Wikipedia:Reference desk/Archives/Science/2009 December 16

= December 16 =

Nuclear Warfare
How much time does it take for nuclear radiation from a bomb to decay to a safe level? —Preceding unsigned comment added by Cborgen (talk • contribs) 00:05, 16 December 2009 (UTC)


 * That's going to depend heavily upon how one chooses to define a 'safe' level &mdash; does one mean a level that won't cause acute, fatal radiation sickness, or a level that won't appreciably increase one's lifetime risk of cancer? Those two numbers are going to be rather different, and they're going to heavily depend as well on the yield and location of the bomb.  (An air burst will generate less fallout than a bomb detonated close to the ground, for instance.)  Finally, do you want to know when it's safe to approach ground zero (the point where the bomb detonated) or just somewhere within a certain radius?  Whether one is upwind or downwind of the blast will make a significant difference to one's safety.
 * All of that pretty much boils down to 'it depends'. You might find the information (and references) in our articles on Nuclear fallout and Effects of nuclear explosions on human health to be helpful, though. TenOfAllTrades(talk) 00:17, 16 December 2009 (UTC)


 * There are a number of radiation threats from a nuclear bomb. We might characterize them roughly as so: The first is the pulse that you get from the weapon itself—which will kill you if you are within a certain radius (the size of that radius varies with the yield of the bomb). The second is the radiation caused by short-lived fission products. This is intense (will kill you if you are near it) but relatively short-lived—this is what a fallout shelter is designed to protect you from (if you stay underground for two weeks, you miss most of this). The third is the long-term radiation caused by other fallout. This is the sort of thing that will not kill you immediately, but over the course of living near it, ingesting it, etc., will raise the background rate of cancer appreciably, cause birth defects, etc. How much fallout is created (both the second and third types here) varies with the yield of the bomb, the design of the bomb (some bombs are "dirtier" than others), and the height of where the bomb goes off. --Mr.98 (talk) 00:53, 16 December 2009 (UTC)


 * It depends on the bomb. There are various kinds of salted bombs to give different radiation half-lives. The worst is probably the cobalt bomb. ScienceApe (talk) 19:16, 20 December 2009 (UTC)

Stars, black holes, and motion...
From an article about a recently re-analyzed black hole - ''The outermost layers of the star are being siphoned by the black hole. The swirling gas forms a hot plasma disk around the black hole before it disappears, and the process emits a lot of X-rays and radio waves.'' The part in bold, can we observe that in motion? If Hubble takes a look at that pair and then looks back a day later - can we see a change? a week/month/year later? It's easy for me to visualize swirling masses of plasma being sucked off/out of a star and into the black hole, but I'm VERY curious about the timescales involved. Surely observation of the process-in-motion is possible with instruments, it being only a question of resolution? Are we there yet? Can we watch this happening or is it still the "snapshot" approach? 218.25.32.210 (talk) 08:05, 16 December 2009 (UTC)
 * It will most likely be a continuous stream, like in this image (which has a white dwarf, rather than a black hole, but the same concept). I think it is mostly an assumption based on the X-rays - they indicate that it is very hot and there aren't many ways of getting things that hot. Gas spirally into a black hole is one of the few things that can. --Tango (talk) 16:35, 16 December 2009 (UTC)


 * The dynamics of accretion disk material can be studied through spectroscopy; the material's velocity gives rise to an observable Doppler shift. See for example Time-resolved spectroscopy of accretion disks in ALGOLS.  -- Coneslayer (talk) 16:48, 16 December 2009 (UTC)


 * To be clear - we don't have any photographs showing this pretty swirling thing happening. We can only deduce that from other data.  Hence, no - the Hubble can't take a pair of pictures and show it happening.  A better question is whether the spectroscopic data shows that change. SteveBaker (talk) 17:43, 16 December 2009 (UTC)


 * There ARE measurements of radiation from the innermost accretion disk of a BH which indicate that there are enormous velocities involved: the amplitude of the radiation had a frequency of several hundred Hz superimposed. That means that matter circles the BH just above the event horizon several hundred times per second, see eg . --Ayacop (talk) 14:40, 17 December 2009 (UTC)

