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Hello. So i'm a highschooler in a physical world science class.
We are studying work (the force required to move something)
Basically i'm confused about this whole subject. does it have to do with gravity? how do we measure force?
Well I was assigned the investigative quesion "Would it require more work to pull a brick along a flat surface or an angled surface?"
If you could give me some info to help me find the answer it would be greately appreciated.
03:14, 11 February 2007 (UTC)03:14, 11 February 2007 (UTC)
It strikes me that the question as quoted is kinda hard to answer. Is it angled up or down? F'rinstance, if it were angled down by, say, 85 degrees, I doubt that very much work would have to be done. Bunthorne04:06, 11 February 2007 (UTC)[reply]
You can't force me to do your work for you. That being said, work is not force, it's force times distance; all else being the same, if you had to move something twice the distance, it would take twice the work. Gravity can be involved in work, but doesn't have to. For example, lifting a weight off the ground requires taking it into consideration. To answer your last question without answering your question, you should ask yourself how much force you would have to use to move that brick. Hint: what forces would oppose the motion? As a last resort, you could read mechanical work. Clarityfiend04:08, 11 February 2007 (UTC)[reply]
Start with Mechanical work. How mathematical is the class? Does it use vectors? The simple answer would be to look at the vectors that oppose movement. Gravity is a downward force. Friction is opposing force in the direction of movement. Tbeatty04:10, 11 February 2007 (UTC)[reply]
I think everyone else missed the meaning of your question:
A force is a push/pull that causes other things to move. The amount it effects things depends on that thing's mass - if you try using the same force against two different things, the thing with a small mass will be accelerated more than something with a big mass.
Work is a change in energy, which is also force times distance - which is not obvious at all, because it's usually not the same "work" we think of. For most high school physics classes, you're studying work from the viewpoint of a single force. For example, the total amount of work you do against(or get from) gravity is equal to the force of gravity times the distance you moved against(or with) gravity. Even though "holding a book up without moving it" would feel like "work", no total work is being done against gravity, so you probably would still consider it 0 work against gravity. Moving the thing side to side also isn't work - you're not moving it opposed to / with gravity.
So your "investigative question" is too vague - which work? Work against gravity? Work against friction? Note that gravity has a single direction, so the direction of your distance is important (you can only count the up-down distance for gravity). Friction acts in every direction that you might try to slide (as long as you don't lift off the surface), so the whole path's distance counts if you're calculating work by or against friction. —AySz88\^-^01:23, 12 February 2007 (UTC)[reply]
Way to confuse the poor kid! OK - so work is force times distance. So if you have to pull the weight twice as far - it's twice as much work. If you have to pull it with twice the amount of force then it's twice the work. OK - so now we know what work is. How about force? Well, there are lots of different forces. To get a block to move along a surface, you have to provide enough force to overcome friction and to overcome gravity and to actually accellerate the block. If the ramp is level - then gravity doesn't enter into the picture - so the forces you need depends only on the friction and however much accelleration you are giving it. In these kinds of school-level problems they probably assume that the block only has to move exceedingly slowly - so we can ignore the effort to accellerate it from a standstill (that would be F = m.a - force equals the mass times the amount of accelleration). For the sloping ramp, you also have to include gravity. If you are pulling the block down the ramp - gravity provides some of the force - and depending on how steep the slope is and how much friction there is, you may not need to provide much (or perhaps any) force yourself - at any rate, the force you need has got to be less than for the level ramp. If you are pulling the block up the ramp - then you've got both friction AND gravity to fight - so the forces you need is more than for the level ramp. If you are pulling the block over the same distance in all three cases then we know that the work required will be more for pulling the block uphill, less on a level ramp and less still on a downward ramp. This is good because it's exactly what you'd expect. If your job is hauling bricks around - you know it's less work to pull them downhill than uphill...I mean...DUH! In the real world - there are lots and lots of other annoying considerations - friction is generally less when something is moving than when it's stationary - that 'accelleration' cost isn't negligable - the amount of friction depends on the slope of the ramp - if the block moves. it's going to get hot - and that will change the nature of the frictional forces....all sorts of annoying practical details get in the way of knowing the utterly correct answer - but for school project the don't want that kind of depth. SteveBaker23:18, 12 February 2007 (UTC)[reply]
I would like to edit the heading for the 'Grey Matter' page; Grey should be spelt Gray as it is pertaining to a colour and not a family name. I have managed to edit all the 'greys'on the rest of the article but the title of the article as well as a diagram seem to be un-editable. How can I change it? Or can you change it please? Thank you.— Preceding unsigned comment added by 137.122.26.206 (talk • contribs)
So in just about every action movie ever made, some guy holds a gun to some guys head while guns are pointed at his own head. The premise of the conflict is that if the criminal is shot, even in the head, and even if he sees no sign beforehand, he'll have time to shoot his own victim before he dies. Is this at all realistic? Couldn't one of the poeple in this situation just pull the trigger without showing any sign beforehand and be certain he couldn't get shot back? I don't know which other desk might be better suited for this question.
