Wikipedia:Reference desk/Archives/Science/2010 December 9

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December 9

Math problem

I'd ask this at the Math reference desk, but I'll ask it here for two reasons:

1. The math reference desk is stagnant and

2. It's probably not that a difficult problem.

The problem states:

Your chores for the week are to cut the grass, wash the car, clean your room, clean the garage, and shine your shoes. You are to do 1 chore each day from Monday through Friday. You can do a chore on whatever day you want, except that you must wash the car either Thursday or Friday. In how many different ways can you perform the chores?

To solve the problem, I'd take 4!, 4!, 4!, 4!, (the days of the week for the 4 chores) and 1! (the days of the week for the other chore), and add those together to get 17!. Then, I'd take that 17! and divide it by the four 4!s for something like:

17×16×15×14×13×12×11×10×9×8×7×6×5×4×3×2×1

---

4×3×2×1×4×3×2×1×4×3×2×1×4×3×2×1

And would end up with 1,072,071,000 different ways. That seems a bit excessive. Where, in my work, did I go wrong? Thanks in advance, Albacore (talk) 01:42, 9 December 2010 (UTC)

It's only 2 × 4! = 48. There are two choices for when to wash the car, then for each, four choices of when to cut the grass; for each combo three choices for when to clean the room, and so on. The reasoning underlying your solution is completely opaque to me. Looie496 (talk) 01:54, 9 December 2010 (UTC)

I concur: 48 possibilities. On Monday through Wednesday, you can't wash the car, so you have only 4 chores available. That means 4 options on Monday; 3 on Tuesday; 2 options on Wednesday. On Thursday you can either wash the car or pick whatever other chore you haven't done yet - so 2 choices; and on Friday, you have no options left. So, 4×3×2×2×1 = 48. Nimur (talk) 01:56, 9 December 2010 (UTC)

I also agree with 48, but I'm curious as to why you think the Math Desk is stagnant. I just came from posting a rather elaborate answer there, source code included, so I can assure you that posts there do get attention. StuRat (talk) 08:03, 9 December 2010 (UTC)

Regarding your reasoning: you cannot add factorials in the way that you do. Consider:

$5=1+1+1+1+1=1!+1!+1!+1!+1!=5!=120$

which I think is false for obvious reasons. There are also not 4! possibilities for each day (other than Thursday/Friday). There are 4 possibilities for the first day; for each of those four, there are 3 possibilities for the second day, and so on. Repeating this for all days is why you use the factorial. The mistakes you made suggest that you need to spend a lot more time practicing combinatorics problems. --superioridad ^(discusión) 01:10, 10 December 2010 (UTC)

Albacore, I think you've over-thought this and abandoned your intuition in favour of applying a method you don't quite understand. It's good that you reality-checked your answer and spotted it was way off: well done for that. I really feel you need to look back at the earlier, easier examples and explanations in your book/notes, and remember how the reasoning works! Maybe do a few problems without using the factorial symbol or button, to remind yourself what numbers you're actually using and why. Then, start seeing where you can use the factorial to speed things up. 86.161.208.185 (talk) 01:52, 12 December 2010 (UTC)

Reproduction and rate of erythrovirus B19

Hello everyone. I've been looking around for how Parvovirus B19, the virus that causes fifth Disease, reproduces, and at what rate. Nothing has come of. Do any of you happen to know? Buggie111 (talk) 03:29, 9 December 2010 (UTC)

This recent review article suggests that the cell cycle of the virus is unresolved. However, there is a section on replication and there are a number of references that you could track down to find out what is known about it. --- Medical geneticist (talk) 14:30, 9 December 2010 (UTC)

Potential and kinetic energy

Gravitational potential energy depends on the reference frame or base against which you measure h the height int he formula U=mgh. However you can choose a reference frame arbitrarily so if I were to drop a rock into a river 20 metres below me (pretend I'm on a bridge) rather than choose the surface of the water as is the logical choice I could in theory choose the point where I let go as the base and get a negative potential energy, but this is nonsense. I could also choose the airplane flying above me or the Great Wall of China, for that matter, and get equally nonsense results! It obviously doesn't make sense that a real quantity changes just because you change something on a piece of paper, but that seems to be what the formula implies! What is wrong? 24.92.84.83 (talk) 03:37, 9 December 2010 (UTC)

