Wikipedia:Reference desk/Archives/Science/2007 November 13

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November 13

Technology and Depression

Does the more engrossing technology becomes translate to the more depression society feels as a whole? (Forgive the grammar.)

Living in our parent's house, no post-secondary education or real job experience between us, my brother and I were absolutely entranced with World of Warcraft. "Real" decisions with all their complications were so far removed from the crisply cut and dry work/rewards system in game. I alone spent 7 months of 16-hour grueling days rising to the rank of Grand Marshal of the Alliance and I loved it. We lived every waking hour in WOW for 2 years. Our parents eventually gave us an eye-opening ultimatum and he killed himself with a plastic bag and duct tape about a year ago. I was in a psych unit for about 2 months after. I know now what horror and true self-loathing feel like.

In my heart I know this is a sign of things to come for society as a whole. I'd like to know if depression and technology are correlated. Sappysap 02:13, 13 November 2007 (UTC)

There are arguments that they are, but I don't believe so. Depression is fairly clearly genetic, and has existed in some form for all of history. Suicide, of course, being the worst-case fate of depression, has also been in existence and I don't believe suicide rates have increased or decreased significantly with time, though certainly environmental factors have played a role. I have no significant sources with which to back this, though, other than the fact that "melancholy" was a diagnosis since at least the early 17th century. SamuelRiv 03:14, 13 November 2007 (UTC)

First you need to clear up what exactly do you mean by depression. There are many subtypes of clinical depression, some of which include atypical depression, and dysthymia. Each clinical type will have certain characteristic features; so people with dysthymia may be less prone to committing suicide than people in a major depressive episode.

Correct me if I am wrong, but your hypothesis is that technology allows people who are lonely and/or need human contact but lack it to withdraw into artificial worlds where they get only superficial human interaction. This leads to increasing loneliness and eventually to suicide or total withdrawal from society?

Also when you use the word technology, do you mean only modern technology? If not, then we would need to analyze the impact of earlier technological advances like electricity and the printing press (among many others) to see if they provide support to your hypothesis.

What would be key to supporting your argument is that technology heightens the impact of someone who would be prone to withdrawal from human interact, because I would imagine there is and always has been a certain subset of people who are lonely but still take actions to further isolate themselves.

As to SamuelRiv's claim that depression is "fairly clearly genetic", I do not agree. First it is unclear what that claim even means. Minimally, it would mean there is a genetic component, which does seem to be the case. But the more natural interpretation is that he is claiming it is almost entirely due to genetic factors, which would need some serious evidence to justify. This issue also points to the need to clear up what exactly you are talking about when you use the word "depression", since each clinical subtype is likely to have different roles a genetic component would play.--152.2.62.27 13:00, 13 November 2007 (UTC)

I also disagree that depression is a mainly genetic phenomonen, I have always believed that it is mainly due to the way one has been brought up combined with the life experiences they have been through, ie somebody that was emotionally neglected as a child and/or bullied will take a major battering to their self esteem which is definitely a factor in causing depression. These psychological scars can take years to heal, that is if they ever heal completely. On the flip side, somebody with a more fortunate upbringing is less likely to suffer from these issues BUT there are many exceptions to this and I think there may be a genetic element too...HOWEVER this may be psychological - if a parent suffered from depressive conditions then they may 'rub off' on the child.

Going back to technology, I think it CAN cause depression but it can also be a godsend. For example a depressive person could spend hours "wasting their life" (in the eyes of some people) playing video games but then again this is usually a distraction and definitely doesn't mean that without this activity, the person would be a raving extrovert. On the flip side, I think the internet can be very helpful as people with mental illnesses including depression can find out information about their condition and discuss it with other suffers which is often very cathartic. People may be willing to engage in others online when they wouldn't otherwise.

A third argument is that it is not technology itself that is causing depression but the way society has become. People nowadays are generally more insular than they were say thirty years ago, many people don't know their next door neighbours any more and the general perception is that things are unsafe and there are a lot of people around who cannot be trusted. This could (arguably) be caused partyly by technology, for example people not bothering to talk to each other because they are more interested in watching the TV or gaming while before the advent of these solitary pleasures, most pastimes were more social, take the fact that a lot of families no longer sit around a table for meals, instead choosing to eat in front of the TV. Take also the fact there are a lot more single person households than ever before, this causes isolation but I cannot see any attribution to technology except for perhaps the learning of antisocial trends from television. GaryReggae 15:51, 13 November 2007 (UTC)

Though it is out of date you might fight Oswald Spengler an interesting read—it's not about technology, per se, so much as it is about modernity. But in any case, people have wondered for years (since the 19th century, anyway) if the creation of new technological means of representation and life have "sucked the soul" out of modern human interactions, encouraging depression, vice, etc. I myself am doubtful of a straightforward relationship—first, I don't believe there probably were "good old days" where everybody was chummy and warm and happy (most people are sons-of-bitches no matter what time or place you live—but that's the misanthrope in me talking), and second, I'm unconvinced that technology itself is responsible for the sorts of changes people are talking about. When a family eats its dinners in front of the television rather than together at a table, it isn't the television that has compelled them to do so; it's just as easy not to have the TV on during dinner (even easier as time goes on, with the ability to time-shift shows). As for depression—that's a complicated issue, but I don't see any obvious technological links there. If it wasn't the game, who isn't to say it wouldn't have been some other form of escapism? --24.147.86.187 01:03, 14 November 2007 (UTC)

• Sappysap, I could point you to a ton of Wikipedia articles, such as Uncanny Valley and Escapism. But ultimately, there's still a lot of research to be done before we can say for sure how artificial worlds affect psychology. In a situation like yours, I doubt any answer from us volunteers at Wikipedia will be completely satisfying (especially ones like me who aren't even pre-med students!). You may want to consider the possibility that this is your life calling, enroll in your local college, and get yourself a degree in pyschology. If that's not an option, at least ask your psychologist (or someone you like from when you were at the ward) to refer you to one of their colleagues who specializes in technology, so that you can do some meaningful research. --M@rēino 14:42, 14 November 2007 (UTC)

I don't think it's the technology. If you'd spent two years doing nothing but...I dunno...reading medieval poetry or learning to ride a unicycle - the end result would be kinda similar. MMPORG's are fun - they're fine for a couple of hours of entertainment a few days a week - they help you unwind - but in the end when you finally chuck it in - you haven't achieved anything. Heck you said it was "grueling" - so you weren't even getting the relaxational benefits. But it's pretty clear that making your entire life (16 hours a day, 7 days a week for 2 years) happen in a place some dumb game designer (like me - I'm a game designer) dreamed up is not a healthy thing. I mean, 11,000 hours at minimum wage would buy you a small house or a pretty decent sports car! The problem here isn't the technology - it's the sheer waste of human effort in doing it in the first place. I know where you're coming from - I'm closing in on 10,000 edits over 3 years to Wikipedia. That's an addiction that I recognise - but it's a lot less than 10,000 hours - probably more like 1,000 hours - but still, it's a serious chunk out of my life. The difference is that it doesn't feel like time wasted - there is a positive result - the accumulation of all human knowledge where almost anyone can read it...that's something worth spending your time on. The technology of Wikipedia and that of an MMPORG are not all that dissimilar!

So it's not the technology...it's what you use it for and to what extent you let it take over your life.

I've come across horror stories of MMPORG addiction too (although nothing like on this scale) - we had a neighbour who's smart "straight-A's" teenage son spent 50+ hours straight (no sleep, no food) wrapped up in Everquest. When his parents found out, they took away his computer promising to return it after one week. He then came over to my house, smashed a window, broke in and stole my wife's laptop. Sadly, being a sleep-deprived teenager, he forgot to steal the battery charger and phoned us up the following day to see if we happened to have a laptop charger he could borrow...which certainly raised some suspicions! When we got the laptop back, yep - Everquest had been mysteriously installed on it. But that incident blew away any trust his parents had. He wound up in a downward spiral that put him in military school then downtown Bagdad when he should have been in college.