70.108.199.13004:57, 11 February 2007 (UTC)[reply]
At point-blank range, the only way both people would end up dead is if they pulled their triggers at almost exactly the same instant. If one had already pulled the trigger, there would be no way for the other to get his off on time, unless the separation was only a few thousandths of a second after (or even shorter). − Twas Now ( talk • contribs • e-mail ) 05:40, 11 February 2007 (UTC)[reply]
I always thought it would cause them to simultaneously spasm some muscles, while collapsing. But, I confess to not having actually seen many people with guns be shot to the head. [Mαc Δαvιs]X (How's my driving?) ❖ 16:23, 11 February 2007 (UTC)[reply]
You don't necessarily die (or lose consciousness) instantly when shot by a gun, particularly with small calibers. In fact, people have taken bullets to their heads and walked around for hours afterwards. It all depends on the path the bullet travels. (The scene at the end of Fight Club, in other words, isn't entirely ridiculous.) That being said, I wouldn't want to depend on living long enough to fire the bullet. The situation is just a small-scale version of Mutual Assured Destruction, and movie-makers should be allowed some level of artistic license, no?
If that's not satisfying enough, I image one could do a few calculations. To find the answer, we would ultimately have to take various factors into account. The sound of the bullet travels at the speed of sound (350 m/s); bullets travel faster than sound, while in the air, but would slow down considerably upon hitting the skull. Then, one would have to take into account the time it takes for the brain to lose consciousness, and compare that to the time it takes to pull the trigger (which, considering that it would be a near-instinctual process, travels through nerves' electrical impulses - which is near the scale of the speed of light rather than the speed of sound). In short, I don't think it's crazy to imagine that if one person pulled the trigger in a movie's Mexican standoff, the person who pulled it would be at great risk of dying as well. zafiroblue05 | Talk22:07, 11 February 2007 (UTC)[reply]
I should have looked it up, you're right - except that rather than moving much slower than the speed of sound, action potentials move at around the same speed (100-1000 m/s, compared to sound's ~350 m/s) - so it's still up in the air. zafiroblue05 | Talk03:03, 13 February 2007 (UTC)[reply]
When the bullet hits someone in the head, the impact and the pain will throw him off balance for a short period of time, at least. This should give the victim some time to react -- WikiCheng | Talk05:25, 12 February 2007 (UTC)[reply]
Any info we provide about such topics is of course only for general interest and movie discussion, and is not intended to be practical advice for personal defense purposes. Wikipedia says simple reaction time is about 150 milliseconds for him to respond to the impact of the bullet or the sound of the shot by a trigger pull. (I have personally seen 100 msec simple reaction time). If the standoff participants are 10 feet apart, the bullet will travel ten feet in about 1/100 second, which is 10 msec. In the standoff described, if there were no possibility that the other gunman would agree to stand down, and he was going on like a movie badman about how he he was going to kill you right after he finishes his page of plot exposition dialogue, in other words you are completely sure he is eventually going to shoot you, and you have a sufficiently large caliber gun to completely mush his brain, I would shoot him through the brain and duck out of the way while emptying the gun at him. Negotiation is far preferable, but with his brain destroyed he cannot make an intentional trigger press, and odd are better any reflexive finger twitch depressing the trigger would fail to hit you than if he were allowed to fire a carefully aimed shot through your own brain. Of course you would say something to induce him to relax before you fired. Say "I'm giving you to the count of three to put down your gun. One.." (he relaxes), you shoot. You might also remark that he's going to have a hard time shooting you, because the safety on his gun is engaged (made you look). You would not want to give a visible "tell" that you were about to shoot. In other words, if you lose a lot at poker, being such a stone cold killer in the standoff is not going to work out for you. If it is not a standoff but just a person aiming a gun at another person, a judo instructor said that if the gunman is within easy reach, as is usual in movies and TV for composition and dramatic purposes, a trained martial arts fighter has a chance (but by no means a certainty) of grabbing the gun hand and sidestepping or turning so that when he pulls the trigger you are no longer in front of the gun. Then the martial arts expert can open the big can of whupass. But this requires a movement executed in the aforementioned 100 or 150 milliseconds, and even he said that he would not attempt his if it were a simple robbery. Just give him your watch and your wallet and don't wind up a dead hero. The average citizen would not succeed in the disarming attempt. A smart gunman would stay out of reach, and try to get his victim in an offbalance posture to prevent any sudden lunges (I guess that's why cops bark instructions for suspects to get in certain offbalance positions.) Edison17:54, 12 February 2007 (UTC)[reply]
