as those with exemplifying qualifications". I am CEng FIEE and do not have a Masters degree. My BSc is in Physics. --Phil Holmes (talk) 11:33, 27 February 2010 (UTC)

Radiometric dating of dinosaur fossils

After reading Radiometric dating and Dinosaur, I don't really understand how dinosaur fossils are dated. (We have no specific dinosaur fossil article.) Is the age of the fossil inferred from radiometric dating of the rock that it's embedded in? Which radiometric dating technique is used? If I understand correctly, the mineralization of a dinosaur bone does not sound like an event whose date can be determined. Comet Tuttle (talk) 19:08, 26 February 2010 (UTC)

My ballpark understanding was that they measure the amount of Nitrogen-14 to Carbon-14 in the fossil, knowing $\mathrm {~_{6}^{14}C} \rightarrow \mathrm {~_{7}^{14}N^{1+}} +e^{-}+{\bar {\nu }}_{e}$ has a half life of 5730 years. I could be wrong on some details of what I just said.20.137.18.50 (talk) 19:15, 26 February 2010 (UTC)

I don't think carbon dating can be used. All the bone is replaced with minerals, and I believe the fossils are too old for carbon dating to be effective, anyway. Comet Tuttle (talk) 19:23, 26 February 2010 (UTC)

(EC) Radiocarbon dating only works for things that lived within ~60000 years. For dinosaurs, one of the other methods described in Radiometric dating would be required, although I'm not sure which one it is. -- Flyguy649 ^talk 19:26, 26 February 2010 (UTC)

This answer suggests the rocks above and below the fossil are dated to set a bracket for the age of the fossil. The isotopes used are uranium-238, uranium-235 and potassium-40. -- Flyguy649 ^talk 19:32, 26 February 2010 (UTC)

I'm still not understanding something here. I understand carbon dating, but that's out. I'm confused as to what exactly is the date that's measured by the other methods described in Radiometric dating. The potassium-argon dating article clearly states that it measures when the liquid rock solidifies, and I assume that Uranium-uranium dating and Uranium-lead dating measure the same thing. (Those two articles should be clearer for the sake of laymen like myself. For all I know, the dating methods are measuring how long it has been since the uranium was ejected from a supernova.) If I'm correct and these are the types of techniques being used, then dating the dinosaur fossils seems really inaccurate. All that's being measured is a layer of nearby molten rock that solidified, which doesn't necessarily correspond to the eon that the dinosaur lived; all you might get, it seems to me, is a maximum age (from the layer beneath the fossil) and a minimum age (from any layer on top of the fossil, if the fossil was lucky enough to have magma run on top of its rock layer without destroying the fossil). Comet Tuttle (talk) 20:09, 26 February 2010 (UTC)

The supernova idea is completely off the mark. The Potassium Argon Method measures the time since the formation of the rock. You are right that what one gets is a maximum and a minimun age but that can be enough to give an accurate estimate for the age of the fossil in most cases. There is no need for lave to run over the fossil. Ashes from volcanic eruptions can be used and fortunately large volcanic eruptions that leave an world wide imprint on rocks happen often enough for the method to ve quite accurate. Dauto (talk) 20:22, 26 February 2010 (UTC)

What we're dealing with here is chronostratigraphy, which basically combines biostratigraphy (ages based on fossil assemblages - if you have lots of different types of 'bugs' you can often be very precise) with geochronology, based on radiometric dating. The International Commission on Stratigraphy works out type sections of each subdivision of the geological time scale, complete with 'golden spike' to mark the boundaries. Once you have pretty much all the Phanerozoic (since the start of the Cambrian) sedimentary rocks dated in this way, then you need to tie it to an absolute time scale, which is where all the various radiometric dating techniques come in. You then need to search the world for some place where your rock unit contains lava flows, that are clearly interbedded with the sediments, take samples and date them and then eventually you have a time scale that allows you to say "this dinosaur lived 84 million years ago" or whatever. Mikenorton (talk) 20:49, 26 February 2010 (UTC)

Of course, the two work hand in hand, nobody actually waits for the ICS before they go out to date lavas The biostrat and the geochronology are both being constantly refined, which can mean major geological boundaries occasionally shifting around by a few million years or more. This can be a bit perplexing if you've just come up with a model for the timing of an orogenic event based on a mix of radiometric ages and fossils, although generally the changes sort out more problems than they create. Mikenorton (talk) 21:10, 26 February 2010 (UTC)

