Wikipedia:Reference desk/Archives/Science/2009 September 14

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September 14

Identify string in my foot

during my weekly martial arts practice (white belt), I noticed that I feel some sort of string (probably a ligament) being tensed/stretched whenever I shift my weight and land on one of my feet. Could you identify that string? I think it's between the Calcaneus and the metatarsus. Why do multi-classing nerd/jock is so difficult in real life.--121.54.2.188 (talk) 01:35, 14 September 2009 (UTC)

We cannot give medical advice. Please see a doctor. Baseball Bugs ^{What's up, Doc?} carrots 01:48, 14 September 2009 (UTC)

I don't really see this as a medical question since no problem has been described -- the big "string" down there is the plantar fascia. If you mean something else, the picture in that article might help you. Looie496 (talk) 01:58, 14 September 2009 (UTC)

I wasn't soliciting any medical advice, rather, I was seeking anatomy advice. It seems that you'll discover new muscle groups every time you practice (and they also hurt afterward to remind you that they exist:)). Thanks Looie, though it seems to be thicker than I thought. Maybe it's one of those digital slips attached to it.--121.54.2.188 (talk) 02:23, 14 September 2009 (UTC)

Possibly the peroneus longus tendon. Axl ¤ [Talk] 09:37, 19 September 2009 (UTC)

Can you strain your tongue?

Is it possible to strain your tongue muscle the same way you can with other muscles? Are there any muscles that can't be strained/pulled? Spiderone 07:29, 14 September 2009 (UTC)

In principle, I don't see why not, but given it's only "attached" on one end I'll venture that it's very unlikely to happen by accident.--Leon (talk) 08:44, 14 September 2009 (UTC)

I don't know about you, Leon, but my arms and legs are all attached at only one end (I wouldn't enjoy looking like a pretzel), and I strain them quite often. --KageTora - (영호 (影虎)) (talk) 16:42, 15 September 2009 (UTC)

Absolutely. I've done it. --Pykk (talk) 09:36, 14 September 2009 (UTC)

Really? OK I won't ask how. Spiderone 12:43, 14 September 2009 (UTC)

Sounds like a job for the cunning linguists at WP:RD/L! I don't know if that's exactly the same kind of strain, but it certainly happens, and was even memorialized in an episode of Seinfeld, IIRC. Matt Deres (talk) 16:17, 14 September 2009 (UTC)

So, entendre aside, it's at least as plausible as dying from licking envelopes. ~ Amory (user • talk • contribs) 17:20, 14 September 2009 (UTC)

Licking envelopes can be harmful to ones health Googlemeister (talk) 18:09, 14 September 2009 (UTC)

"Are there any muscles that can't be strained/pulled?" - The brain is apparently a muscle, and I've never heard of anyone straining or pulling it, except metaphorically. No. --KageTora - (영호 (影虎)) (talk) 17:02, 15 September 2009 (UTC)

I'd like to pursue KageTora's comment, "my arms and legs are all attached at only one end, and I strain them quite often"

My intuitive reaction would be that at the time you strained a muscle in your arm/leg, both ends were probably quite firmly attached to something you were lifting, pressing against, or in some other manner immobile -- not just flapping around loosely like a tongue.

True? --DaHorsesMouth (talk) 23:23, 15 September 2009 (UTC)

Not exactly. I don't usually lift things with my legs, yet there have been times when I have pulled a muscle somewhere in them whilst running, for example. --KageTora - (영호 (影虎)) (talk) 23:43, 15 September 2009 (UTC)

DaHorsesMouth was generalizing and being evidently gave an oversimplified answer seeing how specific you are being. When both bones to which a muscle is attached (origin + insertion, such as the trapezius being attached to both the occipital bone of the cranium and the scapula) move farther apart in relation to each other at the same time that very muscle is being contracted (such as during a car crash, as in our trapezius example), the muscle will be strained. Because the genioglossus and hyoglossus and styloglossus, the three muscles that, when combined, compose the majority of the tongue by both mass and volume, are attached to only one bone (genial tubercles of the mandible, hyoid and styloid process, respectively) they effectively cannot contract against resistance to result in a strain. DRosenbach ^{(Talk | Contribs)} 02:05, 16 September 2009 (UTC)