The sign of the energy is arbitrary to your perspective. If you chose the surface of the water as the "zero point" and thus have "negative energy" compared to choosing the bridge as the reference point, then all of your measurements would be sign-flipped, and the end results would be the same. Its even more arbitrary than that; absolute energy is somewhat meaningless as well, what you are usually measuring is ΔE, not E. What that means is that once you define the "zero point" for your coordinate system, all measurements become relative to that, and ΔE is invarient in absolute value, and the sign is arbitrary, but consistant across whatever system you use. For example, lets say you define the airplane overhead as your reference point. Now, the two points against which you measure the potential energy of the rock are a) the bridge and b) the ground below it. The potential energy is still calculated against that distance; so even if neither of those two points is defined as "zero potential energy", the value of ΔE is the same in all reference systems. Even more significant, the various laws of conservation of mass/energy apply even relativisticly, so that the total ΔE measured by raising or dropping the rock is the same regardless of your relative velocity, etc. --Jayron32 03:59, 9 December 2010 (UTC)

Potential energy can indeed be negative. Potential energy, like many other concepts in physics, may be viewed as a way to describe a system. Whether it's a "real" quantity is ultimately a matter of definition but it's certainly a very practical quantity for making computations of real events, for example predicting how objects will move in relation to a given point when something is done to them. By the way, the strength of a gravitational field varies with location, meaning that g in mgh is not a constant, so the formula mgh is an approximation. It works well for a small bridge on a large planet, but not in some other cases. PrimeHunter (talk) 04:10, 9 December 2010 (UTC)

In real calculations on energy, only the change in potential energy is used (as Jayron mentioned above), so the starting point is irrelevant. The same applies to temperature, where zero can be taken to be any point at or above −459.67°F. Dbfirs 08:44, 9 December 2010 (UTC)

Let's imagine there is an object of weight W newtons on the floor, and I am using the floor as the datum for determining potential energy PE. The object on the floor has potential energy of zero. Now I raise this object above my head to a height of 2 metres above the floor so that its potential energy is 2W Joules. The change in potential energy is:

PE2 — PE1 = 2W — 0 = 2W

So in the course of raising this object above my head its potential energy has changed by 2W. This is positive so its potential energy has increased.

Now let's imagine I am using the ceiling as the datum for determining potential energy. The ceiling is 3 metres above the floor. The object on the floor has potential energy of W(— 3) or — 3W Joules. When I raise this object 2 metres above the floor it is 1 metre below the ceiling so its potential energy is W(— 1) or — W Joules. The change in potential energy is:

PE2 — PE1 = — W — (— 3W) = — W + 3W = 2W

Despite the different choice of datum the change in potential energy is still an increase of 2W. Dolphin (t) 11:34, 9 December 2010 (UTC)

The OP is correct when he says "It obviously doesn't make sense that a real quantity changes just because you change something on a piece of paper". The natural answer to his question "What is wrong" is that potential energy isn't a "real quantity". Potentential energy cannot be measured. Only changes in potential energy can be measured. For advanced readers: The answers in this thread are knowingly or otherwise ignoring any general relativistic implications which make that question much trickier than those answers here would let one on. 71.101.41.73 (talk) 16:16, 11 December 2010 (UTC)

technology run wild stories

I just got an email from a friend linking a fox news report about how credit cards with RFID now make it so easy now to be ripped off by someone simply walking past you and scanning your CC with a RFID reader in the street or a crowd. And surprise! The guy promoting this scare story runs a company that sells wallets which are "scan proof" *sigh*. So I'm working on a blog post about how this is just another "technology run wild" story, and I'm using microwaves and mobile phones and the internet all as examples of technology that was going to "ruin the world" and it got me thinking, I'm quite certain that TV and probably even radio would have had the same type of technophobe movements, but that was before my time, even the printing press was going to ruin the world wasn't it? I wonder if anyone knows about any references relating to this? Stick to my question please, i don't need any more other suggestions for my story, it's pretty much done, this is just the extremely concise version. Vespine (talk) 03:49, 9 December 2010 (UTC)

See Luddite. —Bkell (talk) 04:05, 9 December 2010 (UTC)

Wow that's totally one of those things I SHOULD have known.. I sort of knew how the word Luddite was used, but had no idea how specifically it applied to exactly this situation, great ref thanks.Vespine (talk) 04:52, 9 December 2010 (UTC)

And the form of the word "neo-Luddite" is often applied to modern technophobes. StuRat (talk) 07:57, 9 December 2010 (UTC)

Earliest driven wheels

When were drive wheels or driving wheels invented? In other words, what was the historically first application of applying force to the wheels of a vehicle to provide the necessary propulsion (rather than, say, pulling the vehicle with a horse)? —Bkell (talk) 04:17, 9 December 2010 (UTC)

I thought it might have been a bicycle, but the first mechanically-driven bicycle was invented by Kirkpatrick Macmillan in 1839. Previous bicycles were basically scooters; they moved by pushing your feet against the ground. The first working model of a powered locomotive was built by either by John Fitch in 1794 or by Richard Trevithick in 1802-1804. Since the bicycle is the simplest human-powered drive wheel, and the locomotive the earliest (apparently) engine-powered drive wheel, my bet is on either Fitch's of Trevithick's locomotive. --Jayron32 04:50, 9 December 2010 (UTC)

• Before railway locomotives there was Cugnot's steam carriage. That was 1769. --Anonymous, 21:39 UTC, December 11, 2010.

