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

= May 9 =

Neanderthal genes in modern humans
I read here that I may be partially Neanderthal. What I don't understand is how it is possible that you could only have a small nonzero Neanderthal contribution to the human genome. I mean, the scientists are surprised that it seems to be as large as 1% to 4% instead of a much lower number or zero. But how can any nonzero Neanderthal contribution to someone's DNA be less than an entire chromosome? Count Iblis (talk) 00:23, 9 May 2010 (UTC)
 * Chromosomes come in pairs, and the two members of a pair can swap parts via the process of chromosomal crossover. After a sufficient number of generations you get a very high level of mixing, even between parts of a single chromosome. Looie496 (talk) 00:35, 9 May 2010 (UTC)
 * I think, emphasis on that, that they are referring to the original DNA that is left in your DNA. Clearly when the Neanderthal mated with a Homo sapien, they're contributing chromosomes, but over time this DNA is going be diluted with new DNA from other Homo sapiens. I think I explained that badly. In short, the 1% is the remaining original Neanderthal DNA. Regards, --— Cyclonenim | Chat 00:37, 9 May 2010 (UTC)
 * Yeah, Looie explains it better. Regards, --— Cyclonenim | Chat 00:37, 9 May 2010 (UTC)
 * What confused me (speaking as someone who almost failed high school biology), is that since we have 46 chromosomes, I would have thought that the smallest (non-zero) amount of neanderthal DNA we could posses is 1/46th. Does this "chromosomal crossover" that Cyclonenim mentioned mean that two parents can have more than 2^46 different possible offspring?  24.68.41.132 (talk) 02:55, 9 May 2010 (UTC)
 * Yes. Nil Einne (talk) 08:41, 9 May 2010 (UTC)
 * The first-generation offspring will have 23 chromosomes that are "all-neanderthal", and 23 chromosomes that are "all-modern-human". The number of possible neanderthal chromosome combinations that the neanderthal parent could transmit to the child is a lot greater than 223, due to recombination between the two variants of neanderthal chromosomes that the neanderthal parent has inherited from the neanderthal grandparents. Likewise of course with the human parent. When the hybrid child grows up, and starts producing reproductive cells, there will again be recombination between the pairs of chromosomes, but now the recombination will occur between a neanderthal-derived chromosome and a modern-human-derived chromosome, resulting in chromosomes that are partly derived from neanderthals and partly derived from modern humans. If the hybrid's children continue breeding with modern humans, the neanderthal contribution to the genomes of their offspring will be diluted. However, if some of the neanderthal genes turn out to be advantageous, natural selection will ensure that these are not lost. --NorwegianBluetalk 09:57, 9 May 2010 (UTC)

Thanks for the answer everyone! (I should have kept biology on my high school curriculum, I guess). So, how fragmented will the Neanderthal genetic material be in the modern population? I roughly estimated this as follows. Suppose that I have a Neanderthal gene on one of my chromosomes, orginating from a hybrid that lived 60,000 years ago. Then, if I understand this article correctly, on average two genes separated by a million base pairs will have a P = 1% probability of being separated in the next generation. So, the probability of a gene that is L*10^6 basepairs away not being separated after N generations is (1-P*L)^N, which is valid for L not much larger than 1. If I take N = 3000 and equate the probability to 1/2, I find L = 0.023. So already at a distance of 23,000 basepairs there is a 50% chance of separation. Count Iblis (talk) 14:56, 9 May 2010 (UTC)


 * Natural selection is not random. 67.243.7.245 (talk) 15:16, 9 May 2010 (UTC)
 * Yes it is! SteveBaker (talk) 16:09, 9 May 2010 (UTC)
 * This is actually a misconception about natural selection. The process wherein DNA mutations occur is random, as is the combination of genetic information from one's parents.  However, as DNA encodes physiology and behavior, these two random process have consequences in the environment, which means genetic information is subject to nonrandom processes of selection.  Over time, the genes that encode for physiology and behavior that work best in a given environment persist at a greater frequency than those that don't; this is natural selection.
 * Bringing it to the subject at hand, this means that if any Neanderthal genetic information has environmental consequences (including those from the social environment), then there would be non-random influence on populations descending from such intermixing. For example, if the Modern-Humans who came to Europe after Neanderthals had very dark skin, then they would have difficulty producing vitamin D; relevent genetic information from the native Neanderthals, who surely would have already acquired lighter skin, would thus be favored over generations. — Æµ§œš¹  [aɪm ˈfɹ̠ˤʷɛ̃ɾ̃ˡi]  19:17, 9 May 2010 (UTC)
 * So a random frog mutates and gets the gene for eternal life and for being irresistable to frogs of the other sex - but is eaten by your pet cat just one hour before it reached maturity and passed this remarkable gene onto the next generation. OF COURSE IT'S RANDOM!!! SteveBaker (talk) 23:51, 9 May 2010 (UTC)
 * Does eternal life mean that the frog is more likely to survive and pass on its genes in this particular environment? I don't think so, but even if it did, it's not a guarantee. Advantagious changes usually only give a slight advantage over others.  If natural selection were random, then there would be nothing pushing species to adapt to their environments.
 * Keep in mind that when there are no selective pressures on a given set of DNA, it is subject to genetic drift, which is also random. — Æµ§œš¹  [aɪm ˈfɹ̠ˤʷɛ̃ɾ̃ˡi]  01:22, 10 May 2010 (UTC)
 * (Edit Conflicts) But in that example, Steve, natural selection hasn't occurred, because it never had a chance to. The selection happens over generations, after one or more frogs have (semi-randomly) mutated that gene and successfully introduced it into the frog gene pool, when nature/the environment (including pet cats) operates on it (or more accurately its phenotypic expression(s)) and selects for or against it. Individual carrier frogs may well be subject to random accidents (ones not related to the genes' effects), and others to random 'good fortune', but the effects of these will average out in the gene pool as a whole, so the selection pressure on that gene is the opposite of random. Compare it to Monopoly where, although individual moves are affected by the limited randomness of dice scores (e.g. you can't score 1 or 13), a skilful player will still usually beat less skilful ones in the long run, though he/she might still be wiped out by unusual events. 87.81.230.195 (talk) 01:49, 10 May 2010 (UTC)


 * Steve, your frog example is not random. it is stochastic. These are two different things. Natural selection is not random. Dauto (talk) 05:38, 10 May 2010 (UTC)
 * I suggest you look up the word "stochastic" in the dictionary. Wiktionary provides just one meaning - it says "stochastic: Random, randomly-determined.".  Randomness that tends to average out in the long run is still random.  There is still always the possibility that a thousand species of genetically perfect dinosaurs get wiped out by a random meteor strike. SteveBaker (talk) 16:20, 10 May 2010 (UTC)
 * If, over thousands of generations, a species changes to adapt to its environment, then that change itself can't be random. At the individual level, the advantage of beneficial genes is much smaller than it is at the group level over generations.  Your examples seem to assume that environment is irrelevent when it comes to fitness.  In your frog example, the most advantagious genes are the ones that make frogs cat resistant.  In your dinosaur example, the meteor strike has altered the environment so drastically that the genes that didn't fit the new environment didn't survive.  When the environment changes drastically and none of the individuals in a species have genes that favor that environment, extinction is a lot more likely. — Æµ§œš¹  [aɪm ˈfɹ̠ˤʷɛ̃ɾ̃ˡi]  18:05, 10 May 2010 (UTC)


