Wikipedia:Reference desk/Archives/Science/2011 January 15

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January 15

The abundance of chemical elements in the planets of Solar System?

I would like a table of datas. Give reference if possible--125.214.246.65 (talk) 05:51, 15 January 2011 (UTC)

Does Abundance of the chemical elements adequately fit your needs? Dismas|^(talk) 06:39, 15 January 2011 (UTC)

Not yet, I want more data about specific planets.--125.214.246.65 (talk) 07:17, 15 January 2011 (UTC)

Well, the individual articles for the planets contain information on their composition. Dismas|^(talk) 07:40, 15 January 2011 (UTC)

Be careful with the data! Except earth all other planets are estimates. Deep drilling and seismic experiments are necessary to improve the estimates, but the deepest hole ever made on an other planet is 30cm deep and only the moon received seismometers. The spectroscopy of the surface gives only limited informations on the interior of differentiated body. --Stone (talk) 08:58, 15 January 2011 (UTC)

But some components shown in these articles were compounds, not elements.--125.214.246.65 (talk) 14:19, 15 January 2011 (UTC)

Compounds are combinations of elements. At least in some cases, the evidence cannot even distinguish a pure element from a compound containing it (if the test is for properties of the element/nucleus/atom rather than the overall form of it). And in other cases, the atoms aren't even in a stable elemental form (plasmas and such) even if they are conceptually the pure element (rather than a compound). DMacks (talk) 18:04, 15 January 2011 (UTC)

Hydrogen is probably the most abundant element in the solar system by mass, including metallic hydrogen. Little is known about the cores of ice giants (Uranus and Neptune), and they may in fact contain solid diamond [1]. ~AH1^(TCU) 02:32, 18 January 2011 (UTC)

Sulfuric Acid

Can sulfuric acid be made anhydrous with the use of anhydrous copper(II) sulfate as a dessicator? --Plasmic Physics (talk) 09:03, 15 January 2011 (UTC)

I have not directly searched for detailed information on the question, but as far as I can remember, anhydrous copper(II) sulfate is not commonly used as a desiccant and probably not a so powerful desiccant. Where do you intend to add the dessicant ? Directly in the bottle of the H₂SO₄ to "dry" or spread in a cup installed in a close dessicator beside another open recipient containing H₂SO₄ ? I do not advise to directly add the anhydrous copper sulfate, nor any other desiccant, into the sulfuric acid. It will likely partly dissolve into the sulfuric acid and contaminate it. What is the aim pursued in doing so? When asking a question to a chemist, it is always good to inform him about the pursued objective to receive a well founded advice and not only a technical and perhaps inadequate answer.

Safety: remember, concentrated H₂SO₄ and oleum are also strong oxidizers and can violently react with many chemicals. I remember to have transformed cellulose kleenex in black carbon char when trying to clean spills of concentrated sulfuric acid on the bench in the lab. Regards, Shinkolobwe (talk) 17:06, 15 January 2011 (UTC)

Look also at oleum production and at sulfur trioxide or disulfuric acid. Oleum was also historically produced from the distillation of iron sulfate. Similarly, it could perhaps be concentrated by distillation over a powerful desiccant such as anhydrous magnesium sulfate. Magnesium strongly retains 6 water molecules in its hydration crown. Shinkolobwe (talk) 17:40, 15 January 2011 (UTC)

Copper sulfate is used as a dessicant in a few specific types of organic-chemistry reactions (acetal formation for example). I don't know a measure of its absolute strength, but in my experience it's harder to dry copper sulfate than it is to dry other inorganic solid desiccant (molecular sieves, silica, or indicator-doped drierite). Sulfuric acid is a much more common a very strong dehydrating agent and dessicant for all sorts of materials. DMacks (talk) 18:00, 15 January 2011 (UTC)

In my opnion, the harder it is to dry a desiccant, the more effective it is. I chose the desiccant to be a sulfate, to prevent reactions taking place with the sulfuric acid, excluding decomposition. I want to try adding sulfuric acid over anhydrous copper sulfate, followed by vacuum filtration through a teflon buchner funnel. The end goal is to produce an acid with sufficient concentration to react with KNO3 to produce nitric acid for distallation in a retort.

I know I can simply buy the concentrated acid, but this is part of my process, I do things the hard way. Some people collect stamps, I collect chemicals, solids only, but sometimes I need a little liquid to make a solid. I am not a novice, I have studied chemistry both formally and informally for many years now, only two years at university though.