Condensation Between Propanoic Acid and 3-Aminopropanal
Hello. How is the main product of the condensation between propanoic acid and 3-aminopropanal named according to IUPAC nomenclature? I think that the aldehyde group in 3-aminopropanal does not react and naming the main product 3-formyl-N-propylpropanamide is ambiguous since the aldehyde group can be on the alkyl chain or the 'amide' chain by the naming alone. Is the main product a double-branched amine or a single-branched amide? Thanks in advance. --Mayfare (talk) 09:15, 16 December 2009 (UTC)
 * First you have to figure out the structure (what part(s) react, what the connectivity of the product is), then worry about how to name it. When you have branched-branches, there are specific ways to express what is branched off of what, for example, saying "3-formylpropyl" or "3-formylproanamide" to explain which 3-carbon chain is formylated. Also be careful to use hyphens or parentheses to separate groups "N-propyl-propanamide" means "propyl on N of propanamide" whereas "N-propylpropanamide" sounds like there is a "propylpropanamide" on the N of something else. DMacks (talk) 09:53, 16 December 2009 (UTC)

Total weight of all insects in the world
If you gathered all the insects in the world together, how much would they weigh in total? --OpenToppedBus - Talk to the driver 11:47, 16 December 2009 (UTC)
 * Some background to the question. I've seen lots of sites saying things like "the total weight of insects on the planet is more than all other species put together" or "the total weight of insects destroyed by spiders in a year exceeds the weight of the human population". Last night I heard Johnny Ball claim that for every human on the planet, there are enough insects to equal the weight of seven African elephants. But I can't seem to find any evidence to back any of this up. --OpenToppedBus - Talk to the driver 12:41, 16 December 2009 (UTC)
 * Our article biomass (ecology) contains estimates that the total mass of ants is 9–90 times the total mass of humans. I don't know what proportion of insects are ants. Algebraist 13:00, 16 December 2009 (UTC)
 * Ant says that ants "may form 15–25% of the terrestrial animal biomass". Algebraist 13:03, 16 December 2009 (UTC)
 * And it looks like ants may be about half of insect biomass. So let's say insects are roughly 20-200 times the biomass of humans. Worldwide, an average human is roughly 50kg, so there's 1,000 - 10,000kg of insects for each of us. Even taking the top end, that would still be barely more than an elephant each of insects. So it looks like Johnny Ball was wrong about that last night, as about so much else. --OpenToppedBus - Talk to the driver 14:01, 16 December 2009 (UTC)


 * According to this page: http://www.entsoc.org/resources/faq.htm?/print#triv1 there are about 10^19 (10 quintillion) insects in total. Multiply by whatever you think is the average weight. Mytg8 (talk) 14:22, 16 December 2009 (UTC)

Atoms
why are atoms colourless, despite colour originally containing atoms has colour?

colour or paint is made from various compounds.Molecules combine to form compounds.a moleculeis formed by the combination of atoms.As we know that atom retains its identity throughout chemical change.therefore atoms should have colour —Preceding unsigned comment added by Omkar2510 (talk • contribs) 12:54, 16 December 2009 (UTC)


 * Colour comes from the interaction of a substance with light. Exactly how it interacts with light depends on the structure of the material, and in particular on the legal state changes for electrons in the material. These are different for individual atoms, molecules, and crystals. But yes, atoms do preferably emit and absorb light at certain frequencies - see emission spectrum. In that sense, they do have a colour (but one that is different from most of the compounds it forms, and different in non-trivial ways). --Stephan Schulz (talk) 13:39, 16 December 2009 (UTC)


 * The OP is claiming that since paint has color and is made of atoms than atoms must have colors as well. I just want to point out the fallacy in this logic with an analogy: Humans have thoughts and are made of cells. Does that imply that cells must have thoughts? Dauto (talk) 14:26, 16 December 2009 (UTC)
 * We've got articles on logical error: fallacy of division or fallacy of composition, depending on how you look at it. --Sean 15:34, 16 December 2009 (UTC)