Wow! Locomotives predate pedal bicycles! I wouldn't have guessed... Vespine (talk) 05:26, 9 December 2010 (UTC)

I wouldn't have either. But History of the bicycle confirms that. --Jayron32 06:49, 9 December 2010 (UTC)

Hmm, I hadn't thought of bicycles. Thanks. Tricycle#History mentions a tricycle made in 1680 that was powered by hand cranks—I assume that must mean it had a driven wheel. —Bkell (talk) 06:59, 9 December 2010 (UTC)

If you include toys, you might find older spring-powered wheel-driven vehicles. StuRat (talk) 07:53, 9 December 2010 (UTC)

I'd be interested in hearing about such toys, too, if anyone can find references to early examples. —Bkell (talk) 08:05, 9 December 2010 (UTC)

The idea of having driven wheels dates to long before that. See the Italian physician Guido da Vigevano designs, which made use of cranks and gear wheels. Crank_(mechanism)#Middle_Ages Maybe the Romans did the same.--Aspro (talk) 14:47, 9 December 2010 (UTC)

Um, humans have had mill wheels for many many centuries. It occurs that by driven wheel you might mean more specifically a wheel which turns as a means of propulsion? Googlemeister (talk) 15:56, 9 December 2010 (UTC)

It's not clear to me, but it seems likely that at least some kinds of pedalo (human driven paddle boat) probably have some form of drive wheel in them. Our article claims that Da Vinci drew one, though I have no idea what sort of pedalo it was or even he was drawing something that already existed or if it was a more hypothetical exercise, like his helicopter. Might be worth looking into, though. Matt Deres (talk) 17:48, 9 December 2010 (UTC)

Hero of Alexandria, who probably lived 10-70 AD, invented a wheeled vehicle powered by a falling weight, as described on this web page. I can't imagine that Archimedes (a few hundred years earlier) would have failed to think of this, but I can't find any explicit description -- unfortunately he himself didn't consider his mechanical inventions important enough to write about. Looie496 (talk) 22:11, 9 December 2010 (UTC)

Is there any evidence he built a working model, or was it all just speculation on his part or the part of those hundreds of years later who wrote about him without much by way of historical record? Edison (talk) 15:52, 10 December 2010 (UTC)

Hero/Heron also made a steam engine, with a rotating sphere, as described in our aeolipile article. This certainly could have been attached to a drive wheel on a vehicle, but I don't believe it ever was. StuRat (talk) 17:03, 10 December 2010 (UTC)

swiss watch

sir,the back side of this watch reads,

                                     cd 3ATM water resistant
                                     christian Daniel
                                     24k gold plated
                                     hardness crystal
                                     swiss Qartz
                                     10MIC     
                                     No.001235

pls let me know the price and manufaturer of this watch.

something i found about this on www.trademe.co.nz but not the informaton which i want with help of google.com —Preceding unsigned comment added by 121.243.158.100 (talk) 04:47, 9 December 2010 (UTC)

Honestly, there's no way that anyone at Wikipedia could appraise this watch. You need to find a reputable jeweller or appraiser if you believe the watch to be truly valuable. --Jayron32 04:52, 9 December 2010 (UTC)

They aren't very valuable -- produced in large quantities and probably worth well under 50 US dollars. Looie496 (talk) 06:38, 9 December 2010 (UTC)

Note that gold plating is different than "made of gold." The actual quantity of gold present is virtually 0 (we're talking micrograms, if even that). You can bet that even a $1 digital watch has a minuscule bit of gold-plating in some of its electronic connectors. Nimur (talk) 17:46, 9 December 2010 (UTC)

Evolution

How ocean and land formed? —Preceding unsigned comment added by 125.21.50.214 (talk) 06:35, 9 December 2010 (UTC)

Our History of the Earth article covers this. Looie496 (talk) 06:39, 9 December 2010 (UTC)

And Geological history of Earth also has some more details. Most of the time when the earth was forming in its current form, with land and oceans and all that, occured during the Hadean eon. --Jayron32 06:47, 9 December 2010 (UTC)

Note that evolution is only concerned with how life progresses after it has come into being. The origins of the first life (on Earth) is abiogenesis, the formation of the first landscapes on Earth is geology, the formation of the early Earth is in the realm of cosmology, astronomy, and cosmogony. Are you implying a reference to Kent Hovid's challenge? CS Miller (talk) 09:23, 9 December 2010 (UTC)