I distilled nitric acid in this way before, but I used 4.6 M sulfuric acid instead. This gave a dissapoiting 5 mL of pure nitric acid. I know it was nitric, because I spilled a single drop on the hot plate and it instantly rusted it, underneath the paintwork, causing it to bubble. --Plasmic Physics (talk) 21:20, 15 January 2011 (UTC)

If you are equipped with a distillation apparatus, why not to simply remove the water from the mid-concentrated sulfuric acid by distillation ? To remove water with a solid desiccant would require a large molar ratio desiccant/sulfuric acid and is not an effective technique. The vacuum pump will also require a protection trap to avoid to inadvertently aspire sulfuric acid or an aerosol into the costly pumping mechanism. Better also not to use a water trump directly connected to the buchner vessel to avoid a possible and hazardous backlash of water from the trump into the concentrated sulfuric acid. Take care. Just by curiosity: To what synthesis has to serve the nitric acid you need to produce ? Shinkolobwe (talk) 22:17, 15 January 2011 (UTC)

I tried distilling the sulfuric acid before, but the process gives off too much corrosive vapour. Is there a different way then, that excludes boiling off the water? I intend to produce bismuth nitrate, neodymium nitrate, and lanthanum nitrate. --Plasmic Physics (talk) 03:07, 16 January 2011 (UTC)

I'd really discourage anyone from the experiments I can imagine after reading this discussion. Because the actual question–seems to me–is how to achieve the goal without proper laboratory equipment and technique.
Anyways, feasible methods for making a concentrated sulfuric acid out of dilute sulfuric acid are: 1) distilling water off, 2) adding SO₃ or oleum.
Dessicants are used for removing of traces of water from the medium they're not migrating into. If the initial sulfuric acid in question is dilute, then by pure logic it isn't impossible to obtain a concentrated copper sulfate solution in relatively more concentrated sulfuric acid. But it would take unreasonable amounts of copper sulfate and the result will definitely not be anhydrous.
On the other hand, it doesn't take 100% H₂SO₄ to displace nitrates in the desired fashion. Dynamic equilibrium will produce more of HNO₃ being steadily removed by distillation in the retort. Legate of Skai (talk) 14:20, 17 January 2011 (UTC)

The problem with copper sulfate (or sulfates in general) is the basicity of the SO₄^2- ion. With H₂SO₄ being a strong acid, it's very likely that the H₂SO₄ will simply protonate the SO₄^2- ion to form 2 HSO₄^- ions (hydrogen sulfate), so basically you will kind of neutralize your sulfuric acid instead of concentrating it (for the same reason you can't use NaOH/CaO to dry H₂SO₄ even though those are also quite hygroscopic. You'll get a nice explosion though with sulfuric acid splattering around). The only possibility of drying it chemically which comes to my mind would be using an acidic drying agent such as phosphorous pentoxide, but I have no idea how to seperate it from the acid afterwards. This leaves the addition of SO₃ but this is 1) difficult to produce and 2) difficult to handle. Setting up a contact process at home just to get some better yield of conc. HNO₃ is slightly overkilling it IMO.--178.26.171.11 (talk) 19:47, 19 January 2011 (UTC)

OK, anhydrous is ideal, but not required.

Legate of Skai: What if the same amount of copper sulfate is reused? Add anhydrous copper sulfate to the sulfuric acid untill the hydrated copper sulfate precipitates. Filter out and dry the ppt so that it may be reused. Repeat untill maximum concentration is reached.

178.26.171.11: I don't understand how copper sulfate would neutralise sulfuric acid. Even though I have a chemical vendor at my disposal, it is key that I source my reagents myself; phosphorus pentoxide is even harder to obtain than quality sulfuric acid.

