Wikipedia:Reference desk/Archives/Science/2011 September 10

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September 10

vision a dimensional world

After I've been explained, that the number of dimension I see depends on the number of D I live in, I wonder: Does the brain learn to separate the Ds? Or is it born with this knowledge? furthermore, if I'll be able in the future to signal my brain not via my eyes, a signal of 3D and nD will I be able to see shapes at those high Ds? Let's include colors at that point, if I would take, a signal of nerves of a bat, which assuming see heat, will I'll be able to see a new color? — Preceding unsigned comment added by Exx8 (talk • contribs) 00:14, 10 September 2011 (UTC) Exx8 (talk) —Preceding undated comment added 00:12, 10 September 2011 (UTC).

The visual cortex of a healthy brain is likely inborn with the ability to figure out it is in a 3D world, even if it lacks the ability to see in 3D (as many people cannot, who for various reasons have monocular vision, though they as far as I know always still know that there is depth, even if they cannot easily perceive it), assuming it develops in someone who can see at all. The separation may not be immediate since birth, but it seems inborn as part of the brain's development. This is my vague memory from a seminar on vision I took a long, long time ago.

Colors are separate and problematic — it's hard to know how new qualia would look like by definition. Imagine trying to discuss what "red" looks like (as opposed to "blue") to someone who has been blind since birth. --Mr.98 (talk) 01:17, 10 September 2011 (UTC)

If you haven't already read it, Nagel's What Is it Like to Be a Bat? deals directly with some of these issues (including your specific example). Your question about sensing dimensions is an interesting one which I haven't heard before. I'm admittedly very far from an expert in this field so take this with a grain of salt, but I too would imagine that the brain is hard wired to navigate in a three dimensional world. I don't think the brain possess the structures required to comprehend more than three dimensions. Our perception of depth comes from more than just binocular vision and is probably for the most part innate. On the other hand the brain is very plastic and who knows, if it was fed 4D imaging somehow, it might adapt to navigate. --Daniel 01:48, 10 September 2011 (UTC)

Both the eye and the brain are organized in terms of two-dimensional fields of sensation. The representation of the eyes in the brain also incorporate ocular dominance columns which compare the two eyes, and other tricks meant to give a three-dimensional impression. The problem with 4D is that too much stuff is dumped onto this 2D field. I don't think you can see 4D with a human brain in any very effective way, though doubtless in some simplified situations you could learn to get an intuitive sense of it. Wnt (talk) 06:06, 10 September 2011 (UTC)

On the other hand, the cat visual cortex experiments from a while back suggest that the visual cortex isn't naturally born with the final organization, but is shaped by visual stimuli at early age so that neurons which respond to impossible stimuli are pruned out, by apoptosis, for instance if the cat is kept from seeing any vertical lines during its early age, when mature it will appear to be unable to make them out when you try to train it to respond to them for food, but will be able to be trained to respond to horizontal lines; and vice versa, etc. The end result is that mammals (not the experimented cats) end up with a terrific system for detecting edges, corners, T-intersections of one line over another, and thus objects in the physical world, partly by the system being trained by the world. So, how would it adapt to other dimensional stimuli? and if it could, could our cognitive systems handle the input? who knows.Gzuckier (talk) 05:45, 12 September 2011 (UTC)

Orange Juice Solidifies in Fridge

Hello. Why does my fresh carton of Tropicana sometimes solidify in the refrigerator? Thanks in advance. --Mayfare (talk) 03:09, 10 September 2011 (UTC)

It is sitting in a part of the fridge where it is cold enough to freeze. --Jayron32 03:12, 10 September 2011 (UTC)

This often happens in old inefficient refrigerators with poor insulation. The thermostat regulated fridge makes sure the fridge doesn't get too warm. An inefficient fridge achieves this by letting a colder coolant circulate and parts of fridge then get too cold (below freezing outside the freezing compartment).

The newest efficient fridge models can have the opposite problem. They are so efficient that the freezing compartment can defrost if the outside temperature drops below 20°C. Count Iblis (talk) 04:05, 10 September 2011 (UTC)

You can find several sites on the web that say the freezing point of orange juice is below the freezing point of water, but here we have a report from a seventh grader who actually went to the trouble of measuring the freezing temperatures of several juices and found that they were all right around 1.67 degrees C. That's not far below the temperature that refrigerators aim for -- they try to get close to freezing but not actually hit it. Looie496 (talk) 04:52, 10 September 2011 (UTC)

Refrigerators are supposed to be kept between 2 and 5°C. So, if your orange juice is freezing, you can set the fridge to be slightly warmer. 86.163.0.200 (talk) 11:27, 10 September 2011 (UTC)