Not strictly true. We often use the term "evolution" to be shorthand for "biological evolution", but evolution just means "gradual change over time". See Formation and evolution of the Solar System, Evolution of the British Empire, etc. The geologic changes the Earth has experienced over time could quite properly be described as evolution. --Jayron32 13:32, 9 December 2010 (UTC)

While this is true, there are, as you know full well, also people who use "evolution" as shorthand for "godless communist soulless science used by people who will burn in Hell for denying the existence of God". TomorrowTime (talk) 14:04, 9 December 2010 (UTC)

people for whom the preferred shorthand term is "insane".--Jayron32 15:16, 9 December 2010 (UTC)

See also plate tectonics and Miller-Urey experiment. ~AH1^(TCU) 03:40, 11 December 2010 (UTC)

Cube planet

I know this is highly unlikely, but a question higher up piqued my interest. If there were such a planet, cube shaped, with an earthlike mass how would walking along it be? Would it be like the center of each panel is the bottom of a valley, and walking towards edges would be like walking uphill, since the points and edges would be furthest away from the center of gravity? Gunrun (talk) 11:19, 9 December 2010 (UTC)

Yes, with the corners being further from the center than the edges. More significantly, though, the atmosphere won't be evenly distributed. If we suppose a cube planet with an average radius equal to Earth's, then the corners will be, ballpark, a thousand miles above datum. The middle of each face will be, ballpark, several hundred miles below datum. Contrast with the Earth, where Everest is less than 6 miles high, and the Dead Sea just a few hundred feet below datum. As such, the atmosphere would distribute toward the low spots, leaving the high spots in vacuum. Most of the surface will not support human life (Note that the low spots may have too much atmo for life, leaving a ring-shaped habitable zone on each face). Additionally, you'll get some weird gravity effects going from high to low by virtue of that significant an altitude change. — Lomn 14:19, 9 December 2010 (UTC)

I would think that the gravity would not be pulling you straight down when you were near the edges, but at a 45 degree angle down and away from the edge. That might mean you are going to have a very difficult time approaching the edge, since gravity will be creating a lot of drag on your forward progress. Googlemeister (talk) 14:36, 9 December 2010 (UTC)

It's just like climbing a 45 degree slope. Hard work, but far from impossible as long as the surface allows some decent grip. --Tango (talk) 02:23, 12 December 2010 (UTC)

I had a thought after posting this. Wouldn't the shape of the planet affect the shape of its gravitational field? Your explanation (and my initial thoughts) seem to work assuming the gravity is a sphere eminating from the center of the cube, is this how it works? I'm assuming this cube planet would have a cube core and mantle, as well as the crust here, and the densities would be similar to earth. Gunrun (talk) 14:35, 9 December 2010 (UTC)

There is a book with this as the premise. 'Children of Chaos' and 'Mother of Lies' by Dave Duncan. The planet is actually dodecahedral in the books, and the books are not really about that, but the geography is treated very well, including an appendix discussing the science (and the places where he had to take some license). Ariel. (talk) 17:13, 9 December 2010 (UTC)

The gravitational field wouldn't be spherically symmetric, but it wouldn't be hugely different from being spherically symmetric. In particular, at the center of a face, the center of an edge, or at a corner, gravity would pull you precisely towards the center of the planet. Anywhere else, gravity would pull you in a direction that's mainly towards the center of the planet, but somewhat towards the nearest corner. Red Act (talk) 17:43, 9 December 2010 (UTC)

That's right: I plotted the equipotentials and guess that the local gravity vector is never more than 10° or so away from the center-ward direction. --Tardis (talk) 20:13, 9 December 2010 (UTC)

Threads like this are why I treasure Ref Desk. You do not see this in the NY Times. Edison (talk) 06:43, 10 December 2010 (UTC)

The Bizarros live on a cube-shaped world. 93.95.251.162 (talk) 18:33, 10 December 2010 (UTC) Martin.

