Wikipedia:Reference desk/Archives/Science/2009 July 4

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July 4

what sort of atomic radius is this?

If I take a mole of some element, measure its volume, then divide by 6.024 * 10^23, and then find the radius of the sphere with that volume, what am I measuring? Van der waals radius? —Preceding unsigned comment added by Em3ryguy (talk • contribs) 01:16, 4 July 2009 (UTC)

I'm not sure you are measuring anything useful. Any substance is going to have a space between its atoms in any phase of matter; your calculation includes this extra-atomic space as part of the radius of the atom itself. Even assuming you had a perfect crystal at absolute zero (which is an impossible to reach state), you still have to consider packing efficiency. In any real solid, the atoms are going to still obey kinetic molecular theory in that they will vibrate in place; and this motion will also add volume to the theoretical perfect packing required for your calculation to work. See Van der Waals radius#Methods of determination for real experiments used to calculate the Van der Waals radius. I am pretty certain that most modern chemists would use X-ray crystallography to find such values today. Also, the Van der Waals radius is something of a meaningless curiosity. It supposes that atoms are hard spheres; they are not, and do not behave as such under any conditions where their radius is likely to be important. --Jayron32.talk.contribs 01:28, 4 July 2009 (UTC)

Useful or not, though, there is a technical term for this radius: mean interatomic spacing, or crystal lattice atomic spacing (depending on the type of matter, you might or might not have a crystal lattice). Take a look at this material science textbook, for example. Nimur (talk) 04:00, 4 July 2009 (UTC)

The real reason the most shell fish should be kept live until cooking

I'd like to read a accurate and somewhat scientific description of the reason that most shellfish are kept alive until cooking. I haven't been able to find much on or off Wikipedia. ike9898 (talk) 01:26, 4 July 2009 (UTC)

Live lobsters do not generally rot. Dead ones might, if improperly stored. Edison (talk) 03:39, 4 July 2009 (UTC)

No, it's a good question from the OP because other dead animals we eat aren't kept alive until throwing them into the cooking pot. Tempshill (talk) 04:27, 4 July 2009 (UTC)

Only because it's impractical to do so. When kept alive, shellfish are far less likely to escape than other animals.--Shantavira|^{feed me} 04:45, 4 July 2009 (UTC)

Shellfish are kept alive until cooking because there is often no way to distinguish between a recently dead shellfish and a long-dead shellfish. This is especially true of true shellfish like clams and oysters; you generally only can tell if its good to eat if it hasn't rotted, and the only way to tell if it hasn't rotted is to ensure its still alive when you cook it. Most live shellfish ship pretty well on ice, which is why you can get live clams pretty much anywhere (for the right price). Heck, I've seen them in seafood markets in Chicago, which is 1000 miles from an ocean. Crustaceans, which often get classed as shellfish, (though they really aren't), like shrimp, prawn, lobster, and crab are probably safe to eat if frozen alive; in fact lots of shrimp is handled that way. Lobster, on the other hand, is not generally cheaper to ship frozen than alive; unlike shrimp, lobster can survive for a long time out of water, and can survive in a "dormant" state if kept on ice, like dry ice, which is why live lobsters can be shipped to many parts of the country. Crabs are usually cooked and packaged on the boat, in the case of snow crabs or king crabs, which are the kinds of crab you get when you get those crab legs on the chinese buffet. Other types of crab, such as dungeness crab or blue crab are usually only availible dockside. Live crabs are cooked to consume at the restaurant, or cooked and picked and shipped in the form of "lump crabmeat". Unless you are at the shore, you are not going to be able to get raw or fresh crab to cook. --Jayron32.talk.contribs 18:13, 4 July 2009 (UTC)

Original poster here I'm afraid everyone who's answered so far doesn't know much more than I do, and I really think that there is more to it than what has been mentioned so far. I have seen some (unreliable) sources explain that it has something to do with autolytic enzymes that are released when the animal dies which break down the tissue rapidly. I also think I've heard about some serious food safety concerns. Anybody else have input? ike9898 (talk) 01:13, 5 July 2009 (UTC)

I think Shantavira, Edison and Jayron32 have a bit of a point. In particular, most seafood including fish generally has to be fresh and is usually cooked within a day or two of cathing or frozen or otherwise prepared. You do get live fish in restaurants, but for home use, carrying and storing live fish is impractical but live shellfish is usually not. Shellfish may be somewhat worse then fish (I see suggestions they should be cooked within a few hours) so this may not entirely answer the question but I think it's getting there. The fresh is best mantra is of course a common view, and it's not uncommon for there to be live chickens in wet markets but I'm not entirely sure if many people would actually want to eat a beef from a cow slaughtered a few hours ago. Nil Einne (talk) 12:42, 5 July 2009 (UTC)

I don't think your statement about live crabs is entirely accurate. It depends where you live. It's not that hard to buy live crabs in Kuala Lumpur or other areas of the Klang Valley for example which is not that close to the shore [1]. This is probably true in most largish Malaysian cities even ones that are further from the shore (although given its geography, most Malaysian cities aren't that far from the shore). I tried to find evidence for live crabs from Ipoh which is probably further from the shore but couldn't find definite evidence other then from restaurants. It's probably not as easy nowadays as it was in the past given the changing demands, the growth of supermarkets and hypermarkets and the lower popularity of wet markets but I still think it's likely not that hard. And as mentioned in the thread, they may be available in supermarkets and hypermarkets in Malaysia, like Carrefour sometimes. I suspect this is the same in many parts of South East Asia and even more so in East Asia. E.g. this [2] is supposedly in Walmart in China although which Walmart isn't specified... In other words, it depends a lot on the local preference, market and demand. N.B. live fish are quite common in most Malaysian restaurants including cheap coffee shop ones. Nil Einne (talk) 12:28, 5 July 2009 (UTC)