This paper (subscription required, free abstract) from the Journal of Food Science puts the measured freezing point depression of orange juice at between 1 and 2 degrees Celsius (depending on sugar content). That is, the freezing point measured is at -1 to -2°C. I hate to pick on a seventh-grader, but the link doesn't go into much detail about the methodology employed and the measurement technique employed; are we certain that his thermometer was properly calibrated? It is also possible that there ought to be negative signs in his data that were lost when the report was formatted for web display.... TenOfAllTrades(talk) 15:17, 10 September 2011 (UTC)

It seems a control sample (water) would also have been good Nil Einne (talk) 17:08, 11 September 2011 (UTC)

"Dangerous rip currents"

I keep seeing these warnings on the Weather Channel about "dangerous rip currents" associated with Hurricane Katia (2011). Now, my understanding of rip currents, borne out by the article, is that they're only a danger to people who think they are dangerous. I remember as a kid blundering into one of them and being surprised by how quickly I was pushed out to sea, but reasoning that the water can't all be moving away from land and one current of water couldn't stay moving while the rest of the ocean wasn't. After that I staked it out for the whole vacation, using it as a "conveyor belt" to bring me out after boogie boarding in each time. Yet someone else could have died, panicking and swimming furiously toward shore! But then again, I've never actually been in a rip current powered by a hurricane - could they be somehow more malignant? Also, I understand that there are islands in strong currents or perhaps tidal bores where a person could indeed be swept by a current off the far end of the island - nobody calls that sort of current a "rip current", do they? Bottom line: why would the Weather Channel participate in perpetuating such a dangerous misconception? Wnt (talk) 06:22, 10 September 2011 (UTC)

It's hardly "dangerous" of the Weather Channel telling people that things may be more dangerous than they really are. (One could argue "scare mongering".) And anyway, rip currents ARE dangerous to people who don't know how they behave. In my country, Australia, surf life savers seem to need to rescue plenty of such people from rips every summer. Maybe the excessive amount of water moving both shoreward and away from the shore because of a tropical storm's bigger waves do create an extra risk. HiLo48 (talk) 06:33, 10 September 2011 (UTC)

In fact our own article which Wnt read says "Rip currents cause more than 100 deaths annually in the United States. Rip currents cause 80% of rescues needed by beach lifeguards.[5]" Nil Einne (talk) 09:05, 10 September 2011 (UTC)

Indeed, that makes rip currents positively deadly compared to some other ocean-related fears. On average, the United States only sees 16 shark attacks per year, and only racks up about five shark-related fatalities every decade ([1]) but for no good reason everyone is terrified of sharks. TenOfAllTrades(talk)

(edit conflict)As tempted as I am to say something indicating my negative opinion of TWC, it's probably because rip currents actually can be dangerous if you don't know the proper safety procedures to take when caught in one. It would be the same reason the National Weather Service issues Rip Current Statements...unfortunately, there aren't any in effect right now, so there are no examples to view. I'll see if I can dig up an archived example. Ks0stm ^(T•C•G) 06:42, 10 September 2011 (UTC)

Well, they are dangerous to those who don't understand them, children, poor swimmers, and people who cannot swim safely back without getting exhausted. Rip tides are over deeper water than the breakers, so someone who was playing in water they could stand in can stray into one and suddenly find themselves heading away from shore in deep water. They can look calmer than the breakers, so a less confident swimmer might head for that area. For a good chunk of the population, it is better to warn them so they can avoid the danger, and post lifeguards to rescue them. Only once you've achieved general avoidance of riptides for this group is it helpful to talk about how to rescue yourself, because this chunk of the population will mostly not be able to do so. Confident surfers and seaswimmers will acquire their information elsewhere. 86.163.0.200 (talk) 11:39, 10 September 2011 (UTC)

I don't swim but I also question whether they are definitely harmless even to decent swimmers who understand them. It's one thing to understand. It's another to put in to action. Humans are often prone to panic and so even if they theoretically know what to do, if they've never done it before they may still make mistakes, including ones that ultimately prove fatal, when it happens. On the point 'people who cannot swim safely back without getting exhausted' I suspect this doesn't just apply to swimmers who lack the stamina but swimmers who may already be exhausted. In other words, I suspect a good swimmer who knows what to do and does completely the right thing could still have problems in some cases. Remember you have to consider the case when there's no lifeguards on anyone else who could effect a rescue. Nil Einne (talk) 13:56, 10 September 2011 (UTC)

Agreed. Another risk is if you are pushed out into boating channels and struck by a boat or jet-ski, before you can find your way to the on-shore current. Also, isn't it possible that the surface water is all moving off-shore, with only deep currents moving towards the shore ? In such a case, you'd need to hold your breath and dive deep to go towards the shore, and when you came up for a breath you'd get pushed back further out. StuRat (talk) 17:30, 10 September 2011 (UTC)