 * As Stephan notes, the behaviour of matter can be very different when one compares bulk solids with nanomaterials with individual atoms. Consider gold as one example.  What colour is it?  Well, obviously it's gold.  Take a chunk of it; it's gold.  Hammer it out into thin sheets; it's gold.  Put flakes of it in your drink; it's Gold...schläger.  Makes it into tiny beads a hundred nanometers across, and it's...bright red.  Whoops.  Take it down to single atoms in a vacuum, and you'd get the sharp absorbance spectrum that Stephan describes.  Per Dauto, the whole is often very different from the mere sum of its parts; the behaviour of large numbers of atoms tends to be very different from the behaviour of single atoms. TenOfAllTrades(talk) 14:45, 16 December 2009 (UTC)


 * To pick a more commonplace example - carbon is black - right? ... Unless it's a diamond when it's completely transparent. It depends on the crystalline nature of the material - not just what it's made of. SteveBaker (talk) 17:39, 16 December 2009 (UTC)

See HOMO-LUMO gap. The colour behaviour of a substance is determined by its chemical environment, e.g. electron charge density and the relative energies of various electronic orbitals. John Riemann Soong (talk) 18:57, 17 December 2009 (UTC)

combined serial and parallel circuits
In a combination of serial and parallel circuits, such as this one, am I correct to conclude that all three lamps shine equally brightly? My reasoning is that a and b/c are connected serially, so both a and b/c should get half of the voltage of the cell. B and c are connected in parallel, so they each get the full amount, thus all three each get half of the voltage of the cell. (I know this might sound like homework, but I am just trying to understand more about electricity and I have no teacher whom I can ask...) Lova Falk (talk) 14:41, 16 December 2009 (UTC)


 * I'm not going to answer straight out, but I will ask &mdash; how much current flows through each lamp? TenOfAllTrades(talk) 14:47, 16 December 2009 (UTC)


 * As far as I understand, lamp a gets twice the current compared to lamp b and c. Lova Falk (talk) 14:55, 16 December 2009 (UTC)


 * Assuming that all three lamps are identical, yes. Now, by Ohm's law, what does that tell you about the voltage drop across lamp a versus across lamps b and c?  TenOfAllTrades(talk) 15:16, 16 December 2009 (UTC)


 * Do I understand correctly that the resistance of each lamp is the same, irrespective of the way it is connected? In that case, the voltage of lamp a should be twice the voltage of lamps b and c. Lova Falk (talk) 16:03, 16 December 2009 (UTC)


 * Yes, resistance is an intrinsic property of an item (in basic circuit scenarios, anyway). You could also approach the solution by combining lamps B and C into a single unit BC (via the equation for parallel resistance) and then solve the series system of A and BC. &mdash; Lomn 16:16, 16 December 2009 (UTC)


 * Bingo. Sounds like you're on your way to a final answer, now.  From here (and for more general cases like multiple components in parallel or unequal resistances/loads) you'll probably want to have a look at the formulae and derivations in Resistor, Series and parallel circuits, and Kirchhoff's circuit laws. TenOfAllTrades(talk) 16:44, 16 December 2009 (UTC)


 * Thank you! Lova Falk (talk) 17:12, 16 December 2009 (UTC)


 * This is a little late, but I should point out that the resistance of a real lamp is not a constant. The resistance is almost zero when the lamp is cold, but goes up very much when hot. So don't try to calculate the wattage of a lamp by measuring the resistance when off, and using ohms law.