@Jayron ... frozen alive; in fact lots of shrimp is handled that way ... Don't know about your shriping boats, but most of the ones I've seen tossed them into boiling water before they got frozen/ stored on ice.71.236.26.74 (talk) 14:31, 5 July 2009 (UTC)

Many are, but many are not. Because of their fiddly size, ease of spoil, and relative expense, shrimps are processed right on the boat and the processing may be in a number of ways. Besides size sorting, the shrimp may be cooked or not, de-"vein"ed or not, have the tail removed or not, and have the head removed or not. The stuff you find at the local mega-mart is most likely to be cooked because most people aren't interested in doing that part themselves, but the product shipped to restaurants would more likely be in the raw state. Matt Deres (talk) 23:33, 5 July 2009 (UTC)

Dumping

Why do I always need to take a dump about half to one hour after a meal? Do most people do this? —Preceding unsigned comment added by 79.75.91.172 (talk) 02:47, 4 July 2009 (UTC)

It happened to me at one era in my life right after breakfast, otherwise not for many years. It was as automatic as putting a coin in a gumball machine. Edison (talk) 03:38, 4 July 2009 (UTC)

The gumball machine operator may be liable under product description regulations. Cuddlyable3 (talk) 15:11, 4 July 2009 (UTC)

The first half of your question is requesting a medical diagnosis. Please see Wikipedia:Reference desk/Guidelines/Medical advice and User:Kainaw/Kainaw's criterion. The second part of the question can be answered by directing you to a study on defecation frequency. 152.16.59.190 (talk) 03:54, 4 July 2009 (UTC)

Is enquiring about trends in normal bodily functions seeking a medical dignosis? Do most people dump shortly after a meal? Thats all I want to know. Its not medical its statistical. —Preceding unsigned comment added by 79.75.91.172 (talk) 09:57, 4 July 2009 (UTC)

Seeking a medical diagnosis for a perfectly normal and widespread phenomenon? Have you looked at Gastrocolic reflex it should answer your question and confirm the normality of the action. It happens to a lot of people, particularly after the first meal of the day. Richard Avery (talk) 10:01, 4 July 2009 (UTC)

It's a real nuisance when I go shopping after a restaurant meal. —Tamfang (talk) 18:48, 21 July 2009 (UTC)

Well thats exactly the answer I was looking for. Thanks Richard. —Preceding unsigned comment added by 79.75.91.172 (talk) 12:52, 4 July 2009 (UTC)

Maybe a link to this could be added to the article on defecation —Preceding unsigned comment added by 79.75.115.133 (talk) 22:41, 4 July 2009 (UTC)

X-ray background

From Wikipedia, the free encyclopedia, quote: "The X-ray background is occulted by the dark side of the Moon." Is this true? How could it be? Please explain. And, please do make sure, when you replying, you actually do understand that "dark side of the Moon" do not actually refer to the "far side of the Moon". Another quotation: "The far side [of the Moon] should not be confused with the "dark side" (the hemisphere that is not illuminated by the Sun at a given point in time), as the two are the same only during a full moon. ". Vitall (talk) 03:14, 4 July 2009 (UTC)

Please do your own homework.

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. —Preceding unsigned comment added by SteveBaker (talk • contribs)

What lead you to believe that question asking if Wikipedia article statement is true/correct could be homework question??? And, that is unrelated, but where in the world topic of X-ray background radiation are taught in schools? Vitall (talk) 03:58, 4 July 2009 (UTC)

An edit earlier today changed the link in the article so that it pointed to Far side of the moon, which created an incorrect statement. I've reverted that edit. For an explanation of how the dark (unlit) side of the moon appears to occult the background radiation, see the description for File:Moon in x-rays.gif. Actually, the entire Moon occults the x-ray background, but in that image x-rays generated by the sun are reflected from the lit portion of the Moon's surface. 152.16.59.190 (talk) 03:37, 4 July 2009 (UTC)

the entire Moon occults the x-ray background <--- I fully agree with THAT statement. Sort of obvious. So do Sun, Mercury, Venus etc. occults the x-ray background radiation. And, so there is absolutely no need to keep designated Moon statement in X-ray astronomy and X-ray background articles??? And no need to emphasize dark side of the Moon? Vitall (talk) 04:08, 4 July 2009 (UTC)

Point taken. Sentence removed. By the way, Wikipedia is the encyclopedia that anyone can edit. Be bold when you see something that needs to be changed. 152.16.59.190 (talk) 06:32, 4 July 2009 (UTC)

Ohh. Please also check this edit. Should be OK I guess. Vitall (talk) 06:45, 4 July 2009 (UTC)

Now what in the world does X-ray background have to do with the occult? ;-) 76.21.37.87 (talk) 04:35, 4 July 2009 (UTC)

That was also part of the question. When I was reading X-ray astronomy, I came across statement that "observed X-ray background is occulted by the dark side of the Moon." It sort of throw me away - in what sense? "Occulted" just like "covered", "hid"? Or Wikipedia meant something different? And why only dark side? And if that radiation is observed, how could it be "occulted", etc. Vitall (talk) 04:46, 4 July 2009 (UTC)