SteveBaker 19:46, 14 November 2007 (UTC)

Mareino: I desperately wanted to write and publish an article about the surrealism currently going on in games like WOW. There are sweat shops in China, with families on the line, farming WOW gold (I know for a fact b/c I've actually learned a litte phonetic Mandarin from them, even here in hick Virginia, US). They are in so much pain. There are so many "heroes" in-game that have gained their status by giving up on real life completely, much like myself and my brother. Both of our best friends, the guild, dissuaded me from publishing anything after his suicide, as it would dishoner a fallen comrade and the guild as a whole. I know this is irrational, but I guess I agree with them. It's insane to even state, but: you had to have been there with us; through all the guild wars, and the insurmountable dungeons. Of course, none of them really existed but they were as real to me as any war imaginable. That is why I tremble for the future of technology and virtual media. Sappysap 04:16, 15 November 2007 (UTC)

Reduction of Fe(III) in FeCl₃(aq) to Fe(II)

How would I reduce the iron in FeCl₃(aq) to Fe(II) in order to use the indicator ferrozine, which only detects Fe(II), not Fe(III)? Can I somehow use ascorbic acid to do so? What would the chemical reaction be? Could this be undergone by using titration? Chickenflicker-♣ 02:48, 13 November 2007 (UTC)

Edit: What would the reaction between Hydrazine (N₂H₂(l)) and FeCl₃(aq) look like? Chickenflicker-♣ 03:03, 13 November 2007 (UTC)

I've seen hydrazine used as a 2-electron reductant (standard way to prepare Pd(0) complexes is from Pd(II) salts, and also the Wolff-Kishner reduction). Not sure about a good 1-electron reduction mechanism. DMacks 14:39, 13 November 2007 (UTC)

Thinking more technically here, hydrazine is N₂H₄. N₂H₂ (diazene) also exists and is a good reducing agent (via hydrogen transfer, such as in hydrogenation of unsaturated compounds), but it's mighty unstable (the pure material decomposes even around –180°C). DMacks 16:50, 13 November 2007 (UTC)

horizontal component of speed

This is a mix between a maths and physics questions...anyway here goes:

An object is being pulled up a slope by a force at a constant speed of say 3m/s. At the top of the slope is a horizontal plane. What is the object's speed at the beginning of its journey along the horizontal plane? Is it 3m/s, or is it just the horizontal component of velocity (ie. 3cos(angle))? Is the vertical component of velocity wiped off or does it become a part of the horizontal velocity?

    ->>______________________
   -  /
  -  /
    /
   /
  /
 /
/

What about when a ball rolls down a slope and then travels along flat ground? At the beginning of its journey along the flat ground, is its speed just the horizontal component of its previous velocity or is the magnitude of both components of its previous velocity?

                     /
                    /
                   /
                  /
                 /
                /
____________<<-/  

Thanks! D3av 02:53, 13 November 2007 (UTC)

In these types of problems you usually approximate the sharp corner as a tiny rounded corner. Then assuming ideal conditions the ball rolling down the hill will conserve kinetic energy when it hits flat ground. The ball being pulled up a hill will also conserve kinetic energy, but the end result depends on the precise wording of the problem. If you are constrained to be on the surface of the slope, then you move forward with kinetic energy conserved. However, if unconstrained, you fly upwards as a projectile with some initial velocity at some angle. SamuelRiv 03:09, 13 November 2007 (UTC)

So say the one moving down the hill is unconstrained (just freely rolling down). Would you use the horizontal component of velocity or the whole thing?

When you say "with kinetic energy conserved", does that mean that the vertical component of velocity becomes part of the horizontal component?

We had one question in which the object seemed to be constrained as it was being dragged up the slope. The instruction was to use only the horizontal component of velocity when it reached the horizontal plane at the top. Was this a correct instruction? thanks. D3av 03:26, 13 November 2007 (UTC)

As SamuelRiv said, it depends on the precise wording of the problem, in particular how the force is being applied. For example, if there's something pushing on the object from the left (slowly, so that gravity keeps the object from ever leaving the surface), moving at a constant rate to the right,

       ______________________
      /
->|  /
->|*/
->|/
  /
 /
/

then the horizontal component of the velocity will remain constant, and the vertical component will go to zero at the corner. If there's a rope attached to the object that goes up and over the corner (so the corner acts somewhat like a pulley), and the rope is being pulled at a constant rate,

       ______________________->
      /______________________
     //
    //
   */
   /
  /
 /
/

then the component of the velocity parallel to the rope will remain constant, so the horizontal and vertical components will change as the rope's direction changes. If there's something pushing the object in the direction parallel to the slope,

       ______________________
\     /
 \   /
  \*/
   /
  /
 /
/

(...well, I don't know how to make it look like that thing is moving diagonally upwards with ASCII, but anyway) it will actually move faster when it reaches the horizontal surface because the component of the velocity in the direction of the slope will remain constant, but it gets a new perpendicular component when it passes the corner. So it all depends which of these different problems you're trying to answer. —Keenan Pepper 06:27, 13 November 2007 (UTC)

The vertical component won't magically go to zero, regardless of whether the object is being pushed or pulled. Unless the object is held on some sort of track, it will rise into the air above the horizontal plane until gravity brings it back down. Clarityfiend 08:00, 13 November 2007 (UTC)

Unless, as I said, it goes "slowly, so that gravity keeps the object from ever leaving the surface". This is a quasi-static approximation, which is actually very good for everyday objects. If you pull a rubber ball up a ramp with a string, slowly, it doesn't fly into the air when it reaches the top. —Keenan Pepper 23:04, 13 November 2007 (UTC)

Thankyou very much for those answers. They are very comprehensive and clear. I'm still unsure though (as I wrote above) about what will happen to a ball that is rolling down a slope and then continues along a flat surface. (The only forces acting on it are the force of gravity and the normal). Thanks D3av 09:45, 13 November 2007 (UTC)

It depends on the nature of the ball. When the ball hits the bottom of the slope, there is a collision. What happens next depends on what the ball and the ground are made out of. A softer ball might bounce a few times - eventually rolling off along the new direction at roughly the speed it was moving down the slope - much conserving kinetic energy (assuming we ignore losses). What happened was that the kinetic energy temporarily turned into elastic energy from the compression of the ball's material - then that elastic energy was turned back into kinetic energy directed in a different direction. That little energy interchange permitted the velocity vector to change but the kinetic energy to be conserved. A harder ball might simply give up the vertical component of its energy in the collision (so the kinetic energy turns to heat) - continuing to move along the horizontal region at a speed equal to the horizontal component of it's former motion. Without more information, you can't say which it will be.

When the ball is travelling up the slope, the same thing happens. At the top of the slope the ball will continue upwards (because of conservation of momentum) - travel on a free parabolic trajectory and eventually collide with the top of the slope. Once again, the ball will either bounce - causing it's new velocity to be horizontal - with the same speed as it was travelling up the slope - or it'll impact without a bounce - causing it to lose the vertical component of it's former velocity.

With real balls, the behavior is somewhere between the two cases - energy is lost in the bouncing and the actual horizontal speed will be a blend between the old horizontal speed and the old net speed.

SteveBaker 12:26, 13 November 2007 (UTC)

To original poster, remember kinetic energy is NOT a vector. It has no horizontal and vertical components. When kinetic energy is conserved, all velocities must be taken into account. SamuelRiv 14:56, 13 November 2007 (UTC)

But also remember that kinetic energy is NOT necessarily conserved -- it's only conserved if the collision between the ball and the horizontal surface, described by Steve, is elastic. To make this clear, imagine rolling a cannonball down a steeply sloped, almost vertical wall into a lawn. It'll probably stop dead at the bottom and embed itself in the grass. However, the same ball coming down a gentler slope, one that's almost horizontal, will probably keep rolling and may well do it without much loss of kinetic energy. --Anon, 23:32 UTC, Nov. 13.

Thanks for all your help. I see that my maths course has oversimplified the issue and ignores bouncing/heat/other forces etc. I'll just try to follow the guidance of the question! D3av 00:03, 14 November 2007 (UTC)

That's pretty typical of a math course - they aren't really interested in the physics - because it's a math course! However, in this case, it's not the same thing as simply ignoring friction and air resistance for the sake of getting an answer. In this case the outcome will be quite utterly different for an elastic versus an inelastic collision. The issue of whether kinetic energy is conserved or whether conservation of momentum applies is not an insignificant matter. SteveBaker 18:46, 14 November 2007 (UTC)

Well, momentum (unlike kinetic energy) is always conserved, but that's not useful to consider in this problem. Whether or not the ball's speed changes, its velocity certainly does, since it's moving in a different direction; therefore there is a transfer of momentum between the ball and the Earth. --Anonymous, 23:01 UTC, November 14, 2007.

stars come down

hi my question: there is a place in space that stars come down(and it is a holy place...god says in quran)Have you heard anything about it? —Preceding unsigned comment added by 213.207.252.64 (talk) 10:20, 13 November 2007 (UTC)

Could you provide a reference to a specific passage in the Quran? Scripture often contains sections that need to be read very carefully to determine what they refer to exactly. My first guess would be a black hole. (EhJJ) 11:41, 13 November 2007 (UTC)

It's possible that the book refers to meteorites as stars. -- JSBillings 11:43, 13 November 2007 (UTC)

If "come down" means "move deeper into a gravity well", then I suppose you could say that "stars come down" as they fall into a supermassive black hole. Those are found at the centers of many (and perhaps all) galaxies. But there is no one, single place where this happens in the universe - it's likely that there are at least as many of these objects as there are galaxies - and there are billions of those. The nearest one of these things is at the center of our own Milky Way galaxy.