 * One thing that I think we're missing here is that Neanderthal and Modern-Human DNA would be remarkably similar to start with. Heck Chimpanzee DNA and Human DNA is about 94% identical - we'd imagine that Neanderthals and humans would be more similar than Chimps and humans.  So even if we had one complete Neanderthal chromosome - we'd still only have a tiny, tiny percentage of DNA that looked different from a Neanderthal-free modern human. SteveBaker (talk) 16:09, 9 May 2010 (UTC)
 * To quote Wikipedia, "While unable to definitively conclude that interbreeding between [humans and Neanderthals] did not occur, analysis of the nuclear DNA from the Neanderthal suggests the low likelihood of it having occurred at any appreciable level". Source:
 * That 2006 quote is pretty irrelevant now. It relates to the same group, and the same project. The data they had aquired in 2006 did not give evidence for admixture. Today, when more data is available, it turns out that some interbreeding probably occured. --NorwegianBluetalk 20:21, 9 May 2010 (UTC)

Paintballing science
Where I went paintballing they sold pressurized CO2 chambers, as well as more expensive pressurized N2 chambers (used for firing the balls). Why would nitrogen be better than carbon dioxide? Isn't it just dependent on the pressure, which is independent of the gas used? Also, my facemask kept fogging up and I was wondering what could have been done to reduce that. But before that, I'm not too sure why it would fog up...my guess is that it fogs up because the moist air from our lungs (moist because it was raining?) hits the cold visor, causing the air to cool down which makes the water in the air condense. I'm not completely satisfied by this because the air cools down to the same temp as the surrounding air, which can hold its water without too much trouble. I was hopeing someone could help me out with this. But anyways, about preventing the fogging...I heard spitting on the visor helps, but why? Any other useful tips? Thanks. 173.179.59.66 (talk) 01:49, 9 May 2010 (UTC)
 * After looking at some paintball forums, it seems that the nitrogen is stored as a compressed gas, but the CO2 is stored as a liquid. As a gas (nitrogen) expands, it gets slightly cooler, but as liquid CO2 evaporates it gets very cold.  Apparently this makes the gun cold and leads to less consistent shots.  See .24.150.18.30 (talk) 02:27, 9 May 2010 (UTC)
 * As for the fogging mask, smearing a drop of saliva may work, as I know (from experience) that this is useful in SCUBA diving (I'm sure that a fogged up mask is no fun in paintball, but it can be an absolute nightmare underwater!). The fogging is caused by water vapour in your breath hitting a cold surface (the mask) and condensing into tiny droplets.  My theory is that if there is already a thin layer of moisture (saliva) on the mask that is smooth enough to see through, then any drops that form from you exhaling will just become part of that layer and won't affect the light passing through.  Dive shops and maybe ski stores may sell antifog lens cleaners which could help.24.150.18.30 (talk) 02:37, 9 May 2010 (UTC)
 * The other option is to get a better quality mask with vents to allow the moisture out (not an option for SCUBA divers, but it works well for masks used in air - I've used one for skiing and you could really tell the difference if you blocked the vents, not that I can work out why goggles that only covers your eyes, not your nose or mouth, would fog up so much...). --Tango (talk) 02:54, 9 May 2010 (UTC)
 * On preventing your mask from fogging up, I often see people at paintball fields putting shaving cream on their facemask lenses in order to prevent fogging. No idea what the theory behind this is or why it works, but I guess people wouldn't do it if it didn't have some degree of effectiveness. And yes, if you're serious about paintball, definitely invest in a high-quality mask with vents and anti-fog coating on the lenses. --Cerebellum (talk) 03:31, 9 May 2010 (UTC)
 * Smearing the visor with spit can reduce fogging because of the spit's small tendency to act as a surfactant - it prevents the condensate from forming small drops and instead it forms a uniform layer, which you can see through. Other liquids, such as shaving foam, potato juice and detergent do the same thing. The best way of reducing condensation is to use a double-layered mask, which is very common in skiing.  The inner one warms up and so does not suffer from condensation.  The outer one gets cold, but the warm damp air does not reach it, and so it doesn't get fogged either.  Many motor-cycle helmets do the same sort of thing, often with a system called a pin lock.  --Phil Holmes (talk) 09:45, 9 May 2010 (UTC)


 * And least anybody forgets to mention it. Wikipedia -as always- has an article.   Paintball equipment.--Aspro (talk) 12:26, 9 May 2010 (UTC)


 * Also see Anti-fog... Cacycle (talk) 22:11, 9 May 2010 (UTC)

Thanks a bunch! 173.179.59.66 (talk) 02:05, 10 May 2010 (UTC)

Angular momentum transform
If the angular momentum of a particle about the point (x0,y0) is known, how do I calculate the angular momentum with respect to another position, say (x1,y1)? --142.151.129.67 (talk) 02:54, 9 May 2010 (UTC)


 * You can't. They are independent. Imagine you have a particle with mass 1kg moving in a circle of radius 1m centred at (0,0) at 1 m/s - it has a constant angular momentum around (0,0) of 1 Nms. The same particle's angular momentum around (0,1) will depend on its position in the circle, eg. when it is at (0,1) its angular momentum will be zero, when it is at (0,-1) its angular momentum will be 2 Nms. If there was a transformation from one centre to another then a constant angular momentum would have to transform into a constant angular momentum, so clearly there is no such transformation. --Tango (talk) 03:03, 9 May 2010 (UTC)


