Wikipedia:Reference desk/Archives/Science/2008 July 28

Science desk
< July 27	<< Jun | July | Aug >>	July 29 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

July 28

Is there a combination of foods that can explode in a peron's stomache?

Are there any combinations of foods that, although each is individually tasty, eaten succession would explode? —Preceding unsigned comment added by 79.122.77.196 (talk) 04:42, 28 July 2008 (UTC)

I'm going to make a giant leap of faith here and assume that you mean stomach when you ask about "stomache" and by "peron's" you mean person's. These spelling errors aside, there may indeed be some combination of foods that meet your requirements but you'll not likely find help at Wikipedia unless you provide some context that shows your intentions to be benign. Otherwise, find another site to screw around with. -hydnjo talk 05:08, 28 July 2008 (UTC)

Unless you develop a taste for something exotic, I really doubt it. And hydnjo, let's assume good faith. 79.122 is probably refering to the urban legend that soda a "pop rocks" can make your stomach explode. The RefDesk gets much stranger questions that this. Plasticup ^T/C 12:10, 28 July 2008 (UTC)

To explode would require your stomach to be sealed off from the outside atmosphere, but it's not (see belching). --Sean 14:00, 28 July 2008 (UTC)

The stomach's venting doesn't always work (see "torsion" in dogs, cows, humans and presumably all mammals). This happens when the stomach twists and cuts off the normal intake or exit routes - like bending a garden hose sharply to cut off the flow. The condition can prove to be fatal within the hour. 196.2.124.253 (talk) 19:34, 28 July 2008 (UTC)

With the right preparation just "one wafer-thin mint" should do it. Richard Avery (talk) 15:53, 28 July 2008 (UTC)

I doubt there's an explosive combination, but I'm sure you could get something poisonous. Salt is the exact opposite: It's made from chlorine, a poisonous gas, and sodium, which would explode if you ate it. — DanielLC 16:47, 28 July 2008 (UTC)

Perhaps Evita Peron's and/or Juan Peron's stomach would explode if they took enough nitroglycerin pills ? StuRat (talk) 16:48, 28 July 2008 (UTC)

Hi. This doesn't make your stomach explode, but certain combinations of foods will cause food poisoning. For example, I don't have a source for this, but it is a widely known fact (in some countries) that eating persimmons and crab meat together will lead to severe food poisoning. There are also claims that other foods together will cause ill effects, like beef with chestnuts or carrots with turnips, but it's more likely that those might be made up. Thanks. ~AH1^(TCU) 17:59, 28 July 2008 (UTC)

Hi. Here is an example of ill-matched foods (but will not make your stomach explode). Thanks. ~AH1^(TCU) 18:05, 28 July 2008 (UTC)

Are we getting into Culturally specific illnesses like Fan death? 79.66.124.253 (talk) 20:41, 28 July 2008 (UTC)

A confusing post

(header added by hydnjo)

why does compensation point occurs onlu twice a day? —Preceding unsigned comment added by 219.90.105.176 (talk) 04:58, 28 July 2008 (UTC)

Is the teeny article compensation point any guide? Julia Rossi (talk) 05:41, 28 July 2008 (UTC)

Hint: photosynthesis is driven by sunlight - is there more sunlight at noon or at midnight? Franamax (talk) 08:52, 28 July 2008 (UTC)

Well the rate of photosynthesis will increase from zero at night to a maximum in the day, before returning to zero at night. Intersecting the rate of respiration once on the up and once on the down, these two points are the compensation points. Philc 0780 17:42, 28 July 2008 (UTC)

"Compensation"? Why not equilibrium point? 196.2.124.253 (talk) 19:38, 28 July 2008 (UTC)

Dolmen construction

Is this the right desk, but after looking at the article refd above Dolmen and finding a topstone that weighed 150 tonnes (I take it the article may mean 1.5 tonnes), how were they lifted into place? Julia Rossi (talk) 05:38, 28 July 2008 (UTC)