What about cold distillation? Cool the acid solution untill a crust of ice forms, then remove this. Even though the crust contains a fraction of the sulfuric acid, the concentration is different compared to the solution. Repeating this process should gradually change the acid solution concentration. --Plasmic Physics (talk) 09:26, 20 January 2011 (UTC)

It works like that: H₂SO₄ (sulfuric acid) can give away 2 protons. The first stage, where sulfuric acid gives off one proton, yields the HSO₄^- (hydrogen sulfate or bisulfate) ion (here with water as the base):

$\mathrm {H_{2}SO_{4}+H_{2}O\rightleftharpoons HSO_{4}^{-}+H_{3}O^{+}}$

_{Chemical equilibrium on the far right}

The pK_a of that reaction is very low (-3), making H₂SO₄ a strong acid. In the second step, another proton can be removed from the remaining hydrogen sulfate ion:

$\mathrm {HSO_{4}^{-}+H_{2}O\rightleftharpoons SO_{4}^{2-}+H_{3}O^{+}}$

_{Chemical equilibrium on the right at normal pH, rather on the left at low pH}

Here, the pK_a is 1.9, therefore HSO₄^- is of medium to high acidity, making the sulfate (SO₄^2-) ion a weak to medium corresponding base. And if a molecule of sulfuric acid and a sulfate ion meet, following neutralization (or acid-base) reaction will happen:

$\mathrm {H_{2}SO_{4}+SO_{4}^{2-}\rightleftharpoons 2\ HSO_{4}^{-}}$

_{Chemical equilibrium on the far right}

Therefore, if you add copper sulfate to concentrated sulfuric acid, you'll get a solution of copper hydrogen sulfate in sulfuric acid, not quite what you want.

The freezing method will not work too well because the sulfuric acid doesn't have much of a tendency to separate from water (very endothermic) so you probably won't get much of a concentration change between liquid and solid phase and your yield will quickly go towards zero if you have to repeat the process multiple times for just a small bit of separation. This is all assuming you want to get close to 100% sulfuric acid.

But if you just want to concentrate sulfuric acid to let's say 90% (should be enough for your purpose of getting HNO₃), simply heat it and evaporate away the water just until SO₃ fumes start to form (around 300°C). Needless to say that this is quite dangerous because 1) oven-hot H₂SO₄ might splatter around because with rising concentration, viscosity will also rise and change the boiling behaviour and 2) SO₃ fumes are extremely corrosive and toxic, but I just AGF right now and believe that you know what you're doing.

The acid obtained that way should be able to let you distill HNO₃ out of your nitrate salt/sulfuric acid mixture. Just make sure your nitrate salt is dry and doesn't contain crystallization water which besides lowering the yield could also violently react with the sulfuric acid on mixing. KNO₃ is probably the best choice, NaNO₃ is rather hygroscopic and should be freshly dried (by heating to ~200°C) before usage.

Another more "elegant" method would be using barium nitrate and a slight excess of the dilute(!) sulfuric acid which you already have. This will form unsoluble barium sulfate and you can simply filter (a glass wool funnel + vacuum pump would be the best choice) and then distill the nitric acid. 178.26.171.11 (talk) 18:19, 20 January 2011 (UTC)

Thank you, that brings me to my next problem: When I reacted the mixture of sulfuric acid and potassium nitrate, it produced a solid cake of potassium bisulfate which was very hard to remove by mechanical means. Is there a chemical that should break it up, like sodium hexametaphosphate? --Plasmic Physics (talk) 04:52, 21 January 2011 (UTC)

Oh, sorry for the late reply but I couldn't find the discussion easily any more after it was archived. But to answer your question: what about H₂O? According to our article, the solubility is very high. BTW, after dissolving the bisulfate you can recycle it by re-evaporating the water and heat the bisulfate to very high temperatures to make it decay into potassium disulfate + water first and on further heating to potassium sulfate and sulfur trioxide with which you could concentrate your sulfuric acid. 178.26.171.11 (talk) 00:55, 25 January 2011 (UTC)

I tried to disolve it in water, but there was no change in volume of ppt. The cake consisted of micro crystals which, precipitated during distillation. I don't think that I would have enogh potassium bisulfate to recycle into acid.

Is it expected for and acid resistant, round bottom flask to be attacked during this type of reaction? My initial setup included a round bottom flask and a graham condenser, now a retort. After the first use according to this experiment, the flask was observed to have lost 2 mm of its thickness near the bottom. This neww weakness, caused the flask to break when an attempt was made to mechanically destroy the cake. --Plasmic Physics (talk) 06:17, 25 January 2011 (UTC)

Hmm, that seems strange to me... because actually the bisulfate should readily and exothermically dissolve in water, even more so if the crystals are small (higher overall surface area which leads to higher reaction speed). Was the potassium nitrate maybe contamined with other cations, such as calcium or barium? To test it simply dissolve a small amount of the salt in water and add a drop of sulfuric acid - if the KNO₃ was "clean" (i.e. free of heavy earth alkaline metals, lead or any other cations that form insoluble sulfate salts) then there should be no ppt.