 * The second implication is that if you actually made the circuit in the diagram with real lamps, you will not see what you expect. The lamp with less current flowing through it will not be as hot, so it's resistance goes down. Normally more current will then flow (heating it up) - but when it tries, the other lamps get hotter, increasing their resistance. So the current stays low, and the lamp with less current flowing through it will actually by dark, rather than half on. Ariel. (talk) 08:43, 18 December 2009 (UTC)

new planets
I have questions after reading http://news.bbc.co.uk/2/hi/science/nature/8414476.stm but don't know where to look in Wikipedia. (1) On a planet five time the mass of earth, what would the gravity be like? What would it be like to walk on the surface? (2) What are the odds of any given planetary atmosphere being breathable by humans? (3) How long would it take to travel to a planet 28 light years away using current technology? Thank you for helping. —Preceding unsigned comment added by 70.29.47.136 (talk) 15:05, 16 December 2009 (UTC)


 * At 5 times the mass of earth, the gravity would be 5 times as great, and you'd feel 5 times as heavy. See Newton's law of universal gravitation
 * Probably not very great. The existance of plants is prerequisite for an atmosphere to be breathable by humans and other animal life.  Free oxygen is way too reactive to exist unless a system is actively producing it, and on Earth, it is the existance of plant life that creates and maintains the oxygen in our atmosphere.  So, unless plants are already there, there will not be a breathable atmosphere.
 * 24 light years = 2.65 x 1014 kilometers. So you would just need to calculate the maximum speed of a rocket using current technology, and do the math.  -- Jayron  32  15:33, 16 December 2009 (UTC)


 * 1) Gravity depends not just on the mass of the planet, but also the radius of the planet, and is dictated according to the equation Gm/r2, where G is the Gravitational constant, m is the mass of the planet, and r is the radius of the planet. The article seems to conflate size with mass, and gives unclear numbers, so I couldn't say much about what it would feel like there.
 * 2) Who knows, although probably unlikely. Europa has plenty of oxygen, for example.  On Earth, Oxygen came into existence largely as a result of photosynthetic life, which could indicate it isn't likely to occur elsewhere.  Oxygen was toxic to early life, so much so that planets with a breathable atmospheres are terrible candidates for extraterrestrial life.
 * 3) A long time. Voyager 1 is the fastest object that far away at 17km/s, at which rate it would take  493,775 years.
 * ~ Amory ( u •  t  •  c ) 15:34, 16 December 2009 (UTC)


 * The correct formula for the surface gravitational acceleration is Gm/r2. Assuming a rocky planet with density simmilar to earth's density you get
 * gPlanet=gearth(massPlanet/massearth)1/3
 * gPlanet=17m/s2
 * Dauto (talk) 16:37, 16 December 2009 (UTC)
 * Whoops, brainfart. Fixed, thanks. ~ Amory ( u  •  t  •  c ) 17:08, 16 December 2009 (UTC)
 * While Voyager is the fastest object made by man, it was not designed solely to travel as fast as possible. I would imagine that if cost was not much of an issue, we could, with current technology, create a much faster spacecraft, capable of at least 1% lightspeed and probably more.  That would shorten the trip to the order of hundreds or thousands of years.  Not great, but it beats half a million years.  Googlemeister (talk) 16:05, 16 December 2009 (UTC)
 * Quibble: Not fastest.  The Voyager 1 article says:  The current speed of New Horizons is slightly greater than Voyager 1 but when New Horizons reaches the same distance from the sun as Voyager 1 is now, its speed will be about 13 km/s compared to Voyager's 17 km/s.  Comet Tuttle (talk) 18:02, 16 December 2009 (UTC)
 * 0.01c would be very difficult with current technology. I think the only technology we have that could do it is an ion drive and we are very much in the early days of ion drives. Rockets would require far too much reaction mass to get up to that kind of speed and would be completely infeasible. Laser propulsion would be a good way to do it, but I don't think anyone has even made a working prototype of such a system. --Tango (talk) 16:48, 16 December 2009 (UTC)
 * Probably. Best I could come up with back of the envelope is 0.5%c, and it would be quite the engineering feat to get 10,000 solid rocket boosters into orbit and lash them together so that you have 10 useful stages and a final craft weight of only 10,000 lbs.  Also would not be that fun for any human since it would get up to speed in about 20 minutes and people can not pull 4 g (Correction, 4,000 g) for that long can they?  Googlemeister (talk) 17:32, 16 December 2009 (UTC)
 * Also you'd rocket right past that exoplanet. More likely you'd want to burn half your fuel on the way there, then wait thousands of years, then on approach, your descendants would burn the other half to slow down and reach the exoplanet at a reasonable speed so they could enjoy the visit.  Comet Tuttle (talk) 17:56, 16 December 2009 (UTC)
 * Is that 10,000lbs the payload mass or the initial mass of the whole craft? Either way, I don't think it's possible. The Apollo mission payloads were about 100,000lbs, I think, and they were only supposed to keep 3 people alive for about a week. A generation ship (which we seem to be talking about) would need at least 50 people and they would need to kept alive (and sane) for hundreds of years, which would require a much greater payload. If you use more realistic estimates of the payload mass you'll find the amount of rocket fuel required would be completely infeasible, even if you could overcome the engineering problems. --Tango (talk) 18:23, 16 December 2009 (UTC)