 * A good discussion of the mathematics of angular momentum under various coordinate transforms is given in Chapter 9.4 of Classical Dynamics of Particles and Systems ($150 at Amazon). You can probably find this book at a university library.  The derivation is too complex to write here, but can be summarized: "The total angular momentum about an origin is the sum of the angular momentum of the center of mass about that origin and the angular momentum of the system about the position of the center of mass."   This is closely related to the parallel axis theorem, which describes the transform of moment of inertia to other coordinate systems.  I am not sure if Tango's statement above is correct - I think what he means is that the angular momentum can not be easily represented.  In the case of a non-inertial coordinate system, the angular momentum will appear to not be conserved, and will be time-varying.  This is one way to describe Coriolis force, for example - a "mysterious" generation of angular momentum - but all that is really happening is that we are observing conservation of momentum in a system from the viewpoint of a non-inertial frame.  Nimur (talk) 02:05, 10 May 2010 (UTC)
 * It's not difficult to represent, it is impossible. The Coriolis force has nothing to do with it; we haven't changed reference frame. I've shown that given an angular momentum around one point I can find two situations both with that angular momentum around that point but with different angular momenta around a different point. That means you cannot determine the angular momentum around the second point without more information. --Tango (talk) 02:33, 10 May 2010 (UTC)
 * Tango, I think you are incorrect; what you have shown failed to take into account the Parallel Axis Theorem; in other words when you gave your circle example, you did not account for the angular momentum of the center of mass (in the new coordinate system - you moved the origin!). This is where the mysterious 2 Nms has disappeared / appeared - it should have been the angular momentum relative to the new position of the center of mass.  Your derivation was incomplete, and that's why there's the inconsistency - you have performed half of the transform but forgot to transform the system's angular momentum about the center.  If you decompose the vector to the center, r, into (ra + R), where R is the vector to the new origin, and ra is the vector to the true center of mass of the object, and then proceed with your derivation, you will find a new term in R (which describes, in your case, the orbit of the object around an arbitrary circle, and varies with time).  The sum of the angular momenta from the system (M R x dR/dt) and the object's rotation (Iara) will be conserved - that is the total angular momentum of the entire system.  Think of Earth revolving around the sun, and also rotating on its own axis.  The total angular momentum of the system is constant, but if you observe from the point of view of Earth, (which is a moving reference frame), you "forgot" to account for the angular momentum of the whole orbit.  This must be accounted for by computing the center of mass for the entire system, and taking a coordinate transform to your new origin of choice.  In an n-body problem, where the choice of origin is arbitrary, how else would we be able to analyze angular momentum?  It is definitely possible to represent angular momentum as observed from any arbitrary origin; this is done by decomposing the angular momentum into a rotation about the new origin and a rotation about the object's center of mass.  For each arbitrary origin, the numeric values of this decomposition vary.  In the most general coordinate transforms (rotations and translations), we need one or more inertia tensors to describe the system, and can not drop any terms.  Nimur (talk) 15:16, 10 May 2010 (UTC)
 * I'm talking about single particle. The centre of mass is the location of the particle and the angular momentum of the system around the centre of mass is zero. There is only one term. I don't need any theorems since I'm calculating the angular momentum for a specific system using the definition of angular momentum. --Tango (talk) 21:22, 10 May 2010 (UTC)
 * Unless I'm very much mistaken, isn't angular momentum defined within a plane, and not "about" anything? In which case, the  question isn't very meaningful.--Leon (talk) 21:41, 10 May 2010 (UTC)
 * The definition of angular momentum, as given in our article, is $$\mathbf{L} = \mathbf{r} \times \mathbf{p} = \mathbf{r} \times m\mathbf{v}\, ,$$ so given an arbitrary coordinate system, r and v are going to be different. Because we know that angular momentum is a conserved quantity, something has to account for these changes in r and v.  That would be the parallel axis theorem, which is a direct consequence of the definition of angular momentum in generalized coordinates.  Nimur (talk) 07:10, 11 May 2010 (UTC)
 * Correct, and the cross product yields a bivector, or oriented plane segment.
 * The parallel axis theorem allows one to determine the moment of inertia of a body about a given axis if the moment of inertia about its centre of mass is known. From this, if one knows the angular velocity about this new axis, one can furthermore calculate the angular momentum.  That is not the same as being able to ascertain the angular momentum about some new point if all that is known is the angular momentum about some other.--Leon (talk) 07:44, 11 May 2010 (UTC)
 * That is the definition of angular momentum around the origin. The numbers I've quoted above are the magnitude of that vector (my system was planar, so there was no reason to state the direction - it is clearly perpendicular to that plane). Angular momentum is conserved - that means it stays the same if you change the time. It doesn't stay the same if you change the origin. There is clearly a relationship between the angular momentum around one point and the angular momentum around another, but that relationship depends on the details of the system in question so you can't do anything if you are just given the angular momentum. You need to be given the details of the system as well. --Tango (talk) 10:56, 11 May 2010 (UTC)
 * (Hope this helps) A critical oversight in much of the above is that the origin r is in an "inertial" frame, one undergoing no net force. If one considers angular momentum about the axis of rotation, that's fine, and angular momentum is constant in the absence of an external torque.  However, if the origin is anything other than the axis of rotation, but the motion of particles is much as before (the same, but all translated), for the origin to remain an inertial frame (i.e. with no forces or torques acting upon it) there must be some external torque to counterbalance the torque incurred on the origin by holding the rotating body in place.  Hence angular momentum of the system does not "appear" constant, though it is, we have simply neglected some forces!  As an example, consider holding a stick with a spinning wheel ball held on a string, spinning about this end, at the end opposite to your hand.  The wheel ball is exerting a force on your hand, and you are (presumably) counterbalancing that with some force to keep the stick in place.  Make any sense?--Leon (talk) 00:37, 12 May 2010 (UTC)
 * Ok, let's simplify the example. Abandon all forces. We have a single particle moving at constant velocity. It has (at a given moment of time) an angular momentum around (0,0,0) of (1 Nms,0,0). What, given no additional information, is its angular momentum around (0,1,0)? Please give the actual numbers (and show your working). --Tango (talk) 16:05, 12 May 2010 (UTC)
 * Its angular momentum isn't "about" anything! And the general equation, $$\sum_i \mathbf{r}_i\times \mathbf{p}_i$$, is always valid providing that the assigned origin is experiencing no net force.  This is a tacit assumption of the definition.  So we can't "abandon" all forces.  If I spin a ball on a string around my finger, the latter of which is at (0,0,0), the ball is exerting a force on my finger but not a torque, though my finger is still supplying a force to exactly counterbalance that of the string.  However, imagine I spin the ball on a string, attached at one end to a pole, of which I am holding the other end, and my hand holding the pole is at (0,0,0).  The force that the pole exerts on my hand is a torque in this case, and if my hand is to be kept at (0,0,0) my hand must be providing an equal and opposite torque back.  Newton's laws do make assumptions about the frame of reference, and indeed he argued for an "absolute" frame of reference (I can't think of the exact line of reasoning or the exact statement, but it involved this: swish water around a bucket and take the water to be in the rest frame and calculate; you get very different results to what is observed; taking the Earth as the rest frame you get results that tie in MUCH better with experimental data).--Leon (talk) 22:12, 12 May 2010 (UTC)
 * It is about something. Angular momentum is all about rotations. You can't have a rotation without a centre of rotation. Usually we use the origin (that is the assumption implicit in your formula), but there is no requirement to do so (and the choice of origin is arbitrary anyway). Read the second sentence of Angular momentum - note the bit where it says "with respect to some point". When I said abandon all forces I meant assume we have a system with no forces in it, hence the particle has constant velocity (a particle travelling along a straight line will have non-zero angular momentum, as long as the line doesn't go through the point you are working with respect to). --Tango (talk) 23:35, 12 May 2010 (UTC)
 * Yes, we are free to use any point, any non-accelerating point. And your example is weak because the angular momentum is constant in it!  You may point out that in your example angular momentum does depend on the axis of rotation, which I concede is correct, absolute momentum and absolute angular momentum are of little significance (changes wrt to time are significant, not the absolute values); if you account for those forces that mediate the rotation in my example above, the paradox dissolves.--Leon (talk) 08:04, 13 May 2010 (UTC)
 * However, depending on what exactly the crux of the matter is, I may be seeing your point. Yes, a choice of frame does affect the value of angular momentum calculated.  This would also be true for linear momentum (try a constantly moving frame), and in both cases we would be unable to ascertain the new (angular) momentum without more data that the momenta themselves.--Leon (talk) 08:11, 13 May 2010 (UTC)
 * Of course the angular momentum is constant - it is a conserved quantity and I got rid of all forces that could possibly change it. There is no rotation in my example, just a particle travelling along a straight line, so there is no axis of rotation either. The angular momentum tells you what the nominal axis of rotation is (it will be perpendicular to the plane determined by the path of the particle and the point you are working wrt), rather than being determined by it. I don't understand what paradox you are talking about - there is no reason to expect angular momentum to be the same wrt different points. The crux of the matter is the OP's question, which is very clear, and you have now finally agreed that my answer was correct. I really don't understand why you were disagreeing... --Tango (talk) 10:18, 13 May 2010 (UTC)

Boiling and vapor
From my understanding, boiling a solution evacuates solely the water as vapor and causes the solutes to remain in solution. But what happens when all the solvent has been vaporized; is the solute in crystal form along the walls of the pot? My particular question relates to boiling milk. Does the water content boil off and the fats, sugars, etc. remain in solution, albeit more concentrated? What exactly is included in the steam that comes off of a pot of boiling milk?  DRosenbach  ( Talk 03:19, 9 May 2010 (UTC)
 * Yes the steam will be mostly water. The scum left behind will be largely protein and lactose, with liquid butter fat making up the oily component.  The lactose in water will form a syrup, which will boil at a higher temperature, and this will eventually caramelize. Graeme Bartlett (talk) 03:30, 9 May 2010 (UTC)
 * Mostly...what other than water would boil off? And when you say that the syrup will boil at a higher temp, so that would be carbohydrate in gas form?  DRosenbach  ( Talk 03:32, 9 May 2010 (UTC)
 * The vapour consists of the compounds of the liquid in the relation to their partial pressure at the given temperature and that is depending on the vapour pressure of the compound. So at 100°C a little of the fat will be also evaporate, not enough to see it but not zero molecules. With milk I would guess that the vapour is more than 99.999% water. --Stone (talk) 08:42, 9 May 2010 (UTC)
 * Strictly, your original statement is not true. If we dissolve some ethanol in water and then raise the temperature of the solution, the initial vapour will be composed predominantly of ethanol with some water also.  As stone says, the proportion of ethanol to water would depend on their partial pressures at the temperature the solution had been raised to.  So "boiling a solution" does not evacuate "solely the water as vapour".  --Phil Holmes (talk) 09:39, 9 May 2010 (UTC)
 * Well to start with the most volatile substances will boil out first. These would be nitrogen oxygen carbon dioxide.  You would also have noticed a smell, this is also due to volatile substances that have evaporated along with the water. Graeme Bartlett (talk) 12:31, 9 May 2010 (UTC)
 * When you boil milk, it is almost entirely water that evaporates. If you boil it down until almost no water is left, the result is fudge -- an almost solid mass made a bit squishy by the fat content.  (The fudge you buy in stores has a lot of sugar added, and usually other flavorings such as chocolate, but basic fudge is what you get by boiling the water out of milk.) Looie496 (talk) 16:07, 9 May 2010 (UTC)