I don't think this scenario is frequent - at least out article claims that rip currents are strongest on the surface. However, to escape rip currents you may need to swim through breakers even if you do everything right. Have you ever done that? It's certainly doable, but even if you are reasonably experienced in this particular activity, the waves are going to pummel you, and if you don't have the proper skills, they will pull you under, disorient you, and make you swallow a lot of water. This is not just a matter of "knowledge", but of physical skill and experience, not to mention strength and endurance. --Stephan Schulz (talk) 08:52, 12 September 2011 (UTC)

Going back to the original (why the special warning related to hurricane), I suspect the weather system creates rip currents that are stronger and/or closer to shore (or even where there were none before). So it's a warning not just "the waves look bigger and are therefore obviously more dangerous than on a calm day" but also "and there are non-visible more dangerous currents in places you do not usually encounter them". DMacks (talk) 17:37, 10 September 2011 (UTC)

stationary spots

When watching time lapse weather satellite images of a cloud bank along a low pressure wave in many cases you will see along the length of the wave that some very dense clouds are developing from a very stationary spot to form a tail in the direction of the cloud bank along the wave with the spot or point of beginning remaining absolutely stationary. Is there an explanation as to how this spot can remain so stationary such as colder air from above that is traveling perpendicular to the wave dipping down into the wave? --DeeperQA (talk) 12:23, 10 September 2011 (UTC)

Can you point to such a sequence, or could you generate one that can be looked at? It's hard to give a generic answer to this question, but one possibility might be related to the influence of topography: Mountain peaks tend to generate Lenticular clouds which appear stationary when a storm system passes over. Michel M Verstraete (talk) 15:26, 10 September 2011 (UTC).

Agreed. Or the clouds may themselves be released from the ground, either from natural sources, such as volcanism, or from man-made sources, such as cooling towers for nuclear power plants. Also, if the air is supersaturated with humidity, particulates released into the air may act as nucleation sites to cause cloud formation. StuRat (talk) 17:21, 10 September 2011 (UTC)

Try: Bing weather map The low pressure wave extends from the upper band of Nate in the Bay of Compeche across Florida into the Atlantic to the bottom band of former Katia. My local station appears to use a Bing weather map with a cloud or radar view but maybe the NWS satellite will show them as well. They pop up and last from 10 minutes to and hour along the low pressure wave in the wave where it crosses the Gulf of Mexico. 100 miles off Clearwater beach is (was) a good one. --DeeperQA (talk) 18:02, 10 September 2011 (UTC)

A tropical upper tropospheric trough or stationary front, perhaps? ~AH1 ^(discuss!) 15:04, 11 September 2011 (UTC)

Could be. A further description is more like a point where a stick is being poked downward into a the wave stream that results in a moving trail. --DeeperQA (talk) 20:28, 11 September 2011 (UTC)

ethanol producing algae

I've read that there are genetically altered algae that produce ethanol from power plan waste CO2 and sewerage nitrogen and phosphorous. Can this algae be used by individuals to produce ethanol at home? --DeeperQA (talk) 12:45, 10 September 2011 (UTC)

In setups of this sort, the algae do not produce ethanol by themselves. They produce starch and other carbohydrates which can be converted into sugars that can be fermented by yeasts into ethanol.

However, DOW has been experimenting with genetically modified strains of cyanobacteria (innacurately sometimes called blue-green algae, even though they are NOT algae), that are able to directly produce ethanol while carrying out photosynthesis at the same time. The equiptment need is far beyond the resources and technical capability of a home-based individual, though. See [[2]]. Remember that cyanobacteria are NOT algae, regardless of the fact that many lay people mistakenly think so, as does the author of the article I just cited. Dominus Vobisdu (talk) 13:04, 10 September 2011 (UTC)

Mystery bird (finch?)

File:Norwegianfinch.JPG

birdie

Hi, a simple question: can anyone identify this bird?

It lives in southern Norway, but it's not a Norwegian Blue.... The closest I've found so far is the hawfinch, but the bill doesn't seem big enough. Henry^Flower 13:20, 10 September 2011 (UTC)

It makes me think of an immature chaffinch (Fringilla coelebs) that has yet to gain mature plumage. There are some similar ones on Google image search. Richard Avery (talk) 13:40, 10 September 2011 (UTC)

That seems to fit. Thanks! Henry^Flower 14:04, 10 September 2011 (UTC)

What device is this?