In colder climates, one way to do that would be through frost heave. Get the soil at the centre wet, when it freezes, it will lift the stone a little bit - then brace at the outside to keep it elevated. Next summer, pack more earth underneath and get it wet again. I have no idea if they did it that way, but maybe the aliens gave them some tips. Franamax (talk) 07:57, 28 July 2008 (UTC)

And I agree 150 tonnes is a little much. Switching units, rock is 160-190 lb/ft3, so a 20' x 8' by 1' slab would be around 12 tons. To get 150 tons, you need 50' x 9' x 4' - none of the pics look to be quite that impressive. Franamax (talk) 08:12, 28 July 2008 (UTC)

Agreed. Maybe the detail needs a point in the article. Will take it to the talk page. Julia Rossi (talk) 08:58, 28 July 2008 (UTC)

Hi! The article says The largest dolmen in Europe is the Brownshill Dolmen in County Carlow, Ireland.[citation needed] Its capstone weighs about 150 tonnes where as Brownshill Dolmen says The capstone at Brownshill, weighing an estimated 100 metric tons - so there is definately some error there. (though link 2 from that article gives a 'reputed 150 tonnes'.

Well granite weighs more than water and 1cubic meter weighs ~3 ton so 150 ton would be 50 cubic metre (100 tonn 33 cubic metre) giving 5x5x2m (or 3x2x5.5m) both of which seem optimistic estimates. Tens of tonnes seems more likely87.102.86.73 (talk) 11:18, 28 July 2008 (UTC)

If you expand this image then you can see that the stone is ~2x5m on the front face - so maybe the figure is correct.87.102.86.73 (talk) 11:41, 28 July 2008 (UTC)

Following up a bit, the 150-tonne figure seems not unreasonable for the biggest stone pictured in the article. A general method of construction would be to build a long slope to the desired height and drag or use rollers to move stones into place. Vertical stones can be emplaced by digging a pit and tipping the stone in (some crushing of workers may occur, don't try this at home). The cap stone can be simply dragged over top, clear away all the dirt, bob's yer uncle - standing stones. Considering that these were for ritual/religious purposes, it's not unreasonable to think that this vast an effort would be expended over a period of years. Franamax (talk) 10:24, 28 July 2008 (UTC)

More likely method is to use a lever or wooden wedges to raise one end of the capstone by a few inches, then secure this end with wooden blocks and repeat at the other end, thus raising the stone incrementally on a wooden platform. An article on Megalithic tombs built by small teams in British Archaeology suggests this could be done with surprisingly small amounts of manpower - it says a team of 8 could have raised the capstones of the Stonehenge trilithons, weighing 40 tons each. But earthen ramps are also possible - I think there is a theory that many dolmens were originally buried in earth mounds, like passage graves. Gandalf61 (talk) 10:38, 28 July 2008 (UTC)

One theory (for specific dolmens) is that they weren't raised at all - but were dug into below, smaller stones inserted and the rest of the soil removed. This requires the capstones to be already there - erratics.87.102.86.73 (talk) 11:30, 28 July 2008 (UTC)

Thanks to you all for your convincing studies and proofs. Geniuses all (include the dolmen men) and will leave the article in tact.  : ) Julia Rossi (talk) 09:32, 29 July 2008 (UTC)

The book Aku Aku, by Thor Heyerdahl, describes the methods used to raise the monoliths on Easter Island. Basically, yo use a long lever to raise the block a small amount, and then fill under it with small stones. repeat the process many many times. the block rises on a mound of small stones. A team of about twenty men raised a monolith more thna ten feet horizontally, and then tilted it to a standing postition by raising the head end further, taking less than a week to complete the procedure. They then used the same stones to build a ramp, and then used the same procedure to move a "topknot" stone up the ramp and place it on top of the monolith. I used a variant of thsi procedure to extract a neighbor's Chevy Suburban from a ditch. I worked alone, and used pieces of firewood instead of stones. It took about an hour to raise it about five feet. -Arch dude (talk) 02:30, 31 July 2008 (UTC)

which kidney works first?