Plastic&Electric kettles
from what kind of plastic are electric kettle made? —Preceding unsigned comment added by Ha-y Gavra (talk • contribs) 11:04, 9 May 2010 (UTC)


 * PEEK comes to mind but it is expensive. More common high temp. plastics are polypropylene and POM.--Aspro (talk) 12:40, 9 May 2010 (UTC)


 * My kettle says >PP< which I'm pretty sure means polypropylene: the article says this is a common material to make kettles out of. 86.180.48.37 (talk) 19:45, 9 May 2010 (UTC)

Moon distance
how far is moon from the earth —Preceding unsigned comment added by 41.184.96.83 (talk) 12:54, 9 May 2010 (UTC)
 * 384,405 km (from center to center), according to a diagram in the moon article.  DRosenbach  ( Talk 13:04, 9 May 2010 (UTC)
 * Which makes for about 376,296 km (233,819 miles) subtracting out both sphere's radii.  DRosenbach  ( Talk 12:08, 11 May 2010 (UTC)


 * Well, as the moon's orbit is best described by an ellipse there are a few key distances which can all be found in the first few lines of the infobox here. The apogee is the furthest distance it ever gets from Earth and the perigee the closest.  Martlet1215 (talk) 16:55, 9 May 2010 (UTC)

river discharge
What is the relationship between the average discharge of a river and maximum discharge? —Preceding unsigned comment added by Amrahs (talk • contribs) 13:32, 9 May 2010 (UTC)
 * There isn't a relationship between those two figures. Rivers vary: maximum discharge is the largest amount of water that flows through the river in a certain amount of time, that usually happens in winter time. Average discharge is the average (usually mean) amount of water that flows through in a certain amount of time.--92.251.131.97 (talk) 14:24, 9 May 2010 (UTC)


 * Have you read Discharge (hydrology).--Aspro (talk) 14:30, 9 May 2010 (UTC)

National Security Guards or Special Air Service
which one according to u are better; SAS of British army and NSG of India?

thank you —Preceding unsigned comment added by 117.197.245.199 (talk) 13:37, 9 May 2010 (UTC)
 * Well if we compare the performance of the SAS during the Iranian embassy seige and both Iraq wars to the performance of the NSG during hte Mumbai terror attacks I think SAS is better.--92.251.131.97 (talk) 14:21, 9 May 2010 (UTC)
 * The Wikipedia (science) reference desk is not the place to ask for opinions. The best one can possibly do to answer this question is to attempt to list records of each service's performances in similar situations. As the activities of these organisations are largely kept secret, it is quite difficult to attempt to gather empirical evidence of any relevance. 88.90.16.140 (talk) 15:43, 9 May 2010 (UTC)
 * Why isn't a place to ask for opinions? Sure this isn't science it should be in humanities or misc, but we can give opinions.--92.251.131.97 (talk) 16:43, 9 May 2010 (UTC)
 * The reference desks were originally set up to assist people with writing articles. Our opinions don't go in the articles and so the ref desks are about facts not opinions. Theresa Knott &#124; token threats 16:46, 9 May 2010 (UTC)
 * Have you read the bit at the top of the page where it says "The reference desk does not answer requests for opinions"? --Phil Holmes (talk) 17:27, 9 May 2010 (UTC)

Is there much Greek blood in southern Italians?
Approx what % of people from southern Italy have some Greek ancestors?

Approx what % of people from the Po valley have some Celtic ancestors?

thanks--92.251.131.97 (talk) 14:19, 9 May 2010 (UTC)


 * Happy to help, but a little google searching goes a long way! Greeks in Italy...and still looking around for Po valley --rocketrye12talk/contribs 15:11, 9 May 2010 (UTC)
 * Sorry I'm already aware of the history of greek people in Italy so that article doesn't help much, I'm asking how many actual Italian people have greek blood. —Preceding unsigned comment added by 92.251.131.97 (talk) 16:25, 9 May 2010 (UTC)
 * You are going to have to give a better definition of Greek blood. If you just mean a person with an ancestor who has lived in Greece in the past, the number would be almost all of them. Graeme Bartlett (talk) 21:54, 9 May 2010 (UTC)

I'm going to suggest that you **** blood. DNA sequencing is where it's at. You want to look for the allele/haplotype frequencies (or their combinations) for molecular phenotypes that tend to be particular to "Greeks" (though you're going to have to define what type of Greek -- Mycaenean, Hellenistic, etc.?) John Riemann Soong (talk) 07:28, 10 May 2010 (UTC)
 * I think it's rather likely that the OP is simply using 'blood' in the colloquial fashion as used in Kinship terminology to mean related by a common ancestor (i.e. genetically related) rather then literally having anything to do with blood. DNA sequencing is of course simply a method of attempting to measure this relationship. Of course you can use a blood sample for DNA sequencing although it's rare given the difficulty of taking blood compared ot other easier methods of obtaining a DNA sample like a cheek swab. Nil Einne (talk) 08:58, 10 May 2010 (UTC)
 * I agree. And by that definition, I'd bet on 99+% for both "celtic blood" and "greek blood" unless one defines a non-negligible threshold. --Stephan Schulz (talk) 09:16, 10 May 2010 (UTC)

travel speed in space
how fast a spaceship travel in space? —Preceding unsigned comment added by Pedfp (talk • contribs) 14:21, 9 May 2010 (UTC)


 * It would depend on where it's going. It would have to be going fast enough to achieve the appropriate Escape velocity.--Aspro (talk) 14:33, 9 May 2010 (UTC)
 * It's not necessary to reach escape velocity to escape a planet (at least not the escape velocity at the surface), as mentioned at Escape velocity. -- BenRG (talk) 04:03, 10 May 2010 (UTC)


 * Are you asking, how fast can spaceships currently existing travel, or how fast could they travel, or how fast is theoretically feasible with different methods? Because it will be different for each of them. --Mr.98 (talk) 15:03, 9 May 2010 (UTC)