It's pretty clear that the victim doesn't have time to react to the first person pulling the trigger - so the question initially boils down to: Will a bullet travelling through someone's head cause enough damage to prevent an almost instinctive pull of their trigger in retaliation? The fact is, we don't know. Nobody knows because you can't do experiments and there are vastly too many variables to make even an intelligent guess. However, lack of information doesn't necessarily stop this from being a viable scenario. Consider this: So there you are standing with your gun to his head and his gun to your head. You don't know for sure whether he would get a round off if you just pulled the trigger. However, you'd be taking an enormous risk. Furthermore, the other guy clearly thinks the same way - he's saying to himself "If I pull the trigger, there is a good chance I might die". If nobody has fired yet - then clearly both believe there is a big enough risk that if they shoot first they'll die. This is classic deterrence. You just have to provide a sufficiently credible probability of producing fatal retaliation for there to be a standoff. It doesn't matter what the facts of the matter are - only what the participants BELIEVE the probabilities might be. Since even a 1% chance of dying is pretty unacceptable to most people, we can see that this stand-off might actually be quite effective even if it's really unlikely that someone could pull the trigger with half of their brain splattered all over the floor. SteveBaker23:00, 12 February 2007 (UTC)[reply]
Great answer Steve!. I suppose this is what prevents the police sharpshooters from shooting the criminals even if they have a clean shot, if the criminal is pointing a gun to a victim's head -- WikiCheng | Talk10:29, 13 February 2007 (UTC)[reply]
There were a few other tests on missile buses for high altitude nuclear testing but none were on realistic trajectories (and I'm not sure if they were even in standard missile-warhead mating situations). The Soviets tested a number of ABMs in this fashion on realistic trajectories I believe but that's not the same thing as an ICBM at all. In any case this is the specific place on Wikipedia which discusses these tests. For a good book on the lacking of such testing see Inventing Accuracy by Donald MacKenzie. --24.147.86.18723:21, 12 February 2007 (UTC)[reply]
I am suffering from joint pains for a long time.Also I will be having muscle pulls easily.Sometimes, while playing cricket I had serious pain in my thumb and it will swell up and remain black for some days.What may be the probable reasons for my problems.
The blood is not impure, it is just low in oxygen when it is blue. Hemoglobin changes colour to red when oxygen is attached. Note that blood in some other animals is coloured differently, blue or violet or green even GB08:34, 11 February 2007 (UTC)[reply]
I remember reading that some women who are near crying babies sometimes start lactating. For example, wet nurses lactate but don't necessarily have babies at the time. --Wirbelwindヴィルヴェルヴィント (talk) 19:27, 11 February 2007 (UTC)[reply]
Hormonal abnormalities (a pituitary tumor) can cause lactation in women (or men). So it might happen. An unmarried girl might also be a new mother (it does happen). Edison17:23, 12 February 2007 (UTC)[reply]
Nothing is holding it still! All the loose objects in the Universe will be moving and spinning due to the way they were formed. Originally there was momentum or angular momemtum and the earth has retained momentum ever since. It has changed direction as it orbits the sun and galaxy.
There are several ways to answer this. The easiest is to say: "Gravity". The sun's gravity pulls the planets (including the earth) around in their orbits. But it's a bit more subtle than that. If the sun's gravity wasn't there, the planets would head off in straight lines and rapidly disperse all over the place. You might then still ask: "Why does the Earth Move" - but if there is no reference point to measure it's movement against, then for an object travelling in a straight line, you can't quite literally cannot say whether the object is moving such and such speed and in such and such direction - or whether it's standing still and all of the the other objects in the universe are moving in the opposite direction. So it's fairly meaningless to ask why or even if an object is moving in a straight line. Hence my earlier answer: Gravity is what bends those straight lines into the curves that make it clear that one object is moving relative to the other. SteveBaker22:43, 12 February 2007 (UTC)[reply]
According to Kepler the earth moves because the rays of light from the sun hit it in such a way that it is pushed sideways, but also moves round the sun instead of away due to huge magnets in both the sun and the earth:)Hidden secret 713:10, 13 February 2007 (UTC)[reply]
I recently heard about an arguement between two scientists about the level of energy in the Zero point energy field. Dr Steven Weinberg claimed that there was not all that much about, yet according to a recent page i read at physics web the zpf can produce pressure on casimir mirrors in the nanometre range [the least amount of energy between the plates] of about 1 atmopshere thats kgs per cm 2. Wouldnt it take a large amount of EM waves to produce this much pressure. The article isnt loading at the moment on my pc, but look the casimir effect page on this site at the link "casimir effect, a force from nothing" for the page about the 1 atmosphere of pressure.