Good fair bit of information, there, thanks. However, I feel I can point out that it is, however, possible to strain one's tongue when the tongue is contorted into an unnatural position, such as during an seizure. Sorry, I don't want to sound like I am being argumentative, but I think I may be misunderstood as such, so I'll just keep my eye on this thread and only add anything if I think I can (and with citations if possible). --KageTora - (영호 (影虎)) (talk) 09:03, 16 September 2009 (UTC)

If a strain is defined as both micro- and macrotearing of a muscle due to overstretching, I would say that tongues would not necessarily fall outside the realm of "strainable" muscle tissue insofar as one can probably find a situation/method in/by which to overstretch the tongue and produce said tearing. Forcible overstretching of the tongue is definitely a possibility, such as securing the tongue in a particular fashion to prevent the securing medium from tearing through the tongue upon pulling, and then pulling. In physiologic use of the tongue, pain sensation kicks in well before the maximum stretching is reached (just like pain sensation will kick in well before any muscle nears its maximum stretched length) but, as stated earlier, a rapid, perhaps passive (as in an injury, where the pull happens rather than being performed) pull on attached bones can tear a contracting muscle. When I stick my tongue out as far as I can (as I do now as I type), my tongue begins to hurt. I don't think it will tear if I keep pushing it out because I can't exceed its natural length without tugging on it with a second attachment. DRosenbach ^{(Talk | Contribs)} 13:04, 16 September 2009 (UTC)

Fields

Whether earth has got a negative or positive field around it? —Preceding unsigned comment added by 202.164.157.168 (talk) 16:04, 14 September 2009 (UTC)

What kind of field? One with daisies in it? Fences&Windows 17:05, 14 September 2009 (UTC)

The commonest cloud-to-ground lightning is believed to be initiated by a negative-to-positive respectively field. Various theories about how the field arises are described in Lightning.Cuddlyable3 (talk) 19:42, 14 September 2009 (UTC)

But that's cloud-to-ground lightening. Ground-to-cloud lightening is also possible. DRosenbach ^{(Talk | Contribs)} 00:09, 15 September 2009 (UTC)

In case he meant something else, the outer Van Allen radiation belt is composed of positive ions, meaning that the earth, at the outermost levels, has a positive electric field around it; this has nothing at all to do with lightning being that the Van Allen belts are about 20,000 kilometers out... --Jayron32 21:54, 14 September 2009 (UTC)

Someone with a subscription might be able to tell us something about the total net charge from this paper: Discussion on the Earth's net electric charge doi:10.1007/BF01054576. However, the paper (from 1988) says it's a difficult question to answer. AlmostReadytoFly (talk) 13:35, 15 September 2009 (UTC)

Is a 12 gauge sabot shotgun round traceable?

I was wondering because of the plastic brace. —Preceding unsigned comment added by HitmanNumber86 (talk • contribs) 16:22, 14 September 2009 (UTC)

The sabot drops away from the shot during flight before reaching the target. Presumably the used sabot pieces can be traced somewhere on the ground afterwards but the real trace of the sabot's use is the concentrated pattern of shot on the target. Cuddlyable3 (talk) 19:50, 14 September 2009 (UTC)

I think the question is can you trace the shells to an individual gun, like you would with, say, a bullet, not whether a sabot was used or not. I imagine if you can get parts of the sabot, you can trace that to an individual gun (grooves and all that), though I am just speculating. --98.217.14.211 (talk) 19:58, 14 September 2009 (UTC)

The braces on Sabot are made of plastic, not lead or copper. I don't think the pattern would be reproducible on a shotgun. —Preceding unsigned comment added by HitmanNumber86 (talk • contribs) 20:10, 14 September 2009 (UTC)

Most shotgun barrels are not rifled. 75.41.110.200 (talk) 03:20, 15 September 2009 (UTC)

Electric fences for cattle and horses: dangerous to humans?