"The U.S. is capable of placing a sensor near a suspected terrorist hideout that can count the number of heartbeats in a room and gauge how fast they're beating. There are even efforts to understand what a "guilty" heartbeat pattern might be." [3] Imagine Reason (talk) 13:44, 10 September 2011 (UTC)

They might be referring to a ultra high impedance electric field probe. [4]. Another option is to use a sensitive movement transducer to pick up the heartbeat from say the floor. Some customs check points use the latter method. A "guilty" heartbeat pattern just refers to a faster than normal rate that may indicate anxiousness.--Aspro (talk) 14:12, 10 September 2011 (UTC)

I'd be skeptical of any article which describes a satellite image showing tribesmen gathering in a remote area where none should be - the photograph so clear you can see the caliber of ammunition they are carrying. As stated in IMINT#Satellites, there are real and calculable physical limitations to the resolution of satellite imagery, and Modern U.S. IMINT satellites are believed to have around 10cm resolution. -- 49.230.106.194 (talk) 14:35, 10 September 2011 (UTC)

They probably mean a photo by a UAV rather than a satellite. You need good optics even in one of them or else have an easily expendable one quite low down where they can shoot it with a rifle. I'd be surprised if you could use heartbeat detection in the field as I'd have thought it would require quite large equirpment but it certainly sounds worthwhile for detecting illegal aliens in trucks at customs checkpoints. Dmcq (talk) 17:06, 10 September 2011 (UTC)

electric water heater 16A plug to 10A plug -- is this a problem?

I had an electric water heater installed in my bathroom today. It came with a 16A 220V plug, but my bathroom doesn't have a 16A outlet. I can buy a powerstrip that receives a 16A, but itself has a 10A plug. Would using this be a safety hazard in any way? The strip is rated for 4000w, for what it's worth. The Masked Booby (talk) 14:20, 10 September 2011 (UTC)

Yes, it could potentially be a hazard. You might overtax the 10A circuit and set off the breaker. If the breaker is defective something worse might happen. It is possible to start a fire that way. Dauto (talk) 15:43, 10 September 2011 (UTC)

That does sound dangerous. The reason it's a 10A circuit is likely because the wires in the wall aren't thick enough to support a 16A draw. If there is a 10A breaker or fuse, then that will pop. If not, the wires may overheat and cause a fire in the wall. You need to have new, thicker wires routed to create a new 16A circuit. Until that time, you could use an extension cord to connect to a 16A circuit elsewhere in the house, if you have one. However, that will need to be a rather thick extension cord. Ensure that it's rated for 16A/220V. (The longer the cord is, the thicker it will need to be to carry the 16A rating.) Extension cords present a trip hazard, so try to route it where you won't have to walk across it, such as by hugging walls. StuRat (talk) 17:13, 10 September 2011 (UTC)

I'd also like to point out that whoever installed a 16A water heater without ensuring that you actually had 16A service was totally negligent. StuRat (talk) 17:14, 10 September 2011 (UTC)

I would also be concerned about what genius made a power strip with 16A sockets but a 10A plug and I'm guessing without a 10A fuse on the power strip or plug. I believe the OP is in China were I guess such things are not so unusual but they would likely violate numerous standards in many countries (probably even China). Nil Einne (talk) 18:39, 10 September 2011 (UTC)

I'd expect that if the Chinese made something that dangerous, it must be designed for export. :-) StuRat (talk) 01:00, 11 September 2011 (UTC)

So the are usually wired on their own dedicated circuit for higher current then required for the rest of the house. I don't know if that's required by regulation where you live, and if it is, it might not be cheap to retrofit a whole new dedicated high current circuit into your house. Regardless, a licensed electrician would be the best person to answer these questions for you. House wiring is really one of the things you DON'T want to get just "half right". Vespine (talk) 22:38, 11 September 2011 (UTC)

In the US, at least, 220V is typically only used for electric forced air furnaces, electric water heaters, and maybe electric clothes dryers or stoves/ovens. So, see if you have any free 220V outlets near those, and if they are rated for 16A. If so, you might be able to run an extension cord (of the proper rating) from there. However, you may not be able to use your new water heater at the same time as those other devices. StuRat (talk) 01:39, 12 September 2011 (UTC)

aliens

does u.s.a have aliens in their possesion in a area named as sector 7? — Preceding unsigned comment added by 122.173.14.0 (talk) 14:38, 10 September 2011 (UTC)

If we told you, we'd have to kill you. Sorry. But there is discussion of the fictional organization Sector 7 at our page on the Transformers film. --Mr.98 (talk) 14:54, 10 September 2011 (UTC)

The truth is, of the three (ex-terrapats) that were temporarily housed at sector 7, one died due to (unreported) circumstances. The remaining two were re-housed at a newly refurbished section 35 situated in the old area 51 top secret facility in Nevada.190.56.105.3 (talk) 15:26, 10 September 2011 (UTC)

We don't quote stuff from Wikileaks as we can't count it as a reliable source, but I'm sure the truth is out there ;-) Dmcq (talk) 16:42, 10 September 2011 (UTC)