I know what it is but the question still remains, pressures at the nanometre scale on the plates utilised in the casimir effect can reach up to the equivalent of one atmosphere of radiation pressure on the plates, this is because their is so little resistance inside the plates because much of the zpf is blocked out. My question to clarify is: How come, if steven weinburg is correct and the zpf is very weak, the pressures produced are that strong. surely it would have to be a hugely strong field to produce this kind of radiation pressure, even when there is nothing around to resist it. Please actually asnwer with something relevent to the question this time huh?
Causal observer
I would assume if you read the article, you would understand. The zero point energy is the lowest energy state an evacuated space can have. The Casimir effect, and the force being strong works only at the nanometer scale, because ZPF is so weak. It doesn't take much to move a mass so small, and it doesn't take much to push it hard. The Casimir effect happens because, sort of, the energy in the space outside the two plates is enough more than the energy in the space between the two plates. It takes the nanometer range to work, because it is so weak, such as the energy difference of between the plates and outside the plates, that needs to be high. [Mαc Δαvιs]X (How's my driving?) ❖ 17:22, 12 February 2007 (UTC)[reply]
Actually i think you may have misunderstood the article, From what i understand the casimir effect only works on the nano scale because the only way we can make one area of the zpf less energetic would be to cut off certain kinds of Em, so therefore there is less inbetween the two plates than on the outside so they are pushed together, you cannot get 1 atmosphere of pressure from a weak force. The mirrors are small but the force is universal so it pushes the same on everything, the pressure measured on the plates, when calculated [the plates are extremely small] is the equivalent of 1 atmosphere of pressure, so on all things the zpf exertes a pressure in all directions of around 1 atmosphere [i contacted a physcists about this] The casimir effect does not change the strength of the zpf, it makes it weaker in one spot, so currently everything in the universe is being pushed upon by 1 atmosphere of zpe vacuum pressure, the reason we cant feel it , is the same reason we dont feel the weight of air, we have enough pushing out and enough pushing in. So we are back where we started, why does such a weak force produce such a strong pressure, and who is right, the quantum physicists who say it is extremely powerfull or the classical physicists who say it isnt that powerfull?
Causal observer
After looking up on the net, it seems that beers generally have a energy content of 50kCal per Oz, which is roughly 1000kCal per pint. Which seems to me atleast to be an awful lot. I was wondering how they measured these things, is this the enthalpy of formation, or the enthalpy of combustion of the constituent products or something? If so does it inculde the energy of the ethanol, which being a hydrocarbon I assume has quite a lot of stored energy in it, and if so, si the human body capable of breaking down ethanol into CO2 and H20, which it would have to do to release these energies. Because I was (possibly wrongly) under the impression the human body couldnt break down hydrocarbons.
I know how they measure calories for other foods. They send a sample to a lab, the lab burns a set amount of the food and see how much it heats water up by. I'm assuming beers have a high energy content because the alcohol in the beer is burnable. As for the human body breaking down ethanol, ethanol metabolism says:
There is a common misconception that drinking alcohol leads to weight gain. This has never been proven in the literature and is the subject of ongoing debate among experts. There are many complex theories in the literature which hope to explain why drinking alcohol (i.e. high concentration alcohol such as vodka, as opposed to an alcoholic beverage like beer) may not lead to weight gain, but none of the answers are conclusive. It is known that some or even most of the alcohol that is ingested is not catabolized entirely to H2O and CO2. Instead, much of the alcohol that is processed by the body ends up as acetic acid in the urine.