As we all know, most humans have two kidneys. i want to know that which kidney (left or right one)starts to work first & which remains in stand by.Alok2n 00 (talk) 08:04, 28 July 2008 (UTC)

I don't think it works quite like that. Unless I'm mistaken, they both work simultaneously -- it's just that they're efficient enough that we can get by with only one of them if the other one is removed or stops working for some reason. -- Captain Disdain (talk) 09:54, 28 July 2008 (UTC)

Wait, you guys don't consciously choose? Plasticup ^T/C 12:13, 28 July 2008 (UTC)

Well, y'know, kidneys these days. They never do as I say. -- Captain Disdain (talk) 12:41, 28 July 2008 (UTC)

Have a look at Development of the urinary and reproductive organs#Metanephros and definitive kidney and Kidney#Embryology for more information. Zain Ebrahim (talk) 14:05, 28 July 2008 (UTC)

Refridgerators

what are the gases that used in refrigerators?what are they used for? —Preceding unsigned comment added by 117.196.134.20 (talk) 15:55, 28 July 2008 (UTC)

See refrigerant and list of refrigerants.--Shantavira|^{feed me} 16:11, 28 July 2008 (UTC)

That list is free/on us. StuRat (talk) 16:36, 28 July 2008 (UTC)

Groannnnn. 128.165.101.105 (talk) 19:27, 28 July 2008 (UTC)

Maybe we need a page for Hellmanism, the philosophy whose main tenet is, "Keep cool but don't freeze." OtherDave (talk) 23:38, 28 July 2008 (UTC)

Did Hellman work at the Mayo Clinic ? StuRat (talk) 03:00, 29 July 2008 (UTC)

Energy efficiency of a human compared to a robot

If we were to compare Asimo with a human, which is more energy efficient? Like if a human ate two burgers in the morning and Asimo had his batteries charged, then both go for a walk, which one uses their energy more efficiently? ScienceApe (talk) 16:03, 28 July 2008 (UTC)

The average person uses about 2000 Calories per day. That's about 100 watts. This is average, including sleeping. Asimo uses about 25 watts while walking,[1] so he's more than four times as efficient. Sorry, didn't read the article. It's a different robot that they were comparing to Asimo. Asimo uses 1200 watts,[2] so people are about twelve times as efficient. — DanielLC 16:27, 28 July 2008 (UTC)

I'd say the person is more energy efficient in most situations. This is because machinery often wastes so much energy, in the form of heat, that it gets too hot to touch, while people don't. However, machines have an "off" position, where they use little or no energy, while people don't. So, the robot is more efficient when off than the person when not working. If the machinery is electric, there is also the inefficiency of converting some other form of energy into electricity, delivering it, and storing it. Although, in this case, I suppose the inefficiency of human digestion must also be addressed. StuRat (talk) 16:32, 28 July 2008 (UTC)

Stu's comment about digestion makes me think the question is not completely answerable unless the Humans and Robots share a common fuel source, otherwise it's hard to contemplate overall efficiency. For example, the robot might be charged by a solar panel of certain dimensions. Could a human be sustained by food grown on farm land of that same dimensions? Probably not if the human's preferred fuel is hamburgers, but probably so if the human can be fueled by blue-green algae or some other such horrible food. APL (talk) 18:16, 28 July 2008 (UTC)

Perhaps alcohol and/or some organic oils can be used as fuel for both. StuRat (talk) 18:23, 28 July 2008 (UTC)

Haha alcohol! I'd like to see a workforce powered by that, but more importantly when a human digests alcohol, dont they only oxidise it to a ketone or carboxylic acid, and not fully, like in a combustion engine, to CO2 and water. Gving a much smaller energy release. Philc 0780 18:26, 28 July 2008 (UTC)

I thought grain alcohol was converted to sugars and then fully metabolized. StuRat (talk) 22:18, 28 July 2008 (UTC)

I'd say the robot because the robot is just walking, the human is walking, thinking, digesting, pumping blood round the body, processing waste, etc. You design a robot to do one task and it will generally do that one task far better than a human ever could. The good thing about humans is that we are extremely multi-purpose. --Tango (talk) 16:38, 28 July 2008 (UTC)