Furthermore the glass shouldn't have been attacked by sulfuric or nitric acid, you can even store very strong and oxidizing acids such as aqua regia or perchloric acid in glassware. (Strong bases, on the other hand, do attack glass, especially at elevated temperatures) Out of my head the only acid that attacks glass is HF but I don't think that your nitrate was contamined with fluoride (that stuff, especially in its protonated form HF, is highly toxic) so, assuming that you didn't heat the glass to extremely high temperatures (>800°C), I have no idea how this could've happened. So that leaves the question: Is your potassium nitrate pure? Where did you get it from? The only explanation I could think of is that you used toothpaste as a KNO₃ source, this would explain both the unsoluble cake (calcium fluoride) and the etched glassware (hydrogen fluoride). 178.26.171.11 (talk) 22:58, 26 January 2011 (UTC)

Good theory, why didn't I think about it? I used fertilizer grade potassium nitrate, obtained from a hydroponics retailer. What's the chance of this type of potassium nitrate being contaminated with fluoride salts? Could it be purified using recrystalisation? --Plasmic Physics (talk) 04:13, 27 January 2011 (UTC)

OK, but somehow I doubt that any fertilizer bought in a developed country has fluorides in it because of their toxicity and you don't want them in your food later on. Unless your source is dubious of course. I'm not from the US so I don't know about "hydroponics". What does it say on the label, something like "100% potassium nitrate" or is it a mixture? At least here in Germany, you usually only get mixtures instead of pure nitrates, government paranoia might play a role... Did you try to dissolve some of the fertilizer in pure water, and did it work? 178.26.171.11 (talk) 18:00, 27 January 2011 (UTC)

It was obtained by the kilogram, from a bulk dispenser plainly labeled pottassium nitrate. Here in New Zealand, people aren't overly concerned with terrorism; I also managed to obtain ammonium nitrate from the same retailer. I have not tried dissolving it just yet, I'm away from my lab for the time being. --Plasmic Physics (talk) 21:17, 27 January 2011 (UTC)

slingshot effect & Manned Space Travel

How fast can a spacecraft travel using the slingshot effect ? Because I would like to know if the slingshot effect be used for (a) manned space missions around the solar system and (b) to reach other systms ?Scotius (talk) 13:07, 15 January 2011 (UTC)

The slingshot effect is currently an essential part of any travel to the outer planets; see Interplanetary spaceflight#Economical travel techniques. The slingshot effect can change a spacecraft's velocity by up to twice the planet's orbital speed. The orbital speed of Mars, for example, is about 24 km/s. The main disadvantage of the slingshot effect is that there usually aren't planets in an appropriate position for getting to where you want to go.

Manned interstellar travel would require speeds so fast (like a tenth of the speed of light) that the slingshot effect wouldn't be of significant value; see Interstellar travel#Proposed methods of interstellar travel. Red Act (talk) 15:40, 15 January 2011 (UTC)

Eating cold foods

My mom thinks it's bad to eat cold foods such as ice cream on an empty stomach or cold dishes that have not been reheated. I doubt that most people reheat their dishes to what is needed to kill pathogens. Imagine Reason (talk) 14:03, 15 January 2011 (UTC)

Not sure what your question is, but it is certainly true that cooked food should be thoroughly reheated before eating. Very cold food and drink is not good for your stomach, at least according to Chinese medicine, but this is the science desk (ducks)...--Shantavira|^{feed me} 15:02, 15 January 2011 (UTC)

"certainly true that cooked food should be thoroughly reheated before eating". Not so certain, I think, as I sit here and type following the old habit from college days of eating cold pizza for breakfast. 75.41.110.200 (talk) 15:48, 15 January 2011 (UTC)

Reheating will only kill pathogens if you heat the food to somewhere near the boiling point and keep it there for 10 minutes. But that's only necessary if the food has been sitting around for long enough to develop significant levels of bacterial contamination. Looie496 (talk) 17:44, 15 January 2011 (UTC)

Very dubious, since many (most?) prepared foods that people enjoy cold are cooked at some point in their preparation. Even ice cream (traditionally prepared) is cooked before it's frozen. Would you suggest "thouroughly reheating" my cookies & cream? (Even those cookies were once cooked!) Staecker (talk) 21:21, 15 January 2011 (UTC)