"Occult" means hidden. Astronomers use the term "occlusion" to denote the covering, or hiding, of one phenomenon by another. So to say that the moon occults the X-ray background is technically correct, although it differs from popular usage of the term. Maybe a better term to use in this case would be "occluded" rather than "occulted".--TammyMoet (talk) 08:44, 4 July 2009 (UTC)

OCCULT verb: become concealed or hidden from view or have its light extinguished, cause an eclipse of (a celestial body) by intervention[3]. Etymology: From 1533, "secret, not divulged," from Latin occultus "hidden, concealed, secret," past participle of occulere "cover over, conceal," from ob "over" + a verb related to celare "to hide," from Proto-Indo-European base *kel- (see cell). Meaning "not apprehended by the mind, beyond the range of understanding" is from 1545. The association with the supernatural sciences (magic, alchemy, astrology, etc.) dates from 1633[4] a century later. Cuddlyable3 (talk) 15:03, 4 July 2009 (UTC)

Fungi-related food questions

If cooking kills bacteria and fungi, then why not just cook spoiled food instead of throwing it away? —Lowellian (reply) 03:53, 4 July 2009 (UTC)

Spoiled food is not only unhealthy because of the presence of bacteria or other microorganisms. These organisms have chemically changed the food (e.g. by breaking down proteins; consuming starches and sugars; potentially producing waste toxins, etc.). Nimur (talk) 04:08, 4 July 2009 (UTC)

If fungi like mushrooms are used for food (and thus presumably nutritious), then why isn't fungi like mold also nutritious? —Lowellian (reply) 08:11, 4 July 2009 (UTC)

Some bacteria and fungi produce toxins as a byproduct of metabolism Even if you kill the pathogen later by cooking, the toxin is still there. See botulism as an example. Our article on Foodborne illness describes this quite well. For your second question, remember the fungi are as different from each other as plants are. Some are beneficial, some are innocous, and some are deadly. -Arch dude (talk) 12:24, 4 July 2009 (UTC)

Mushrooms are fungi and classified as vegetables, but that doesn't make all mushrooms edible, either. Many species are extremely toxic or psychoactive. There are some fungi that are not mushrooms but still edible, for example the Judas' ear or snow fungus, but I'm not sure what food group they would belong in. ~AH1^(TCU) 23:53, 4 July 2009 (UTC)

Snow fungus says it is a mushroom. Maybe you meant they aren't Agaricomycotina. 71.236.26.74 (talk) 05:09, 5 July 2009 (UTC)

Also, what food group do mushrooms fit under? They're fungi, so they're not fat/oil, fruit/vegetable, wheat/grain/bread, dairy, or meat/poultry, so what are they? —Lowellian (reply) 08:11, 4 July 2009 (UTC)

Mushrooms are the fruiting body of the mycelium, so they are fruit.--TammyMoet (talk) 08:40, 4 July 2009 (UTC)

TammyMoet, mushrooms are indeed known as "fruiting bodies". However it is an unusual stretch to extrapolate this to describe mushrooms as "fruits". The United States Department of Agriculture classifies mushrooms as "vegetables and vegetable products". Axl ¤ [Talk] 11:00, 4 July 2009 (UTC)

To expand on Axl and the U.S. government; fruits and vegetables are not culinarily exclusive categories; there is some overlap. Botanically speaking, a fruit is any seed-bearing body in a plant. Culinarily speaking, a vegetable is any part of a plant which is used in cooking for savory applications. Thus, there are lots of fruits which are used as vegetables. Consider not only tomatos, but also the entire family of squash and eggplants and cucumber and lots of other fruits which are used in savory cooking applications. There are even a few plants which, while techinically not "fruit", take on fruit-like applications in cooking, such as rhubarb. --Jayron32.talk.contribs 12:40, 4 July 2009 (UTC)

The reason I had my doubts about classification of mushrooms as vegetables is that my understanding is that vegetables are defined as plants that aren't fruits or seeds, but mushrooms aren't plants, so doesn't that contradict the definition? —Lowellian (reply) 18:28, 4 July 2009 (UTC)

Well, except that chefs and taxonomists don't necessarily consult each other when working these things out. From a culinary point of view, something you eat is either a "Plant" or an "Animal". That's it. That Linnean taxonomy has changed over the past 100 years to remove fungi from the "plant" kingdom where they were formerly classified, and moved them to their own kingdom on co-equal footing with plants doesn't matter much to cooks. They still use mushrooms in the same manner in cooking as they always have. And they are used as vegetables. --Jayron32.talk.contribs 19:22, 4 July 2009 (UTC)

If mushrooms are vegetables, does that mean that one could survive on a diet in which one consumes only mushrooms in place of other fruits and vegetables? (That is, you would still be eating stuff like bread, milk, meat, etc., but no other fruit or vegetables like carrots, lettuce, bananas, spinach, etc.) —Lowellian (reply) 18:28, 4 July 2009 (UTC)

All these words are informal. Biologically, mushrooms are not even plants -- they are thought to be more closely related to animals, in fact. Looie496 (talk) 18:37, 4 July 2009 (UTC)

To answer Lowellian: Since humans can survive on a diet that contains very little if any plants (as e.g. in the original life style of Inuit or Mongols), I don't think adding mushrooms will hurt. Fruit and vegetables are a very good and convenient source for many nutrients, but not the only one. --Stephan Schulz (talk) 19:02, 4 July 2009 (UTC)