Recently while walking in the english countryside I found the public footpath blocked by an electric fence wire, and had to wriggle commando-style underneath it. Just how dangerous are such electric fences to humans - they are designed to deter cows or horses, which are a lot bigger than us. And if you touched it, what would happen - would it be like a tazer perhaps? 78.146.163.118 (talk) 17:00, 14 September 2009 (UTC)

It's a quick shock, and it will catch you off guard. I touched one nailed to the other side of a wooden fence. I assume it charges a capacitor, it took a second for me to realize what happened. It doesn't hurt. --HitmanNumber86 (talk) 17:14, 14 September 2009 (UTC)

Hitman is right, it's not so bad, just very jolting (har har). Nobody wants to injure their herds, so the fences are designed to be just unpleasant enough to deter. I've used electric fences while backpacking to deter bears from food; the idea is you surround your food stuffs, etc. with a fence, and wandering bears get shocked while sniffing around. As might be expected with a bunch of bored males, we all took turns grabbing the fence, seeing who could hold on the longest. Nobody got hurt, just a little frazzled. ~ Amory (user • talk • contribs) 17:25, 14 September 2009 (UTC)

the longer the electric wire, the lower the shock. Also, if you are standing in mud or water, your shock will be significantly greater. Googlemeister (talk) 18:07, 14 September 2009 (UTC)

We have an electric fence article with some details about various pulse patterns. Googlemeister, why does the length of a conductor affect how much of a shock you get from touching it, and do you specifically mean the whole fence length or "from supply to point of contact" only? DMacks (talk) 20:00, 14 September 2009 (UTC)

Entire fence length. Personal experience, the same device produced a very mild shock when used on a 2.5 mile perimeter fence, and a very strong shock when used on a 150 yard perimeter fence. Additional OR, standing on a tire does not prevent the shock. Googlemeister (talk) 20:15, 14 September 2009 (UTC)

The current has to go down the line, through you, and then back to the grounding rod, which is often back by the charger. If that's a long way or the ground is dry, the resistance will be higher and the current lower. Also, there are usually weeds and things touching the fence which sap the current, so the longer the line the more there are. As an aside, I had to rush out from a dinner party this evening because my 6 pigs had decided to ignore the electric fence and go on walkabout. --Sean 08:42, 15 September 2009 (UTC)

Voltage drop. AlmostReadytoFly (talk) 13:29, 15 September 2009 (UTC)

As an aside, what would actually happen if you were to urinate on an electric fence? In one of Bam Margera's pre-Jackass videos, one of his buddies pees on an EF to test out the old folk adage that suggests that it is not a good idea to do this - and is apparently in great pain afterwards. However, I am aware to the possibility that this may have been staged or exaggerated for comedy purposes. --Kurt Shaped Box (talk) 20:11, 14 September 2009 (UTC)

MythBusters did it. I don't know the result. Googlemeister, Brainiac: Science Abuse prevented shock from a fence by getting participants to stand on styrofoam. —Preceding unsigned comment added by Vimescarrot (talk • contribs) 20:21, 14 September 2009 (UTC)

Mythbusters comprehensively busted it - proving that it's impossible to create a constant stream of urine - it always breaks up into droplets so there is no constant path for the current to follow. SteveBaker (talk) 20:36, 14 September 2009 (UTC)

Hmmmm. Not even if you hold your dick about two inches from the wire (as seen on the aforementioned vid)? Did they test that? --Kurt Shaped Box (talk) 20:57, 14 September 2009 (UTC)

I believe they busted it with peeing on an electrified third rail. They showed it is possible if you kneel down and have a really huge... yeah. Anyway, If you pee on an electric fence, you could pee on the part right in front of you, so the urine wouldn't fall as far, and thus would move slower and be less spread out. — DanielLC 03:48, 16 September 2009 (UTC)