Under normal circumstances, such a planet would approach hydrostatic equilibrium. Imagine the simultaneously reduced (further distance from core) and elevated (forces pulling you toward centres of three planes) gravity while you stand on one of the corners. ~AH1^(TCU) 03:38, 11 December 2010 (UTC)

Yogurt

My EasuYo made yogurt turns to water after 24 hours what am I doing wrong I've followed manufactures instructions and also their help line advice without success.13:44, 9 December 2010 (UTC)86.133.163.19 (talk) 13:44, 9 December 2010 (UTC)

Added a title. Vimescarrot (talk) 13:46, 9 December 2010 (UTC)

Water is one of the byprotducts of the lactobacillus cultures that make yogurt, well, yogurt. The lactobacillus eats the sugars in the milk and produces water as one of its waste products. This is normal and to be expected. The same sort of water build-up occurs with sour cream and other cultured milk products. If the package is kept in its original seal, the lactobacillus remains somewhat dormant, so you tend not to see it until after the package is opened and it is exposed to air. Its completely harmless, and a normal part of what being yogurt is all about. You can, to your preference, either stir the watery part back into the yogurt or pour it down the drain. --Jayron32 14:00, 9 December 2010 (UTC)

The metabolic water produced from the milk sugars is going to be absolutely minuscule compared to the 90+% water content from the original milk itself. My understanding is that the liquid seepage you see from cultured dairy products is due to the broken protein matrix contracting and exuding the liquid, in a similar fashion to how yogurt cheese is made. As you said, some liquid seepage from the yogurt is completely natural. However, I get the impression that the poster has a different problem, where their yogurt doesn't just exude liquid, but turns completely to liquid. I'm not sure why it would turn to liquid after a day - are you sure it was completely set to begin with? Also, is it completely liquid, or just thinner than you would like? Usually commercial makers add stiffeners (like gelatin, starch, agar, etc.) to yogurts to increase the solidity. For homemade yogurts, the usual recommendation is to add additional dry milk powder to the milk to increase the protein content and thus the stiffness of the yogurt. Another issue for liquidy homemade yogurt may be that your yogurt cultures are sick/dead, or that the temperature of incubation was off. -- 174.31.212.34 (talk) 17:40, 9 December 2010 (UTC)

Think the OP is simply expecting it to look like it does when bought ready made. Commercial yoghurt is cultured in large vats first were the whey separates. Then the liquid whey gets thoroughly mixed back in before going to fill thousands of smaller pots. If the OP just waits until the culture has fully converted all the milk, then take a fork and stir thoroughly, he will find it takes on the same smooth consistency of shop bought yoghurt. Let it stand overnight in the fridge and it will set a little and look like he expects it to look. Mix it too early and it will separate a little again. I prefer it in its natural unmixed form so as to enjoy the contrast of flavours. If the OP hasn't ever seen or tasted home-made yoghurt before, this might look really odd and the taste rather unexpected and strange.--Aspro (talk) 23:29, 9 December 2010 (UTC)

Store-bought yoghurt often also contains starch or gelatine to "improve" the texture. --Stephan Schulz (talk) 00:19, 13 December 2010 (UTC)

Makeup for the eyes in the form of airbrush

I mean in a Bladerunner kind of makeup. An airbrush that you spray over your eyes forming a black stripe. Mr.K. (talk) 13:46, 9 December 2010 (UTC)

Airbrushes are used for makeup. It is not easy to do it on yourself, but it is used for professional work for photo-shoots and movies. Do you have a more specific question? -- kainaw™ 13:54, 9 December 2010 (UTC)

But, are airbrushes used over the eyes? Even if they are closed, is that a safe procedure? Mr.K. (talk) 13:56, 9 December 2010 (UTC)

Yes. What is the safety concern that you have? Are you assuming that they are using extremely high-pressure spray guns designed to blow a person's eyeballs through their skull? They use just enough pressure to spray a little tint about 2 inches from the nozzle to the skin. Spray-on tanners (which are also applied to the face) likely use more pressure than that. -- kainaw™ 14:02, 9 December 2010 (UTC)

I was more concerned about any effect of the makeup entering in contact with the eyes (or in this case, the tanning solution). Mr.K. (talk) 14:07, 9 December 2010 (UTC)

You close the eyes first, obviously. Lots of makeup is applied on the eyelids and around the eyes. The airbrush used is not high pressure. --Mr.98 (talk) 15:36, 9 December 2010 (UTC)

Well, yes, if you look at it this way. It's just a different way of applying makeup, which is often applied near the eyes and that has been tested thoroughly in the eyes of some animals before... 17:34, 9 December 2010 (UTC)

Wikipedia has an article on airbrush makeup.--Shantavira|^{feed me} 17:53, 9 December 2010 (UTC)

"You know you're wife's getting old when ... she needs to use power tools to apply her make-up." :-) StuRat (talk) 01:37, 10 December 2010 (UTC)

"You know you're getting old when ... you start talking about herself in the third person." 86.161.208.185 (talk) 11:19, 10 December 2010 (UTC)

Nasal saline solution

In many places, I see the recommendation to use non-iodized salt to make a nasal saline solution at home. Why is it different to iodized salt? Mr.K. (talk) 14:04, 9 December 2010 (UTC)