I find it amusing that a black hole might be considered a holy place. Good and useful things (like stars and planets) go in - nothing (including light or information) comes out! But really, any sufficiently vague and ancient statement (with likely translation errors and a wide laxity of interpretation) can be made to fit some real thing. I don't think I'd read anything into this statement beyond that whoever wrote it didn't know what a star really was and thought that the idea of the seemingly immutable stars 'falling' was a dramatic image. Spotting an occasional meteor and (incorrectly) assuming that was a 'falling star' might serve to reinforce that. Sadly, with what we now know, it really doesn't make any sense.

SteveBaker 12:07, 13 November 2007 (UTC)

Stars falling can refer to the yearly Leonid meteor shower which offered a particularly vivid display on November 13, 1833 bringing quite a few people to thinking it was the end of the world. Not that this particular event would be mentioned in the Qur'an but maybe a very early occurence of this same phenomenon. Keria 14:26, 13 November 2007 (UTC)

Meteor showers such as the Leonids and Perseids are not localised to one place though. Every place on the earth and the moon is peppered by them at one time or another. So this couldn't be a specific holy place - except by the very broadest interpretation. SteveBaker 14:56, 13 November 2007 (UTC)

The representations of the meteor shower often show a sort of fountain of stars as if there was an origin to these moving lights. I'm not sure what the questioner means by "that stars come down" but if it's from I could definitely see a 7th century scholar/mystic (or Muhhamad!) seeing a holy fixed source of stars in space in the apparent origin of these showers. Keria 15:55, 13 November 2007 (UTC)

Yes - both the Leonids and the Perseids appear to come from one place in the sky - in fact their names come from the constellations that they appear to radiate from (that point is called "the radiant"). The reason is that these showers each come from tiny bits of an ancient broken-up comet that have spread out into a gigantic loop around the sun. The earth moves through each of those streams once a year - which is why the debris appears to come from a fixed spot in the sky year after year - over the exact same couple of days each year. But the problem with this theory is that this 'radiant' effect means that the 'shooting stars' radiate outwards - they are all heading off to different places - not falling to the same place. SteveBaker 23:51, 13 November 2007 (UTC)

• The Qur'an also says that Mohammed flew from Mecca to Jerusalem on a magical flying horse, so you should probably take the unrealistic parts as metaphor. --Sean 19:06, 13 November 2007 (UTC)

The authors of the quran can't possibly be intending to refer to black holes, since its authors lived long before anyone knew anything about black holes!

I did a search on "stars" within English translations of the quran.[1] There are only about 15 hits, although that number varies by translation. There are a couple passages that look like what you might be referring to:

Al-Waqi’a 56:75 refers to what is translated in the Yusuf Ali translation as "the setting of the stars". The Muhammad Asad translation, however, uses the phrase "coming-down" instead of "setting". (The Muhammad Asad translation also uses "parts" instead of "stars", so the meaning of that passage is particularly unclear.) If this is the passage you are referring to, then no, there is no place where stars literally come down. They merely appear to come down, to the west of wherever you happen to be located, due to the rotation of the Earth.

Al-Hajj 22:18 talks about the stars "bowing down" before god, or "prostrating" themselves, depending on the translation. And Al-Hajj is about the pilgrimage to Mecca, which is certainly considered a very holy place among Muslims. If this is the passage you are referring to, then no, no such bowing motion is observed in stars, even when they appear to be over Mecca. MrRedact 19:30, 13 November 2007 (UTC)

Thanks! Excellent work! That's pretty much what I thought - vague translations of an uncertain source...but it's nice to see it confirmed. SteveBaker 23:51, 13 November 2007 (UTC)

This is mona who asked the question,at the first place I want to thank you because of your answers then I want to excuse you because of the vague question(my own language is not EN so it is expected,I'v studied the persian translation and it was wrong)I want to correct it: The exact passage:I swear to sites of stars(75) if you know it's a big sworn(76) my question:where are sites of stars and why are they so important that GOD swear them

Alabama? --Trovatore 00:02, 14 November 2007 (UTC)

What snake a specific book refered to

I'm looking for a king of snake who may havg from a tree over the Amazon river or one of its tributaries, would look like a vine to someone in a canoo going under that tree, and can swim away if grabbed off the tree. I looked in Category:Reptiles of South America, but couldn't find it - the Emerald tree boa looked promissing, but I couldn't find evidence of it being able to swim. Od Mishehu 11:29, 13 November 2007 (UTC)

Perhaps an anaconda? (EhJJ) 11:38, 13 November 2007 (UTC)

I don't think it hangs from trees. In the book in question (Magic Tree House book called Afternoon on the Amazon), Jack tries to get to the river bank by pulling on a "vine" that turns out to be a snake. Od Mishehu 11:47, 13 November 2007 (UTC)

It doesn't look like the author actually used real animals in her books, but just made up a convenient fictional snake as a plot device. -- JSBillings 13:27, 13 November 2007 (UTC)

I'm afraid I don't understand the question. It was a fictional snake. --Milkbreath 12:01, 13 November 2007 (UTC)

Indeed, it's difficult to see why a snake would hang above a river unless it could swim. But why would it do that? It's not as though tree snakes normally go fishing.--Shantavira|^{feed me} 13:52, 13 November 2007 (UTC)

I thought all snakes could swim, no? I know hedgehogs can. Lanfear's Bane | t 16:46, 13 November 2007 (UTC)

Condensation and Deposition

I need to measure the rate at which Condensation and Deposition (physics) occurs at standard temperature and pressure from a particular inorganic vapor with a melting point in the 1000s K produced by combustion. I have had no luck so far. How do physicists and chemists measure condensation and deposition? As a biologist without a whole lot of physical chemistry, I haven't even been able to figure out where to begin. I am hoping that there is a formula but I fear that these rates are determined only empirically. Thanks for any help. Spc303 12:59, 13 November 2007 (UTC)

You'll want to look up the phase diagram for your substance and then calculate the vapor pressure, I believe (not an engineer). Deposition is a much harder problem, and I'm afraid I don't know the physics of that phase transition, but for condensation you may also need to look up the constants a and b in the Van der Waals equation, but not for these formulas. Here's the relevant equations: for the change in the number of particles going from liquid to gas phase, we have

$\Delta N={\frac {16\pi \gamma ^{3}}{3n_{l}^{2}}}(k_{B}T\log(p/p_{0}))^{3}$

where $\gamma$ is the surface tension (in erg/cm³), $n_{l}$ is the number of molecules per volume of the liquid, and $p$ is the vapor pressure defined by

$p=p_{0}\exp(-L_{0}/RT)$

and $L_{0}$ is the last thing to look up is the latent heat of vaporization per molecule. Note T is temperature, kB is Boltzmann's constant, and R is the gas constant. SamuelRiv 14:53, 13 November 2007 (UTC)

Biofuels

Hi there, I have a question about Biofuels which I can't find a meaningful definitive answer to online. Where I work, we have a well used old 2002 reg Citroen Berlingo van (diesel) and I have been toying with the idea of using waste cooking oil as fuel as we have tons of the stuff available. According to some sources I have seen (for example [2]), engines need to have conversion kits fitted, such as adding an extra fuel tank or heating the fuel, however I have read many cases of people just putting used cooking oil (once it has been filtered) straight into their fuel tank - not neat but making sure they have half a tank of standard mineral diesel as well as half a tank of veg oil, ie 1 50:50 mix. Would this work or damage the engine? It's located in the South of the UK so unlikely to be susceptible to really cold temperatures. It's probably too much hassle to get the engine modified but if we can use it as it is then great! The van is a bit of a 'rattler' so it's not exactly the end of the world if it needs repairing after any 'experiments'.

Disclaimer: I know we need to pay fuel duty tax if we use more than 2,500 litres a year and obviously we would monitor this and ensure that we do not go over this amount so this is NOT a legal question, just a technical one. GaryReggae 15:36, 13 November 2007 (UTC)

There are two problems you need to deal with:

• Filtering
• Cold weather

Typical cooking oil has lots of bits of food in it - and sometimes some dissolved water - and it absolutely must be carefully filtered before you put it into the tank of your car. I'm told that coffee filters work - but they fall apart and clog before you get enough oil through them. You need something more robust if you are going to be doing this for any length of time.