It is clear that ethanol can be used as fuel for the body. As the article states, the major pathway for alcohol metabolism is ethanol -> acetaldehyde -> acetate -> acetyl (complexed with Coenzyme A) --> phosphorylation of ADP. This is not debated. What is debated is how much of each of these chemicals follow through in the reaction pathway before excretion. That some of the intermediates are excreted, and that a small fraction of the alcohol is metabolized by enzymes other than ADH-Σ (the type that usually deals with ethanol) serves to reduce the amount of energy one gains from ethanol. However increased ethanol in the diet tends to upregulate production of ADH (by smooth ER hypertrophy), which will increase the energy gained by creating more reaction opportunities before excretion. tucker/rekcut18:58, 12 February 2007 (UTC)[reply]
I doubt anybody knows the exact explanation, but why is that in the world more boys are born than women? In some countries, such as China, selective abortion in rural areas may explain this fact but a similar incident can be observed in European countries such as Italy or Spain, where I doubt that hypothesis has any basis. Any ideas? --Taraborn13:27, 11 February 2007 (UTC)
PS: Maybe this would fit better at the Humanities desk :P[reply]
There are three separate phenomena of interest related to newborn sex ratios (M:F):
The "baseline" or "natural" ratio of a healthy human newborn population appears to be about 105:100. The mechanisms are not known with certainty but are probably multiple. However, by late childhood, because of differential mortality, the ratio is close to even. By later in adult life, because of continued and accelerated male mortality, the ratio shifts markedly in favor of females.
It has been noticed that in certain conditions, such as war, the ratio shifts slightly in favor of males. This suggests there may be many subtle biological factors that differentially influence either fertilization success of Y spermatozoa or implantation success. Pheromones, coital frequency, anyone's guess.
A major cause of differential newborn sex ratios in some Asian countries, especially India and China, is simply selective abortion of female fetuses or infanticide of unregistered female births. This has resulted in imbalances as high as 120:100 in some populations of those countries (and obviously the governments are trying to discourage this). Attempts by parents to "choose sex" occur in western countries by a variety of mechanisms, but have a much smaller effect on the sex ratio because the methods are less effective than abortion or infanticide, because a smaller proportion of parents try to do it, and because not all prefer males. alteripse15:30, 11 February 2007 (UTC)[reply]
Blue: More women than men/Red: More men than women.Russia has more women than men (0.94 males per female, see Demographics of Russia); one of the theories put forward are that during the famines which occured in the Cold War, the low food levels were somehow detected by the body, and produced more female babies than male babies (in theory, a population of 1 male and 99 females can produce 99 babies in 9 months, but with very little genetic diversity, while a population of 99 males and 1 female could only produce 1 baby in nine months, but the genetic diversity of the population could be maintained far longer. As competition would be fierce for resources in a famine ridden country, in order for your genes to survive, you'd want to produce as many children and grandchildren as possible, hence the big female ratio is chosen). Now that Russia has more stable agriculture and access to food imports, the balance has shifted back to 1.06 males per female. Incidently, the population is one of the causes of mail-order brides. (See alsoSex ratio) Laïka16:55, 11 February 2007 (UTC)[reply]
A current hypothesis as to why females are ever so slightly selected against has to do with the differences in the X and Y chromosomes. Most simply, the X chromosome is larger than the Y chromosome, and more of a target for detrimental mutations (more base pairs = more of a chance that at least one BP will mutate). This is complicated greatly by the observations that women in fact have duplicate X chromosomes, that one is heterochromatized in the barr body, and by the existence of the Xist gene, but the difference, AFAIK, comes down to how much genetic material is present that, if destroyed, will decrease viability. tucker/rekcut16:49, 11 February 2007 (UTC)[reply]
A very simple hypothesis, widespread among laymen, is that the male sperm swim slightly faster because they carry a lighter load, and therefore win the race slightly more often... They only have to lug along a small Y chromosome instead of one of the much larger X chromosomes, and the smaller amount of DNA can be packed into a slightly smaller sperm head, which decreases viscous drag. --mglg(talk)02:08, 12 February 2007 (UTC)[reply]
Don't offer nonsense under the guise of "other people say". The chromosomes themselves make up such a small fraction of the weight of a spermatozoon that that "explanation" is like saying this locomotive will beat that one because the engineer weighs 190 lbs instead of 200 lbs. alteripse05:11, 12 February 2007 (UTC)[reply]