Would that mean it is foolish to design a robot in the image of a human, as it will only fail to be as versatile, however well it performs at individual tasks. Philc 0780 17:39, 28 July 2008 (UTC)

Humans (and other organisms) do have an "off" switch. It only gets used once. Recycling organic matter is a technology predating Homo Sapiens. Humans (and other...) are self replicating, whilst the production cost of robots is significant. Humans (and other...) are sustained by solar enery. Mind you, this is the Science Desk... --Cookatoo.ergo.ZooM (talk) 18:41, 28 July 2008 (UTC)

Production costs of humans are significant - I'm sure you've seen the statistics for the cost of bringing up a child. Humans work indirectly from solar energy, just like pretty much everything else. Where do you think the energy in oil comes from? --Tango (talk) 19:09, 28 July 2008 (UTC)

I pointed out that humans (and other organisms) are sustained by solar energy, implying that heat, food, fossil fuels (and pretty much everything else) is the result of the proximity of a star. I fail to see your point, Tango. --Cookatoo.ergo.ZooM (talk) 20:00, 28 July 2008 (UTC)

My point is that both humans and robots are powered the same way, so it's irrelevant to the question. --Tango (talk) 22:01, 28 July 2008 (UTC)

The problem is that it's quite difficult to convert the energy used by a robot and a human both back to the amount of sunlight used. However, if you have them both use a common fuel, like alcohol or other biofuels, then it isn't necessary to convert back to sunlight, you only need to compare the amount of the common fuel each uses (to perform a given amount of work) in order to determine the relative efficiencies. StuRat (talk) 22:15, 28 July 2008 (UTC)

That only works if the common fuel is an ideal fuel for both technologies. Otherwise you could swing the results either way you wanted by choosing an inappropriate fuel. APL (talk) 00:28, 29 July 2008 (UTC)

Those cost of bringing up a child statistic usualy involve bringing them up in a rich nation like USA. Presumably it'd be cheaper to manufacture your humans in a third world country. As a bonus, that's probably where 90% of the robot would be manufactured. APL (talk) 21:06, 28 July 2008 (UTC)

Common cold and sense of smell

Hi, sometimes when I catch a cold, my sense of smell gets altered in a strange way. In such cases, I always sense a characteristic smell which reminds me of pills or medicine, but I can't put my finger on what specific drug it might be. The smell gets stronger as the cold progresses, and I've never had the same sensation when I didn't have a cold.

Is there a place where I can read how the common cold affects the sense of smell? I want to see if other people experience the same thing too. (As a side note, when I get that smell, I almost always find that my nasal mucus gets dry and flaky easily as well.) --Kjoonlee 18:59, 28 July 2008 (UTC)

I can support your observation. Humans are not so bad when it comes to differentiating smells. Richard Feynman has fascinating anecdotes on that in his Surely You're Joking, Mr. Feynman! (holding a newly bought bottle from a dozen in your hand, or a book from a bookshelf, then trying to sniff it out). I'm currently testing privately the hypothesis that I can smell when someone has a (viral) cold. Support for this comes also from the fact that some viruses have distinct odors. --Ayacop (talk) 09:25, 29 July 2008 (UTC)

Panadol and fever

My question is, does panadol help cure fever or does it simply relieve the sensations of the patient while the body heals it's self?Bastard Soap (talk) 19:56, 28 July 2008 (UTC)

It relieves symptoms. This can be a benefit because sleeping helps the body heal itself, and can sleep easier when you aren't being kept awake by coughing and difficulty breathing. ~Amatulić (talk) 20:00, 28 July 2008 (UTC)

Hmm, no, it lowers the temperature (I can testify to that ;). From the article:

"The mechanism by which paracetamol reduces fever and pain is still a source of debate.[9] The reason for this confusion has largely been due to the fact that paracetamol reduces the production of prostaglandins — pro-inflammatory chemicals." -- Aeluwas (talk) 21:09, 28 July 2008 (UTC)

Yes, but fever is a symptom of the illness, so lowering the temperature is just reliving a symptom. It doesn't directly do anything to combat the illness, you would need some kind of antiviral drug for that in most cases (and there aren't any of those for most viruses as I understand it). --Tango (talk) 21:57, 28 July 2008 (UTC)

Not only that, but Panadol's lowering the temperature may actually prolong the illness, since a fever is the body's response to combatting bacterial or viral threats. According to Fever#Usefulness of fever, a fever has some important functions in the healing process.