A factor that makes these fences *much* less dangerous than one you might put around your average concentration camp is that they only give a short pulse about once or twice per second. That gives you the ability to let go, which you might not otherwise be able to do. The bottom line is that nobody's trying to impose the death penalty on their livestock for thinking about leaving home. --Sean 08:45, 15 September 2009 (UTC)

There is leakage of current through all the insulators, so a longer fence run has more parallel leakage paths to drain of the electricity besides through the person touching it. The voltage drop through a long wire would be more of a factor the larger the current flow, but the idea of a fence charger is to have a high voltage and a small current,to startle and inflict pain unlike prison camps in movies which have electric fences intended to electrocute escapees. If one fence insulator has a resistance of, say 50 megohms to ground, then 10 would have a combined resistance of 5 megohms (the leakages would be in parallel). If your resistance (including dry boots) was .5 megohm, then the leakage through the fence would be a small factor and you would get a pretty good jolt. If there were 100 insulators on a long fence, the leakage resistance would have a combined resistance of .5 megohm, the same as yours (hypothetically) and half the jolt would go through the fence leakage to ground. One cannot rule out some idiot wiring a fence to the power mains, so treat anything electrical with caution. Edison (talk) 14:31, 15 September 2009 (UTC)

If the shock does not hurt, then how does it deter large animals? 78.147.99.119 (talk) 19:18, 15 September 2009 (UTC)

Something does not have to hurt to be unpleasant. Your muscles involuntarily clenching is not all that fun even if it does not hurt in a traditional sense. Googlemeister (talk) 21:13, 15 September 2009 (UTC)

It definitely hurts - but it's brief and does no lasting damage. You could push the fence down if you were even moderately determined...and so can cows. The fact is that they are trained to avoid these fences - so the issue rarely comes up. My great uncle used to turn off his fences to save electricity - and the cows still wouldn't touch them. SteveBaker (talk) 21:22, 15 September 2009 (UTC)

Anecdote: we tried growing sorghum in our area in the '70's. Not sure if it's what this wiki calls sorghum, it looks like corn but grows faster and no ears of corn. Cows love it and it was great fodder. Our mistake was to plant it next to a pasture enclosed by an electric fence. It just tasted too good, and the fence meant nothing to the cows. So we cut down anything within a cow-reach of the fence - too late though, we'd provided a positive stimulus and nothing we could do would stop them anymore, except evacuate the pasture. Rounding up animals through 8-foot-high crops - ahh, good times! :) Franamax (talk) 21:54, 15 September 2009 (UTC)

This is actually a classic example of conditioning. Skinner even mentions it in Walden 2 with sheep—teach them it is electric a few times, and you don't have to have it turned on ever again. I imagine in practice there would need to be some occasional reinforcement (much less adaption for the fact that the herd changes over time). --98.217.14.211 (talk) 00:14, 16 September 2009 (UTC)

(e/c with SB, who says much the same more briefly :) The other factor is that for any animal, large or small, a completely unexpected experience in what had seemed like a familiar environment enforces an aversive response, i.e. they just never try it again. Especially with cows (and pigs), exploring is often done with the nose, which is especially sensitive, so they learn the lesson very quickly. I'm a large animal, and I learned very quickly to not touch an electric fence ('tho I learned that you can use a piece of grass to explore the pulse gradually, you need these techniques to find out whether the fence is really working). In fact, once all the cows have sniffed the fence once, you don't actually need to power it anymore, fence controllers with dead batteries aren't all that uncommon.

And I would indeed say it hurts! The first time, it feels like the end of the world. This is especially true of those fencers, usually AC plugged in at the barn as opposed to DC-battery remote fencers, which I believe sense the voltage at the "other end" and boost their output to overcome line-loss due to grounding. The mild experiences I've seen reported above are perhaps the result of touching battery-powerd fences. For a good AC-powered fence, you're getting into bee-sting territory, but without the swelling.