I doubt that ice cream is still frozen when it reaches the stomach. And if it is, it certainly won't keep frozen for a long time. Quest09 (talk) 22:34, 15 January 2011 (UTC)

Please ask her as to whom she heard this from – (and report back here). It may be due to an old wives' tale based on and accurate observation but one which got cause and effect arse about face. Rather than me attempt to explain why, the article pagophagia will indicate why your doctor asks people she suspects of having iron deficiency anaemia how often they eat ice-cream/ eat ice cubes etc. An' remember: Mom may not always be right but she is never, ever wrong! --Aspro (talk) 22:53, 15 January 2011 (UTC)

The point is probably to prevent the cold food from lowering the core body temperature and not to kill any pathogens (why microwave your ice cream?). There could then be a correlation between body temperature and the immune system etc. but any such effect is likely to be slight. ~AH1^(TCU) 02:23, 18 January 2011 (UTC)

If I recall correctly from my SafeServ training the requirement for reheating is 165* F internal temperature, a far cry from boiling, but still fairly hot. According to the US FDA that would kill most bacteria in seconds [[. There were also rules for holding at a temperature lower than that, around 140, for a longer period, if I recall but it's been long enough I've forgotten. Remember 40-140* F is the danger zone, food kept in that range at all (including while it cools)

Yes it's true that most foods served cold are cooked at some point, however it's key to note that a factory making food products or a good chef takes steps to limit potential bacteria growth. 40-140* F is the danger zone, the temperatures that allow for the best possible bacterial growth, and the longer food lingers there the greater the risk. Foods cooked then cooled are usually best stored in a manner that prevents bacteria from entering and cooled quickly as practical (using icewater baths around the bowl, for instance). Leftovers are best kept hot and then cooled quickly. Heating to 165*F (a far cry from boiling but pretty darn hot) should kill most bacteria instantly according to the FDA, so why take a chance, heat leftovers well and you'll be far better off for it, even if the risk is small, it's not absent altogether. 65.29.47.55 (talk) 03:09, 19 January 2011 (UTC)

Half-life

According to half-life, will a given substance eventually decay into nothing? I know the conservation of energy, it will be in the form of energy, but will the substance cease to exist at some point? I believe this is true according to the 2nd law of thermodynamics and heat death, but I just want to confirm it. Also does all matter have a half-life? ScienceApe (talk) 14:59, 15 January 2011 (UTC)

Half-life is usually referring to radioactive decay. If a radioactive nucleus decays, it will split into decay products - usually one or two smaller nuclei, plus a number of elementary particles and some energy in the form of photons. So it will not decay into "nothing". Stable nuclei do do not decay on normal time scales. In particular, it's an open question if H⁺ (the proton) decays at all. If it does, the ultimate end product is electromagnetic radiation. --Stephan Schulz (talk) 15:11, 15 January 2011 (UTC)

Yeah I know it doesn't decay into nothing, that's why I said it will be in the form of energy, but... If I were to use an example, you have a block of iron. Will the iron at some point cease to exist as iron? Will at some point, it will exist entirely as photons? ScienceApe (talk) 17:32, 15 January 2011 (UTC)

According to Proton#Stability, the decay product of a proton is still a particle not just radiation (see also Proton decay for discussion of theoretical possibilities, including that it might actually be stable). A sample of the element iron is not going to change into "not iron"--it is composed of several isotopes, of which ~94% of the material is indefinitely stable, the remaining ~6% is usually called "stable" too, but technically is an isotope that decays to chromium with a half-life of 3x10²² years. DMacks (talk) 17:43, 15 January 2011 (UTC)

Hmm, but the first possibility mentioned is that it decays into a neutral pion and a positron. The pion would most likely decay into two photons; the positron could meet up with an orphaned electron. So at a first glance it appears you could still wind up with just radiation. (Maybe the probability of positron/electron encounters goes down so fast, in the increasingly empty universe, that that reaction never goes to completion, not sure about that.) --Trovatore (talk) 21:19, 15 January 2011 (UTC)

So a chunk of iron floating in space will always exist? I thought according to heat death, all the matter in the universe will eventually decay into photons? ScienceApe (talk) 18:48, 15 January 2011 (UTC)