Cold weather is the more serious matter. Veggy oil is not very runny at cold temperatures - that can strain your fuel pump and clog the system up pretty badly. A proper conversion kit uses a secondary tank of straight diesel to start the car - then picks up waste heat from either the exhaust or the hot water entering the radiator to heat up the fuel lines coming from the veggy oil tank so you can switch over to veggy oil a few minutes after you've started the car when everything is nice and toasty. If you lived here in Texas, you could probably not bother and just stick filtered oil straight into your car, switching to straight diesel for a few weeks in the depths of winter - but in the South of England - no, you're going to need a heating system of some kind at least 6 months of the year. Certainly you can experiment. I don't see the harm in mixing diesel and oil - although I suppose they might settle out if not mixed leaving the oil being the first thing that enters the system on a cold weather start. If I were you, I'd start in the summer when things should work OK even if you're not really getting it right - and as the weather gets colder, and the thing gets harder to start - or just won't run at all - then you need to switch to diesel (at least in some mixture). But before you do any of those things, you need to figure out a good way to filter your oil.

SteveBaker 16:37, 13 November 2007 (UTC)

This is something you need to ask a sharp mechanic (like the Car Talk guys) if your mind is really set on it. A car has internal computer regulators of the octane level of your fuel, among everything else including the integrity of the fuel-air mixture that is injected into the engine. It may be that this regulation will cause serious consequences to your exhaust or fuel efficiency, and will certainly damage the engine if you're using just any old cooking oil. The engine is designed to self-clean and lubricate, and it only can do this if the nature of the exhaust of the fuel you put into it is predictable. Cooking oil almost certainly burns different and probably is dirtier than gasoline, causing premature wear in your pistons. I would definitely recommend against it for all these reasons. But please, if you feel you must do it, ask an actual mechanic. SamuelRiv 20:37, 13 November 2007 (UTC)

Whaaat??? Your answer is almost 100% wrong! We're not talking about gasoline. This is a diesel engined vehicle. The whole business of octane levels are relevent to gasoline powered cars because you need to avoid knocking - but diesel engines are quite different in that regard because they rely on the fuel detonating without a spark. The idea of running diesel cars on left-over cooking oil is extremely well understood (see Vegetable oil used as fuel for example). Thousands of people have done it - it works (although without engine modifications - it only works well in warm climates and with older vehicles). The issues are the ones I've described. The issues you bring up are not a problem.

The onboard computer (the ECU) doesn't "regulate the octane level" - how the heck could it possibly do that? There isn't some separate 'octane supply'! What it generally consists of is a small microphone that detects the onset of 'knocking' in the engine due to insufficient octane in the fuel. The computer then alters the fuel/air mixture to avoid the damage that might result. But that's all irrelevent here because this isn't a gasoline engine!

SteveBaker 23:45, 13 November 2007 (UTC)

A computer that can arbitrarily alter hydrocarbon composition of a mixture would be impressive, indeed... -- 128.61.20.199 13:35, 14 November 2007 (UTC)

Thanks for all your ideas. I will do some more research on conversion kits and suchlike. It seems another option is to add methanol to the veg oil to thin it down but I don't know how well that works. GaryReggae 12:36, 14 November 2007 (UTC)

A big problem with methanol is that it is an organic solvent, so you'd want to talk to someone who knows organic chemistry before subjecting the various fuel lines and seals in your engine to that. It may also react with or give the fuel undesirable properties, but I don't know enough chemistry to say. Anyway, running diesels on veg oil works beautifully from my experience, just make sure you take the precautions that Steve mentioned. -- 128.61.20.199 13:32, 14 November 2007 (UTC)

Yeah - absolutely. Even the 10% ethanol that they are putting in gasoline these days can seriously mess up the engine of an older car. I don't know about methanol - but the results could be similar. Ethanol dissolves rubber and some kinds of plastics. On modern cars they've been aware that ethanol was likely to be added to gasoline for many years - so modern cars have seals made of plastics that don't dissolve - but I've noticed all sorts of seal failures in my 1963 Mini since we switched over to E10 a year or two ago here in Texas. There are other considerations too - ethanol produces acidic residues from combustion that can destroy the viscosity of non-synthetic motor oils, it also conducts electricity so E25 ethanol (coming soon to a gas station near you!) can short out the fuel gauge sending unit or your fuel pump on some older cars.

I don't know which (if any) of these things also applies to methanol - but unless you know for sure that methanol has been sucessfully tested by a lot of people - I wouldn't use it. Mix your veggy oil with regular diesel fuel by all means - but getting creative is not advised! SteveBaker 18:39, 14 November 2007 (UTC)

"From the Fryer to the Fuel Tank", by Joshua Ticknell is the standard book I read on the subject. (try to get the second or any later edition). Also, ReNew magazine published in Aust has articles occasionally. Lots of considerations. The price of old chip oil in rural victoria has gone up in the last 5 years as a result of conversions.Polypipe Wrangler 20:14, 14 November 2007 (UTC)

PMTs, APDs, DUV, and other exciting acronyms

Anybody happen to know roughly what the window of time for detection of a Cherenkov radiation event in a particle detector is? This statement in the first paragraph of the photomultiplier tube article struck me as possibly outdated and unsubstantiated:

"Semiconductor devices like avalanche photodiodes have replaced photomultipliers in some applications, but photomultipliers are still used in most cases."

Modern APDs (like the ones my group produces for UV wavelengths) approach the gain afforded by PMTs and enable single-photon detection with enormously lower cost, power requirements and physical size, but higher reliability, bandwidth, speed, quantum efficiency, etc. The advantages of APDs for this application seem to be somewhat confirmed by a presentation given by a KEK scientist about a next-generation Cherenkov radiation based neutrino detector, though his avalanche gain figure is outdated (gain over 10⁴ is quite doable with APDs these days).

The only area I can think of where PMTs might still hold an advantage regards the window of event detection. I have no idea how much time you have to detect the photons emitted from a Cherenkov event, nor exactly how PMTs are operated (are they run continuously or sampled?). Typically high-gain APDs are operated in Geiger mode--they are biased below breakdown and then pulsed into breakdown wherein they operate at very high gain for the duration of the pulse. While this pulsed/sampled mode of operation can be run reasonably fast (several MHz with pulses on the order of tens of nanoseconds with a good device), I don't believe it's possible to operate very high gain APDs in breakdown continuously for risk of various destructive breakdown mechanisms. The higher quantum efficiency of APDs may make up for the possible deficiency of Geiger mode measurement, but I have no figures for PMTs to compare with.

So enough rambling, here are my burning questions: How long do you have to detect Cherenkov radiation? Presumably the data coming from these neutrino detectors is sampled for digital processing, but how quickly? Are the photomultiplier tubes operated continuously? Does the whole system operate in a sort of "triggered" mode, operating the photomultipliers continuously and waiting for a detection? What percentage of "missed" events is considered acceptable? Does anyone happen to know what the quantum efficiency of a PMT typically is? (I'd guess it's mostly limited by the front-end optics and the quantum efficiency of the photoelectric effect in the photocathode) Have I lost more than 90% of the readers of this page by now? Would I be better off learning Japanese and calling up the KEK to ask my questions? :)

Thanks for any insight you can provide.

Bonus: Impressive picture of the innards of the Super-Kamiokande detector. Notice the two technicians and the size of each photomultiplier tube (enhanced by the big lens assembly). Now consider the infamous Super-K PMT implosion accident which took out over half of these in a chain-reaction, and the motivations for using micron-sized APDs rather than enormous evacuated tubes become apparent!

-- mattb 16:42, 13 November 2007 (UTC)

IceCube operates continuously with triggered events. Trigger timestamping is at <5 ns, and the resulting current pulses are digitized at 250 MHz. The size of pulses allows one to discriminate events running from 1 to ~200 photons per 15 ns. Quantum efficiency for those PMTs is typically 20%, with practical efficiencies with a single photon closer to 10%. Many events involve more than 1 photon. For IceCube like environments, you also need to operate at -40 ° C. Dragons flight 17:44, 13 November 2007 (UTC)

Ah ha. So the necessary detection window is around 15 ns, and the detector needs to be able to distinguish ~1-200 photons in that time? That's a lot shorter window than I thought. Continuous linear operation with high gain is indeed a big boon.. You can operate APDs continuously in their linear (low gain) region and outpace the PMTs, but high-gain Geiger mode operation is necessary for single-photon detection and isn't nearly as fast... With a 15 nanosecond window for counting photons, I don't think current APDs alone could cut it, since the shortest Geiger pulse I've seen an APD used with is a bit under 1 ns. Even with good active quenching, there's a necessary 1 or 2 ns delay before the APD can be reset for another count, so if you're right there's no way they could distinguish between 200 photons in 15 ns using Geiger mode... Realistically, I don't think Geiger mode is even practical for periods in the tens-of-picoseconds range; generating timing pulses that short is difficult, and quenching fast enough is even more difficult. That explains the interest in hybrid detection schemes using an APD in linear mode... Thanks for the info. -- mattb 18:51, 13 November 2007 (UTC)

This page indicates a maximum 30% quantum efficiency and overall gain up to about 10⁸. DMacks 17:47, 13 November 2007 (UTC)

Free-falling

When we fall, we fall at a rate of approximately 33 feet per second, per second.