Also I've never seen an electric fence used to confine horses, in my experience there is way too much risk that they will surge forward in panic and wreck the fence. Horses can often act in counter-intuitive ways... Franamax (talk) 21:34, 15 September 2009 (UTC)

Electric fences are often used to confine horses, actually. You can use them inside another more substantial fencing, such as inside a hedge or a post-and-rail paddock fence (for example) to give additional security, or on their own to fence off parts of a field for strip grazing. Horses can be quite clever though. They can often sense when a fence isn't on and take advantage. One pony I know realised that when he was wearing his rug, it didn't hurt and he ducked underneath it. Serraching for equine electric fencing will give lots of examples.

Slightly off-topic, but electric fences don't have to be designed to deter large mammals – see "Battery Brainwave" at http://www.newscientist.com/article/mg18625051.800-feedback.html for one aimed at slugs and snails. Delightful. Adrian J. Hunter^{(talk•contribs)} 07:03, 17 September 2009 (UTC)

significant variation in the number of genes between homologous chromosomes

So ... I'd like to prove one of my professors wrong. I envision that it is quite frequent (like say 1/30, 1/50, etc.) for an individual to have homologous chromosomes with differing number of genes. (And by genes, I mean coding regions -- so if I have an extra copy of the same gene, that counts as an extra gene.) So let's say one homologous chromosome would have 1402 genes, and another would have 1403. I'm looking for situations like that (and these situations come about through gene flow).

Such variable frequency must be quite common -- the allele frequency can be kinda low (like 0.5-5%) so I'll get my points back, but not ridiculously low. (Higher frequency the better.) In this case, it would be like having one gene to no gene at all (so effectively the extra gene would be sort of X/0, but this is on an autosome). a Can anybody give me some examples? John Riemann Soong (talk) 18:02, 14 September 2009 (UTC)

The number of HLA-DR beta genes varies among alleles, as outlined in our article. Another example, which I don't know for certain, but still... I'd be very surprised if every individual had the exact same number of olfactory receptors. And then there's the blood group antigen Rh(D). Rh(D) negative individuals lack the D gene (or, more rarely, have a nonfunctional one). --NorwegianBlue ^talk 20:25, 14 September 2009 (UTC)

You could probably make the point that genome-wide copy number variation (CNV) accounts for a great deal of genetic diversity between individuals. Many CNV regions are considered polymorphisms in that they are present in a significant fraction (>5%) of individuals, and some CNV regions contain genes. My favorite paper on this topic right now is here --- Medical geneticist (talk) 23:17, 14 September 2009 (UTC)

I would agree with you and suggest that the burden of proof is really on your professor. It's quite ridiculous to assert that homologous chromosomes possess the identical number of genes -- that may very well be why they are called homologous and not identical. As homologous chromosomes derive from each parent, I think an example to be shown is not when there would be a different number of genes, but rather an instance of the same number of genes (that's what I meant by switching the burden of proof). Genetic variation is not only quality of alleles but also quantity of alleles (as in co-dominance/epistasis) -- in a sense, quantity translates (pun not intended) into a difference in quality. DRosenbach ^{(Talk | Contribs)} 00:07, 15 September 2009 (UTC)

Hmm -- that being said, I don't know if it would be significant -- depends on your definition. Quantitatively significant it might not be. DRosenbach ^{(Talk | Contribs)} 00:12, 15 September 2009 (UTC)

Let's not throw out the basic fundamentals of genetics just yet, ok? I think it is still safe to assume that the majority of genes follow the "one copy from each parent" pattern and that two copies is the general rule... not to mention the most correct answer on a test. Of course, as indicated above, there are exceptions to that rule (which is part of what makes genetics so interesting) but for the purposes of teaching fundamentals of genetics lets stick to the rules. The OP's Prof may have been a bit too dogmatic in his/her teaching, but that might have been totally appropriate for the level of the course. --- Medical geneticist (talk) 14:00, 15 September 2009 (UTC)

If the gene in question is subject to inhibition at either the transcription or translation levels, or the gene product is subject to a neutralization pathway - any of which are regulated by concentration of the gene product - then isn't the CN immaterial? Concentration-dependent inhibition is inhibition and reaches equlibrium. Yes, that may be beyond the scope of what the prof is trying to teach, just asking for my own benefit. Franamax (talk) 22:40, 15 September 2009 (UTC)

bird acrobatics

Small birds seem extremely good at flying around and not running into things (besides windows and cars) because they are incredibly nimble in flight. What kind of acceleration do these small birds reach, as human fighter pilots can hit 8-9Gs on much larger turns. Googlemeister (talk) 20:23, 14 September 2009 (UTC)