No. In the standard model, conservation laws such as that of the baryon number prevent ordinary matter from turning entirely into photons. There is nothing in the concept of heat death that requires everything to become photons -- it just says that the universe will end up with the highest possible entropy; states that are not possible to reach do not count even if they would have had even higher entropy. –Henning Makholm (talk) 21:10, 15 January 2011 (UTC)

I seem to recall that in an old revision of the heat death article it stated that only the "lowly photon" is what will remain of the universe or something like that. I guess it was wrong. ScienceApe (talk) 21:22, 15 January 2011 (UTC)

The hunk of iron is space may eventually fall into a star, or black hole, and suffer the consequences of that. The black hole could then give out Hawking radiation. An alternative is that the future von Neumann machines will find the iron and covert it to one more of itself. Graeme Bartlett (talk) 21:42, 15 January 2011 (UTC)

"Lowly photon" will happen only if protons do actually decay. But there is currently no evidence for that, only some theoretical ideas. Ariel. (talk) 00:08, 16 January 2011 (UTC)

The entire universe can make a transition to a state of exact supersymmetry. Count Iblis (talk) 01:56, 16 January 2011 (UTC)

So if proton decay is in fact true, then it's safe to say that everything eventually should evaporate into photons at some point? ScienceApe (talk) 05:33, 16 January 2011 (UTC)

No, neutrinos and electrons will stick around. Ariel. (talk) 18:44, 16 January 2011 (UTC)

Radioactive decay typically ends at the stable isotope, but proton decay is theorized to occur at 10³³ years. ~AH1^(TCU) 02:21, 18 January 2011 (UTC)

RADIAL TYRES

Why radial tyres are called so.. I know they are different from Bias tyres as radial tyres has steel wires and Bias tyres have nylone tyres . . but I don't understand what it has to do with the word "RADIAL". I also want to know what are radial tubes.. — Preceding unsigned comment added by Amit.meer (talk • contribs) 15:44, 15 January 2011 (UTC)

The choice of materials is a separate issue from the terms you are asking about. We have an article cleverly titled "radial tyre", which discusses the geometric construction difference between the bias and radial reinforcement types. I'm not sure what the specific term "radial tube" means. DMacks (talk) 17:31, 15 January 2011 (UTC)

(edit conflict) Radial tire explains that they are called radial because of the radial direction of a layer of cords for reinforcement. According to the article, radial tires have radial ply AND bias plies, while bias tires only have bias ply. Though radial tires usually incorporate steel reinforcement, the term seems to apply to radial ply nylon-reinforced tires as well. SemanticMantis (talk) 17:38, 15 January 2011 (UTC)

Momentum

Why is momentum abbreviated p? I realize that m is traditionally used for mass but some letters are used to abbreviate more than one thing, and why was p in particular chosen, rather than any other letter? Merci .24.92.70.160 (talk) 17:55, 15 January 2011 (UTC)

I think its pretty arbitrary, but as you note you COULDN'T use "m" for momentum, since "m" stands for mass and mass and momentum often appear in the same equations. If you find "m" being used for something else, it would be in some other system which did NOT use mass. There are other arbitrary letter associations as well, for example "q" for "heat energy" and "U" for "internal energy" and things like that. I assume that the people who chose them first usually try to pick a letter which makes sense (like "F" for force), but in places where there is likely to be conflict, they may just choose something completely random and unlikely to be confused with other symbols. --Jayron32 20:16, 15 January 2011 (UTC)

In some languages (such as German or Danish), momentum is called "Impuls". There's a p there right at the beginning of the stressed syllable. In pre-Newtonian mechanics and kinematics, the word of choice for the concept seems to have been "impetus". –Henning Makholm (talk) 21:21, 15 January 2011 (UTC)

Just an observation and speculation: Sommerfeld, in vol. 1 (Mechanics) of his lectures on theoretical physics, uses G in all the chapters on the Newtonian formulation of mechanics and only switches to p when he gets to the Lagrangian and Hamiltonian formulations. Is it possible, that p has been introduced because it makes a nice symmetrical pair with q? Similarly, G could have been used because it follows F, making $F={\dot {G}}$ a pretty formula. One would have to look through older books of mechanics to see what was actually used there. --Wrongfilter (talk) 11:55, 16 January 2011 (UTC)