 * The fastest manned spacecraft was Apollo 10 on it's return journey from the moon when it hit 39,900 kilometers per hour (24,791 mph) in 1969. That's also the fastest that any human has travelled.
 * The fastest any human built unmanned spaceship has ever gone (and actually, is continuing to go right now) is the New Horizons probe that NASA has sent to Pluto. It is travelling at 16.3 kilometers every second which is 58,500 kilometers per hour, 10.1 miles per second or 36,400 miles per hour.   It will gradually slow down as it gets closer to Pluto.  The long-term fastest spacecraft are the Voyager probes - which are going fast enough to escape the sun's gravity completely - around 34,000 miles per hour.
 * The fastest practical (just!) spacecraft we've ever designed would be the Project Orion nuclear bomb-powered spacecraft that in some versions would be theoretically capable of reaching about 10% of the speed of light and getting us to the nearest star in about 1000 years. An Orion-class spacecraft would (by necessity) be simply gigantic - with a massive 'pusher plate' about 20 kilometers across!  You could house an entire city full of people inside - but it would be ruinously expensive to build - and if you launched it, it woudl make the area for hundreds of miles around its launchpad uninhabitable due to radioactive fallout - possibly causing a 'nuclear winter' scenario and ending life on earth in the process!  However, if mankind had to leave the Earth in a hurry for some reason of impending catastrophy (a major earth-killing asteroid impact, for example) then it would perhaps be feasible to build such a thing in order to save a half million people and save some vestige of humanity for the future!
 * There have been highly theoretical concepts for anti-matter powered spacecraft that would permit speeds up to 80% of light speed...but we have no idea how we could build such things in practice.
 * The fastest that any spaceship (or anything else for that matter) could theoretically go is just a hair short of the speed of light...no matter how it's built or propelled, that's a "final" limit because no physical object can travel at the speed of light.
 * SteveBaker (talk) 15:52, 9 May 2010 (UTC)
 * An Orion drive's pusher plate doesn't have to be that big. According to the designs, the plate wouldn't be wider than the ship itself, and the ship's diameter can be less than 50 meters. ScienceApe (talk) 20:25, 9 May 2010 (UTC)
 * And it could be built in orbit or (if small enough) launched into orbit using a conventional rocket, thus avoiding radioactive fallout at ground level. 67.170.215.166 (talk) 08:22, 10 May 2010 (UTC)
 * The problem is that Orion only works if it is exceedingly heavy. It has to survive being pounded by nuclear weapons!  Not just once - but hundreds or even thousands of times!  That makes building it in orbit impractical.  I suppose you could possibly build it on the moon using locally mined materials though. SteveBaker (talk) 11:49, 10 May 2010 (UTC)


 * Well at least an equation tells us that we'd need infinite energy to go the speed of light, but I'm not so sure. I mean I bet all the "empty space" inside atoms will turn out not to be empty at all (maybe that will solve the dark matter speculation). Science is pretty primitive even now considering what it may be in even 100 years, never mind 1,000 or if/when humans have colonized other planets.--92.251.131.97 (talk) 16:29, 9 May 2010 (UTC)
 * Matter not reaching the speed of light is more of a structural requirement of our current understanding of physics. It's not just an issue of taking too much energy.  In order for the laws and the speed of light to be the same in all inertial frames, things just can't work such a way that matter can move like that. Rckrone (talk) 17:04, 9 May 2010 (UTC)
 * And dismissing what is pretty solid science on the basis that there might be a magical workaround is, well, magical thinking. It's possible that Magiconium exists and somehow allows us to break the basic rules of special relativity, but there's absolutely no good reason to think so. --Mr.98 (talk) 18:45, 9 May 2010 (UTC)
 * The equations of special relativity have been exceptionally well tested. When you sling material around in something like the Large Hadron Collider, you can see how it takes increasingly large amounts of energy to get asymptotically closer to lightspeed - precisely in accordance with that equation.  There is no realistic possibility that the equation is wrong.  Sad and annoying though it is - the speed of light is an exceptionally well researched "hard limit" that's not going to be exceeded by conventional means.  We're down to "wormholes" and "warp drives" and other highly unlikely technologies that 'get around' the lightspeed limit...but those are all very dubious science and stink of wishful thinking rather than scientifically derived possibilities. SteveBaker (talk) 11:49, 10 May 2010 (UTC)
 * Not saying you are wrong here Steve, but the same things were said about aircraft and the speed of sound through the 20s and early 40s. Aren't all scientific principals in theory only one experiment (ok, not counting verification experiments) away from being discarded?  Googlemeister (talk) 13:34, 10 May 2010 (UTC)
 * That was an entirely different matter. They already knew that (for example) a rifle bullet or the tip of a bull-whip could go faster than sound - the issue wasn't a fundamental one.  The doubt was that an airplane could be controlled above the speed of sound.  Also, they hadn't done any experiments to try this on a small scale.  We've done precisely that.  We've found that the amount of energy it takes to accelerate a bunch of electrons or protons from (say) 90% of the speed of light to 99% of the speed of light is less than it takes to get from 99% to 99.9% - which in turn is less than it takes to get from 99.9% to 99.99% and so on.  The rate of increase exactly fits the equation.  Also, the idea that an aircraft couldn't fly faster than sound was based on gut feel and untested ideas.  The speed of light limit emerged from simple mathematics and solid experimental evidence - and every single subsequent experiment (and there have been an awful lot of them) has backed it up to the hilt.  Nobody is saying that it's absolutely, utterly impossible that we're wrong about this - that would be unscientific - but what we can say is just how improbable it is that we're wrong given the vast pile of evidence we've amassed since the theory was put forward that precisely match the predictions it makes.  It's hard to express a measure of 'believability' here...um...I would say (for example) that it is more likely that the Simulation hypothesis is true than that Special relativity is wrong. SteveBaker (talk) 22:54, 10 May 2010 (UTC)


 * That's a great answer, Steve, but how would traveling at .1c get you to Proxima Centauri in 1000 years? Or did you mean nearest stars? — Æµ§œš¹  [aɪm ˈfɹ̠ˤʷɛ̃ɾ̃ˡi] 19:02, 9 May 2010 (UTC)
 * You don't just take off from earth, turn on the hyperdrives and instantly start going at 0.1c. The Orion would slowly gain speed up to 0.1c halfway through the trip - then have to turn around and slow down to zero by the time they get to their destination.  Anyway, that number came from our Project Orion article...I suppose it might be wrong. SteveBaker (talk) 23:30, 9 May 2010 (UTC)
 * Assuming uniform acceleration, the average speed is half the maximum speed. 4ly/0.05c=80 years. I'll go and check that article... --Tango (talk) 01:48, 10 May 2010 (UTC)
 * Ah, I see what happened. You took the time from the original paper and the speed from the later studies - they were different versions of the design and had different speeds. --Tango (talk) 01:51, 10 May 2010 (UTC)
 * I don't follow the logic behind Project Orion. The relativistic rocket equation is m ∂α/∂t = −β ∂m/∂t, where α is the ship's rapidity, m is its proper mass, β is the exhaust velocity, and t is proper time. Integrating gives mi / mf = eΔα / β. Plugging in Δα = 0.1 (10% c) and β = 30 km/s gives mi / mf ~ 10434, i.e., you need 10434 times as much fuel as payload to get to that speed, or 10868 if you want to slow down again at the other end. That's assuming a perfect cylindrical explosion and perfect elastic reflection off the reaction plate. The article's source for the claim of 0.1 c is "Cosmos" by Carl Sagan, which does say "Orion and Daedalus might travel at 10 percent the speed of light"—with no source or justification, of course. In context it's easy to believe that Sagan just made the number up. The other problem with traveling at 10% c is the particles of the interstellar medium striking you at a speed of 30,000 km/sec and a rate of maybe 10,000 per square meter per second. The whole thing seems to me quite ludicrous. -- BenRG (talk) 04:03, 10 May 2010 (UTC)
 * Our article gives an exhaust velocity of up to 30,000 km/s, not 30 km/s. --Tango (talk) 18:10, 10 May 2010 (UTC)


 * You need to be careful with speeds in space - you need to specify what they are relative to. I think your Apollo 10 speed is relative to the Earth and your New Horizon's speed is relative to the Sun, so they aren't directly comparable. --Tango (talk) 21:31, 9 May 2010 (UTC)
 * Again, I read the numbers from their respective articles. You can check them for yourself. SteveBaker (talk) 23:30, 9 May 2010 (UTC)
 * I'm not saying the numbers are wrong, just meaningless without specifying the reference frame. --Tango (talk) 01:48, 10 May 2010 (UTC)
 * I understand perfectly what you're saying - and I agree with you 100% - I'm just asking that you don't shoot the messenger! I'm merely repeating what the respective articles say.  Unless the Guinness Book of Records sets a standard for what speeds have to be relative to...all bets are off.  The Earth orbits around the sun at 67,000 mph - which is more than the numbers specified for any of those spacecraft - so it's hardly an insignificant matter!  FWIW, I'm pretty sure the Apollo 10 numbers are relative to the Earth where the New Horizons and Voyager numbers are relative to the Sun - but I can't tell for sure. SteveBaker (talk) 11:49, 10 May 2010 (UTC)