I guess the question I was more asking was, do fungi, from a nutritional rather than culinary perspective, serve the same purpose for the human diet as plants? Do they provide the same sorts of nutrients? That is, could they substitute for the nutrients that plants provide? —Lowellian (reply) 06:19, 5 July 2009 (UTC)

Hmmm. This depends on the role fruit and vegetables play in the diet. If you follow modern recommended dietary standards (as in the food pyramid) and replace all fruit and vegetables with mushrooms, then I suspect you will miss some essential nutrients. But I also suspect the same is true if you replace everything with spinach... --Stephan Schulz (talk) 07:24, 5 July 2009 (UTC)

Not quite true, it would just involve quite a bit of food chemistry. In the days before the internet there was a study (NASA?) that said humans could live on vats of fungi and algae if an adequate number of varieties were selected and chemical reformulation was employed to produce those chemicals that could not be harvested directly. 71.236.26.74 (talk) 08:23, 5 July 2009 (UTC)

Well, yes, if enough chemistry is involved, you can probably live off a shovel of Earth and a bucket of sea water ;-). --Stephan Schulz (talk) 08:34, 5 July 2009 (UTC)

I think it's questionable if you really need milk or meat if you have a well chosen diet. Also bread and other grain/s /products, while not usually considered vegetables or fruits, do come from plant sources. And some fungi are more suitable meat substitutes (being high in mycoprotein) then fruit/vegetable. E.g. Quorn is made primarily from fungi and some single cell protein proposals are primarily fungi Nil Einne (talk) 21:18, 5 July 2009 (UTC)

Viewing solar eclipse

With a solar eclipse coming on the 22nd of this month I wanted to ask what precautions I must take while viewing the eclipse (i.e. wear sunglasses etc.). And will it be visible here in Pakistan? Thanks. —Preceding unsigned comment added by 116.71.61.65 (talk) 06:06, 4 July 2009 (UTC)

See Solar eclipse of July 22, 2009 and Solar eclipse#Viewing. By far the safest way to view a solar eclipse is by projecting the eclipse onto a flat surface and viewing the projection. NASA has a web site dedicated to eye safety during solar eclipses. 152.16.59.190 (talk) 06:41, 4 July 2009 (UTC)

See same question (Archives June 26 "Solar eclipse") > no. 14 welder's glass/goggles are safe and should be available in Pakistan. Don't use any lower grade though. 71.236.26.74 (talk) 15:26, 4 July 2009 (UTC)

This map shows the local circumstances of the eclipse; you can click on your location to view these. The total part of the eclipse will not be visible in Pakistan, but the partial phases can be seen close to sunrise in the early morning. The "magnitude" tells you how much of the sun is being covered (multiply the number by 100 for the percentage). ~AH1^(TCU) 23:38, 4 July 2009 (UTC)

Physics questions

My science textbook sucks, and I have some questions about motion (Note: please don't oversimplify anything):

1. When I lift an object, I'm converting some form of energy inside me into kinetic energy (to lift the object) and then to gravitational potential energy. When I drop the object, the energy is converted back into kinetic energy. Once the object hits the ground, where has the energy gone? There's no kinetic energy.
2. My textbook states "... when he [a weightlifter] holds the barbell above his head, no work is being done on the barbell. The weightlifter is applying an upward force equal to the weight of the barbell, but there is no movement in the direction of the upward force." This doesn't sound right - surely work is being done to counteract gravity and prevent the barbell from falling?
3. If I hit a very small brick with a very large force (sideways, so the brick may move), my hand will hurt because of Newton's 3rd law. If I hit a brick wall (made of the same material) with the same force, will my hand hurt the same amount?
4. $E=mc^{2}$ states that objects at rest have energy. The Energy article states that energy is "the amount of work that can be performed by a force", but the energy of an object at rest can't be used by any force, can it?

Thanks in advance. --wj32 ^t/c 07:51, 4 July 2009 (UTC)

A1. The answer is that when you drop the object down, either it bounces back up again if its something like a ball, or it makes a big sound. It scatters all the air and dust particles in the area and slightly heats up the ground. The energy is dissipated in doing all these small small tings.

But most of the energy ends up as heat, both in the object and in whatever it hits. -Arch dude (talk) 11:33, 4 July 2009 (UTC)

A2. Somebody owes Steve a dollar. Well, you're confusing force with energy. The weightlifter needs to keep supplying force in order to counter gravity, but he does no work, and ideally, spends no energy, keeping it up. Work is force times displacement. Since he just has to hold the weight in place, he doesn't do any work.

Give the poor sweating weightlifter a rest and replace him with a six-foot-tall rack to hold the barbell. Does the rack doo work? No. -Arch dude (talk) 12:12, 4 July 2009 (UTC)

A3. Yes, your hand will hurt the same amount. The thing is, it is easy for us to give more force to a stationary brick wall than a movable brick, because it just, well, moves away. However, for equal impulsive forces, you will hurt the same.

The physics of injury is complicated, so we cannot give a real-world answer. But a simplified answer is that hitting the wall will hurt more, because the "hurt" depends on how much ofhe energy of teh blow is absorbed by the hand. when the brick can move, much of he energey is converted into kinetic energy in the brick, while the rest is absorbed as pressure waves in the hand and the brik, withthe pressure waves eventually dissipating as heat. With the wall, none of he energy is converted to kinetic energy, so more of it is availble to hurt your hand. -Arch dude (talk) 12:12, 4 July 2009 (UTC)

A4. Read Nuclear Energy. That is the very same energy which can be used to blow up a city.