Is this an 'exponential' rate? Do we fall at a speed that is increasing 'exponentially?'

Peter Lamont —Preceding unsigned comment added by 72.39.249.249 (talk) 17:09, 13 November 2007 (UTC)

Free-fall due to gravity on earth is at an acceleration of about 32 feet per second, per second. Note it's "per second per second", so it's not a simple speed of falling. What it is, is how fast the speed increases: every second of falling, one is falling at a speed 32 feet/second faster. It's a linear increase in speed because gravity is an approximately constant force in this situation. DMacks 17:15, 13 November 2007 (UTC)

Neglecting air resistance and the changes in gravity due to distance and (eventually) relativistic effects - then over time:

• Your accelleration is constant (at roughly 9.8 ms^-2 - which is about 32 feet per second per second).
• The speed with which you are falling is increasing linearly.
• The distance from your starting point is increasing exponentially (but not in the mathematical sense of e^x).

SteveBaker 17:38, 13 November 2007 (UTC)

Actually, that's not an exponential increase. For the speed to increase, then we need to have $v(t)=ab^{t}+c$. The velocity, is, in fact, given by $v(t)=ct+v_{0}$ where c is a gravitational constant (32 ft/s² or 9.8 m/s²) and v₀ is the starting velocity. Donald Hosek 17:43, 13 November 2007 (UTC)

The technical term for this type of increase is "increasng polynomially." That's faster than "increasing linearly" but slower than "increasing exponentially." in this case the ploynomial is of order 2 (the square,) so we can also say is is O(n²) -Arch dude 17:51, 13 November 2007 (UTC)

• And therefore we can alternatively say "increasing quadratically". --Anon, 23:35 UTC, Nov. 13.

(ec) Let's run successive integrals: constant acceleration, linear speed, quadratic position. DMacks 17:52, 13 November 2007 (UTC)

see also Big O notation. -Arch dude 17:53, 13 November 2007 (UTC)

Also note that terminal velocity will tend to put a cap on acceleration if you give it enough time (and aren't in a vacuum) GeeJo ^(t)⁄_(c) • 18:18, 13 November 2007 (UTC)

Length of US State Coastlines

I saw a VERY cool entry called "List of Countries by Length of Coastlines"

And naturally wanted to extend that to our 50 States and was hoping to find that already computed somewhere .....

Has it been?

Many thanks! Drew

(Email address removed to prevent every spammer sending you 'special offers'... Skittle 18:45, 13 November 2007 (UTC))—Preceding unsigned comment added by 71.160.50.240 (talk) 17:58, 13 November 2007 (UTC)

I did some digging, but I have yet to find such a list. In fact, I'm having trouble finding the info state-by-state. We could compile a list and create a Wikipedia page with it if we ever find the facts. --Milkbreath 20:12, 13 November 2007 (UTC)

You could start here - just the Atlantic and Gulf coasts. Cheers Geologyguy 20:17, 13 November 2007 (UTC)

Here are the rest. Cheers Geologyguy 20:18, 13 November 2007 (UTC)

It's also traditional to bring up How Long Is the Coast of Britain? with these questions, emphasizing that states like Maryland can see a particularly large coastline versus what might be expected. — Lomn 20:19, 13 November 2007 (UTC)

One source (see the section "Coastline and Shoreline" and note the disclaimer about being "generalized" and made from "small scale maps" -- meaning the figures probably err on the side of being too small)

NOAA has been creating high-quality shoreline datasets recently. These would probably provide the most accurate info on coastline length currently available. I'm not sure if they have finished building the datasets yet, or whether one would need to use something like GIS to get a list of coastline lengths by state. The main webpage for the project is here (NOAA's Coastal Geospatial Data Project). The project includes far more than just coastline data. A page specifically on shoreline data is here (NOAA Medium Resolution Shoreline, GIS Data). More info here. There's tons of pages and info on these NOAA pages, so it might take some effort to figure it out. Still, the best coastline length data would be from there. I looked at the two PDFs linked above and it looks like they use NOAA data, but not quite the most recent data, athough I could be wrong. Pfly 21:10, 13 November 2007 (UTC)

This is a famous introductory problem in fractals. Basically, there is no such thing as an "accurate" estimate of coastline length, as the coastline is self-similar to multiple scales. Any measure of coastline must state what the length of the "meter stick" used to measure is. Otherwise, you get a length that approaches infinity with increasing precision. Two separate books will quote two wildly different figures because of this. SamuelRiv 23:47, 13 November 2007 (UTC)

You could give the length as a function of how close you looked, couldn't you? Also, what if you took the difference in area covered in high tide and low tide, and divided that by the average distance between the high- and low-tide shores? — Daniel 00:34, 14 November 2007 (UTC)

Coastlines are fractal-like and self-similar among some scales. But when measuring at a scale fine enough to be affected by tides, for example, the analogy of coastlines as fractals breaks down. The problem then is one of defining what to measure -- low tide line, high tide? Also, rivers often widen into estuaries and merge with the sea without an obvious demarcation. At the kind of measuring scales common to things like the NOAA projects, these are the problems with coastline measurement, not fractals. Plus, when measuring at this fine scale, differences of coastline definition (where in the tide zone to measure, where to delineate estuary vs. sea, etc) are not going to result in the kind of wildly divergent figures one finds at the larger scales where fractal issues arise. In short, the idea that coastlines are fractals is only true under certain conditions and scales -- coastlines are not fractals in the strict, mathematical, absolute at all scales notion. Like most real world phenomena, mathematical analogies break down at some point, once you look close enough. I suspect the fractal coastline thing has been a little too much of a good introductory example for explaining fractals. But remember the difference between pure mathematics and the real world. Coastline lengths do not "approach infinity" as measurement precision increases. Just go to the beach with a centimeter-long ruler and take a stab at measuring the coastline and you'll find the problem isn't so much that you are measuring something fractal but more that you keep getting your feet wet as the waves wash up; and what time is low tide today anyway? (sorry for being overly critical, the fractal coastline thing is a minor pet peeve of mine!) Pfly 07:35, 14 November 2007 (UTC)

It is fractal to many orders of magnitude. Obviously it is not a perfect fractal. But let's say you were on a rocky coastline, not a sandy one, so tides wouldn't affect things too much. How do you measure into the crevasses of each rock where water gets in with a centimeter ruler? The point is that there's practically no such thing as an "true" coastline measurement. SamuelRiv 13:14, 14 November 2007 (UTC)

I absolutely agree - there is no correct answer to this question - it depends ENTIRELY on how precise your ruler is. The difference between measuring with a 10m ruler and a 10cm ruler is huge - it's not going to be a 1% error or even a 10% error - the length you end up with could easily double when you measure around every little inlet and rock rather than taking 'short cuts' across them. Truly, we should not be encouraging people to look at the results of this or that survey - the correct answer to this question is that the length of a coastline is unknowable. SteveBaker 18:30, 14 November 2007 (UTC)

I understand the inherent problems, but I still don't see the harm in making a list of US states ranked by coastline length based on a measurement system standardized for the nation. Just make sure to point out the uncertainty of the figures and the vagueness of what a coastline is in the first place. That's what is done with lists of rivers by length (or ought to be done!), even though measuring river lengths has the same problems as measuring coastlines. Pfly 22:44, 14 November 2007 (UTC)

User:Milkbreath made the page yesterday List of U.S. states by coastline. Perhaps the participants in this thread may want to add the appropriate caveats and links to fractals. Cheers Geologyguy 22:48, 14 November 2007 (UTC)

Squinty eyes focus better

I notice my vision has changed since I originally got my glasses (I'm near sighted). However, I notice that if I slightly squint my eyes while wearing glasses, my view is much clearer. What's the science behind this? Am I slightly reshaping my cornea by squiting or is less light passing through causing my iris to have a higher "f stop" or something entirely different? --24.249.108.133 18:13, 13 November 2007 (UTC)

First, we have to get this out of the way... This is not a medical diagnosis. Some Wikipedia users feel that any description of the human body is a diagnosis and delete both the question and answer out of fear of someone suing Wikipedia.

It is very common to have extremely clear vision when you first get a new pair of glasses (or contacts) and then it blurs. Being nearsighted, your eye is basically too long for the lens. When you get new glasses, the muscles learn to relax and make it just a tad longer. By squinting, you do two things - you put pressure on the lens and you shorten the length of the eye. Combined, this corrects the nearsightedness problem. Of course, you can't squint hard enough to correct any significant level of nearsightedness - so walking around squinting all the time is not an option to wearing proper corrective lenses. -- kainaw™ 18:19, 13 November 2007 (UTC)

Kainaw, could you please avoid dragging meta-discussions onto the desks? Particularly argumentative misrepresentations of people's motivations.