Power to weight ratio

I'm a little unclear on this. Is having a high power to weight ratio good for heavy vehicles? Like if an engine has a low power to weight ratio it might be ok on a light vehicle, but it would be terrible on a heavy vehicle? ScienceApe (talk) 18:46, 15 January 2011 (UTC)

Define "good" and "terrible"? What parameter are you measuring? Fuel consumption? Acceleration? Towing/loading capability? Engine and drive-train wear and tear? A lower-power engine might use less fuel (depending on the gearing and how much you need to stomp on the accelerator pedal), and could put less strain on your drive-train simply by not being very powerful. On the other hand a more powerful might use more fuel to the advantage of loading capability and acceleration. Paradoxically a more powerful engine could reduce drive-train wear and even fuel consumption simply by having the luxury of longer gearing. But we need to knowwhat you're after to properly answer the question. Zunaid 19:25, 15 January 2011 (UTC)

Here's the article, of course: power-to-weight ratio. I'd draw your attention to "Different designs trade off power-to-weight ratio to increase comfort, cargo space, fuel economy, emissions control, energy security and endurance." Perhaps ignore the "energy security" part - I have no idea what that's trying to say - but "fuel economy" sounds particularly important.

• Pointlessly heavy engine: bad.
• Engine which is heavier to yield some benefit besides power: good.
• Engine which, heavy or light, leaves the vehicle underpowered: bad.

On the whole I think the power-to-weight ratio of the engine matters less in heavy vehicles, like trucks, since they would be hauling a cargo anyway (perhaps a cargo of engines?) - it matters a lot if the vehicle is relying on lightness for performance, where the weight of the engine accounts for a large part of the total weight of the vehicle. 213.122.57.166 (talk) 19:40, 15 January 2011 (UTC)

I guess the problem is I don't really understand what the ratio means (I read the article but I don't understand it). So if a vehicle has a high power to weight ratio does that mean it has high acceleration? Does it have high velocity? If you have a car and a heavy truck that both use the same exact engine with the same power to weight ratio, what will the performance be like on the car and the heavy truck? ScienceApe (talk) 21:20, 15 January 2011 (UTC)

I think you might be confused by the fact that the term "power to weight ratio" is used both to describe engines and vehicles. When it is used in reference to an engine, it is a ratio of the engine weight to its power (Engine weight/Power). When it is used in reference to a vehicle it is a ratio of vehicle weight (including the engine) to power (Vehicle weight/Power). Engine power to weight is used to compare the power density of propulsion units and doesn't directly relate to a individual vehicle's performance. Vehicle power to weight is part of determines a vehicles acceleration. If you have a car and a heavy truck with identical (vehicle) power to weight ratios they will likely have similar performance (compare a 2000kg car with a 200 hp engine to a 4000kg truck with a 400 hp engine), but it wont be identical. Power to weight ratio is only a rough estimate of actual real world performance for a number of reasons. For one thing the "power" figure is the peak output of the engine, the output varies depending on the engines RPM (some engines produce a broad band of power over the RPM range some do not). Other factors effect acceleration and top speed, such as aerodynamics, rolling resistance and drive train efficiency. --Leivick (talk) 21:34, 15 January 2011 (UTC)

As another example:Back in the days when engines where expensive to replace, a diesel engine locomotive would have big heavy slow revving engines (say 500 hp) which would last tens of thousands of hours between major overhauls, whilst piston engined fighter aircraftwith high revving (say 500 hp) engines would be much lighter but need rebuilding after only about 500 hours. So the power to weight ratio was an rough and ready way to guess what the engine was best suited for -but not very scientific, as it was dependant on how well the engine was manufactured. For instance: during the second world war, the British government gave the USA all the Rolls Royce technology, because their engine had such superior power to weight ratios and durability. However, US did not machines the components to Imperial inches and that meant the parts were not interchangeable.--Aspro (talk) 23:21, 15 January 2011 (UTC)

I'm curious, Aspro. I know that there is a US survey inch, but I can't imagine manufacturers using this rare "inch", nor can I imagine Americans using metric units at that time. Were American machine tools just less accurate? Dbfirs 09:38, 16 January 2011 (UTC)