I think it depends what you mean by 'u.s.a .... in their possesion'. I'm sure there must be aliens in various areas in the US named sector 7 (e.g. [5] [6] [7] [8]). I wonder if there is even some prison with something called (or located in something called) sector 7 with aliens in it? Do these count as 'in possesion' of the USA? Nil Einne (talk) 18:33, 10 September 2011 (UTC)

Some searching found [9]. My random guess is they did detain aliens in sector 7 before (it sounds like Plantation Key Detention Center was technically in sector seven although it seems possible they would have detained aliens in The Roth Building or in their cars anyway). It seems they no longer have sector 7 [10] [11] so I guess they don't have any now (although sector 7 existed in 2005-2008 at least it seems, the 223MB PDF confirms that for 2007). These [12] may still have a sector 7. However I wouldn't recommend calling [13] to find out if they currently have any aliens in their posession in sector 7 as I don't think they'll be pleased. Of course neither of these are federal authorities, but I would personally still considered them part of the wider US government. Nil Einne (talk) 19:33, 10 September 2011 (UTC)

Is this also the science fiction desk? ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:17, 11 September 2011 (UTC)

Wrong indenting? All of what I wrote is accurate AFAIK. Nil Einne (talk) 16:28, 11 September 2011 (UTC)

Speed in circular orbit

What is the speed an object has if it is in circular orbit around a bigger object ?

I HAVE found the answer online, and done the math miself, arriving at sqrt(MG/r). However, http://en.wikipedia.org/wiki/Circular_orbit#Velocity has a formula that includes the mass of small body. Is it wrong, or am I missing something ?

Cold Light (talk) 16:15, 10 September 2011 (UTC)

The speed of the orbiting object depends both on its own mass (m) and the mass (M) of the larger object around which it evolves. Michel M Verstraete (talk) 16:27, 10 September 2011 (UTC).

That's true, but not really helpful. The original poster is wondering why that is the case, and what causes the difference between his derivation and ours. In practice, the mass of the smaller body must be taken into account if it is an appreciable fraction of the mass of the larger body. In other words, the effect is imperceptible for small things like satellites, astronauts, and space stations (M+m is very nearly the same as M), but it does matter for things like the Moon.

If you have one object in orbit around another, it doesn't actually circle the exact center of the larger object; instead, both objects orbit the system's center of mass (the barycenter). (This is a consequence of the conservation of linear momentum.) For small orbiting objects, the barycenter is nearly at the center of the larger object, so it's reasonable to approximate its orbit that way. In the extreme case of two objects with equal mass, they will both circle a point halfway between their centers. I imagine that your derivation of the orbital speed assumes that the smaller object circles the center of the larger object, which is where the discrepancy between the two formulae arises. TenOfAllTrades(talk) 16:34, 10 September 2011 (UTC)

I agree with the above responses. I'm guessing you've used the general form of circular motion and Newton's law of universal gravitation, in which case if you want to rationalise the problem the 'height' of the satellite is only close enough to radius of its orbit for satellites with a small mass. Your derivation is the standard one for the "satellites, astronauts, and space stations" mentioned above, certainly at the level at which I think you're working. Grandiose (me, talk, contribs) 17:27, 10 September 2011 (UTC)

What are those cylinders on top of the C4?

In this photo, http://en.wikipedia.org/wiki/File:Eod2.jpg you have C4, and then the guy is inserting blasting caps into the C4. At first I thought the cylinders on top of the C4 were the blasting caps, but now I know that they are not. So what are they? ScienceApe (talk) 17:32, 10 September 2011 (UTC)

On the page Plastic explosive, the caption for this picture says "C-4 plastic explosive (the off-white rectangular blocks) being used to destroy unexploded artillery components." So I guess that's what they are. Mikenorton (talk) 18:23, 10 September 2011 (UTC)

They look like artillery components, at least superficially — like the rear end of a shell, without any bullet attached. --Mr.98 (talk) 19:03, 10 September 2011 (UTC)

The brass cylinders on top of the C4 look like 40mm grenade blanks but they could also be large artillery primers. There are also flat round brass disks in the sand under the C4 that could also be artillery primers for a calibre that uses modular propellent charges rather than a cased round such as 105mm or 155mm. The black object with the screw thread in the top left corner looks like a shell or mortar bomb fuze, the only words on the label that are readable are "Cartridge" and "Scot Inc.",the word "expired" has been written over the label with a black marker. Roger (talk) 16:26, 11 September 2011 (UTC)

does cost of a food product turn people on or off

Removed request for opinions

There is an automobile ad touting the most leg room per dollar of any car on the road. (or something like that). Would you be enticed to buy a food product if it touted having the least or the most number of calories from fat, carbohydrates and protein per dollar of any other food product on the market? --DeeperQA (talk) 19:27, 10 September 2011 (UTC)