I think that new Stephen Hawking Into The Universe with Stephen Hawking show, where he goes on about how it may be possible for us to travel forwards in time, mentions the fastest spacecraft created. You can watch it on Youtube if you haven't seen it already, or are interested. There are three episodes. I think he mentions how fast the Voyagers are speeding out of the solar system. An awesome show, I think.--Brianann MacAmhlaidh (talk) 08:38, 10 May 2010 (UTC)


 * I know - I watched it on TV a few nights ago and winced when they said that. The Voyager probes have been the fastest man made spacecraft for almost 30 years and are very famous for that fact - so it's written up that way in an awful lot of books and web sites.  The New Horizons spacecraft only beat their record fairly recently after some slingshot manouver or other and it will soon slow down again, leaving the two Voyagers as the 'current' fastest once more.  Both Voyagers are also slowing down of course - they are still influenced by the sun's gravity, albeit feebly.  It's perfectly possible that Hawking's show is out of date (when was it written?) - or simple inadequately fact-checked at the time. SteveBaker (talk) 11:36, 10 May 2010 (UTC)

What does crab use to dig a hole ?
I'd like to know what crab uses to dig a hole on a beach or muddy area. Thanks. —Preceding unsigned comment added by Srkim793 (talk • contribs) 18:29, 9 May 2010 (UTC)


 * I would imagine they all use the same method as this, even if their digging into river banks etc.Ghost Crab digging a hole Fort Desoto Beach, FL--Aspro (talk) 19:09, 9 May 2010 (UTC)

Invention of high blood pressure medication
I am trying to find when the first high blood pressure or hypertension medication was first developed, who developed it, was it meant to be used for this disease or was it found to be better for another. I found who invented the blood pressure test, but for life of me can't find anything on the medication. Thank you so much —Preceding unsigned comment added by 166.183.201.189 (talk) 18:43, 9 May 2010 (UTC)


 * Do you mean modern synthetic pharmaceutical drugs? Herbalists have used plants that achived the same effect long before the start of 'recorded'  history; so that date is not available.--Aspro (talk) 19:17, 9 May 2010 (UTC)


 * Hypertension has only been recognized as a disease since the 19th century, so I find Aspro's assertion that it's been treated intentionally since before recorded history to be unlikely. Drugs used to treat high blood pressure are known as Antihypertensive drugs, of which there are many types.  There's no history on that page, which is unfortunate.  The hypertension article only briefly mentions a history, giving a couple of the people who first recognized that it existed. Buddy431 (talk) 19:36, 9 May 2010 (UTC)


 * Where did I state that they knew they where treating high blood pressure? Were the Ancient  Egyptians  able to explain the metallurgical science behind adding tin to copper, which enabled them to defend their empire? Or was it just obvious it was harder? Can bumble bees fly, although they are aerodynamically ill equiped to do so? --Aspro (talk) 20:08, 9 May 2010 (UTC)


 * Back up your assertion then. Find a natural substance used for a long time that lowers high blood pressure. Buddy431 (talk) 20:18, 9 May 2010 (UTC)


 * Reading both articles further, a major type of High Blood Pressure medication is ACE inhibitors. There is some History both on that page, as well as the more technical article ACE inhibitors drug design.  The first article indicates that the first ones were discovered in the 1950s and 1960s, but it wasn't until 1981 that the U.S. Food and Drug Administration approved captopril for use. Buddy431 (talk) 19:42, 9 May 2010 (UTC)


 * According to this article: "Until the late 1940s, treatment for hypertension had been largely limited to sedatives, nitrates, or the complete surgical ligation of the sympathetic nerves running alongside the spinal cord. Over the course of the ensuing decade, four entirely new classes of antihypertensive drugs emerged in a dizzying array of branded combination preparations. Though high levels of adverse effects at initial dosages limited their widespread use, by 1958 these antihypertensives had more or less displaced surgical treatment for acute hypertensive crises." The four classes Greene identifies are ganglionic blockers (the "earliest class of specific antihypertensives"), hydralazine ("thought to neutralize an unknown “pressor substance,” is still in use today at lower dosages"), rauwolfia compounds, and veratrum alkaloids. That's all pretty much Greek to me, but the article itself is about Diuril, which was initially a diuretic, but later found to be a good anti-hypertension drug, and is credited as radically changing the way hypertension is treated (e.g. treating chronic and symptomless cases rather than just acute ones). Greene also has a book out that talks a lot about this. --Mr.98 (talk) 19:47, 9 May 2010 (UTC)
 * Here's another good, easily read (though unsourced) article about the history of Hypertension treatment: . The site is all about high blood pressure, and looks reasonable (though I'm no medical professional).  They say that Veratrum alkaloids were the first types of drugs, used as early as the 1930's, but that their effectiveness was limited (they were also quite toxic). Buddy431 (talk) 19:54, 9 May 2010 (UTC)


 * Another thought. Just as different herbs would be given to treat different causes of high blood pressure, so different synthetic drugs would been developed to treat those different causes too. So, perhaps you need to defocus from the major symptom, to each of the  underlying causes. Let me know of what's not clear and I'll rephrase it (we don't give medical advice so I'm playing-safe  here) --Aspro (talk) 19:55, 9 May 2010 (UTC)


 * Please back up your assertions. Hypertension's only been recognized recently, and people generally couldn't have treated a risk factor that they didn't know existed.  While it's true that some high blood pressure drugs were developed from natural products (like ACE inhibitors from the venom of Bothrops jararaca), I can find no mention that these substances were used medicinally before the heavy research done starting in the 1950s.  It appears that different substances were tested more or less at random in a guinea pig to find one that worked.  Treating high blood pressure with straight up snake venom would have been very bad, and unless you can find a citation asserting so, I won't believe it. Buddy431 (talk) 20:16, 9 May 2010 (UTC)


 * “unless you can find a citation asserting so, I won't believe it.” I'm not posting here to prove to you, or anyone else. OP 166.183.201.189 has asked a question, and a good one; and I am doing what I can, to guide them to the answer that their looking for. I am 'aware' that some countries have adopted teaching methods, where students end up demanding  that they get spoon fed, like some,  cuckoo chick in a nest. However,  if they expect regurgitated factoids from me, without context,  and without doing any of the slog themselves -  then  can take a running jump.--Aspro (talk) 21:16, 9 May 2010 (UTC)


 * As soon as Google 'cardiaca' I get Motherwort, the next thing that comes to mind is kidneys, so what comes up - dandelions. Oh silly me, of course! The real reason why they recorded these things in these old books is that Dr Who left a copy of the BNF and some modern  herbalists books!--Aspro (talk) 20:18, 9 May 2010 (UTC)
 * I'll grant you motherwort: it appears to contain a mild vasodilator that can reduce blood pressure, and it was (and is) used for heart problems (among other things). You also make a good point about diuretics (what dandelion's were used for), in that they can be used to treat hypertension.  However, most modern diuretics used to treat hypertension (like Thiazide) are actually effective in lowering blood pressure even at levels below which they increase urine production.  I'm sorry I was so dismissive of you, but I'd really appreciate it if you could give examples, rather than make bold assertions without backing them up. Buddy431 (talk) 20:50, 9 May 2010 (UTC)


 * For those that find this discussion difficult to fathom (and this should not be a discussion page but a ....). A manufacture can only get pattern protection  for something that has been invented. A patted treatment can be sold for thousands of times its cost to manufacture. Even after spending millions on drug development, it still makes more money than it would if it manufactured a generic drug.  So what is a manufacturing company going to do ?