Please be WP:civil and do not characterize responses as "nonsense." One issue I did not see is the reported greater survival for low birth weight female infants, as related by a pediatrician and as seen at [1]. Why are male babies more fragile?Edison17:15, 12 February 2007 (UTC)[reply]
In fact, dear Alteripse, the human genome contains about 4 billion bases, at about 660 daltons each, which adds up to about 4 pg of pure DNA. A human sperm head is about 3x3x5 microns in size, and therefore weighs around 50 pg. Thus the pure DNA itself constitutes almost 10% of the weight of the sperm head! Furthermore, the extremely tight packing of the DNA in the sperm head is created with the help of packing proteins, mainly protamine, which contribute considerable weight to the chromatin (the packed protein/DNA complex). Thus the chromatin indeed constitutes a major part of the bulk of the sperm head, and the amount of DNA is a determining factor for the size, and thus the viscous drag, of the sperm head. There has been an extremely strong evolutionary pressure towards compact and fast sperm, due to sperm competition when a female is inseminated by more than one male (which is standard behavior in chimpanzees - no comment about humans...), and this has led to elimination of most non-essential cellular components in sperm. I share your concern that too many Reference Desk answers are offered "from the hip", but please save your most dripping levels of indignation for cases where your own footing is any more solid than that of the person you are complaining about... --mglg(talk)20:09, 12 February 2007 (UTC)[reply]
By the way, in the cattle industry there is a lot of money to be had if you can figure out a better way to sort bull sperm into male and female, and the difference in the amount of DNA is large enough that there is research going on trying to use phase microscopy methods to tell male and female bull sperm apart based on total dry-mass content. --mglg(talk)20:35, 12 February 2007 (UTC)[reply]
Do the math. The second sex chromosome is 1/46th of the chromatin, and the difference between the weights of 46XX and 46XY chromatin is at most about 1%. If the chromatin is 10% of the sperm wt, then you are looking at a 0.1% difference in weight of X sperm and Y sperm, which is a fairly tiny amount to explain a >5% differential success rate. The explanation is indeed nonsense. alteripse01:26, 13 February 2007 (UTC)[reply]
I hate to be picky, but sperm are haploid, and thus only carry 23 chromosomes... Furthermore, the 10% figure I gave above was just a lower bound to get the point across, it ignored all hydration water as well as the packing proteins etc.; in reality the chromatin is the dominant content of the sperm head. Therefore a better starting estimate of the volume difference would be 1/23, or about 4%. This is substantial difference, and it gets enhanced by the fact that the outcome of the race is determined not by the speed of a single sperm but by the speed of the fastest of a few hundred million independent sperm; for statistical reasons this has a much narrower distribution, so a small change in expected speed can have a large effect. Please understand that I am not arguing that this theory is necessarily true, I am only explaining its plausibility in the hope that you might reconsider whether your initial, eh, categorical dismissal was appropriate. --mglg(talk)02:49, 13 February 2007 (UTC)[reply]
"there is research going on trying to use phase microscopy methods to tell male and female bull sperm apart based on total dry-mass content." Wow! I never heard of female bulls before. Edison05:31, 13 February 2007 (UTC)[reply]
Is there anything known about the dynamics of singele molecules orbiting the sun? Mechanically there should be no difference to larger bodies, but radiation pressure from the sun should have a major and probably, due to different emission lines, a selectiv effect.
(after eit conflict) The sun's solar wind would push any orbiting gas or dust out to the heliopause, well beyond the outermost planets, in short timescales in astromical terms. The solar wind is powerful enough to significantly erode the atmospheres of the inner planets, apart from the Earth, which is protected by its magnetic field. Gandalf6117:58, 11 February 2007 (UTC)[reply]
Yellowing of plastics commonly results from exposure to the invisible form of radiation labeled, Ultra-Violet light. This yellowing phenomenon is technically called photo degradation and is common to many objects subjected to Ultra-Violet raditaion. An example of that is the yellowing of a newspapers left in sunlight. Some plastics are more resistant, such as the Acrylics. On the other hand, a stabilizer, actually a photo-absorber, can be added to the chemistry. After a while, even plastics, rubbers, etc., with these photo-absorbers will degrade, too. Photo-absorbers are like a balloon. Whereas a balloon can only hold so much air before it "degrades" by exploding, these photo-absorbers also become exhausted over time. Micro-wave ovens, as you likely know, are a form of radiation. Ken 64.231.90.17617:18, 11 February 2007 (UTC)[reply]
Ken, are you saying that the microwave's own radiation can cause the plastic on its outside to yellow? Or would the yellowing be primarily from sunlight? --Lph19:44, 11 February 2007 (UTC)[reply]