Indeed, one traditional way to get over a flu more quickly is to "break" the fever by helping it along (sweating under blankets, sitting in a steam bath, etc.) rather than suppressing it. That's not always safe to do, though. ~Amatulić (talk) 22:14, 28 July 2008 (UTC)

For the record it's also worth noting that if elevation of the body's temperature goes unchecked, the worst fevers can result in permanent brain damage and even death. Often, symptoms are treated because they are uncomfortable aspects of the illness but they can also be dangerous facets of the condition that need to be addressed for the patient's safety. --Shaggorama (talk) 07:49, 30 July 2008 (UTC)

How to answer this child's question

"Why does a mirror reverse only right and left, but not up and down?"

I have an answer inadequate for a child about how reflectance works, but I want to know what others would say. ~Amatulić (talk) 20:00, 28 July 2008 (UTC)

Basically, the mirror doesn't reverse anything. It only appears to reverse left/right because your hands are mirror images of each other. Looking at the mirror and moving your right hand, the hand on the right from your point of view moves. If you move your head, your head moves toward the top of the mirror. Your right hand only looks like your left hand because of your own internal symmetry. We don't have top/bottom symmetry, so it doesn't appear to flip that. (I'm sure there are probably better explanations out there, probably with pictures, too). --Bennybp (talk) 20:38, 28 July 2008 (UTC)

It exchanges back and front, not up and down. It's like looking at your twin, reaching the right hand to shake it. 93.132.186.140 (talk) 20:44, 28 July 2008 (UTC)

If you hold a piece of paper up to a mirror so that it is facing you and not the mirror, you will have to rotate it 180 degrees for it to face the mirror. Typically, we rotate things horizontally because we have an instinctive desire to keep things "right-side-up", however, nothing is stopping you from rotating it vertically.

Essentially, the reason the page is backwards is because you turned it around to face the mirror. If you turned it over then the left side of the page would still be on the left, however the page would be upside down.

Words printed on a clear piece of plastic make this more obvious. APL (talk) 21:00, 28 July 2008 (UTC)

That doesn't sound exactly right... the mirror image is projectively symmetric, not rotationally symmetric. You cannot rotate the object in any way to make it look like the mirror image (printed text would be a perfect example of this). Nimur (talk) 22:35, 28 July 2008 (UTC)

To be clear, the image is always flipped. It's chirality is always reversed. But the question was why left-to-right and not up-to-down. The answer is that there is no intrinsic difference. It just depends on how you're holding it. APL (talk) 00:21, 29 July 2008 (UTC)

As 93.132 says, mirrors reverse front and back. We define left and right in terms of which way we're currently facing, which means they get reversed too, whereas we define up and down with respect to Earth's gravity and that isn't changed. If you are facing the mirror which is north of you and point east, your reflection will also point east, since east and west don't depend on which way you're facing. --Tango (talk) 21:07, 28 July 2008 (UTC)

there are mirrors that reverse up and down. Tell your young friend to look into a spoon and describe what they see. --Shaggorama (talk) 22:09, 28 July 2008 (UTC)

I think it's fair to assume the OP was talking about planar mirrors. Obviously different shaped mirrors do different thing. --Tango (talk) 22:11, 28 July 2008 (UTC)

(Blush) As a physicist, I should have known better than to omit a key detail like that. Yes, I was referring to mirrors that exhibit a virtual image, which include planar and convex mirrors, and concave mirrors when you are closer than the focal length. A child wouldn't understand a conjugate mirror either. Tango's answer, confirming the anon's answer, said it best. Thanks. ~Amatulić (talk) 22:22, 28 July 2008 (UTC)