E=MC² operates when mass is converted to energy, typically in a nuclear reaction. This is separate from any kinetic energy or gravitational potential energy. However, there is no feasible way to convert ordinary mass conpletely into energy. We know how to convert a tiny percentage of some kinds of mass into energy, as in a nuclear reactor or a thermonuclear detonation. -Arch dude (talk) 12:12, 4 July 2009 (UTC)

Happy to help... Rkr1991 (talk) 08:37, 4 July 2009 (UTC)

"there is no feasible way to convert ordinary mass completely into energy"... what about matter-antimatter collisions? It's feasible (though fortunately not easy, given the relative lack of abundance of antimatter). --98.217.14.211 (talk) 13:03, 4 July 2009 (UTC)

Thanks for the answers. I'm still confused about No. 2. Doesn't the weightlifter use energy to exert a force? After all, if I push against a wall I'm using energy to exert a force against the wall even though it's not moving... Where does that energy go? --wj32 ^t/c 08:48, 4 July 2009 (UTC)

Exerting a stationary force does not use energy, and a stationary force does no work. The barbell could be supported at the same height on a pillar or by hanging it from the roof - no energy is used by the pillar or the cable. You could lean a heavy object against the wall - again, stationary force, no work done on the wall, no energy being used. Your body uses energy all the time, to keep your heart beating, your lungs breathing and to keep you warm. Sometimes some of this energy might do external work - when you are lifting a weight, or pushing a moving object, for example. But all the energy used by the stationary weightlifter or the stationary wall pusher is wasted as heat - none of it does external work.

For extra credits, analyse the energy and work in a frame of reference in which the weightlifter or wallpusher are not stationary - put the weightlifter in a lift moving vertically at constant speed, and put the wall and wallpusher on a train travelling on a straight track at a constant speed. Gandalf61 (talk) 09:07, 4 July 2009 (UTC)

Just to add to what Gandalf said, when a weightlifter holds a dumbbell in the air, he is not doing work in the scientific sense of the term. It may look as if he is, with all the sweating and gurning, but muscular effort is not the same as work. Muscles are designed to be most efficient for running and throwing, and other dynamic activities. Under those conditions, the force they exert is cyclic, so they get a chance to recover between contractions. They are not designed for exerting large forces over long periods of time. From what I can gather from a quick look at muscular contraction, a muscle fibre contracts when it receives a command, and then starts to relax whether you want it to or not. The biochemistry of muscle fibres means that they cannot stay contracted indefinitely. If you tell a muscle to exert a constant force, what actually happens is your nervous system triggers one bunch of muscle fibres; and then, when they get tired, it triggers another bunch. Eventually it runs out of fresh fibres and has to start re-using the first bunch again. This is why muscles twitch under these conditions. The muscle is thus consuming chemical energy all the time just to stay in one place. --Heron (talk) 10:13, 4 July 2009 (UTC)

Thanks for all of your responses! I think I understand it now... --wj32 ^t/c 10:23, 4 July 2009 (UTC)

I disagree with Rkr1991's answer to question three. Try punching a punching bag as hard as you can. Your hand might sting a little. Now try punching a brick wall as hard as you can. [Medical advice: don't actually try that.] In both cases, the change in momentum (impulse) is the same. The movement of the punching bag allows the impulse to be applied over a longer period of time, which means that the maximum force applied is lower. It's the maximum force that determines the pain (and severity of injury). In your scenario where the small brick moves, the impulse is applied over a longer time period. Axl ¤ [Talk] 11:32, 4 July 2009 (UTC)

By impulsive force, I mean a large force applied over a very short period of time. Punching the punching bag increases the time of application of force, because it deforms, or as I said, moves away. So, it doesn't hurt as much. I stand by my statement. Rkr1991 (talk) 12:42, 4 July 2009 (UTC)

A soft hand is incapable of delivering a mathematical impulse function. Cuddlyable3 (talk) 14:46, 4 July 2009 (UTC)

We may never be able to exert the ideal impulsive force, but the closer we get to it, the more it hurts is my point. A shorter sharper strike is more painful on the hand than a longer drawn out strike. Rkr1991 (talk) 04:51, 5 July 2009 (UTC)

Since your question is now answered you might find this article [5] on hitting bricks interesting. 71.236.26.74 (talk) 15:51, 4 July 2009 (UTC)

Thanks for the link. --wj32 ^t/c 02:47, 5 July 2009 (UTC)

Black or white parasol?