 * As an aside: Charles Dickens died from an apoplectic fit from which he never recovered, (I only say this because I have read an  original copy of the Times newspaper giving the details) . In those days, surgeons where the top rank of the medical profession but herbs were an anathema to them. --Aspro (talk) 20:43, 9 May 2010 (UTC)
 * Actually, physicians, not surgeons, were still "the top rank" of the medical profession in 1870. Surgical training had only become integrated into medical school a few decades earlier. The first appendectomy in England had not yet been performed and Joseph Lister was early in his career. alteripse (talk) 00:27, 10 May 2010 (UTC)

Using oil from an oil spill
Is it possible to use oil from an oil spill? I know it's mixed up with seawater and other gunk, but can't they use some chemical process to extract the oil and then refine that into petroleum products like gasoline? ScienceApe (talk) 20:19, 9 May 2010 (UTC)


 * Yes. They refine it and sell it like normal. Ariel. (talk) 21:40, 9 May 2010 (UTC)


 * When crude oil comes out the ground it is mixed up with salty water, grit and all sorts of stuff. It just costs so much more to recover and process  it when it floats about on the ocean. The ancient Greeks however,  found it very profitable  to collect.--Aspro (talk) 21:41, 9 May 2010 (UTC)


 * Oil-water emulsions are routinely collected and re-refined, mostly from the water used to wash the tanks on oil tankers (which is illegal to dump into the ocean, obviously, because of toxic hazards). The oil-water mixture first has to be desalted and dewatered to prevent corrosion problems downstream -- where I work, we use electrostatic desalters to get rid of the salt, and multi-plate oil separators to separate out the water (these would also get rid of the grit).  Generally, oil collected from an oilslick in the ocean (or from washing tankers) has to be desalted/dewatered in a separate unit from the oil that comes from the well, because the former contains a lot more salt and water; once desalted and dewatered, though, it can be fed to the same atmospheric distillation unit as the oil from the well.  So yeah, it's very much possible to collect and refine oil from an oil spill. 67.170.215.166 (talk) 08:37, 10 May 2010 (UTC)

Why don't our severed limbs/appendages just grow back?
Say if I was to lose a finger, or a hand, or part of my arm or something in an accident. Why doesn't the missing part just grow back eventually, considering that the flesh and bones of our bodies have the ability to repair themselves? --95.148.104.246 (talk) 20:33, 9 May 2010 (UTC)
 * The body is limited in its ability to regenerate since that same ability is what leads to tumours. Finger tips often do grow back, but that's about it for humans. Some reptiles have more impressive regenerative abilities (I don't know if they have more cancer - I think they have shorter lifespans, which probably reduces the extent of the problem). There is some information on this topic here: Regeneration (biology). --Tango (talk) 21:38, 9 May 2010 (UTC)
 * Evolutionarily that's not strictly true. While this is one factor, a more prominent one is that it would be so unlikely for you to lose a major body part in your everyday life, so if you had genes for regeneration, it wouldn't give you enough of an advantage over people who don't so that they would be perpetuated. If you do lose a limb, you would probably die before it could regenerate (without the miracles of modern medicine, that is), so you couldn't reproduce. So evolutionarily, the major genetic changes that would allow this have no incentive to occur. 22:49, 9 May 2010 (UTC) —Preceding unsigned comment added by 68.248.227.1 (talk)
 * That's only true if such a trait were a "major genetic change." I think the question may be "since our cells were able to grow themselves into arms and legs once before, why can't they do so again?" I assume that there's some epigenetic explanation for why this ability is switched off after development is complete, but I don't know if that's true. &mdash; Sam 76.118.181.97 (talk) 23:05, 9 May 2010 (UTC)
 * Yes, it's to do with stem cells differentiating into different tissue types. In mammals, that is a one-way process. --Tango (talk) 23:15, 9 May 2010 (UTC)
 * I do not agree that fingertips grow back in humans. I have seen individuals who lost a fingertip and 50 years later it had not grown back even a little bit. Some non-mammals, like amphibians, can regenerate limbs. Edison (talk) 23:49, 9 May 2010 (UTC)
 * I said often, not always. See the article I linked to above, it has a section on finger tips. --Tango (talk) 01:39, 10 May 2010 (UTC)
 * Humans notably can regenerate their livers from only a part of it. Buddy431 (talk) 00:21, 10 May 2010 (UTC)
 * It's my understanding that humans actually have the ability to naturally regenerate limbs but this ability is lost as we age. A fetus that loses a limb can regrow it.  Children under five years who lose fingertips can regrow them without trouble.  After this, tissue regeneration only takes place over small distances (about 1 cm).  There are ways being developed that use this natural ability to regenerate tissue to grow new organs. this video and this video talk about applying this in medicine.  — Æµ§œš¹  [aɪm ˈfɹ̠ˤʷɛ̃ɾ̃ˡi]  01:10, 10 May 2010 (UTC)
 * Conversely however, I scratched my cheek just after I was born and it never healed even slightly! 86.7.19.159 (talk) 21:57, 10 May 2010 (UTC)

We can regenerate organs. We just have to find the correct combination of cellular signals to the turn the system back "on". It's quite about evolutionary incentives. Our liver cells are under constant daily attack from oxidants and all sorts of toxins we eat. So naturally they have been selected to regenerate quite often. John Riemann Soong (talk) 03:22, 10 May 2010 (UTC)
 * The primary error is in your premise -- you ask why we cannot regenerate if we can repair. They are too entirely separate things, although obviously somewhat related.  It's sort of like asking "I don't understand why we can't fly if we can walk!"  DRosenbach  ( Talk 14:27, 10 May 2010 (UTC)

Are vacuums in Europe more powerful than in North America?
In North America the practical limit to a household motor is 12-13 amps, i.e. 1300-1500 watts. But in Europe the voltage is higher, plus they have high amp ring circuits. So do they make the vacuums with more powerful motors? Ariel. (talk) 22:12, 9 May 2010 (UTC)

My vacuum is a Henry. A very popular brand in the UK. It has a 1200W motor and plenty of suction. I don't see why anyone would need any more. Theresa Knott &#124; token threats 22:15, 9 May 2010 (UTC)


 * Things like kettles are faster boiling in Europe. My kettle's 3,100 watts. With vacuums there is little point in higher power. --Aspro (talk) 22:29, 9 May 2010 (UTC)
 * Wouldn't a higher power vaccum be harder to push? 68.248.227.1 (talk) 22:39, 9 May 2010 (UTC)


 * I don't live in Europe but of course a substanial portion of the world uses 220-240V outside of Japan, Taiwan, most parts of the Americas (some of which do use 220-240V as well) and a few other various places File:Weltkarte der Netzspannungen und Netzfrequenzen.svg and NZ isn't one of those places.
 * Anyway my vacuum which is this (I think) is rated 2,300W. However it has a powerhead and at a guess 100-200W at least would be for that when in use. It also has adjustable power. It got good reviews from the NZ Consumer's Institute at the time I think (I didn't read them but the person who purchased it did I believe) which was about 4 years ago, I think the powerhead is good for pet hairs and the like, as well as especially dirty carpets (probably good since I don't vacuum as much as I should). See also.
 * Note that that also says "Vacuum cleaners are often promoted on the basis that more watts equals better performance. This is not true. Our tests show no relationship between performance and rated watts.".
 * I can easily find 2000W vacuums.
 * Two of those are in the UK i.e. Europe, so the answer to this question is apparently some are. For the NZ ones, you can see the price of those with higher power ranges from cheap to expensive so although a primarily a gimmick, it appears many do it. And the answer to the wider but unasked question about places outside Europe with 220-240V is also some are.
 * Nil Einne (talk) 08:19, 10 May 2010 (UTC)