On another note, that wearing glasses causes vision to worsen is certainly not proved, according to my (actually trained) optician. It could be, but the evidence isn't there yet. (Also, I'd always heard that squinting was more to do with creating a smaller aperture than reshaping the eye, but I'm not so sure about that one). Skittle 18:44, 13 November 2007 (UTC)

I haven't considered it a "meta-discussion". There is no discussion. Some users delete questions and answers without any discussion at all. If you know of a better way to stop the problem, feel free to make a suggestion. I should note that this isn't an issue with one user and one question. I've seen many questions deleted by many users and I've seen no means of controlling the problem. -- kainaw™ 20:26, 13 November 2007 (UTC)

The relevant discussions have been had on the talk page many, many times. When people consider a removal contentious, they tend to discuss it on the talk page and determine consensus. The reasons for deleting medical questions and advice are explained at some length all over the talk page archives, as well as at a user subpage (I forget which one). If you have a problem with a particular deletion, please bring it up on the talk page and explain why you feel that particular deletion was inappropriate. If necessary, you could also place a message on the talkpage of the person who carried out the deletion. If you have further to say on this topic, please take it to the talkpage. Skittle 15:23, 14 November 2007 (UTC)

Squinting also has the effect of making your depth of focus deeper, similar to how a pinhole camera doesn't need a lens. --Mdwyer 20:28, 13 November 2007 (UTC)

I think that's a big part of it, and another part may be that "stopping down" the lens (using only the central part of it) reduces aberrations. Photographic lenses certainly benefit from being stopped down (until diffraction becomes important), and I would expect the relatively simple optical design of the eye to likewise benefit. -- Coneslayer 20:39, 13 November 2007 (UTC)

So in short, the answer is yes, squinting is like increasing your eyes' f-stop (i.e. narrowing their aperture). --Anonymous, 23:37 UTC, November 13, 2007.

AND the change in eye shape, as kainaw mentioned. I think they are both significant. --Mdwyer 03:42, 14 November 2007 (UTC)

So it's a bit of both, eh? Interesting. (BTW, I wasn't seeking medical advice. I was just curious about this phenomenon.) --24.249.108.133 18:14, 14 November 2007 (UTC)

Doot dooo do doo doo. Before the scourge of corrective lenses were foisted upon me, I found that rubbing my eyes would cause the blackboard to swim into focus for a few seconds. This was primarily the deformation of my eye. The amount of deformation needed to change (correct?) focus is incredibly small, as I discovered later when eye surgeons started poking at my eye with ultrasonic probes -- the focus would change in a little donut-shaped zone around the probe. It was a truly strange experience. As a child, I didn't know about the pinhole effect, yet, though, so I didn't get to try that one. It must be a common occurance, though, since teachers are told to watch for their kids rubbing their eyes, as it might be a sign of poor vision. --Mdwyer 01:45, 15 November 2007 (UTC)

Hard Coupled Forces Problem

Imagine a train consisting of N frictionless cars following a locomotive that is accelerating the whole train forward with acceleration of magnitude a. Assume that the first car behind the locomotive has mass M. If the tension in the coupling at the rear of each car is 10% smaller than the tension in the coupling in the front of the car, what is the mass of each sucessive car as a fraction of M? What is the total mass of the cars in terms of M? [Hint: You might find it helpful to know that 1+x^2+x^3+...+x^n=(1-x^n+1)/(1-x).] —Preceding unsigned comment added by 164.107.244.238 (talk) 18:59, 13 November 2007 (UTC)

How is it that you're in a position to give us a hint, yet unable to solve the problem yourself? -- Coneslayer 19:45, 13 November 2007 (UTC)

There's a good chance the hint was provided with the original homework question. — Lomn 20:14, 13 November 2007 (UTC)

It was in the original homework problem and you don't need to be a jerk coneslayer

Nor do you, and Coneslayer posed a fair question. Anyway, as noted above, the Reference Desk will not do your homework for you. If a particular part of the problem is causing difficulty, then perhaps we can assist or advise. — Lomn 20:29, 13 November 2007 (UTC)

(To anyone else who has solved this: it's quite neat, isn't it?) To the OP:We could certainly help you better if you told us what you have attempted and what you are having trouble with. No-one's likely to just post a full solution, and we can't give specific advice without knowing the problems you're having. For example, are you familiar with Newton's second and third laws of motion? Algebraist 20:42, 13 November 2007 (UTC)

When setting the accelerations equal

A1=A2

you can say

F1/M1=F2/M2

(T1-T2)/M=(T2-T3)/(M*x)

T2=.9T1

T3=.9T2=.81T1

so

(1-.9)=(.9-.81)/x

so x=.9

thus each car is ninety percent the mass of the car in front of it.

The total mass

Mt = M(1+x+x^2+...+x^(N-1)) = 10M*(1-.9^N) —Preceding unsigned comment added by 164.107.244.238 (talk) 21:44, 13 November 2007 (UTC)

Well done! Unfortunately, you have made a slight error: your arguments do not apply for the Nth (ie rearmost) car, which is not being pulled from behind. You need to consider this case separately. Algebraist 21:49, 13 November 2007 (UTC)

The condition in the question(T(K+1)=.9T(K)) will never apply to the last car. If it did, every tension would have to be zero. —Preceding unsigned comment added by 164.107.244.238 (talk) 22:33, 13 November 2007 (UTC)

Indeed, that condition must fail at the bacl. Thus the last car experiences a forward pull of TN=0.9^(N-1)T1, and no rearward pull at all. Algebraist 22:37, 13 November 2007 (UTC)

Is there a landmass under Antartica and the Arctic?

If the ice and snow melted, would you see dirt/rock? 64.236.121.129 20:09, 13 November 2007 (UTC)

Antarctica is a continent and thus has a landmass. The Arctic has no such continent. — Lomn 20:14, 13 November 2007 (UTC)

Antartica, yes. Arctic, no. —Preceding unsigned comment added by 134.84.156.80 (talk) 20:15, 13 November 2007 (UTC)

By the way, not even all of Antarctica is "under" any ice; there's a certain amount of bare dirt. Not a lot of it, but some. See Dry Valleys. --Trovatore 20:18, 13 November 2007 (UTC)

So is the Arctic just a big slab(s) of ice floating on water? 64.236.121.129 20:20, 13 November 2007 (UTC)

Generally speaking yes. See Arctic. And just for fun, you might want to read up on USS Nautilus (SSN-571). --LarryMac | Talk 20:27, 13 November 2007 (UTC)

If you look on Google Earth, there is actually nothing at all shown at the North Pole (the Arctic) while there is a large icy landmass at the South Pole, the overall earth imagery must have been taken during the Northern hemisphere summer months as the Arctic virtually all melts away...of course while it's summer at the North Pole, it's winter at the South, hence why the Arctic is not there but there is loads of ice at the Antarctic. GaryReggae 20:42, 13 November 2007 (UTC)

Antarctica is actually (at least) two landmasses. --Carnildo 23:42, 13 November 2007 (UTC)

Geography of Antarctica doesn't say that. On the other hand, Geography of Greenland does say that Greenland, the largest Arctic island, may really be three islands. The thing is that the ice sheet (in both places) is so thick that it's hard to measure what's under it. --Anon, 23:49 UTC, November 13, 2007.

The sub-ice configuration of both Antarctica and Greenland is fairly well known, and has been published in National Geographic atlases for years. One version for Antarctica is here. Cheers Geologyguy 23:58, 13 November 2007 (UTC)

The problem is that the land is pushed down under that mass of ice. If the ice ever melted, the land would tend to 'spring back' - increasing the height of all of the terrain. On the other hand, if that amount of ice were ever to melt, the sea level rise would inundate more of the antarctic continent. Between those two counteracting effects and our relatively poor view through all of that ice, I think it's hard to guess the precise result. SteveBaker 01:45, 14 November 2007 (UTC)

Uplift totally wins. The isostatic rebound at equilibrium would be roughly 35% of the ice thickness, which at places means more than 1 km of uplift. The sea level rise due to melting is only ~70 m. Discounting small islands, both Antarctica and Greendand would be a single land mass after accounting for the uplift. Dragons flight 16:06, 14 November 2007 (UTC)

Wow! That's a lot! I'd have guessed the rebound was only tens of meters...but a whole kilometer of uplift! Amazing! SteveBaker 18:24, 14 November 2007 (UTC)

Values around 300-350 meters of rebound are well documented in the Hudson Bay and Scandinavian Shield areas [3] - I couldn't find a ref quickly but I agree with Dragons Flight that values more than 1000 m are known. Cheers Geologyguy 21:08, 14 November 2007 (UTC)