Iodine will sting your nasal epithelium, so using non-iodized salt makes it less painful. Rockpocket 14:45, 9 December 2010 (UTC)

But is this stinging only painful or also unhealthy? Mr.K. (talk) 16:01, 9 December 2010 (UTC)

If it's approved for use as food, it's probably not going to be too harmful applied in moderation to nasal tissue, but you never know, so you should ask a doctor, especially if you intend to use it a lot. WikiDao ☯ (talk) 17:50, 9 December 2010 (UTC)

No, it's just curiosity. But how is the nasal epithelium more sensitive than your mouth or throat? Is it another type of skin? Mr.K. (talk) 18:08, 9 December 2010 (UTC)

They're similar but different tissues, we probably have articles about them. WikiDao ☯ (talk) 18:21, 9 December 2010 (UTC)

The nasal epithelium is not inherently more sensitive, but it is sensitive to different things. It is particularly senstitive to zinc, for example. So avoid nasal spays or washes containing zinc if you value your sense of smell. Rockpocket 18:41, 9 December 2010 (UTC)

In a food related example, pepper in your food isn't very annoying, but inhaling pepper dust sure is. StuRat (talk) 01:33, 10 December 2010 (UTC)

Some people are hyposensitive to iodine, so giving more iodine through a nasal wash, added on to that ingested through food, could cause a reaction. Having said that, iodine hyposensitivity is fairly rare, and iodized salt contains very little iodine, so most people should be able to use iodized salt for nasal washing without any problems. Physchim62 (talk) 20:44, 9 December 2010 (UTC)

I'm guessing you mean hypersensitivity rather than hyposensitivity -- "hypo" means smaller than usual. Looie496 (talk) 22:15, 9 December 2010 (UTC)

I used to make a nasal solution with about a cup of water, ¼ teaspoon of iodized salt, and ¼ teaspoon of either baking soda or baking powder (I can't remember which it's been long enough...whichever one Arm & Hammer makes in an orange box), and it didn't sting or anything beyond the feeling of having water forced through my sinuses, and actually worked quite well. This was actually done with the advice of a doctor, as the main thing I used it for was to help prevent colds from turning into sinus infections, so I don't see that iodized salt versus uniodized salt makes that huge of a difference. Ks0stm ^(T•C•G) 18:41, 10 December 2010 (UTC)

Tapiridae (Tapirs)

Could Tapiridae be surviving Meridiungulata or Notoungulata? Has anybody reputable put forth such a theory? --Eu-151 (talk) 14:33, 9 December 2010 (UTC)

I probably shouldn't respond to a question I know so little about, but the fact that tapirs were common in North America for a long time before the Great American Interchange would seem to argue pretty strongly against the idea. Looie496 (talk) 22:24, 9 December 2010 (UTC)

Another improbable planet question

Say you had an earthlike planet, and one day a thick layer of extremely dense material (say... uranium or plutonium or something) evenly covered the planet, enough of it to increase the mass of the planet by 1/4 or something. Disregarding the utter havoc this would cause to whatever was on the surface what would this do to the planet (and ignoring the possibility of the crust collapsing under the weight of this new material (if thats even possible?)), regarding its gravity? Gunrun (talk) 14:38, 9 December 2010 (UTC)

If you cover the Earth with 1.5x10²⁴ kg of either uranium or plutonium, critical mass will trivially be met, and the planet will explode in an enormous nuclear explosion. Regarding its gravity, the post-planetary vapor will easily achieve escape velocity, so the vapor will not coalesce back into a planet. Red Act (talk) 16:15, 9 December 2010 (UTC)

Actually, this rather flip answer isn't necessarily correct, depending on the isotope being used. U-238, in particular, isn't fissile. Red Act (talk) 17:08, 9 December 2010 (UTC)

Ignoring the nuclear effect described by red act, and just dealing with the added mass, The effect would be to increase the effect of gravity by the same proportion as the new mass added would. Oh, and weight IS the effect of gravity, the two terms are synonyms, so if the new material would cause crustal damage in some way, the only mechanism it would cause it by would be gravity. In otherwords, if you increased the mass of the planet by 1/4 it would exert an increase of 1/4 in the effect of gravity on nearby objects. --Jayron32 16:17, 9 December 2010 (UTC)

Yes, but the surface gravity would not increase as fast, as the radius of the planet also increases with the cube root of the mass increase, assuming the new material is the same density as the existing material. CS Miller (talk) 16:21, 9 December 2010 (UTC)

The increased radius of earth would be about 150km (for uranium) so the mass would increase 25% and the distance would increase about 2.5%, so the gravity would end up 20% higher (on the surface) Googlemeister (talk) 16:28, 9 December 2010 (UTC)