Apparently, A = v2 / r ... if we consider the airspeed of an unladen swallow as 11 metres per second, and postulate a turn with radius 1 metre, the acceleration will be 121m/s^2, which (divide by 9.8 m/s^2) is about 12g. However I'm not sure what the minimum radius of turn would be for such a speed, but this gives you a ballpark estimate. Double the radius and you halve the g force. --Tagishsimon (talk) 23:35, 14 September 2009 (UTC)

An African or European swallow? :-) --Phil Holmes (talk) 09:00, 15 September 2009 (UTC)

The Spine-tailed Swift is the fastest bird in level flight - 105mph or so (47 meters per second). A Peregrine Falcon can hit 175 mph (78 meters per second) - in a steep dive. But I'm pretty sure that neither of them can turn sharply at that speed. I'm kinda skeptical that a swallow could do it at 11 meters per second either. I've had an occasional small bird get stuck inside my garage - and they are definitely struggling to turn within a 20' wide garage at anything much faster than their stall speed. However, birds should be pretty good at withstanding g-forces. The strength of muscles and tendons depends on their cross-sectional area which increases as the square of the size of the animal - but the mass of the animal increases as the cube of the size. Because the actual force they have to resist is mass times acceleration, their small mass and proportionately thicker muscles and tendons should mean that smaller animals are much stronger, pound for pound than larger animals. Add to that that birds are highly evolved for minimum weight and you start to understand why a bird should be easily able of withstanding accelerations that would squish a human. SteveBaker (talk) 02:05, 15 September 2009 (UTC)

Speaking from personal experience here - and it's difficult to gauge the speed when they're flying in a living room or the confines of a small aviary, Budgerigars seem to be able to turn (but not quite literally) on a sixpence. --Kurt Shaped Box (talk) 08:03, 15 September 2009 (UTC)

This page claims that a Peregrine Falcon hits about 25 G pulling out of its dive. (Of course, that's in a different axis than turning in circle.) -- Coneslayer (talk) 14:13, 15 September 2009 (UTC)

G-related: here is an interesting article on how woodpeckers manage to survive 1200 G decelerations as they beat their heads against the wall. --Sean 08:53, 15 September 2009 (UTC)

Wood has a bit of 'give' in it. I wonder what happens to the brains and skulls of those confused woodpeckers which earnestly attempt to hammer metal poles? --Kurt Shaped Box (talk) 17:57, 15 September 2009 (UTC)

The linked article describes high-speed photography footage showing that they give a few test taps before hitting the gas. --Sean 21:09, 15 September 2009 (UTC)

These don't look like test taps to me. The woodpecker seems to be all 'WTF?' after each drilling attempt. --Kurt Shaped Box (talk) 00:35, 17 September 2009 (UTC)

Anicent plotting of landmass

We have a GPS can track the position of the plates is moving, then we should know the plates plate is moving. Is GPS tracking the plates from 5 years ago and now using magnifying glass the way we tell the positions of plates moving? For the places and exact lattitude of continents, could it be a mistake saying is 100 million years ago, tip of South Africa was 60 South? Could plotting the plates long time ago make a error bar?--209.129.85.4 (talk) 21:12, 14 September 2009 (UTC)

You idea is correct, GPS is used to track the movement of land masses. Over large time scales you have to assume that the rate of movement can be predicted to be the same as now. This will not precisely be the case, and other techniques can be used, such as dating the ocean floor, by looking at magnetic anomaly stripes, or preserved direction of magnetic particles in rocks of that age. Graeme Bartlett (talk) 21:54, 14 September 2009 (UTC)