In my mind, as I suspect with many other people, "health" and "cost" priorities are more or less separate. I either buy the best value, or a health decent-value alternative. Knowing the calories straight off (I'm guessing the overall "calories" is probably most likely to catch on) helps, but kcal/£ (in my case) doesn't seem to mean anything to me. A low-cost healthy option (the best) will score the same as a high-cost unhealthy one (the worst). I would then need further thought to determine which, which I imagine undermines the point. Grandiose (me, talk, contribs) 19:32, 10 September 2011 (UTC)

In places where people are starving, they absolutely would like to get the most calories for the money. However, in the West, we seem to get way too many calories, so getting more of them cheaply isn't a priority. For comparison, consider water. If in a desert, you likely are willing to pay for any water you can get. But, during a flood, you probably aren't willing to pay for plain water, but if it is "pure, mountain spring water", or some such nonsense, then maybe you would pay, even then.

Of the categories mentioned, I'd rather pay for calories from protein. Calories from "good fat" might be next, if you could specify vegetable fats, such as olive oil. StuRat (talk) 19:39, 10 September 2011 (UTC)

I tend to pay for flavor per dollar: That is, food that tastes better tends to be worth a premium over food that tastes bland or unappetizing. I suspect many people have a similar policy. --Jayron32 19:42, 10 September 2011 (UTC)

I would definitely pay to get a tasty chili or stew with a complete protein (all essential amino acids) and plenty of dietary fiber with low fat and carbs. Most chilies and stews have relatively high fat, but those which include beans and corn do usually have complete proteins. 69.171.160.254 (talk) 20:09, 10 September 2011 (UTC)

? Have you assumed you are eating vegetarian chili or stew? If it has meat in, surely it already has all essential amino acids? And if it hasn't, eating the beans with rice will cover you. But then I've never bought chili or stew from a shop: I don't think it would occur to me that you wouldn't make it yourself. 86.163.0.200 (talk) 20:34, 10 September 2011 (UTC)

Ending discussion. This isn't a factual science question. The science reference desk is not a forum for discussion of opinions. As it says at the top of this page, "The reference desk does not answer requests for opinions…. Do not start a debate; please seek an internet forum instead." —Bkell (talk) 22:43, 10 September 2011 (UTC)

gravity vs Centrifugal

Can a Centrifugal force of a body at some condition be stronger than its own gravity? Exx8 (talk) —Preceding undated comment added 20:04, 10 September 2011 (UTC).

Yes, very easily. It almost always is for spinning everyday objects like bicycle wheels or soccer balls. However, this holds for small objects which are mostly held together by chemical bonds. For large bodies that are mostly held together by gravity (e.g. planets or stars), such a situation would tear them apart. --Stephan Schulz (talk) 21:05, 10 September 2011 (UTC)

Some asteroids are reported to have negative gravity on their surfaces due to their centrifugal force. FWIW 67.169.177.176 (talk) 04:22, 11 September 2011 (UTC)

so the surface of those asteroids is not peeling off only because of the chemical bonds of the material with itself/the core below? --Lgriot (talk) 12:27, 12 September 2011 (UTC)

All asteroids have gravity. Centripetal force is needed to attach to the surface of a spinning asteroid. Cuddlyable3 (talk) 15:14, 15 September 2011 (UTC)

radioactive isotope heated to a plasma

Hi, suppose we heated an element, isotopically pure uranium 235 say, up until it was 1 million degrees and observed an x percent decrease in halflife. Then we heated a similar sample upt to 2 million degrees and saw a y percent decrease in halflife. Wouldn't be reasonable to conclude that nucleons inside the nuclei were getting more feisty and becoming more likely to tunnel out? Could we assign a temperature to the nuclei? (This is a followup question to one a couple weeks ago.)Thanks. Rich Peterson199.33.32.40 (talk) 20:37, 10 September 2011 (UTC)

Under normal conditions, the state of the nucleus is the ground state. For the picture of "nucleons inside the nuclei were getting more feisty" to become valid, the temperature must exceed the spacing between the energy levels by an order of magnitude, but 10^6 K is only 86 eV, which is ice cold as far as nuclear physics is concerned.