 * Aside from anything else - the news of these new exo-planets only appeared a few days ago. Wikipedia is an encyclopedia - not a newspaper - and while we often do create articles as the news breaks - there is no particular guarantee that this will happen for any given story.  That said:


 * It's tempting to suggest (as others have) that at 5 times the mass of earth, the gravity would be 5 times as great - but gravity gets weaker as you get further from the center of gravity of the planet - so a planet that's 5x the mass of the earth that's made of something very dense (lead maybe) would be smaller than a planet of the same mass that's made of (say) aluminium. Hence the gravity on 'planet lead' would be greater than on 'planet aluminium'.  So it's not just the mass - the density matters too.  Without knowing both numbers, we have to make a kinda guesstimate that maybe the density of these new planets are similar to earth - and if that were the case, the gravity definitely wouldn't be 5x larger because the planet's diameter would be the cube-root of 5 times (1.72 times) larger.  Since gravity drops off as the square of the diameter - we're looking at gravity that's 5/(1.72x1.72) = about 1.7 times more than here on earth.  So if you weigh 150lbs here - you'll weigh 255lbs there...that's a lot - but soldiers walk around with 80lb packs on their backs and they seem to manage - so the gravity wouldn't stop you from living there - but it would make life uncomfortable.  However, you wouldn't want to have to wear a massive spacesuit or anything like that.  But this depends on the density of the planet.  If it's lower density than earth, the gravity could easily be earth-normal.  If it's even slightly denser than earth - we're in deep trouble!


 * Free oxygen is unlikely to exist there unless there is active plant life or something somewhat similar. Oxygen is a fairly reactive element and will combine with all sorts of materials in the soil and would probably eventually disappear unless continually renewed somehow.  It's possible this happened on Mars where the reaction of oxygen with iron in the soil created the red color...rust.  So it's likely that without life, there would be no free oxygen.  However, there might be CO2 or water vapor - and you could maybe carry some kind of battery-powered gizmo that would convert CO2 or water vapor to oxygen and thereby survive so long as you have power.  Also, we could maybe seed the atmosphere and soil with plant life before we go there (See terraforming) so that there would be a nice oxygenated atmosphere ready by the time we got there.  This has been proposed for Mars.  So we could use Mars for practice and maybe we could colonize these planets later.


 * SteveBaker (talk) 17:34, 16 December 2009 (UTC)
 * It is very difficult to say what kind of terraforming would be required. Different planets differ from Earth in different ways. For Mars, we need to warm it up and massively increase the atmospheric pressure. For Venus, we need to do the exact opposite. For this newly discovered planet, who knows? Creating the oxygen is probably the easy bit (as you say, we just need to seed the planet with plants), it will be adjusting the pressure and temperature that is hard. As long as the terraforming can be done without people being present, then there is a factor in our favour - unmanned terraforming probes could be sent much faster than the colonists, since the probes could survive much higher accelerations. That should mean that the colonists could arrive to find a nice habitable planet waiting for them having had hundreds of years for a balanced and reasonably diverse ecosystem to develop. --Tango (talk) 18:15, 16 December 2009 (UTC)