Well, you can't really use a standard GPS unit - they only have an accuracy of around 10 feet (at best). Even an actively slipping fault like the San Andreas is only moving about an inch a year - so you'd have to wait at least 120 years before your GPS would be reliably indicating the motion. It is possible to use GPS - but you need some kind of a 'base station' in addition to the satellites. With that, you can get precisions down to a few inches - but you'd still need a few years worth of observations to see anything happening. SteveBaker (talk) 01:47, 15 September 2009 (UTC)

Of course the accuracy figure for the standard GPS unit refers to the accuracy of a single reading (once all satellites are acquired, plus WAAS etc). I would think, that if you left the GPS receiver in a single spot and averaged the lat/long figures over, say, a 24-hour period, you should be able to do considerably better, because fluctuations would average out. What the maximum improvement factor is, though, I couldn't say. --Trovatore (talk) 03:33, 15 September 2009 (UTC)

Steve is referring to differential GPS and it is indeed used for that purpose. 70.90.174.101 (talk) 04:21, 15 September 2009 (UTC)

Our GPS article has a careful breakdown of the causes of error. SteveBaker (talk) 21:16, 15 September 2009 (UTC)

As for monitoring stuff like faults they use other sensors (besides GPS), such as laser based instruments. I'm thinking they also use them to monitor volcanoes. 66.133.196.152 (talk) 09:27, 16 September 2009 (UTC)

Afterimage to work

How come when I try to look at green object I've try this few times for 60 second I count in my mind the white is still white green is still green. How long should I hold one color like blue to see blue color wear out? Will 5 minutes work? Usually when i look things for two minutes the afterimage is still the same.--209.129.85.4 (talk) 21:14, 14 September 2009 (UTC)

That's odd...are you sure? Usually, when people stare for even 30 seconds at some brightly colored image - then glance quickly over to an all-white surface, the after-images are in the reverse color of the original. So if you were staring at something green, then the after image should be magenta (a kind of purpleish-pink). If you stare at something red, the after-image is cyan (greenish-blue) - and if you stare at blue, the after image should be yellow. SteveBaker (talk) 22:27, 14 September 2009 (UTC)

yes, I'm positive no kidding. I've try look at computer screen is blue i concentrate on it for 60 minutes, blue is still blue, white is still white. i've try looking at someting lime green for 60 seconds lime green stays lime green white stays white. Do I have to look the color closely and take them close exactly at my eye, or I can look few inches away to do this lab right?--209.129.85.4 (talk) 21:35, 15 September 2009 (UTC)

Here's a commonly-cited one that uses the U.S. flag and doesn't even require you to look away - just stare at for a minute or so, click on it, the reverse-colored flag disappears in favor of a white surface, and thus the proper-colored flag will appear as an afterimage.[1] Baseball Bugs ^{What's up, Doc?} carrots 23:34, 14 September 2009 (UTC)

Fatigue colors ? Aha no article!--79.75.88.222 (talk) 01:01, 15 September 2009 (UTC)

Maybe its called Retinal fatigue ? Oh dear no article again!--79.75.88.222 (talk) 01:08, 15 September 2009 (UTC)

Try something simpler! Afterimage, Neural adaptation, Afterimage on empty shape. SteveBaker (talk) 01:35, 15 September 2009 (UTC)

It may seem obvious, but make sure you keep your eyes fixed on one exact spot. like a fixation dot in one of the classic afterimage samples, like the American flag. Then look at a white surface to see the complementary colors appear. Edison (talk) 14:17, 15 September 2009 (UTC)

Walking v. jogging for fitness

Is it possible to say that X minutes of walking are equivalent to Y minutes of jogging from a fitness point of view? Or similarly for distances? For example, might walking for 60 minutes increase your fitness as much as 10 minutes of jogging? 89.243.184.30 (talk) 21:58, 14 September 2009 (UTC)