Fine, make the plasma temps 10million and 15 million then. Or if those aren't right, change them to what might work.Rich (talk) 21:31, 10 September 2011 (UTC)

A nucleus with "feisty"(physicists use the word excited instead of feisty) nucleons is called a nuclear isomer. Their excitation energy vary alot depending on the particular isomer but should be in the range of 100keV and above in most cases so a temperature in the range of 1 billion K or more would be required. Dauto (talk) 22:24, 10 September 2011 (UTC)

Okay thankyo both. So sppose we heated U 235 or some appropriate element to 1 billion or so Kelvin and observed an x percent decrease in half life. Then we measured y,z etc percent changes in half life at other high temperatures.Should we then be able to assign a temperature to the interior of the nuclei?Rich (talk) 23:37, 10 September 2011 (UTC)

Assuming equilibrium, everything including the nuclei will be at the same temperature - but it doesn't make sense to talk about the temperature of an individual nucleus (you can talk about the temperature of a collection of nuclei) because each nucleus is either in its lowest energy state or it is in one of the possible excited states. Either way the state is well defined. To talk about temperature you must have a collection of microscopic states lumped together in one macroscopic state so that entropy can be defined. Then, through the relation between entropy and energy temperature can also be defined. Dauto (talk) 00:13, 11 September 2011 (UTC)

Most useless element

Which element is the most useless? --76.211.90.74 (talk) 22:15, 10 September 2011 (UTC)

I would guess the Transactinide elements, as they don't exist for very long and have only existed in labs that we know of. Their use is , at the moment, only academic, as in teaching us about certain properties of elements and particles. Mingmingla (talk) 22:48, 10 September 2011 (UTC)

Which stable element is the most useless? --76.211.90.74 (talk) 23:22, 10 September 2011 (UTC)

Europium, maybe.  Card Zero  (talk) 23:49, 10 September 2011 (UTC)

OK, I'm prepared to be seriously impressed :-) -- HOW did you know that, or did you really research it in an hour and a half? --DaHorsesMouth (talk) 00:16, 11 September 2011 (UTC)

I just looked at the elements I knew of no use for, and jumped at the one which said it had few commercial applications compared to other elements. Like the article (and Trovatore) says, it's used in helical fluorescent light bulbs, which are arguably very useful (and also arguably less useful than incandescent or LED lamps, but that would be starting an argument) ... and CRT screens, though obsolescent, still have some use in them (I'm using one now).  Card Zero  (talk) 00:58, 11 September 2011 (UTC)

Actually, as it turns out, the mantles I was thinking of are made of cerium (and thorium) and the magnets are neodymium. Oh well. --Trovatore (talk) 05:55, 11 September 2011 (UTC)

It's ... interesting that you backed that up with a link showing that Europium is actually very useful. Magnets, lamp mantles, what do you want, egg in your beer?

I think if the question has a really defensible answer (which I sort of doubt) it would have to be an element that is very expensive, but very little different from a much less expensive neighbor in the periodic table. Rare earths are a good candidate because there are a lot of them with very similar properties. But I certainly wouldn't pick europium. Maybe scandium or ytterbium? I'm really just guessing; there could be important uses for those too. --Trovatore (talk) 00:21, 11 September 2011 (UTC)

Or may be Hafnium? Dauto (talk) 23:55, 10 September 2011 (UTC)

It's apparently used in the the Penryn (microprocessor) which is used in the MacBook Air. Again, opinions of its utility vary.  Card Zero  (talk) 01:12, 11 September 2011 (UTC)

That's funny, most transactinides are useless, and their only use is to make heavier, less usefull ones. Plasmic Physics (talk) 03:18, 11 September 2011 (UTC)

There are useful as stepping stones to the Island of Stability, see here, and from there we can try to move on to the next island. Count Iblis (talk) 03:36, 11 September 2011 (UTC)

Indium has relatively few uses, like scandium, but that's mostly because they're so hard to isolate. As far as my candidate for the most useless element, I'd pick rubidium. 67.169.177.176 (talk) 04:26, 11 September 2011 (UTC)

But it's used in Rubidium-strontium dating, so not so useless. Mikenorton (talk) 08:47, 11 September 2011 (UTC)

Rubidium is used in atomic clocks. I would also dispute indium, I recall using it in microwave stripline circuits. SpinningSpark 22:36, 11 September 2011 (UTC)

Administratium? DMacks (talk) 20:15, 11 September 2011 (UTC)

I was going to say Unobtainium but it turned out to be very valuable indeed. --DeeperQA (talk) 21:04, 11 September 2011 (UTC)

The one on the back left of my stove, which burned out a decade ago. Gzuckier (talk) 05:34, 12 September 2011 (UTC)

Artificial gravity

How fast would a 1 km radius circular spaceship have to spin to have the equivalent of Earth gravity? --76.211.90.74 (talk) 22:16, 10 September 2011 (UTC)

The formula for the artificial gravity generated that way is $a={\frac {4\pi ^{2}r}{T^{2}}}$ where $a\,$ is the artificial acceleration, $r\,$ is the radius and $T\,$ is the period of the rotation. plugging $a=g=9.8m/s^{2}\,$ and $r=1000m\,$ you get $T={\frac {2\pi r^{1/2}}{a^{1/2}}}={\frac {2*1000^{1/2}\pi }{9.8^{1/2}}}=63.47s\,$ which is a little over a minute. Dauto (talk) 23:48, 10 September 2011 (UTC)