"vascular" headaches
Why is vascular headache an outdated term? The page implies that no headaches are now thought to be related to blood vessel swelling. This is at odds with the page for cluster headache which says "The intense pain is caused by the dilation of blood vessels". 81.131.54.224 (talk) 16:08, 16 December 2009 (UTC)
 * The article has no sources and the "outdated" edit was made by an IP with no explanation, so the reason for it is not entirely clear. My understanding based on a quick scan of the literature is that (1) "vascular headache" is no longer an officially recognized diagnosis; (2) the theory that migraine is caused by vascular changes has been called into question; however (3) there is no doubt that some headaches are caused by vascular effects; and (4) the term "vascular headache" is still pretty widely used. Looie496 (talk) 17:53, 16 December 2009 (UTC)
 * OK, I'm going to insert your (3) into the article. 213.122.38.14 (talk) 19:31, 16 December 2009 (UTC)
 * Don't forget about temporal arteritis -- can lead to blindness.  DRosenbach  ( Talk 02:53, 17 December 2009 (UTC)

p-phenylenediamine
does p-phenylenediamine degrade when exposed to air and/or light? if so how much? —Preceding unsigned comment added by 74.65.3.30 (talk) 18:30, 16 December 2009 (UTC)
 * Oh yes it does. It goes black! Just like most aromatic amines - air oxidation, a bottle of aniline will be a brown liquid - it's colourless when distilled, as soon as air gets in it starts to turn yellow. The phenylenesiamines are even easier oxidised, which is why derivatives of them are used as developing agents in photography - see C-41 process.  Ron h jones (Talk) 22:27, 16 December 2009 (UTC)

CHARGED BLACK HOLES
Hello,

I read and come to know that black holes do have charged and react to external charge, but how not known to me. Can you tell me please. —Preceding unsigned comment added by Harshagg (talk • contribs) 19:03, 16 December 2009 (UTC)


 * If you drop something with an electric charge into a black hole - the hole retains that charge...or if something with an electrical charge collapsed to form a black hole - then it could happen that way too. Of course there is the practical question of what we could imagine that was large enough - and charged enough to do that - so the idea of electrically charged black holes is still kinda theoretical.  I don't think anyone is seriously saying that we have any reason to suspect they are out there to be found. SteveBaker (talk) 19:08, 16 December 2009 (UTC)


 * Steve, the honest answer is that we don't know whether any blackholes out there actually have a non-neglible charge. But I find it perfectly believable that they might have a small but non-negligible net charge. Dauto (talk) 19:47, 16 December 2009 (UTC)
 * The problem with black holes having a charge is that, if they can have a small charge, they should be able to have a large charge. Highly charged back holes have naked singularities (see Reissner–Nordström metric). Apparently there is something to do with supersymmetry that fixes this problem (by making higher charged block holes impossible), but I've never really understood supersymmetry. --Tango (talk) 20:36, 16 December 2009 (UTC)


 * Somebody asked this before, but I'm not sure I find the answer fully satisfying: Since electrical charge is communicated via photons, how can we detect that the hole is charged? If I drop, say, 10e30 naked protons into the hole, how can I detect this charge? --Stephan Schulz (talk) 20:27, 16 December 2009 (UTC)


 * You ask how can you detect the charge of the protons. The answer is 'The same way you can detect the mass of the protons'. The electric charge of the protons don't get destroyed by the blackhole. it becomes part of the blackhole. Dauto (talk) 02:15, 17 December 2009 (UTC)
 * Ok, so how to do we detect the mass of the protons? If nothing can escape the event horizon, that includes photons and gravitons. My understanding is that the (virtual) photons or gravitons are created at, or just outside, the event horizon, rather than inside it, but I'm not entirely convinced about how that works. --Tango (talk) 03:24, 17 December 2009 (UTC)