If you are given only the color of parasols, which is more likely to keep you cooler, black or white one? I think black color absorb more light (at least in the range of visible frequency), so it tends to keep you less cool than white one. Like sushi (talk) 07:53, 4 July 2009 (UTC)

Depends on the infra red transmission of the material. Visible light is not infra red and therefore does not warm you. —Preceding unsigned comment added by 79.75.91.172 (talk) 10:00, 4 July 2009 (UTC)

No, this is wrong. Visible light will indeed warm you unless you are a perfect mirror. Where else would the energy go? Or why would a monochromatic laser in the visible be able to burn holes through some material (including "you" ;-)? --Stephan Schulz (talk) 11:38, 4 July 2009 (UTC)

The human body preferentially absorbs infra red. It penetrates to a couple of inches into the body. Other wavelengths do not penetrate and therefore are not absorbed. —Preceding unsigned comment added by 79.75.115.133 (talk) 22:37, 4 July 2009 (UTC)

It's correct that infrared is absorbed quite well. But it is not correct that other wavelength "do not penetrate" and therefore "are not absorbed". Hold a strongish flashlight up against your hand in the dark to see visible light partially penetrating through a part of your body. And even if a given wavelength is only absorbed on the surface, it will still heat the body. Light does not penetrate most metals (due to the electrons ability to interact with nearly all photons), but an iron skillet in the sun will still heat up. --Stephan Schulz (talk) 23:09, 4 July 2009 (UTC)

As far as I understand it, light colours reflect more light and tend to keep you cooler if you are wearing them? The opacity of the material may also be a factor. Exxolon (talk) 12:23, 4 July 2009 (UTC)

Would a two-tone parasol that is white (or even better: aluminised shiny) on the top and black on the inside surface keep one coolest? It could be made reversible for use as a warming umbrella in Winter. Cuddlyable3 (talk) 14:30, 4 July 2009 (UTC)

This general question came up some time ago, in the guise of whether light or dark clothing keeps you cooler. As I recall, there were arguments in both directions, and it was never really solidly resolved. Looie496 (talk) 16:44, 4 July 2009 (UTC)

I think black on the bottom has pros and cons. On the one hand radiation reflecting up against the bottom won't reflect back down and hit you, but it's not as if that energy just disappears. The parasol will convert it to heat, some of which will reach you anyway by conduction through the air and radiation. Rckrone (talk) 18:05, 4 July 2009 (UTC)

Thats a new twist to the discussion. —Preceding unsigned comment added by 79.75.115.133 (talk) 22:23, 4 July 2009 (UTC)

That's not a twist, but a Savannah. --Stephan Schulz (talk) 08:05, 6 July 2009 (UTC)

Work of objects in inertial motion

I asked here some time ago (in fact, on may 20) about the definition of "work", and have come to know that the same amount of force applied for the same time could mean different work done.

I was browsing today's questions and noticed something.

As the work is defined as Force*Displacement, (although I don't know much about what displacement is), it seems that objects exerting some force, but stationary in a reference frame are doing some work for an observer in a diferent reference frame. As any two parts comprising an object are attracted to each other through gravity, and if the object does not change shape, there is a force to cancel out the effect of gravity, and as the object is displaced (right?), objects in an inertial motion seem to do work. Is it right for objects not changing velocity to be doing work?

Like sushi (talk) 13:11, 4 July 2009 (UTC)

It's not possible for objects to exert a force on each other while remaining stationary. If you push against a wall you will accelerate away from it unless you brace yourself against something that's ultimately attached to the wall, like the floor, in which case the sum of the forces against the wall and floor will be zero. There's no net work being done with respect to any reference frame. Objects do self-gravitate and there is a force opposing that, but the sum of the gravitational and opposing forces is zero. -- BenRG (talk) 13:44, 4 July 2009 (UTC)

(ec) Let us first see what Displacement is. It is a vector, having direction pointing to the direction of the overall motion, and magnitude the length of the shortest line joining the initial and final points. Please read the relevant the relevant article for further clarifications.
We need to get one more point clear. Say I push you (literally). The force is applied by me and you move. The product of my force and your displacement is work. So Work done by an object A on another object B is the force applied by A to push B multiplied by the displacement moved by B. I have to say, it should be a dot product, but never mind.
it seems that objects exerting some force, but stationary in a reference frame are doing some work for an observer in a different reference frame. Yes, that's right. Work is dependent on reference frame, that is, it depends on how you look at it.
As any two parts comprising an object are attracted to each other through gravity, and if the object does not change shape, there is a force to cancel out the effect of gravity, and as the object is displaced (right?), objects in an inertial motion seem to do work. I think you are a little confused. Objects push other objects to do (mechanical) work, not themselves. However, you can say gravity (earth) does work on these objects.
Is it right for objects not changing velocity to be doing work? The fact that whether the object that is doing the work is moving or not is irrelevant. What matters is the force applied by the object and the displacement on the object on which work is done. Rkr1991 (talk) 13:50, 4 July 2009 (UTC)

I should have said "objects doing nothing else than inertial motion" for the last one? And (though I may have been confused) as two parts are both attracted to the other by gravity and counter force is by both for both, both parts are applied force and (as the object as a whole is moving) both parts are displaced. That was my point.

Like sushi (talk) 01:42, 6 July 2009 (UTC)

Basically if a moving object isn't accelerating, it must be that the net force on it is zero. So while it's possible for something else to be exerting a force on it in the direction of motion and therefore doing work to it, some other force either from the same source or from somewhere else must be canceling it out and therefore doing the same negative work to it, which is why the kinetic energy of the object stays constant. Rckrone (talk) 19:00, 4 July 2009 (UTC)

So, it is the net force that matters. Then no work done for objects stationlly in a reference frame should be ascribed to zero net force than to zero displacement in the reference frame, because displacement differs in reference frames. Like sushi (talk) 01:42, 6 July 2009 (UTC)

One more thing. Different time lag between two works results in different displacement (not one while the force is applied, but overall displacement) with the same total work, even in the same reference frame. Or can works with a time lag not be added or subtracted directly? If there is no problem with this, I think I need to understand "work" as something that does not reflect total displacement in a reference frame nor force (independently of reference frames) straightly. Like sushi (talk) 05:23, 6 July 2009 (UTC)

You're free to talk about the work done by specific forces, but if you want to consider the total work done by object A on object B, then that's going to be the value corresponding to the net force of object A acting on object B. For example in the case of two halves of a moving object exerting gravity on each other, if you wanted to you could say that half A is doing work on half B through gravity, but at the same time A is doing the same negative work on B through the normal force, and so the total work that A is doing to B is zero.