And, to continue with that, the circumference of a 1 km radium circular space station would be 2(pi)r, or 6.28 km. Going 6.28 km in 63.47 seconds is around 0.1 km/sec. Multiply by 3600 to get 356.2 km/hour. So, docking at those speeds is probably out, except at the hub, but even then you'd still need to match the rotation. StuRat (talk) 15:47, 11 September 2011 (UTC)

A radium space station would be easy to find in the dark, but hazardous to occupy. Edison (talk) 04:28, 12 September 2011 (UTC)

This must be why a much smaller radius with a much taller cylinder would be used to keep circumferential speed reasonable. --DeeperQA (talk) 20:56, 11 September 2011 (UTC)

Well, a rotating ship poses some problems, regardless of speed. But, with each problem comes possible solutions:

A) Docking, as mentioned, could only be done at the hub, which means at most 2 ships could dock at once.

B) Solar panels must always face the Sun, so one side of the ship must remain oriented towards the Sun, and only that side would have solar panels. Due to the rotation, they would need to be attached, not deployed as a larger array. This limits the amount of solar power which can be collected.

C) Directional communication antennas would need to constantly change position, unless they were on the hub and sent signals along the axis of rotation. This might take up one of the valuable docking spots and/or interfere with communications during docking, if the same spot was used for both. An alternative might be to use a non-directional signal to communicate with a non-rotating communication satellite nearby, which would then convey the signal where needed.

D) It would be difficult to reorient the ship, once rotating.

E) It would be necessary to stop the rotation periodically, to perform exterior maintenance. Stopping and starting the rotation would use up fuel. StuRat (talk) 01:26, 12 September 2011 (UTC)

Many of the problems could be solved by combining a non-rotating 'axle' (with docking facilities, Antennae, solar panels etc attached) with a rotating 'wheel'. AndyTheGrump (talk) 01:37, 12 September 2011 (UTC)

Yes, but then you have the problem of how to join the fixed axis with the rotating ring, bearing in mind that it must be air-tight, have little friction, and allow cargo and people to transfer from one to the other. StuRat (talk) 16:53, 12 September 2011 (UTC)

AndyTheGrump ansered your points A, B, and C. My answer to point D is: it's a station, not a ship. why would you want to reorient it? And my answer to point E is: Exterior maintenance can be done the same way that exterior maintenance of a building is done on earth (where gravity can't be turned off). No need to stop the station. Dauto (talk) 04:23, 12 September 2011 (UTC)

Maintenance on the outside edge would be equivalent to working upside down, which isn't normally done on Earth, and it might be at slightly over 1 g, accounting for the thickness of the shell (and assuming you spin it up to 1 g in the interior). As for the need to re-orient the station, you might want to do this, say, when a meteoroid cluster passes, to present the least cross-sectional area and strongest materials in the direction of the oncoming meteoroids.StuRat (talk) 16:50, 12 September 2011 (UTC)

With a 1Km radius wheel, the shell thickness isn't going make much difference to the g force. And I see no problem with having a movable gantry (similar to the type sometimes used for working under bridge decks) for external maintenance purposes. Regarding an airtight rotating seal, I suspect that this might be difficult, but there are several ways to get round this - either not pressurise the 'axle' at all, or have airlocks between the two parts, with an intermediate section, again with airlocks, which can be spun up to 'wheel' speed and docked with the wheel, then undocked, slowed, and locked to the 'axle'. AndyTheGrump (talk) 17:23, 12 September 2011 (UTC)

All of this is adding a lot more complexity, moving parts, risk of failure, weight, and time to each step. StuRat (talk) 03:03, 13 September 2011 (UTC)

Assuming that we are talking about a space station, it's been shown by a study done back in the 70s (when people seemed to believe that spinning stations were just around the corner) that if your station radius is much smaller than a few hundred meters motion sickness becomes a serious problem because of the Coriolis effect. On the other hand, a station radius much larger than a few kilometers becomes too large to be safely held together against centrifugal forces that might tear the station apart.A 1 km radius seems to be the sweet spot between those two problems. See stanford torus. Dauto (talk) 22:46, 11 September 2011 (UTC)

Note that it would still be quite useful to rotate it far more slowly. Yes, it wouldn't be as good for the long-term health of astronauts, but micro-gravity is actually better for getting work done, as you can lift heavy objects, yet don't have to worry about your tools floating away or injuring yourself or damaging equipment by dropping things. StuRat (talk) 01:32, 12 September 2011 (UTC)