The inch the US was using during World War II was what we would now call the "survey inch" (100⁄3937 m), and it was different to to the Imperial inch in use in the UK (approx. 0.9143969⁄36 m). But the Imperial inch was not a stable length standard: it was getting shorter by about 1 ppm every 23 years. I don't know for sure, but I can imagine a problem if Rolls Royce were saying that components had such-and-such dimensions in inches, but were actually machined on tools that were calibrated in metric units (as the metric calibration was already known to be more stable). Physchim62 (talk) 12:52, 16 January 2011 (UTC)

Ah, thanks. I hadn't realised that it was not until 1959 that "inch" meant the same on both sides of the pond. I wouldn't have thought that Rolls Royce would be metric at that time, but I can imagine that engineers adjusted measurements depending on the wear on particular machine tools. Dbfirs 15:46, 16 January 2011 (UTC)

Particle decay

When a particle decays into other particles, it's said that the new particles were created, and that the old particles are destroyed. What observable differences would there be if the original particle was composed of the particles it decayed into? 74.15.137.130 (talk) 19:20, 15 January 2011 (UTC)

Does the article Particle decay answer your questions? --Jayron32 20:11, 15 January 2011 (UTC)

It doesn't seem really enlightening to me, I'm afraid. A possible answer to the original question would be that if the muon was a composite of the electron, antineutrino and mu-neutrino that it decays into, then it sould be possible to understand the difference between the magnetic moment of a muon and an electron as an effect of rotational and/or vibrational effects within the composite muon. In particular, there would be a spin-orbit interaction between the electron's intrinsic spin and its movement within the muon, which would give rise to several different kinds of muons with slightly different masses (and probably other properties, such as decay rate). Such a difference ought to be observable but hasn't actually been observed. –Henning Makholm (talk) 22:04, 15 January 2011 (UTC)

Is that how they discovered that the proton wasn't fundamental? And is the total rest mass in a decay constant? 74.15.137.130 (talk) 22:11, 15 January 2011 (UTC)

No, that was discovered by deep inelastic scattering i.e. shoot small somethings at neutrons and see how they bounce. The total mass - not rest mass - is conserved. To answer your original question, there wouldn't be much difference. Particles are interchangeable and indistinguishable, so there is no difference between "destroy particle, and create a new one" and "keep the original particle". Ariel. (talk) 00:13, 16 January 2011 (UTC)

Many unstable particles can decay into more than one different possible combination of byproducts. Dauto (talk) 06:12, 16 January 2011 (UTC)

Melting and snowing?

It must be possible to do so at the same time. Cause it's doing it right now. How exactly is it possible?

The temperature is around 35-36 degrees Fahrenheit, no wind, and it's a bit cloudy. Hope that info helps. Crimsonraptor | (Contact me) Dumpster dive if you must 20:45, 15 January 2011 (UTC)

Happens all the time. I don't see any reason why it couldn't be cooler higher up in the air and warmer on the ground. Clarityfiend (talk) 20:58, 15 January 2011 (UTC)

It is possible because it is colder up in the clouds where the snow crystallizes. Looie496 (talk) 21:04, 15 January 2011 (UTC)

(Edit conflicts) The ground in winter is typically a little warmer than the air above it - see Heat_pump#Ground_source_heat_pumps - and the air at the height at which the snow forms (possible several thousand feet up) is generally cooler than the air near the ground - see Troposphere#Temperature - so the snow has formed where it is below freezing and is dropping into an environment where it is a little above freezing. 87.81.230.195 (talk) 21:05, 15 January 2011 (UTC)

See also Rain and snow mixed Pfly (talk) 11:23, 16 January 2011 (UTC)

Snow at above freezing temperatures. When the ground is colder than the air above it (thermal inversion), black ice or freezing rain can occur. ~AH1^(TCU) 02:17, 18 January 2011 (UTC)

Sharks living in inland seas?

Are there sharks that live in The Black Sea or Crimean Sea? This is a question ive tried searching for the answer to,with no definitive results. my email is <redacted> —Preceding unsigned comment added by 208.98.220.141 (talk) 22:14, 15 January 2011 (UTC)
Contact information removed, all responses to your question will be posted here. WikiDao ☯ 23:43, 15 January 2011 (UTC)

There is the spiny dogfish, called the Black Sea shark. --Cookatoo.ergo.ZooM (talk) 23:10, 15 January 2011 (UTC)

Also, in the Black Sea, the common thresher shark (Alopias vulpinus) and the angelshark (Squatina squatina). Ghmyrtle (talk) 23:55, 15 January 2011 (UTC)