I was certain that microwave ovens were a form of matter. I guess you learn something new every day. :) One important difference between microwaves and UV is the frequency: the short frequencies in radiation of higher energy than the visible are often able to break chemical bonds directly (ionizing radiation). Microwaves generally only have enough energy to make molecules ring like tuning forks or to set them spinning. --Joel08:30, 12 February 2007 (UTC)[reply]
I've always gotten the impression that the yellowing (or in some cases pinkening) or the plastic in the microwave was due to interactions with colouring in the hot food. I've had brand new plastic bowls go pink after just a couple of times when I've used them to re-heat spaghetti sauce - yet stay clear for months when used to cook peas and beans. No amount of cleaning will get them clear again - so I suspect a chemical change - but I'm not convinced that microwaves were the cause. SteveBaker20:49, 12 February 2007 (UTC)[reply]
Oh wow, I just realized the original question could have been referring to plastic dishes inside the microwave; I thought it was about the outer plastic housing of the microwave itself. --Lph15:39, 13 February 2007 (UTC)[reply]
I can't find anything on Wikipedia on the South-to-North water diversion in China. I found one article here but I want more articles. Normally I turn to Wikipedia's links section, but as I said, I can't find the article. It seems like too big a thing for you all to miss. Thanks. --70.56.231.172 16:28, 11 February 2007 (UTC)
Me again. I'll post this to requested articles as well.--70.56.231.17216:33, 11 February 2007 (UTC)[reply]
You haven't posted a question for us to answer here, so no one is answering! I guess you are actually requesting the article, it seems you know where to go for that! GB05:59, 15 February 2007 (UTC)[reply]
Hi, What is the most suitable method to sterilize a Vitamin , which is needed to supplement a bacterial medium ? Thanking you, --Pupunwiki17:38, 11 February 2007 (UTC)[reply]
I was reading about Einstien's famous equation and was stuck by something.
His equation states that E=mc²
However, if you solve for m you get the equation
However, the equation shows that matter is actually energy divided (?) by the speed of light squared. Is this right? and if it is doesn't that mean that you could "in theory" create something that can have all of it's matter instantly converted into pure energy by destroying it (like fire)? Also, can the principle of nuclear fission and nuclear fusion (smashing one or two radioactive particles together to release energy) be applied to ordinary objects? i.e. firing a neutron or protron at ANY type of atom and getting energy out of it
Even cooler is that when you car is rolling and has regular kinetic energy, it's effective increase in mass is . Simply by moving your car, you get that additional mass. --Tbeatty21:28, 11 February 2007 (UTC)[reply]
Obviously no new atoms or particles are created. The work expended on tensioning the spring is stored as elastic energy in the spring, or more explicitly in the form of electromagnetic field energy between the atoms. To get the flavor of what is going on, think as a simpler example of pulling on the two atoms in an O2 oxygen molecule. The pulling force will very slightly increase the distance between the two oxygen atoms, and increase the total amount of energy in the electric field between the two nuclei and their orbiting electrons. Something similar happens in the spring. The added energy in the electromagnetic field does indeed have a mass (which interacts with the gravitational field just like any mass), and would make the mousetrap read very slightly heavier if placed on a hypothetical very sensitive scale. How much heavier? Well, let's do the math! Let's say tensioning the spring on a small mouse trap requires pushing it by an average force of 10N for a distance of 10 cm. That would store one J of energy, which corresponds to a mass of m = E/c^2 = (1 J)/(3*10^8 m/s)^2 ~ 10^-17 kg = 10^-14 g. Multiplying this by Avogadro's number 6.023*10^23 yields ~6 billion atomic mass units. Since the atomic weight of iron is 56, that is as much mass as 6 billion/56 ~ 100 million iron atoms. More than you'd think, huh? --mglg(talk)01:56, 12 February 2007 (UTC)[reply]
The equation simply shows that mass and energy are interchangeable. The c-squared part comes about because when you convert a very small amount of mass into energy - you get a heck of a lot of energy because c is an ungodly large number and c-squared is insanely big. This explains why atom bombs (which actually convert a tiny fraction of their mass into energy) are so amazingly powerful. Conversely - as others have said...it doesn't matter whether your cup of tea is hot or cold - it doesn't seem to get any heavier. In truth, the relatively small amount of energy in hot tea compared to cold tea results in an utterly negligable amount of extra mass. The speed of light is about 300,000,000 meters per second and squaring that gets you 90,000,000,000,000,000 m2/s2. The amount of heat energy in a cup full of boiling water compared to a cup full at room temperature is about 250 Joules. But that means that the increase in mass of your teacup when it's heated up is 250/90000000000000000 kilograms. (0.0000000000000028kg) - but since (for example) an oxygen atom weighs something like 0.022/Avagadro's number kilograms - and Avagadro's number is another ungoldy huge number (6x1023) or 0.000000000000000000000000036 kg, your tea got heavier by about the mass of 75 billion oxygen atoms.