Soot
Where does the soot come from in my gas fire? —Preceding unsigned comment added by 79.76.139.1 (talk) 22:24, 9 May 2010 (UTC)
 * Impurities in the gas (or impurities that are picked up in the pipes or the mechanisms of the fire itself). If there is a significant amount of soot then something is wrong and you should call a gas engineer out. --Tango (talk) 22:34, 9 May 2010 (UTC)
 * (Edit conflict) Soot is made of small particles of carbon that cool off before they have time to burn. Soot is the same as smoke, only smoke is in the air. When you burn anything with carbon in it, whatever is left that is black is carbon that has not burned. Hope this helps, --The High Fin Sperm Whale 22:37, 9 May 2010 (UTC)


 * Presuming it is mostly Methane and so, it will have carbon atoms as part of its composition. The soot come from incomplete combustion of the gas.--Aspro (talk) 22:40, 9 May 2010 (UTC)

Specifically - if the gas is burning smokily and leaving soot - then there isn't enough oxygen present for complete combustion. Carbon burns in oxygen - if there is carbon left, then it couldn't find oxygen to combine with to make carbon dioxide. That's bad because it also means that there could easily be more carbon monoxide than there should be - and that's a fairly serious health risk. SteveBaker (talk) 23:22, 9 May 2010 (UTC)


 * "Fairly serious health risk" means you could die in a matter of hours. Jc3s5h (talk) 23:27, 9 May 2010 (UTC)


 * Depending on the amount of ventilation - yes, that's perfectly possible. But (for example) candle flames burn very smokily - and produce a lot of carbon monoxide too - but you don't generally have enough of them for that to be a problem.  However, with the amount of fuel that a gas heater gets through - there is ample scope for problems.  I think our OP should get his fire looked at by an expert - there is plenty of scope here for personal danger! SteveBaker (talk) 11:23, 10 May 2010 (UTC)

How do wheels on a cannon affect the force on the cannon ball?
Hi all,

I always thought that wheels on cannons were just the prevent the cannon from, say, ripping out ship floorboards, and that having them "robbed" the cannon ball of some of its energy, but my friend says that the cannon ball actually travels further when the cannon has wheels on it.

I have two arguments. The first is slightly less persuasive even to me: The cannon rolling back requires energy, which is taking energy away that could have gone into the ball. The answer to this is probably: if the cannon is bolted down, this energy probably just goes into changing the spin of the Earth in some minute way, or something.

My second argument is: the ball is propelled forward by expansion of the air between the ball and the back of the cannon, much like a spring pushing two things apart. If you put two marbles on either side of a compressed spring and let go, both would travel a little way. If you fixed one end down and just had one marble, the one marble would travel further when you let go. Right?

So... what's the real science? Thanks! Sam &mdash; 76.24.222.22 (talk) 22:55, 9 May 2010 (UTC)
 * You're right that the ball would move faster if the cannon weren't allowed to move, assuming you didn't just destroy everything. Conservation of momentum requires that when the cannon ball moves forward, something else has to be moving back with the same amount of momentum, but if that object is the Earth rather than only the cannon it will get much less energy out of the interaction (since energy is proportional to velocity squared, while momentum is linear in velocity) and the ball will receive more of it.  That's really the same thing that happens with the marbles, since the fixed end of the spring is also pushing against the Earth. Rckrone (talk) 23:14, 9 May 2010 (UTC)
 * The primary reason for wheels on cannons is to enable the artillerymen to move them around; it's far simpler to limber a cannon than to take it all apart and carry the pieces around on waggons. Nyttend (talk) 01:40, 10 May 2010 (UTC)
 * The tackle on a shipboard gun carriage from the age of sail is a simple form of recoil mechanism that prevents the firing shock from tearing the ship apart. It softens the opposite reaction by spreading the force in time - otherwise, the effect on the firing ship would be similar to the cannonball hitting the opposing ship. It also brings the muzzle of a gun inboard, allowing for reloading without having to haul the gun backwards to get access to the front end of a muzzle-loading cannon.  Acroterion  (talk)  01:48, 10 May 2010 (UTC)

Ok, so let's see if I get this right. Say the the cannon weighs 100 kg and the cannon ball weighs 1 kg. If the total energy of the explosion was 100 Joules, then So the cannon ball travels forward at 10 * 100 = 1000 m/s? Is that right? And the units?
 * From conservation of momentum, we know that the cannon will travel at V backwards and the cannon ball will travel at 100V forward, right?
 * cannon KE + cannon ball KE = 100
 * {(1/2)100 * 1V^2} + {(1/2) * 100V^2} = 100
 * 100 V^2 = 100
 * V = 10

Now we bolt the cannon to the ship, so the whole thing weighs 1000kg. If the total energy of the explosion was 100 Joules, then So the cannon ball travels forward at 0.31 * 1000 = 310 m/s?
 * From conservation of momentum, we know that the cannon will travel at V' backwards and the cannon ball will travel at 1000V' forward, right?
 * cannon KE + cannon ball KE = 100
 * {(1/2)1000 * 1V'^2} + {(1/2) * 1000V'^2} = 100
 * 1000 V'^2 = 100
 * V' = 0.31

Blah, that doesn't work. I just proved that the cannon ball goes slower when the cannon is attached the the ship. Where did I go wrong? &mdash; Sam 76.24.222.22 (talk) 03:40, 10 May 2010 (UTC)
 * I didn't read the whole thing, but as a general rule, you don't get accurate results in ballistics by working with energy, mainly because it's too difficult to model how much of the energy gets dissipated. --Trovatore (talk) 03:44, 10 May 2010 (UTC)


 * Perhaps your friend is getting it mixed up with a trebuchet, which does work better with wheels. This is because the counterweight moves back when the ball moves forward, so it results in the trebuchet moving forward more, and because the kinetic energy of the weight is more down and less backwards, causing it to throw the ball up more too. — DanielLC 05:21, 10 May 2010 (UTC)


 * Without commenting on the math, a gun firing a 32-lb ball weighed about 6000 lb (with another 1000 lb or so for the carriage) with a muzzle velocity of something a little less than 2000 ft/sec (sorry about the units - no metric in those days).  Acroterion  (talk)  11:51, 10 May 2010 (UTC)


 * Little mistake, "{(1/2)100 * 1V^2} + {(1/2) * 100V^2} = 100" should have been
 * ............................................................. (100v)^2
 * so the resulting v should be very small indeed. Maybe about 14 centimetres per second for the cannon, 14 metres per second for ball.

Using energy should work. Keep trying Polypipe Wrangler (talk) 10:49, 12 May 2010 (UTC)

Pitot tube pressure/height
I was reading the article on pitot tubes and I saw that the difference between static and dynamic pressures is used to calculate the airspeed on an aircraft. What I wanted to know (because it's not explained in the article) is how exactly one finds the static and dynamic pressures are measured within the tube. Some sort of spring apparatus? --130.216.46.2 (talk) 23:48, 9 May 2010 (UTC)
 * Pressure sensor. Ariel. (talk) 00:03, 10 May 2010 (UTC)
 * The static pressure is measured by the static sensor. The whole process is better explained at Pitot-static system.  Dismas |(talk) 00:41, 10 May 2010 (UTC)


 * It is the dynamic pressure (or impact pressure) that is relevant to the indicated airspeed of an aircraft, not the difference between static and dynamic. For a low-speed aircraft, the difference between the stagnation pressure and static pressure is called dynamic pressure.  At speeds faster than about 0.3 Mach the difference between stagnation and static pressures is called impact pressure.  Static pressure is conveyed by a conduit to one side of a diaphragm (ie an aneroid capsule or bourdon tube) and stagnation pressure is conveyed (from the pitot tube) to the other side.  The deflection of the aneroid or bourdon is proportional to the dynamic pressure.  The deflection drives the airspeed pointer, and the face of the airspeed indicator is calibrated so that it displays indicated airspeed in units of knots, km/hr, miles per hour etc. See Airspeed indicator.   Dolphin  ( t ) 03:22, 10 May 2010 (UTC)