Take the mirror off the wall and put it on the floor, and if it can take it stand on it. You will see up and down reversed. The mirror in front of you is reversing front and back, not left and right. The viewer is imagining turning around. Graeme Bartlett (talk) 02:39, 29 July 2008 (UTC)

The way I find easiest to explain this is that mirrors don't reverse left and right - rather, other people do. When other people want to look at you, they turn around and face you. There are two ways to do this, by flipping over and doing a handstand (this will reverse their top/bottom in your view, but keep their left/right the same), or, by turning around sideways (this will keep their top/bottom the same in your view, but reverse their left/right). The reason the mirror image seems to be raising the opposite hand to you is because we are accustomed to other people's left/right being reversed when they are facing us. Maelin (Talk | Contribs) 05:52, 29 July 2008 (UTC)

That's an interesting perspective. Thanks. ~Amatulić (talk) 21:53, 29 July 2008 (UTC)

Respiration

Is it fair to say that the primary function of respiration is to regulate PCO2 within narrow limits ~40mmHg? That oxygen isn't because under normal circumstances, O2 capacity of blood is surplus to requirements.

Seeing as we die if we run out of oxygen, I'd they both constitute primary functions. Your statement would be like saying the primary function of the mouth is to consume water, since we carry a greater excess of processed food...Someguy1221 (talk) 21:36, 28 July 2008 (UTC)

I don't think you understand the essence of my question, which relates to control of respiration and not really questioning the function of respiration per se.

As I understand it, the body decides when it needs to breath based on CO₂ levels, not oxygen levels, if that's what you're talking about. Control of respiration should help you. --Tango (talk) 21:55, 28 July 2008 (UTC)

As both supplying oxygen and removing excess carbon dioxide are absolutely essential, it seems quite pointless to call one a primary function and another a secondary function. StuRat (talk) 22:23, 28 July 2008 (UTC)

Here's a thread that may be of interest. Apparently we have bad O2 sensors and good CO2 sensors. The thread's a little long... Franamax (talk) 22:58, 28 July 2008 (UTC)

I just saw a bat!

Hi. I presume this is the appropriate desk to ask questions about animals? Like I said, a rather large bat just flew over me. Are there many bats in the vicinity of Eaglesham? I have seen them before but only very rarely. What sort of species of bat would live in my area? Where do they sleep during the day?; hanging upside-down in trees? Willy turner (talk) 22:20, 28 July 2008 (UTC)

There are bats all over the world. According to the bat article, of all the different species of mammals, about 20% of them are bats. The article will tell you much more if you're curious. ~Amatulić (talk) 22:26, 28 July 2008 (UTC)

Most bat species prefer more protected areas, such as caves or attics of houses, barns, and other buildings. Those that eat insects are quite helpful, while fruit bats just make a mess and vampire bats are downright creepy. StuRat (talk) 22:30, 28 July 2008 (UTC)

There are bats in Scotland. You may look at http://213.121.208.4/pdfs/education/bat.pdf and http://www.first-nature.com/bats/ for the bat species found in Scotland specifically and in Great Britain in general. They are fascinating animals. Enjoy. --Dr Dima (talk) 22:37, 28 July 2008 (UTC)

Thanks. btw, ive heard of bats living in attics before, but is this a very common home for them? To get outside to hunt the house would have to have a hole in the wall or roof. Surely most owners in non-arid areas would notice this as it would result in water getting in? Willy turner (talk) 23:00, 28 July 2008 (UTC)

An aforementioned hole in the wall is usually referred to as a window ;) - just kidding. Now seriously, yes, bats may and occasionally do live in the attic of an occupied house. And yes, they need an opening to fly in and out; but it need not be large. --Dr Dima (talk) 00:44, 29 July 2008 (UTC)