But that uplift takes place over thousands of years (the Hudson Bay / Scandinavian Shield uplift mentioned is still ongoing since the last ice age), whereas the sea level rise could be much faster. Short term, the sea level rise might be the much more important effect. --169.230.94.28 21:57, 14 November 2007 (UTC)

Well, that's got to depend on how fast the ice vanishes. If it all disappeared overnight, I'd agree - but if it melts slowly, then presumably there would be time for the land to start to 'relax' before the flooding gets bad. SteveBaker 02:06, 15 November 2007 (UTC)

Anyone have a 'sub ice' map of Greenland? I'd be interested in seeing that... --Kurt Shaped Box 02:10, 14 November 2007 (UTC)

Guess what! asked and answered right here! To a degree, at least. Cheers Geologyguy 15:41, 14 November 2007 (UTC)

Very cool. Thanks very much. :) --Kurt Shaped Box 23:05, 14 November 2007 (UTC)

I've created a map of what a fully deglaciated Antarctica would look like after accounting for isostatic rebound and sea level rise: [4] Dragons flight (talk) 04:08, 19 November 2007 (UTC)

Hard disk

Is there an associated change of mass, even at the atomic level, when writing data to a hard disk? In other words, does a full disk drive weight more than an empty one? Rockpocket 20:37, 13 November 2007 (UTC)

I doubt it, an HDD is a sealed unit so nothing can 'get in' to increase the weight. All the read/write heads do is rearrange magnetic particles but those particles have to be there in the first place. GaryReggae 20:39, 13 November 2007 (UTC)

(Off on a tangent...) Hard drives aren't actually sealed; there's always some path that allows a slight flow of air in and out of the head/disk assembly, if only to easily accommodate changing barometric pressure.

Atlant 02:33, 14 November 2007 (UTC)

No, you aren't adding or removing particles. You are flipping existing particles upside down. --Mdwyer 20:48, 13 November 2007 (UTC)

Doubtful, even on a subatomic level. Writing to a hard drive does not involve adding or removing matter, and as far as I'm aware, the magnetic potential energy of a full hard drive does not differ from that of a blank one. --Carnildo 23:45, 13 November 2007 (UTC)

Well (blatantly armchairy speculation ahead, beware), when you write to a disk, you're changing its entropy. A disk with all 0's or all 1's on it has less entropy, and is therefore (I think) somewhat less energetically favorable, than a disk with random patterns of 1's and 0's. A disk with highly ordered patterns of 1's and 0's obviously has less entropy, too. It takes energy to create order, and (if what little I know about thermodynamics is accurate) there's a certain energy cost in creating order / decreasing entropy above and beyond (or perhaps I should say below and beneath) the energy required to actually (in the case of a magnetic disk) induce the tightly-polarized little magnetic fields and stuff.

Now, as we all know, devices with potential energy stored in them weigh slightly more, due to e=mc². So does a device that contains an infinitesimal amount of anti-entropic energy in it therefore weigh an even more infinitesimal (less tesimal?) amount more? (I have no idea, and actually I kind of doubt it, because I think I'm mixing apples and apples here, because I'm not at all certain that entropic randomization of an unstably ordered state is an exothermic reaction; rather, it's an inevitable result of other, exothermic reactions. But anyway. Perhaps someone who knows more about this stuff than I do can set me straight.) —Steve Summit (talk) 03:28, 14 November 2007 (UTC)

I think Information entropy and Thermodynamic entropy are different. --antilived^{T | C | G} 05:07, 14 November 2007 (UTC)

I'm surprised that nobody has mentioned the actual effect utilized to store data in most modern hard disks, giant magnetoresistance. GMR does not involve rearranging particle position per se, and I'd be hesitant to foster weird notions about physics by comparing particle spin with macroscopic orientation. Basically GMR enables a large resistance change in material stacks with differing magnetic orientations because it takes more energy to cause electrons to flow through a magnetic domain if their spin is aligned differently than the magnetic moment ordering of the domain. (see ferromagnetism) Back to the original question, any mass gained or lost is totally negligible; macroscopic object masses fluxuate many orders of magnitude more when a dust particle settles on them. (if that's unsatisfying, then the random accumulation of electrical charge may provide compelling reason not to care much about these infinitesimal mass changes) -- 128.61.20.199 13:19, 14 November 2007 (UTC)

Entropy is not a form of energy. Sure, you need non-thermal energy to reduce entropy, but this energy is converted to heat (which can easily leave the hard drive).

The energy, and thus the mass, of the hard drive is probably higher when there are long runs of all 0's or all 1's, compared to alternating 0's and 1's, due to magnetic repulsion between domains of the same orientation. Icek 18:07, 14 November 2007 (UTC)

As far as I'm aware, hard drives use data encodings designed to eliminate such long runs, because long runs tend to result in read errors. --Carnildo 00:18, 15 November 2007 (UTC)

That's interesting, could you point me to some source of further information? Icek 14:51, 15 November 2007 (UTC)

Check out Run length limited encoding. -- Coneslayer 18:43, 15 November 2007 (UTC)

Antenna length

The radio station WUMB has a page here that suggests a way to make a dipole antenna. They say each branch of the antenna should be 30.5" long to optimize your reception of their 91.9 MHz signal. I've read Dipole antenna, which shows a formula of 468/f(MHz) feet, which leads to an antenna length of 0.5223 inches. Can anybody explain how WUMB arrived at their figure, and how this might be generalized to any radio frequency? Thanks, jeffjon 20:39, 13 November 2007 (UTC)

I think you've made a math error; 468/91.9 = 5.09 feet, which is 61 inches (or 30.5 inches for each half). -- Coneslayer 20:43, 13 November 2007 (UTC)

Hmm. The only thing that's better than a math error, is a math error that's so obvious I can't fathom how I might have made it. Thanks. jeffjon 21:36, 13 November 2007 (UTC)

If you're wondering where this seemingly arbitrary number comes from, it corresponds to a little less than half the wavelength (in air). Exactly half the wavelength gives two quarter wave rods, which provides the best resonant properties. However, the antenna input impedance is slightly reactive if its length is exactly a half wavelength, and adjusting the length down a bit causes the reactive part to vanish. Purely real impedances are very easy to match, and conveniently the real impedance of a slightly-less-than-half-wave dipole is fairly close to 75 Ohms. You can, of course, match an antenna with complex-valued input impedance, but it's additional trouble, could involve this beauty, and easier for hobbyists to just cut the antenna to appropriate length. The main reason you want the antenna impedance matched to the line is so the maximum amount of power can be delivered to whatever is at the other end (a LNA, perhaps) instead of being reflected back. As I hinted at above, the dipole configuration mentioned has a real impedance that results in a very low amount of reflected signal due to antenna mismatch when connected to a 75 Ohm line, less than 2%, actually. -- 68.158.1.192 06:53, 14 November 2007 (UTC)

Simplified Electromagnet Math

I'm doing the grade-school experiement of making electromagnets in my kitchen. I'd like to maximize the strength of the magnet while minimizing the copper. It seems like I should be able to plug in all the variables and get an answer. The math is pretty daunting, however! Is there a simplified way to handle this math:

$H\approx 787\ {\mbox{ampere.turns/meter}}$

For sxample, say I've got a .25" thick nail, and I'm going to run 12VDC through it. Is there a way I can draw a graph of turns vs ohms and figure out where the sweet-spot is? In the above math, is 'meter' actually the square-meter cross-section of the nail?? --Mdwyer 21:08, 13 November 2007 (UTC)

Yeah, this is a pretty gnarly optimization problem. It wouldn't be so difficult if you assumed an ideal voltage source (capable of supplying unlimited current) and a linear magnetic material (that reacts the same to an arbitrarily strong magnetic field and never saturates), but that's totally unrealistic. In practice, you're going to be limited by the internal resistance of the power supply (which limits the amount of current you can draw from it) and the magnetic saturation of the ferromagnetic iron core (which limits the magnetic field, at about the amount you quote). Frankly, I think trial and error would be easier to do than a thorough theoretical analysis, and guaranteed to give realistic results. —Keenan Pepper 23:18, 13 November 2007 (UTC)

The theory is straightforward: magnetic inductance is linearly proportional to the number of turns. Increase turns, and you increase inductance. The resistance encountered per length of wire depends on the wire's gauge (look it up), but will be negligible until you approach many hundreds of turns. Basically, from what I recall, we got about 8 ohms per meter of very thin copper wire in one experiment. Magnetic inductance is also directly proportional to current, so at constant voltage, doubling the number of turns doubles the length but reduces current by half, so your net gain is zero. Basically, in the end resistance is not going to be your problem. SamuelRiv 23:54, 13 November 2007 (UTC)

On this page there's a chart showing that, given a fixed voltage, they got a stronger magnet by using fewer turns of thicker wire, but the magnet had heat problems. That just seems backwards to me, for some reason. I always see magnets use lots of turns of really fine wire, but the page seems to suggest that, given a big enough power supply and a way to handle the heat, you should actually be using fewer turns of thicker wire. Am I still missing something? --Mdwyer 03:55, 14 November 2007 (UTC)