Well I think it's going to vary immensely from person to person, depending on their speed, their gait etc, and what you mean by "fitness". If it's calories you're concerned about, most calculators you find seem to hold distance as most important, and modify it somewhat based on speed. This one for instance will let you do either, if you use the distance one and then your speed for the same distance you'll get slightly different answers. It seems to assume if you go faster you burn less calories per mile than if you go slow. As far as increasing your fitness, I'd say running, and specifically holding your heartrate up for a significant amount of time, is likely to be more effective that walking a similar distance. If you're training for a race or something, obviously running is going to be more effective preparation than walking. I find walking to be more boring than running, since you cover less ground in the same time, but I'm sure others would disagree. TastyCakes (talk) 22:11, 14 September 2009 (UTC)

The point about "what you mean by fitness" is very important. My personal prejudice is that running is damaging, at least for me. My joints don't seem to stand up well to it. So if you improve your cardiovascular fitness, but set yourself up for arthritis, what's the net effect on your fitness?

It would be a tough choice, but luckily you don't have to make it — there are lots of cardio options besides running. I'd look into swimming, cycling, or (deadly dull but terribly convenient) elliptical trainers at the gym. Caveat: These are my own personal prejudices; I have no real training and no professional qualifications in this area. --Trovatore (talk) 22:20, 14 September 2009 (UTC)

I go in for dancing instead, I walk to the shops rather than using the car but it only takes about twenty minutes. As a poster pointed out in a previous question if you don't find exercise interesting you may make an overall loss in worthwhile life. With a sociable exercise it is an overall gain, and dance or martial arts are good for exercising the mind as well as the body. If you like walking try joining a walking club, and if you like running you can go in for training for a marathon, but just walking or jogging for exercise?, I can't see the point. Dmcq (talk) 07:55, 15 September 2009 (UTC)

Tai chi chuan and Physical fitness also have sections on fitness as meaning more than replacing fat with muscles. Dmcq (talk)

Walking is certainly better than not walking, but it's unlikely that most walkers elevate their heart-rate enough to have significant cardiac benefits. Running is excellent exercise, but if you're heavy or particularly out of shape, or if you have the wrong shoes or a weird gate then, as Trovatore points out, it can be injurious. Swimming is fantastic exercise, although beginning/returning swimmers overestimate how much effort they're putting in, because they mistake being out of breath due to poor breathing technique to being out of breath due to exertion. For the gym, the elliptical trainer Trovatore mentioned is good exercise (I read Scientific American on it to alleviate the boredom - I find I get about 10% more performance than if I actually concentrate on the exercise), as is the static bike (although beware of knee-side pain, particularly if you are a woman - talk to the gym staff if this proves uncomfortable). And don't discount the gym rowing machine, which is good cardio and uses a wider range of muscles than bike/run/elliptical. -- Finlay McWalter • Talk 09:28, 15 September 2009 (UTC)

I've also seen it said that one ought to do a certain amount of "impact" exercise such as running or woodchopping because it increases bone density and strengthens connective tissue. Running too much certainly causes problems, but I think a modest amount of running is likely to be beneficial. Looie496 (talk) 23:35, 15 September 2009 (UTC)

I can no longer resist and must point out the alternative of playing squash. I've heard (unsourced) rumours that it is the sport with the lowest injury rate and highest cardio expenditure. It's also much more of a fun challenge than tackling an exercise machine. It combines athletic ability with tactics and realization of your own limitations, with an impact limited only by your own expectations of yourself. In a good club, you will always find someone else who plays at your own level. The drawbacks are: according to our article, possible heart attack; from my own experience as a tobacco smoker (inter alia), occasional doubling-over with coughing; again from my own experience, your opponent may run into the wall returning a shot and crack a rib (Bob still doesn't think that was funny! :); and if you possess any sort of massive ego, be ready to get beat by someone you were sure wouldn't be a problem. Also, I've noted that some heavier players complain of knee problems, but I've noted that those people were "heavy-landers", whereas many other players more weighty than myself had good "spring" on their landings and could beat me regularly. They were the ones thinking ahead of the current shot.

All-in-all, as a way to get a good workout, keep up your interest, socialize, and find your peer group in your interest in health - by my lights, it's squash all the way! :) Franamax (talk) 23:39, 15 September 2009 (UTC)