Yes, my mom had bats in her attic (even before she went senile, :-) ). They got in through a plastic attic vent cover, under the eaves, which they had broken. The lack of any insects was appreciated, but not the bat poop in the attic. They were quite difficult to spot because they were silent and black, only came out at night, and generally avoided humans. They usually only gave the impression of a passing shadow. The pest control man repaired the broken vent cover after they had left for the night to hunt. The bats came back home and found that "the lock had been changed". :-) StuRat (talk) 02:54, 29 July 2008 (UTC)

Take they've moved on from belfries because the articles don't have any. You could always talk to the Central Scotland Bat Group, a bat's only help group through here[3]. Julia Rossi (talk) 08:48, 29 July 2008 (UTC)

As mentioned above, bats do like to live in peoples homes, under tiles, any small cracks in walls and of course in the attic. Please note that in the UK all bats are strictly protected and it is illegal to disturb a colony, in any way, see http://www.defra.gov.uk/farm/wildlife/bats/index.htm Jdrewitt (talk) 06:44, 1 August 2008 (UTC)

Why do black things radiate more?

I've looked at black body, in case people are wondering. And I understand the overall argument - "Black stuff absorb more. This means they must radiate more to stay in thermal equilibrium, if they are around stuff of the same temperature. But the amount they radiate is independent of what's around it. So black things at the same temp radiate more." Still, whilst this is an interesting logical proof, I'm wondering if there's any mechanistic explanation - why is it that whatever makes an object reflect light of a certain frequency will also reduce the radiation of that object if heated?--Fangz (talk) 22:54, 28 July 2008 (UTC)

If it would make it easier for you, you can imagine a time reversal. A time reversal of a reflection is also a reflection; but absorption and emission are time-reversals of each-other. Therefore, the probabilities of absorption and emission are governed by the same matrix element (that of electric dipole, usually) in the space spanned by the eigenstates of the electron-in-external-field Hamiltonian. You may read Atomic spectral line article, or - better yet - you may take any of the quantum mechanics textbooks that deal with the subject and figure it out in detail. It is not trivial, so patience is your friend here. --Dr Dima (talk) 00:21, 29 July 2008 (UTC)

question about Uniform circular motion

If a bucket is being swung in the vertical plane by a massless/frictionless rope, and only Tension and Gravity act on the bucket, it can't achieve uniform circular motion.

Am I right? Because when the bucket is horizontal there is a component of force straight down, and the tension is straight sideways thus the acceleration is not pointing to the center. Thanks, I have spent an hour on this. Sentriclecub (talk) 23:51, 28 July 2008 (UTC)

Yes, you're quite right. Algebraist 23:58, 28 July 2008 (UTC)

Thanks, it is rare that a physics professor at MIT slips up although he didn't necessarily exclude that there wasn't a rocket attached to the bucket.

Update Wait then how can a bucket do a circular path? I imagine that the component of (mg) in the direction of the velocity (and thus perpendicular to the acceleration) increases the angular velocity all-while perfectly balancing the F = ma, and since the angular velocity is variable (and so must be either the direction of acceleration or the magnitude or both!)... Resolved Sentriclecub (talk) 01:18, 29 July 2008 (UTC)

I think you've got it, but just to make sure: The bucket can move along a circular path, but not at uniform angular velocity, for exactly the reasons you give. --Tango (talk) 02:23, 29 July 2008 (UTC)

The video was tldr, but the prof may have made the same mistake I made when reading this question, which was reading "uniform circular motion" and thinking "simple harmonic motion". SamuelRiv (talk) 00:27, 31 July 2008 (UTC)

Just to be pedantic, it is possible for the bucket to perform uniform circular motion under the conditions described. We just need to vary the tension force's magnitude and direction. To maintain the uniform circular motion all we need is for the resultant acceleration on the bucket to always point at the same magnitude to a fixed point (the centre of our orbit). In the vertical case the bucket's acceleration isn't only due to the tension but also due to gravity. So, if we were holding the string, we could move our hand around a bit and tug at slightly different strengths to ensure that the resultant force always pointed to the same place. The bucket won't be orbiting your hand but it could be orbiting in uniform circular motion. --Peter Ballett (talk) 22:25, 3 August 2008 (UTC)