 * The event horizon is determined by the condition that the escape velocity becomes the speed of light. Coulomb interaction is mediated by only virtual photons, whose effective speed is not anyhow limited (their propagator is not zero over space-like intervals). So I see no immediate contradiction in probing the electrostatic charge of a black hole. If the charge distribution inside the event horizon were changing, then we (the outsiders) would receive no information on that, as that would be conveyed by electromagnetic waves, i. e. real photons, which surely cannot escape. It would still need some computation (applying quantum field theory in curved spacetime) to prove the relevant part of the "No hair theorem" that the total charge of a black hole is observable, but the charge distribution is absolutely not.undefined&mdash;undefinedPt(T) 06:24, 17 December 2009 (UTC)

Here is another question. We know that gravity is a weak force, compared to the electrostatic force. So it is conceivable that I have a black hole which is charged to a degree that the electrostatic repulsion is larger than the gravitational attraction. Does that mean that the black hole will start bleeding protons? In fact, does this mean that the even horizon is at different radii for negatively charged, positively charged, and neutral particle? The net force (sum of gravity and electrostatic) is certainly different for each... --Stephan Schulz (talk) 08:16, 17 December 2009 (UTC)
 * there is a limit to how charged a black hole can be. the charge to mass ratio is actually way below that of a proton.  The reason would be that to push all those positive protons together takes a lot of energy.  Somewhere someone here worked out the energy to get a mole of protons in a particular apace. Graeme Bartlett (talk) 09:46, 17 December 2009 (UTC)
 * Well, since both gravity and electrostatic force follow inverse-square laws, I should be able to drop protons into a black hole until mass and charge exactly balance out. In that state, the black hole would exert no net force on a proton (but, of course, a lot of force on neutral matter, and an even bigger one on negatively charged particles). --Stephan Schulz (talk) 13:12, 17 December 2009 (UTC)
 * Strong gravity is not inverse-square anymore, see Kepler problem in general relativity: there is an inverse-cube term in the effective potential or, equivalently, a r-4 term in the force law. In addition, curvature of spacetime modifies the Maxwell's equations including the Gauss's law, which would otherwise give the inverse-square behaviour of electrostatic force. I haven't calculated if things actually still happen to cancel and your idea works, but I doubt it.undefined&mdash;undefinedPt(T) 14:49, 17 December 2009 (UTC)
 * After meditating for a while, I suspect it does. Otherwise one could bring in a neutral mass (only subject to gravity), separate the charges, move them out again (now subject to both gravity and electrostatic force), and recombine them. Until I'm mistake, somewhere in the process you have excess energy. --Stephan Schulz (talk) 12:39, 19 December 2009 (UTC)
 * If you interpret gravity as a deformation of spacetime, rather than a force, then it becomes clear that it can't be cancelled out. No matter how strong an electromagnetic field is, it can't make a charged massive particle move along a space-like path and that is required to leave an event horizon. --Tango (talk) 15:52, 17 December 2009 (UTC)


 * In order for the particle to leave the event horizon it could also move backwards along a time-like path which is also impossible. Dauto (talk) 02:04, 18 December 2009 (UTC)


 * As I mentioned above, if the charge to mass ratio gets too great (I think the charge being greater than the mass in the usual dimensionless units) then you end up with a naked singularity, which is usually interpreted as meaning it can't happen. --Tango (talk) 15:52, 17 December 2009 (UTC)

how far we go back things become less accurate?
Do we 100% know where the continents were 300 million years ago or is this just a theory. Because The Future is Wild said Antartica was in the equator 300 million years ago, most show Artartica was in SP at that time. Is this right the further we go back in historical time, the less we know. Do we absolutely know if southern Africa was 75 degs. south 200 million years ago, some model shows southern Africa only 55 degs south at that time.--209.129.85.4 (talk) 22:17, 16 December 2009 (UTC)


