Wikipedia:Reference desk/Archives/Science/2015 January 8

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

January 8

Negative coefficient of friction

I was reading the article on friction in the section about negative coefficients of friction, where it discussed the force of friction decreasing with an increase in the normal force. However, shouldn't these mean that the force of friction is inversely porptional to the normal force? A negative coefficient of friction would mean the force of friction would be negative (which in terms of vectors would mean working in the same direction as the applied force), but it would still increase with an increase in the normal force. Am I wrong here? Thanks, --T H F S W (T · C · E) 04:34, 8 January 2015 (UTC)

I agree with your interpretation. However, as far as I know, that doesn't exist in the real world, while what they described does. One example that comes to mind is ice skating, where the friction is reduced once the pressure builds in the area under the blade and melts the ice, so the skater is then gliding on water on ice. I'd call that a "negative change in the coefficient of friction". Of course, that's more of step function than a continuous case. StuRat (talk) 04:58, 8 January 2015 (UTC)

It is true that there is high pressure under a skate blade, which lowers the freezing/melting point, but it's a myth that this results in melting that makes skating possible. Artificial ice rinks are normally maintained at temperatures below the lowered freezing point. The truth is more complicated and involves other factors. --65.94.50.4 (talk) 05:09, 8 January 2015 (UTC)

Great link. I love it when myths are busted. 196.213.35.146 (talk) 10:08, 8 January 2015 (UTC)

Well, if it was due to friction that the ice melts, then you'd expect it to be very hard to get started moving on skates, since there's no frictional heating until after movement begins. Note that the pressure lowers the melting point, but the increase in pressure also heats the ice. As to the argument that water takes up less volume and therefore the pressure would decrease when it melts, this ignores the fact that the skater will simply sink down by a microscopic amount into the ice, to restore the pressure. StuRat (talk) 19:02, 8 January 2015 (UTC)

I agree that it's a misleading heading. I've put "negative" in quotes until someone comes up with a better way to express the concept. Dbfirs 09:45, 8 January 2015 (UTC)

It may be misleading, but for better or worse a lot of scientific terminology is misleading to the uninitiated, no? That doesn't mean we get to change it on WP, we should be consistent with the literature. The authors of the cited paper say in the abstract "This leads to the emergence of an effectively negative coefficient of friction in the low-load regime." - I don't think it's our place to change the terminology that has been accepted in a Nature publication. It is unclear to me whether your solution of scare quotes is better than calling the section "effectively negative coefficient of friction" - any thoughts from the group? SemanticMantis (talk) 16:20, 8 January 2015 (UTC)

If we quote something which is believed to be in error, we should put "(sic)" after it, to show this. StuRat (talk) 16:58, 8 January 2015 (UTC)

Sure, as a general practice, but I don't believe this specific case to be an error, and in general I would doubt our understanding before I would doubt the findings of Nature paper (sure, they occur, but much more rarely than errors in WP). SemanticMantis (talk) 20:09, 8 January 2015 (UTC)

Note that it is the coefficient of friction that is negative, not the force of friction. The coefficient relates the normal force to the frictional force, and is dimensionless. In this case, the coefficient is negative, because increasing normal force decreases friction (in at least one very special scenario) - read the abstract of the paper cited in our article here [1]. I actually think the article is fine how it was - it makes sense if you've read and understood everything up to that point. SemanticMantis (talk) 16:20, 8 January 2015 (UTC)

(ec) Note that a negative coefficient of friction invariable would lead to a negative frictional force:

Ff = μFn

-Ff = (-μ)Fn

StuRat (talk) 19:00, 8 January 2015 (UTC)

Yes, what SM said. It's not as counter-intuitive as you would first think, because there is adhesion involved; there is a resistance to sliding even under zero normal load. The slip resistance is a constant (adhesion) plus a coefficient of friction times the normal force, and in this case that coefficient would be negative. But this is just decreasing the adhesion under increased vertical load; no one is saying that this could overcome the cohesion and start "pushing" in the same direction as the applied load. --Floquenbeam (talk) 17:02, 8 January 2015 (UTC)

Floquenbeam's explanation makes sense. But, there still can't be a fixed negative coefficient of friction, as that would indeed eventually lead to a frictional force, at high normal force ranges, which surpasses the adhesion force. So, the coefficient of friction must be variable, and only negative at certain low ranges of normal forces. StuRat (talk) 17:05, 8 January 2015 (UTC)