A visitor to a natural history museum admired the Tyrannosaurus skeleton on display and asked an attendant how old it was. "That skeleton is 66 million and 2 years old" he replied. "Can you really date it so accurately?" the visitor asked. The attendant replied "Oh yes, its age was found by radioactive dating when I started work here 2 years ago." Cuddlyable3 (talk) 12:49, 1 March 2010 (UTC)

euthanasia

where can i watch the euthanasia documentary "Mademoiselle and the Doctor" (ideally online) iv looked everywhere and cant find it —Preceding unsigned comment added by Thekiller35789 (talk • contribs) 19:20, 26 February 2010 (UTC)

I found some still images but I think you have to buy the DVD. Cuddlyable3 (talk) 17:10, 1 March 2010 (UTC)

I see a torrent on isoHunt with just 1 lonely seed, so you may be able to get it there. 128.223.131.109 (talk) 23:17, 2 March 2010 (UTC)

Curling

Why would spinning a curling stone cause its trajectory to curve? And how does sweeping act to make the stone go farther? —Preceding unsigned comment added by 173.179.59.66 (talk) 20:11, 26 February 2010 (UTC)

The spinning is probably related to the Magnus effect. The sweeping melts a thin layer of ice, reducing friction. --Tango (talk) 20:16, 26 February 2010 (UTC)

Because curling stones have a three-dimensional lattice structure, so the resultant friction between them causes them to interact angularly. When two stones touch, it's more like velcro clasping, with real pull, than two pieces of paper rubbing together with little effect save for sound. Vranak (talk) 21:51, 26 February 2010 (UTC)

Putting spin on the stones does affect how they collide, but that's not what the OP was talking about. They also curve as they move on their own (like a curveball). --Tango (talk) 21:58, 26 February 2010 (UTC)

It's a bit simpler than all that. Frictional forces are speed-dependent, and when the curling stone is spinning while it slides, one side moves across the ice faster than the other side, and therefore encounters more friction. Looie496 (talk) 22:45, 26 February 2010 (UTC)

If I recall correctly, sliding friction is actually not very speed-dependent (quite unlike air resistance). That doesn't mean you're wrong ("not very" not being the same thing as "not at all") but I'd like to see some support for the claim.

Does anyone here actually have any experience with curling? Say, if the stone is rotating counter-clockwise, does it tend to bend left or right? --Trovatore (talk) 00:02, 27 February 2010 (UTC)

The thing that make curling's physics complicated is that the ice isn't kept smooth like a skating rink - before the match, it's sprinkled with water droplets that freeze and make a surface that's described as "pebbled". The stones are crazy heavy (they are solid granite) - and the contact area is tiny because it's riding on the top of those frozen droplets and the underside of the rocks are concave. With a lot of weight and a small contact area the pressure on the ice is huge. That pressure increase causes the ice to melt and allows the rock to slide on water - not ice. So the spinning must be having a hydrodynamic effect as well as "normal" friction. The spinning will cause all sorts of complex interactions with the thin liquid layer. The fact that pressing hard on the brushes has an effect on the path of the stone shows that water on the surface of the ice makes it more slippery and the amount of water make a huge difference to how the stone moves. SteveBaker (talk) 02:24, 27 February 2010 (UTC)

I curl, and when the rock is spinning clockwise, it curls to the right (from the point of view of the thrower). Sweeping the ice in front of the rock makes it curl less. My theory is that the side of the stone that is rotating with the direction of travel makes more water (from melting the pebbles) and this makes it move faster. The side that is rotating against the direction of travel makes less of a water layer to slide on is impeded more by the pebbles. This may explain why the sweeping the rock makes it curve less - some of the pebbles are already melted, so both sides of the rock experience a (more) similar environment.24.150.18.30 (talk) 04:09, 27 February 2010 (UTC)

The March 2010 issue of UK science magazine Focus has an article on Winter Olympic physics, including a section on Curling. The rotation of the stone is very slow - arund 3 or 4 turns during it's journey across the ice. The sweeping of the ice smootghs, and slightly melts the ice, making the stone travel further whilst curling less. Most curl is towards the end of the trajectory, when forward movement is at it's slowest. The water melted by pressure from the stone at the front is dragged around the circumference of the stone, further reducing friction. —Preceding unsigned comment added by 194.205.143.136 (talk) 14:48, 28 February 2010 (UTC)

FIDEL CASTRO TRAVELS TO THE USA

1960 doesn't seem like such a long time ago

fidel has come to the usa many times but there are no articles about the trips that i can find. can anyone give me a link or 2? —Preceding unsigned comment added by 67.246.254.35 (talk) 22:23, 26 February 2010 (UTC)

I'm not sure why you posted this to the science refdesk — the miscellaneous refdesk would be a better place.