Uranium star

Would it be possible, if you had one solar mass of something very heavy and radioactive, such as Uranium, Plutonium, or Radium, or even something lighter, like lead or bismuth, would if be possible to have a star creating energy via fission? And I know the improbability of this, I was just wondering. --T H F S W (T · C · E) 17:19, 9 December 2010 (UTC)

Probably not at that size. It seems to me you have two possibilities: The temperatures and pressures would (may?) be high enough that any fission products would be fused back together, stealing any energy. Or the fission would proceed but the resulting neutrons would cause other atoms nearby to also fission, making a chain reaction that would blow the whole thing apart. Cool question though. Ariel. (talk) 17:48, 9 December 2010 (UTC)

Have a look at stellar nucleosynthesis. Based on our understanding of the way that heavy elements form, it's not probable that any star could ever reach a state where uranium is a significant constituent, by mass, volume, number of particles, or any other metric. After reaching iron fusion by "burning" lighter elements like Oxygen, the star typically can not produce heavier elements in great quantity. The neutron- and proton-capture reactions are not very efficient. So, for a star to have even a small quantity of uranium present, it necessarily requires a much larger quantity of hydrogen fusion. Uranium may fission in any star of sufficient size and age, but it will never be the dominant energy-production reaction. Also, you might have a look at fission bomb related articles: one of the biggest problems with fission of uranium is that it is often explosive. While this sounds obvious, consider the problem more carefully: a property of uranium is that once it reaches a critical density, the fission reaction occurs explosively at a speed too fast for the metal to hold together. Then the material explodes, blows apart, and the critical density no longer exists and fission can not be sustained. That's why fission bomb designers have to spend so much effort on carefully arranging parts, and working on "explosive lenses" and "neutron reflectors" and so forth. It's pretty unlikely that a natural nuclear fission reactor could ever exist. (Insert your favorite Soviet/Cold-War conspiracy-theory about West African geology here). Nimur (talk) 18:02, 9 December 2010 (UTC)

You guys pointed out that one problem is that it is explosive. This is true, but you must remember that it has to combat the massive pull of gravity. Hydrogen bombs are explosive as well, but the sun has 1,400,000,000,000 of them going off every second. I am not wondering if it would explode (trust, me it wouldn't; collapsing is more likely), I am asking if such a star could have a maintainable, energy-giving, stable reaction. --T H F S W (T · C · E) 18:40, 9 December 2010 (UTC)

I think I wasn't quite clear: the problem isn't merely that fission is explosive - anything energetic will cause expansion. Hydrogen fusion is also energetic, and also causes expansion. The problem is that specifically for uranium fission nuclear chain reaction, the rate of material expansion is faster than the rate of neutron production. This means that there will not be enough neutrons to sustain the fission over long enough time-scale to "actuate" all the uranium. The relative rates of expansion and neutron-production are "hard-coded" into the atoms, due to the energetic nature of uranium fission. (On these time-scales, size-scales, and energy-scales, gravity isn't even in the picture, so to speak - we're talking nuclear-force here, and worried about relative positions and velocities of atomic nuclei). So it is physically impossible to fission a lot of uranium unless something forcefully contains the nuclear reaction. This has been a known problem since the early 1940s, and requires an engineering solution (neutron reflectors, implosion triggers, and so on). It is unlikely that an "engineering solution" like that could naturally exist. Put another way, "we don't see a lot of uranium fission in stars, it will never be the dominant energy-production reaction." Nimur (talk) 19:17, 9 December 2010 (UTC)

THFSW: With fusion the reaction is self regulating - if it runs too fast the density goes down, slowing the reaction. Not so with fission, which has a positive feedback loop instead, and the faster it fissions, the faster it induces more fission, till the whole thing goes at once. Nimur, I'm not sure what you mean by "the rate of material expansion is faster than the rate of neutron production" - are you saying it will expand too far for it to sustain a reaction? I'm not sure that's true, unlike fusion which requires density, with fission all you need is some neutrons, and as long as you have sufficient depth of material that some neutrons inevitably will impact on the uranium, it will sustain, even at low density (at least until it expands large enough that the neutrons decay before they hit anything). i.e. that your uranium "cloud" has enough atoms in it that in any random line 10/14 light minutes long through the cloud there is at least 1 uranium atom (this is oversimplified obviously, you have to take into account cross section, etc.) Ariel. (talk) 20:06, 9 December 2010 (UTC)