Yes, as SM already stated above, it's an "effectively negative coefficient of friction in the low-load regime." It's variable, and no one is saying it continues this behavior at higher loads. --Floquenbeam (talk) 19:07, 8 January 2015 (UTC)

I've changed the wording to an exact quote of the source, but left the scare quotes in the heading. Will this suffice?
I've also moved the section to its proper place after the table of conventional positive values. Dbfirs 19:41, 8 January 2015 (UTC)

Thanks, that's better. I personally don't like the scare quotes, because it seems to me the coefficient is literally negative under the specific regime studied. But I don't feel strongly enough about it to argue or change it :) SemanticMantis (talk) 20:06, 8 January 2015 (UTC)

If I'm not mistaken, it still opposes the applied force. The proportion of opposition gets smaller as more force is applied. I think a hydrofoil can be modeled as a negative coefficient of friction. Motion lifts it, drag is reduced, efficiency increases. It can be modelled other ways with more conventional positive coefficients but it's not as cool. --DHeyward (talk) 02:49, 9 January 2015 (UTC)

Snow Extrusion

Though the illustration might not demonstrate it very well, I'm interested in the natural phenomena of when snow forms an extended extrusion-like structure growing from behind of objects like trees or fences. I assume it has something to do with wind, but how exactly does it work? 176.14.195.212 (talk) 12:36, 8 January 2015 (UTC)

This is a very interesting question, and it turns out to be very difficult to describe and understand all the things snow and wind can do! Here are some nice descriptions of how snow interacts with snow fences - [2] [3]. I don't know of a specific term for the influence of trees/fences on snow, but Penitentes is a word for spire-like snow formations - these form from wind but also due to interactions with temperature and dew point. The general concept that would apply to snow/fence interactions is Aeolian_processes (sometimes spelt 'Eolian'). These processes can also cause pattern formation and have can have some degree of self organization. Behavior of snow due to wind is an active area of research, and there is a lot we still don't understand. Here is an example of a recent empirical paper on snow/wind dynamics [4], and here is a nice overview from the USGS [5]. There are also lots of simulation models and theoretical mathematical treatments, but those can get pretty heavy and might not be that useful for a general audience. An unrelated but interesting water 'extrusion' is ice spike. Both this and the photo you show (and the formation of snow flakes themselves]] are examples of dendritic growth.

If you have any more specific questions I might be able to find more suitable references. In the meantime, let's all enjoy these beautiful photos that we get searching /eolian snow pattern/: [6] or simpley /penitentes/ [7] :) SemanticMantis (talk) 16:06, 8 January 2015 (UTC)

Winter mountaineers in UK call this "rime ice" but we have an article on the same subject called Hard rime. On the hills, you can determine the prevailing wind direction (useful for avalanche prediction) because the ice grows into the wind. See Avalanche, the Basics - Part 2: Staying Safe; "Rime Ice is caused when super-cooled water vapour hits a freezing object (rocks, fence posts etc). Counter-intuitively, rime grows into the wind rather than away from it, so the direction it points is another handy indicator of recent wind direction" (just over halfway down the page). I rather like this picture. Alansplodge (talk) 16:32, 8 January 2015 (UTC)

Just what kind of person would be so easily impressed as to take such an interest in structures that arise out of snow anyway... Snow talk 11:22, 9 January 2015 (UTC)

The kind of people that try to avoid having your electricity supply or mobile phone cut off in Winter, or ensure cars going across bridges don't get impaled by falling icicles. Or any people who are generally interested in strange phenomena. Dmcq (talk) 12:00, 9 January 2015 (UTC)

@Dmcq: Just so you don't walk away with the wrong impression of me, can I trouble you to take a look at my full user name? Snow talk 16:28, 9 January 2015 (UTC)

Ha, until any sarcasm punctuation becomes commonly available, this is a situation where a sarcasm tag might have helped (e.g. '/s', '</s>', '/sarcasm', etc. :) SemanticMantis (talk) 16:45, 9 January 2015 (UTC)

I had considered that, but the problem is that it wouldn't necessarily have delineated that the sarcasm was self-effacing and I hoped that keeping my name at full scale would make that point more clearly. Oh well, just a reminder as to one of numerous reasons I don't attempt humour here as a general rule. :) Snow talk 02:47, 10 January 2015 (UTC)

Human only (or close) diseases.