But anyway — he has? You mean, since 1959? That strikes me as somewhat implausible. What is your reason for believing this? --Trovatore (talk) 23:57, 26 February 2010 (UTC)

Googling castro visit un shows he visited in 1960 (pictured), 1995, and 2000, at a minimum. Comet Tuttle (talk) 01:17, 27 February 2010 (UTC)

Oh, I see. Well, the UN is extraterritorial; it isn't US soil. I suppose he has to pass over US soil to get there. That's not exactly what I think of when I hear of him "coming to the USA", though. --Trovatore (talk) 01:48, 27 February 2010 (UTC)

The UN has an agreement with the US that it will allow people to enter the US when visiting the UN. I think they are usually restricted to NYC if they wouldn't otherwise have been allowed in. --Tango (talk) 02:52, 27 February 2010 (UTC)

It's sort of considered international territory, though United Nations Headquarters discusses how the territory is still subject to US law. And in any case Castro certainly had to land at a US airport and get driven on US roads to reach the UN territory. Comet Tuttle (talk) 04:16, 27 February 2010 (UTC)

where can i read about the trips —Preceding unsigned comment added by 67.246.254.35 (talk) 12:40, 27 February 2010 (UTC)

Dark Energy not needed if gravitation converges to r instead of r^2 for large r ?!?

From the Dark energy article:

It is of interest to note that if the equation for gravity were to approach r instead of r2 at large, intergalactic distances, then the acceleration of the expansion of the universe becomes a mathematical artifact,[clarification needed] negating the need for the existence of Dark Energy.

I am a layman when it comes to physics of that kind, but that seems a bit too simple for not everybody to jump on it.

What would interest me, though, how would an alternative formula for large-scale gravitation look like? Is that where Einsteins cosmological constant comes in? —Preceding unsigned comment added by Xsansara (talk • contribs) 22:32, 26 February 2010 (UTC)

There are theoretical physicists investigating the possible reformulations of gravity that follow something other than an inverse square law at large distances, but they haven't got anything particularly convincing yet. --Tango (talk) 22:43, 26 February 2010 (UTC)

That sentence was added anonymously with no edit comment and no cited source (in 2008!). I removed it. -- BenRG (talk) 23:48, 26 February 2010 (UTC)

It wouldn't be hard to write down a function that falls off like 1/r² for small r and 1/r for large r – for example, 1 / (r r_o tanh (r/r_o)). It's easy to modify Newton's laws along those lines and see what happens—it wouldn't cause accelerating expansion, though. Modifying the force law in general relativity is much harder because in GR the 1/r² falloff isn't an independent assumption. It's an inevitable consequence of, roughly speaking, the fact that the surface area of a sphere is 4πr². You could try making one dimension of space loop around itself (making a cylinder) at some large scale; then at even larger scales space would effectively have one less dimension and the falloff might (I'm not sure) become 1/r. But that kind of looping has already been searched for and ruled out. The cosmological constant is broadly similar to making space hyperbolic instead of flat. In hyperbolic space, the surface area of a sphere is 4π r r_o sinh (r/r_o) 4π r_o² sinh² (r/r_o), which would give you a falloff law like 1 / (r_o² sinh² (r/r_o)), which is approximately 1/r² for r << r_o but goes exponentially to zero for r >> r_o. The cosmological constant actually goes repulsive for large enough distances, so this isn't a perfect analogy, but it is the right idea. -- BenRG (talk) 00:34, 27 February 2010 (UTC)

Thanks für clearing that up for me. I figured that for curved space-time the laws of gravity change on large distances, but to r seemed a bit too simplistic.

That the cosmological constant is equivalent to a hyperbolic geometry of space-time is very helpful in picturing its effects. Maybe it should go into the article (or that on the cosmological constant). --Xsansara (talk) 06:03, 27 February 2010 (UTC)