As for the time thing, work is given by W = F.d only in the case of a constant force. If some displacement takes place in the absence of any force, then the work being contributed at that time is zero. For work based on a time dependent force you would need something more like $W=\int {{\vec {F}}(t)\cdot {\vec {v}}(t)dt}$ where v is the velocity of the object. Rckrone (talk) 08:22, 6 July 2009 (UTC)

Thank you for replying. I think I understand that total force and work is zero in the case. But about the time lag, I don't mean not-constant force. I am thinking about two series of works by constant force which are done with a time interval.

Like sushi (talk) 11:17, 6 July 2009 (UTC)

I misread the formula. And two works by constant force with a time interval can be treated as a single streak with the formula?

Like sushi (talk) 12:19, 6 July 2009 (UTC)

Work can not determine the displacement nor force, but displacement and force can determine the work.

Like sushi (talk) 01:15, 7 July 2009 (UTC)

I'm not sure, but is mass needed to deteremine the velocity when the work and force are given? I think it is not needed to deteremine the force when the work and velocity are given.

Like sushi (talk) 02:53, 7 July 2009 (UTC)

Need help identifying this... thing

Anybody have any idea what this is? I think it might be some kind of fungus, but I don't know the first thing about biology. Thanks, –Juliancolton | ^Talk 16:36, 4 July 2009 (UTC)

Looks to me like a bunch of insect pupae where somebody has cut through the cocoons. Hard to make out, though. Looie496 (talk) 16:41, 4 July 2009 (UTC)

It looks like they've removed something flat that was laying on top and thus exposed the puppae. -- Brangifer (talk) 16:46, 4 July 2009 (UTC)

Ah, that makes sense! Indeed, there had been a tablecloth on top of it all winter long. –Juliancolton | ^Talk 17:48, 4 July 2009 (UTC)

Definitely insect pupae, the question that now remains: pupae of what? Could be some sort of potter wasp or mud dauber. The only way to be sure (or at least have a better id chance) is to take some pupae and put them in a container to allow them to hatch and attempt to id the imagos. We'll await a further post. Richard Avery (talk) 17:40, 4 July 2009 (UTC)

I'll try that. –Juliancolton | ^Talk 17:48, 4 July 2009 (UTC)

Can't get a clear image, but you might want to have a look at Lichen particularly the Xanthoparmelia picture. It's not exactly that, but maybe similar.71.236.26.74 (talk) 18:17, 4 July 2009 (UTC)

What term would describe this

In a medical article Im working on, I need a term to describe the coining of new syndromes... This has to do with Rumination syndrome, where a recent paper proposes the characteristics, diagnosis criteria, causes, and potential outcomes of the disorder in adults (Where the syndrome until now was considered a disorder of infancy).

Is there a term to describe this process of formally and medically assigning the symptoms and causes of a disorder/disease? -- ʄɭoʏɗiaɲ ^τ _¢ 17:47, 4 July 2009 (UTC)

These phrases begin as neologisms. After publication, the phrase gradually pervades the medical literature and comes into general use. Axl ¤ [Talk] 19:15, 4 July 2009 (UTC)

I think the word nosology is something close to what you want. Looie496 (talk) 00:25, 5 July 2009 (UTC)

Rogue pulsar

I am reading a science fiction book and the "antagonist" is a "rogue pulsar." In the story, there is a rogue pulsar travelling through a solar system and it threatens to destroy a planet and it's civilization on its way by. Is this possible? Can pulsars travel through space? —Preceding unsigned comment added by 216.154.19.192 (talk) 19:07, 4 July 2009 (UTC)

Well, pulsars are just rotating neutron stars, and should have a similar distribution of position and velocity as their progenitor stars. That means that yes, they travel through space and might come close to other stars and solar systems. But they are not traveling in some kind of controlled manner, and the chances of an encounter with any given star systems are miniscule. Also see this article. What is the title of the book in question? --Stephan Schulz (talk) 19:18, 4 July 2009 (UTC)

I'm guessing the book in question is Star Trek: Mere Anarchy.[6] Red Act (talk) 04:04, 5 July 2009 (UTC)

You might consider checking out the book Death from the Skies, which has a chapter dealing with how a black hole might destroy the Earth. A rogue neutron star (of which a pulsar is a subtype) would behave quite similarly to a stellar-mass black hole, so it should be a good approximation. Additionally, pulsars generating X-rays or gamma rays could damage a planet from farther away -- in this case, disruption of the Earth's ozone layer is the primary mechanism. To directly answer the question, though, yes, stars move relative to one another. Most roughly orbit a galaxy in the same direction, but some could orbit in retrograde (likely as a result of a previous close encounter with another star) and a galactic collision would greatly raise the odds of a stellar encounter -- though the odds remain extremely low. Proper motion is the measure of a star's motion relative to the Sun. Barnard's star, for example, will move two light years closer to the Sun over the next ten thousand years. — Lomn 03:37, 5 July 2009 (UTC)