Regarding "expanding so fast..." - yes, this causes a premature termination of the chain-reaction. Have a look at the discussion of Little Boy's inefficiency. Technical reports are available in the references and further reading sections of our Little Boy article. I recall a specific paper, published in 1945, that outlined the detailed physics calculations, but I can't find it at the moment. I think it used to be posted from our Nuclear Weapon Design article. In any case, you can perform and verify the calculations yourself by solving the collision kinetics of the fission-product. Nimur (talk) 20:36, 9 December 2010 (UTC)

A star is rather larger than Little Boy. It could expand a lot and still work. Or more specifically the uranium in Little Boy is not very thick, so when it expands neutrons will pass right through without hitting the uranium, but in a star - even an exploded star - the uranium would be thick enough that the neutron will still hit something. Ariel. (talk) 20:41, 9 December 2010 (UTC)

I suppose. I know less about the physics of stars than the physics of nuclear weapons. I should probably spend more time outside. Nimur (talk) 21:33, 9 December 2010 (UTC)

In any case, it strikes me that you'd want something that was more like a star-sized reactor than a star-sized bomb. It seems like of odd to assume that the uranium in question is HEU like Little Boy. I'm imagining some kind of ideal uranium-boron mix or something else that would be somewhat self-moderating and self-restricting. --Mr.98 (talk) 22:28, 9 December 2010 (UTC)

Stars that try to make heavy elements in great quantities end up with these elements decaying back to lead making a lead star. (we need an article) Even so the star s nowhere near 100% lead, but still contains hydrogen and helium, and some other elements. Graeme Bartlett (talk) 20:58, 9 December 2010 (UTC)

Here are a list of references to papers that talk about lead stars. Red Act (talk) 21:25, 9 December 2010 (UTC)

I made a wee stub for it. --Sean 22:24, 9 December 2010 (UTC)

Heavy radioactive elements are most often created in the shock waves and extreme temperatures of Type II supernovae, and any fission energy produced may contribute to the supernova. ~AH1^(TCU) 03:31, 11 December 2010 (UTC)

Neutron stars can collide, resulting in fission-like liberation of energy and mass. Ginger Conspiracy (talk) 05:30, 11 December 2010 (UTC)

touchscreen technology

how to make touch screen? — Preceding unsigned comment added by Djmyemailid (talk • contribs) 18:10, 9 December 2010 (UTC)

We have an article on Touchscreen technology. There are a variety of them, all of which seem difficult to make from scratch. Perhaps easiest is to buy a company that already produces them, or just buy one of their products if you only want one. WikiDao ☯ (talk) 18:18, 9 December 2010 (UTC)

There are touchscreen overlays you can purchase if you have an existing display that you want to add touchscreen capability to. -- kainaw™ 19:20, 9 December 2010 (UTC)

Here are instructions for one way to build your own touchscreen. Red Act (talk) 21:18, 9 December 2010 (UTC)

Electromagnetic induction and capacitance may be of background interest. Ginger Conspiracy (talk) 05:36, 11 December 2010 (UTC)

do magnetic interactions EVER change the magnitude of velocity of a charged particle

I am told that magnetism and electromagnetism are basically two sides of the same force. Now I believe I understand classical mechanics, but I don't get how the magnetic and electromagnetic forces are unified (through relativity?). If the force is unified, how does this reconcile with the observation that magnetic forces don't ever seem to change a charged particle's velocity (not in a circuit)? Can Faraday's law be applied to a beam of electrons? John Riemann Soong (talk) 21:29, 9 December 2010 (UTC)

Magnetic fields produce a force that does no work on a charged particle (by definition, the Lorentz force is perpendicular to the motion, so F_B · v is zero and thus energy is totally conserved). So in the "purist" interpretation, the magnetic field itself does not change the energy of the particle. In complex situations, such as a dense plasma, changing the particle trajectory affects things like viscous drag, collision rate with other particles, and so forth. In that case, the magnetic field can affect the particle energy, but only indirectly by steering it into a different direction, where other effects (electrodynamic, or even just thermal/collisional effects) actually negotiate the energy exchange. Nimur (talk) 21:39, 9 December 2010 (UTC)

From a relativistic perspective, the "electric field" and "magnetic field" are just different aspects of the electromagnetic field tensor. See Covariant formulation of classical electromagnetism for a unified treatment of electromagnetism. Red Act (talk) 04:03, 10 December 2010 (UTC)

A time-varying magnetic field is equivalent to an electric field, as per the Maxwell-Faraday equation. This is used for deflecting electron beams all the time. Although the electron beam in old cathode-ray oscilloscopes commonly used electrostatic deflection, the beam in modern CRT monitors and TVs all use magnetic deflection, i.e., the beam is deflected by a time-varying magnetic field generated by electromagnets; see Cathode ray tube#Overview. Red Act (talk) 05:46, 10 December 2010 (UTC)