It surprises me too. I think we underestimate how much less an atom weighs. If you ask somebody to guess (not calculate) how much would 75 billion oxygen weigh, I suppose their answer would be in grams or fractions of a gram -- WikiCheng | Talk07:32, 12 February 2007 (UTC)[reply]
Er. I know very little about relativity theory, but the above calculation seems to be a misunderstanding or misapplication of Einstein's equation. If my understanding is correct, you can only apply this famous equation to situations in which special relativity applies (significant fractions of light speed). In classical situations where you are considering invariant mass, I don't think you can apply E=mc^2. E=mc²#Background seems to support this. Remember that E=mc^2 is only a special case; it's part of a more general equation:
Anyway, someone who actually knows something about relativity should be able to clear this up. -- mattb@ 2007-02-12T07:29Z
Actually, it's the other way around. The Equation E = m.c2cannot be used at high velocities. Here's why:
Relativity theory defines the total energy of a particle as:
Which is perhaps analagous to the definition of mechanial energery in classical mechanics (potential + kinetic energy). Relativity also defines Erest (the rest energy) and Ekinetic (the relativistic kinetic energy) as follows:
For resting masses Ekinetic = 0 and Etotal becomes mc2. However at significant velocities, the relativistic kinetic energy cannot be ignored and the formula loses its "conventional" form. —LestatdeLioncourt14:19, 12 February 2007 (UTC)[reply]
Questions like this and the answers are fascinating, but to clarify, please state where the extra energy is stored. It is confusing when you state the energy as "equivalent to x number of atoms of oxygen or iron created." Is the energy in fact stored, in the mousetrap example, in the preexisting atoms of the spring, without any new atoms created, so each of so that each metal atom in the spring (which might weigh a few grams in total) increases its mass by some fraction? Could someone make a reasonable assumptiion of how many iron atoms (or other alloyed elements)store the energy and by what percent the mass of each increases? Ditto for the boiling water, where the number of water molecules is easier to determine. Thanks. Edison17:07, 12 February 2007 (UTC)[reply]
Please first reread my answer above. No new atoms are created. In the case of the mouse trap spring, the energy is stored in the form of electromagnetic field in between the atoms (really between the nuclei and the electrons). This field has gravitational mass, because it contains energy. In the case of the hot water, the energy is stored in the form of kinetic energy of the water molecules – they move around faster. Because of their increased velocity, their relativistic mass increases, and therefore their gravitational mass. The invariant mass, or rest mass, of each molecule stays the same, however. --mglg18:29, 12 February 2007 (UTC)[reply]
I'm currently reading Philip Caputo's A Rumor of War, a story about the author's experiences in 1960s Vietnam. He occasionally makes reference to an animal called a "rock ape." My question is, what exactly is a "rock ape"? Is it like a gibbon, or a macaque, or what? The rock ape wikipedia article redirects to a webcomic and offers no help. 68.252.189.13323:45, 11 February 2007 (UTC)[reply]
A google image search of "rock ape" gives me a Barbary macaque (not an ape, but eh? a doubt a lot of GIs/Vietnamese knew the difference in the 60s) not sure about the etymology. --Cody.Pope 00:04, 12 February 2007 (UTC) Ah, yes now I see their range is all wrong. I'd still bet on a macaque over a gibbon, since they rarely leave the trees to hang-out in and around rocks. Also, given that Europeans already called one kind of macaque an ape, I'd bet a trend at large is in play. --Cody.Pope00:36, 12 February 2007 (UTC)[reply]
Nah, I was simply saying that superficially all macaques look pretty much the same (compare this guy to this guy) also from the text of the book it is pretty clear they're talking about a mischievous living animal (also calling it a monkey once or twice). To me that's a macaque. Also, check out this list of monkeys/apes that live in Vietnam. --Cody.Pope01:46, 12 February 2007 (UTC)[reply]
Hello.
I asked my Doctor where you can donate your body to Science when one passes over to the other side, and the Doctor told my wife and I that it was done through the University of Oueensland.
Can you please send me your documents that you need to do this proceedure.
Here is my address which is a temporary address before we move into our new home in Burpengarry in five months time.
<removed address>
Regards
Richard and Moyra Papworth
(edit conflict)This is Wikipedia, the online encyclopedia that anyone can edit, not the University of Queensland. It is likely that you found our article on the university and belived we were the school itself. I can assure you that this is not so, and that the aforementioned university will most likely need to be contacted for this matter. Also, Wikipedia cannot offer legal/medical advice, and this question involves both. - AMP'd02:25, 12 February 2007 (UTC)[reply]