Yes. The magnetic field is proportional to the current flowing through the wire, and the current is limited by the internal resistance of the power supply and the resistance of the wire itself. The way to make a strong magnet is not only to have many turns, but also thick, low-resistance wire, and something to pump a heck of a lot of current through it. The resistive cores of the most powerful magnets in the world look like this: Florida-Bitter coils. See how much metal there is? The only holes are for cooling, because they have to pump dionized water through the magnet incredibly fast to keep it from overheating. —Keenan Pepper 06:13, 14 November 2007 (UTC)

Geez... Given the insane power and cooling requirements for those 35 T magnets, I'd think a high-temperature superconducting coil would be more effective. -- 128.61.20.199 12:47, 14 November 2007 (UTC)

They do use superconducting magnets as well, but the problem with that is that superconductors tend to expel magnetic fields (the Meissner effect), and if the field becomes too strong, the superconductor changes phase into a normal, resistive state. So there is a fundamental limit to the strength of a superconducting magnet, and the strongest magnets are in fact hybrids, with a resistive core inside a larger superconducting magnet. —Keenan Pepper 15:50, 14 November 2007 (UTC)

If I were faced with the design problem you stated, I would want to know the characteristics of the 12 volt DC supply. Is it like a car battery, which could supply hundreds of amps for a short while, or is it like a lab power supply, which might be current limited to a few amps or a fraction of an amp? As for heat problems, how long does the magnet have to stay energized?Many electromagnetic devices are designed for temporary or intermittent operation. As for the wire, magnet wire is made with a thin coating of insulation, so that more turns can be placed closer to the magnetic core than would be the case for, say doorbell wire or wire intended for 120v or 240 v AC service. Joseph Henry in the 1830's created the first truly powerful electromagnets (able to lift thousands of pounds) by using many turns of insulated wire. One thing he did was to place several windings on the same core, in parallel, so that each winding had a low resistance and therefore could carry relatively high current, but the multiple windings added their magnetizing effects. A nail makes a poor electromagnet compared to a u shaped or horseshoe shaped core, where the pole pieces are in proximity. Edison 20:27, 14 November 2007 (UTC)

BINDEEZ

Hi, I need a FULL ingredient list for Bindeez/ aqua dots —Preceding unsigned comment added by 68.193.21.179 (talk) 21:30, 13 November 2007 (UTC)

Since they're not considered food, there's little chance of a publicly-available breakdown of what goes into the product. — Lomn 21:33, 13 November 2007 (UTC)

Pretty sure it would be a trade secret. bibliomaniac15 A straw poll on straw polls 23:21, 13 November 2007 (UTC)

Unless it was patented. But I don't see any obvious patent claims, so maybe it's just a trade secret. --24.147.86.187 01:13, 14 November 2007 (UTC)

If you're interested in the "date rape drug" aspect, see our article on it Bindeez, which describes what chemical was substituted for the one that was supposed to be there. --24.147.86.187 01:13, 14 November 2007 (UTC)

Two identical humans

Two identical human both "weights" 70 kilograms.

They both jump off a high place (simultaneously). One of them is attached to a frictionless pulley and to a 65 kilograms counterweight. Assuming no air resistance, will both of them hit the ground at the same time? Will the one with the counterweight survive the fall if the height is deadly for the other human? 202.168.50.40 23:21, 13 November 2007 (UTC)

More homework, is it? We don't do homework. --Anon, 23:51 UTC, Nov. 13, 2007.

I'd be a bit surprised if none of us do homework. :P —Tamfang 23:59, 13 November 2007 (UTC)

Some of us don't even have homework :-p Someguy1221 00:20, 14 November 2007 (UTC)

But for the OP, this is a very straightforward problem; draw out your force diagrams, and calculate the acceleration (including direction) of everything involved. Someguy1221 00:22, 14 November 2007 (UTC)

And they 'll both die since no air resistance implies no air so they'll asphyxiate. Donald Hosek 00:24, 14 November 2007 (UTC)

When I was designing problem sets (for the one class I taught that required them) I really wanted to kill off a few of our hypothetical astronauts and things like that, just to see if the kids were paying attention. --24.147.86.187 01:16, 14 November 2007 (UTC)

I think they both hit the ground at the same time because for the human with counterweight, his/her mass is effectively only 5 kilograms (70kg - 65kg = 5 kg). But Gallieo says that both masses will fall at the same rate in Vacuum, so they must both hit the ground at the same time regardless of their different mass. 202.168.50.40 03:46, 14 November 2007 (UTC)

Hmm, interesting no one has picked up on the important principle here. Try a reductio ad absurdum thought experiment. Suppose that the second man weighs 65 TONS instead of kilograms. Now would they both hit the ground at the same time, because Galileo said that in a vacuum objects will fall at the same speed? Hmmm…ponder ponder….

The answer is that the second man will fall much more slowly because of the inertia of the 65 kg weight he has to move. Inertia of massive objects is very important in these considerations. For example people think that because objects are weightless in space, you can just pick up a 1 ton block and throw it around like a beach ball. Wrong. The block doesn’t want to move. You would still have to exert a lot of effort to get it going. Try this next time you are down at an ice rink. Get your skates on and put some castor type skates on a grand piano. Now try moving the piano around the rink (get permission from the manager first). You will find that it’s very difficult to start and stop the piano, and to steer it. It will feel like the piano is trying to waltz YOU around the rink. Similarly, while heavy objects will fall at the same speed in a gravitational field (let’s just forget about air resistance, it’s negligible for massive objects), they don’t behave the same way if something is trying to stop them. That’s why a large asteroid will make a huge hole, while a little meteorite won’t. The second man can be said to weigh only 5 kg, and he would feel like he only weighs 5 kgs when he is wearing his counterbalance on the ground. But does that mean he will fall with the same speed as the 70 kg man, because different weights fall at the same speed (a la Galileo)? No, because the second man is, to use the old-fashioned terminology, expending most of his “potential energy” doing the work of lifting the 65 kg weight upwards, while the first man can covert ALL of that potential energy into kinetic energy. And I eat my homework. Myles325a 05:28, 14 November 2007 (UTC)

Geez guys! Can't someone figure this out properly?!?! It's easy!

Once the rope goes tight - there are two forces on the guy - gravity pulling downwards - the tension on the rope pulling upwards. Gravity wins but only just so there is an almighty jerk and the pull on the rope is such as to let the guy fall and the weight start rising - we can all imagine that.

But what's going on? The appeal to Galileo's (fictitious) dropping weights off of the tower of Pisa is utterly incorrect here. Galileo's result is something of a coincidence: The force due to gravity is proportional to the mass of the object (F=mg) - but the accelleration of an object due to gravity is given by F=ma. Because F=ma and F=mg then for an object dropped off of a leaning ancient monument, we're talking about the same force so we can equate those two force formulae: mg=ma and therefore g=a - which is what Galileo noticed. But if the mass you are accellerating isn't the same as the one that gravity is yanking on, then all bets are off...and that's what's happening here.

However, when there are ropes and weights and all sorts of other stuff going on, things are a bit more complicated. Gravity is pulling on both the man and the weight - but in opposite directions (from the point of view of the man) so the total force due to gravity is F=(mass_man - mass_weight).g (proportional to the difference of their masses)- but the accelleration of the pair requires both of them to be accellerated so F=(mass_man + mass_weight).a - the sum of their masses. In this case, we are accellerating 70+65=135kg with the gravitational force due to 70-65=5kg. So, now we're accellerating 135/5 = 27 times slower than the guy with no rope. SteveBaker 02:02, 15 November 2007 (UTC)

Note that only the acceleration is what has been reduced, given that we have no limits on the height of the original fall, save that it is "deadly for the other human" you can still end up with a velocity which would be fatal. If the hight of the fall began at the minimum fatal distance than we would expect our the tethered adventurer to survive, however from a height of X time that minimum deadly height (and here the OP should do his homework, the means to work out a solution were given in some of these replies) the final velocity would be deadly.

Blade metallurgy chart

I've been searching for a chart that shows the properties of differant metals found in blades including iron and titanium alloys. I can't seem to get anything showing many metals or numarical properties that I'm searching for. I would like to know strength hardness and efficiency of hardening on the alloys, not just this can hold an edge better then that. —Preceding unsigned comment added by 71.102.37.149 (talk) 23:41, 13 November 2007 (UTC)

How about this one? http://www.crkt.com/steelfct.html It's missing some- for instance, one of my favorites is 154 CM, and it isn't included because CRKT doesn't use it. DeepSkyFrontier (talk) 23:27, 18 November 2007 (UTC)