 * We don't 100% know much of anything, when you get down to it, especially something like where the continents were 300 million years ago. We have estimates and projections and simulations and assumptions that probably put us in the right ballpark, but then again, they might be wrong in some major way that we don't know at the moment. I would probably suggest that the issue is not "the less we know" but "the more uncertain our knowledge becomes" the further backwards we extrapolate, but that isn't universally applicable. --Mr.98 (talk) 23:20, 16 December 2009 (UTC)
 * The margins of error on things like the positions of tectonic plates do get bigger as we go further back, that is correct, however the error is pretty small. I think there is some disagreement on the details of what happened before Pangea formed, but after that is all seems pretty clear. We have a good discussion of the history of Antartica here. I think 300 million years ago is recent enough that we are fairly sure about it. Africa and Antarctica were both part of the supercontinent of Gondwana 200 million years ago, so they would have been quite close together. About 500 million years ago, Antartica was somewhere near the equator, it moved to the South Pole by around 360 million years ago. I'm not sure what The Future is Wild said, but either you have misunderstood it or they have made mistake - it's probably just a mistake about the time, it's 500 million years ago, not 300. --Tango (talk) 23:33, 16 December 2009 (UTC)
 * That said for Antartica to be green again like it was 300 million years ago. No mistakes--209.129.85.4 (talk) 19:24, 17 December 2009 (UTC)
 * This didn't say anything about the past, they just said Antartica would be green and hot, and it will be back on equator in 100 million years. I doubt if our article made a mistake on time about Tropical Antarctica.--209.129.85.4 (talk) 19:41, 17 December 2009 (UTC)


 * The best information has come from core samples taken by drilling into oceanic plate and examining magnetic alignment. However, oceanic plate is constantly being newly created and the older plate destroyed by the very process that causes continental drift.  The further back one goes, the less plate of that age there is still in existence.  Past the Mesozoic it is all gone and the information becomes much more sketchy.  Sp in ni  ng  Spark  11:45, 17 December 2009 (UTC)

ICP-Chapter 11 Review-Motion
Please help me with these questions. I'm studying for finals and need to know the answers to these questions.

Please please please answer the ones you know(: please and thankyou! you will be of big help to do so. —Preceding unsigned comment added by Kaimorgan16 (talk • contribs) 23:01, 16 December 2009 (UTC)


 * The standard disclaimer:




 * applies here. Key phrase "Please attempt to solve the problem yourself first." You'll get better responses if you make some effort to tell us why you're having answering the questions on your own. -- 128.104.112.87 (talk) 23:05, 16 December 2009 (UTC)


 * I hid the myriad questions, since we won't be answering them in that form. --Tardis (talk) 23:08, 16 December 2009 (UTC)
 * That is far too many questions. You need to actually try and answer them yourself and come back with those ones that you really are stuck on. Try looking up the topics in Wikipedia. For example, question 3 is answered in our article on speed (there is a whole section on it). --Tango (talk) 23:11, 16 December 2009 (UTC)


 * Quoth the teacher "Distance is a measure of length" ??? -- Finlay McWalter • Talk 23:49, 16 December 2009 (UTC)


 * I'm curious as to what age this would be for. And, the answers are simple and basic - the student not knowing the answers implies that they have not attended class or at least been ignoring the teaching. Wikipedians should not encourage this behaviour by answering the questions. 78.149.247.13 (talk) 10:36, 17 December 2009 (UTC)

ANNs
There is a very good Excel spreadsheet example of the Perceptron method in the article but the subtitles are incomplete and need additional entries which I plan to remedy shortly. Does anyone know where I can find a similar Excel spreadsheet example of an XOR capable layered ANN and one that displays all of the computations for Backpropogation? 71.100.0.206 (talk) 23:17, 16 December 2009 (UTC)


 * ANNs in Excel is easily found via google. This one for instance: I'm guessing that this one could be easily modified to show the underlying computations. EverGreg (talk) 08:35, 17 December 2009 (UTC)
 * VBA is easy to find, especially those with a price tag. I'm not looking for a VBA applications program but only an example with the computations for each column at the top as in the Perceptron example. 71.100.0.206 (talk) 17:02, 17 December 2009 (UTC)


 * I'd be interested in finding something similar for a radial-basis function, and other AI algorithms. Unfortunately I do not understand or use C. 78.149.247.13 (talk) 11:21, 17 December 2009 (UTC)