Doesn't proper motion usually mean angular (vs radial) motion? —Tamfang (talk) 18:58, 21 July 2009 (UTC)

Yes, the book I referenced is, indeed, Star Trek: Mere Anarchy. Thanks everyone. With science fiction, one can never tell just how accurate things are. I'm still trying to get my transporter to work properly . . . —Preceding unsigned comment added by 216.154.19.192 (talk) 04:32, 5 July 2009 (UTC)

eB luferac taht uoy t'nod esrever eht xulf roticapac! --Stephan Schulz (talk) 07:14, 5 July 2009 (UTC)

A similar event is in the (deep) backstory of Greg Egan's recent novel Incandescence – which is set in the galactic core, where stars are closer together and thus near encounters are more frequent than around here. —Tamfang (talk) 18:56, 21 July 2009 (UTC)

Is there a term for this psychological phenomenon?:

Let's say you're feeling angry or depressed about something which you know, rationally, is petty, selfish, insignificant, and just not worth that kind of emotional reaction(like a comment on the Internet denouncing your favorite books, or being denied a small treat you weren't really entitled to). But rather than admit to yourself that you're getting worked up over that, you tell yourself that your negative feelings are actually caused by something else-- perhaps something(like, say, governmental corruption) of greater significance to the world but with less personal import than the aforementioned petty thing. Has this form of self-deception been documented and named? If so, is it common? 69.224.113.202 (talk) 19:08, 4 July 2009 (UTC)

Some form of rationalization I guess, but there may be a more specific term. Abecedare (talk) 19:14, 4 July 2009 (UTC)

Sounds more like Transference, which is about changing the locus of negative feelings to reduce psychic tension. --Jayron32.talk.contribs 19:16, 4 July 2009 (UTC)

That's close, but the technical term is displacement. Unfortunately that article is only a stub, but there's a lot of literature about this phenomenon. Looie496 (talk) 19:48, 4 July 2009 (UTC)

It seems to me that displacement is concerned with changing the target of your anger (or other negative emotion), while rationalization is changing the supposed reason for the anger. So the OP's query could go either way, depending upon whether he says, "I am not angry at the parking meter that ate my quarter; but at the town mayor for his incompetence" (displacement) or "I am not angry because of the loss of 25 cents, but because it represent governmental incompetence" (rationalization). Of course, ill-feelings are more diffuse than such linguistic analysis, so the distinction in this instance may be somewhat pedantic. May be relevant, though, if one is writing a novel and needs to use the exact term to prevent those &%@$*# reviewers (who never could write anything worthwhile themselves) from nitpicking it apart. :-) Abecedare (talk) 21:03, 4 July 2009 (UTC)

Could what I describe be classified, then, as a kind of rationalization through (attempts at) displacement? 69.224.113.202 (talk) 22:12, 4 July 2009 (UTC)

Also, just a small clarification: what I had in mind is that the real and presumed reasons for negative emotions may be completely unrelated-- as I tried to convey with the examples of (possibly imagined) personal slights and government corruption. 69.224.113.202 (talk) 23:23, 4 July 2009 (UTC)

A question of comfort

When I leave the shower cubicle for the cooler bathroom, immediately I feel cold across the arms and whole upper body. Though it takes only a few minutes to towel the upper half completely dry, whereupon the discomfort disappears, the way of doing this makes a subjective difference. I find it better to remove all of the water from one part before dealing with another, as opposed to quickly towelling most of the surface water from the whole area then dealing with the residual dampness a part at a time. I know that the water is conducting my body heat to the surrounding air - is this exactly the same as heating the water to evaporate it? In terms of overall heat loss, will it make any difference what towelling procedure is adopted, assuming water is being removed at a constant rate? I'm assuming that any model of the physics involved should ignore body heat generated by towelling effort, including local frictional heating.≥86.146.175.89 (talk) 19:40, 4 July 2009 (UTC)

Your trunk may tend to be more sensitive than your extremities with regards to heat changes; for good reason. Since all of your vital organs are there, your body would probably react more negatively to even slight temperature changes than would your arms and legs, which while important, from a survival point of view, are more expendable than say your heart or lungs. --Jayron32.talk.contribs 19:45, 4 July 2009 (UTC)

Not sure about the answer but if you wipe yourself down before leaving the shower-cubicle you'll drive off quicker with less wetness for the towel to pick up. A few 'swipes' of your hands down your arms, legs and front (back is a bit tough!) gets rid of a lot of the water and makes it quicker to get dried in my experience. ny156uk (talk)

I'm not sure it matters how much volume of water is on your body so much as how much surface area of your body is wet. In other words a thin film of water after a quick pass may have nearly the same cooling effect as the large drops before any toweling. So being thorough might be the most efficient in terms of losing the least heat. Rckrone (talk) 20:30, 4 July 2009 (UTC)

Our article on evaporative cooling deals with a device rather than the human body, but the principle described is the same. There are parts of your body that have a lot of sweat glands and there is always some evaporation there so your body's sensors are expecting that. Any excess is perceived as cooling and will thereby trigger a counter reaction e.g. shivering, constriction of small blood vessels and raising of hairs (goose bumps) Water is actually an insulator, so large drops may take longer to cool you than a thin film because some of the evaporative cooling is used to cool the water drops rather than your skin. That's one of the reasons why misting feels cooler the finer the mist is. The other is more surface area. 71.236.26.74 (talk) 01:13, 5 July 2009 (UTC)