According to their articles, both Smallpox (Variola Major) and Polio are found in nature only in Humans. (It apparently is possible to give smallpox to a primates, but no cases have been observed in the wild). Any idea where I could find a list of diseases that (like these) are more or less restricted to humans?Naraht (talk) 15:54, 8 January 2015 (UTC)

Would you include mental diseases ? I imagine many of those are human-only, as more complex brains are prone to more complex disorders. StuRat (talk) 16:55, 8 January 2015 (UTC)

Lots of zoo animals develop mental disorders, Stereotypy_(non-human) covers one common symptom. Even stuff like schizophrenia, which seems very human-centric by definition, is studied by Animal_models_of_schizophrenia. It's not clear to me if it's fair to say these rodents and primates "have schizophrenia". SemanticMantis (talk) 20:03, 8 January 2015 (UTC)

Unfortunately, Wikipedia seems to be a little deficient here - there are 219 entries in Category:Zoonoses, but only 216 entries on List of infectious diseases. Ideally, the diseases the OP is looking for would be those which are on the second list but not the first. Tevildo (talk) 19:57, 8 January 2015 (UTC)

If you're willing to include parasites, then several of these are human-specific, e.g. Louse#Lice_in_humans. SemanticMantis (talk) 20:03, 8 January 2015 (UTC)

You can look at Eradication of infectious diseases as a human infectious disease can only eradicated if it is human specific. Ruslik_Zero 20:21, 8 January 2015 (UTC)

That's a very good point, thanks! However, Malaria is on that list, and it definitely lives in mosquitoes as well as humans, though it doesn't give them the same symptoms. SemanticMantis (talk) 20:36, 8 January 2015 (UTC)

Plasmodium can not reproduce in mosquitos and humans are the only secondary hosts. Ruslik_Zero 20:54, 8 January 2015 (UTC)

That's not necessarily true; you could eradicate it both in the human population and any other carrier populations (though unfortunately for members of that other species, the likely approach would be (and has at times historically been) to eradicate the carrier population itself. In any event, it's quite likely there's never truly been a human-specific pathogen; all infections to which humanity has been subject have arrived in the human population through zoonoses at some point or another, and there's no way of knowing with any degree of certainty that there's even so much as a single pathogen which entered the human ecological niche and then mutated into a form that has never since infected another member of any other species. Certainly there might have been cases where the only per-existant animal vector(s) disappeared to extinction, but these are surely a great rarity as well. Non-communicable diseases are of course another matter entirely, but I rather suspect Naraht was excluding them by principle. Snow talk 11:18, 9 January 2015 (UTC)

This is a difficult topic especially regarding viruses. There are closely related viruses that have different effects on hosts. SIV and HIV are closely related. AIDS is mostly human though. Rabies in bats is quite the different story for humans. The flu moves through swine, avian and human hosts that all contribute and affect each differently. I suspect the "human only" might be symptom related rather than antibody related and it would surprise me a bit to find a symptomless carrier in insects, birds or mammals that are simply unknown. --DHeyward (talk) 04:26, 10 January 2015 (UTC)

Beaver poo

Mice don't "hold it in" but rather urinate and defecate more or less constantly. Is this also true of larger rodents, such as beavers ? StuRat (talk) 17:25, 8 January 2015 (UTC)

According to [8], beaver droppings are found in the early morning at the water's edge. I don't know if that means they do it overnight and it's only found when the sun rises, or if they do the business at first light, but it implies they hold it in through daylight hours. Mogism (talk) 17:31, 8 January 2015 (UTC)

Thanks. Anyone else ? StuRat (talk) 06:32, 12 January 2015 (UTC)

Are siblings the most genetically similar people to a person?

And if so... if one sibling is smart, and their sibling(s) is/are dumb, does that not demonstrate that the factors that determine smartness and dumbness are predominantly environmental and not genetic? 69.121.131.137 (talk) 22:32, 8 January 2015 (UTC)

As for the first question, apart from identical twins, the closest genetic relationship is that between two siblings or that between a parent and his/her child. A child has inherited 50% of its genome from each of its parents, so is 50% identical in its genome to either parent. Likewise you can calculate that two full siblings share on average 50% of their genome.

As for the second question, no, that does not logically follow, at least not from just a few cases. Children can inherit different genetic material from their parents, and so they need not share between them any of the genes which influence intelligence. In fact theoretically they need not share any genes. On average, they share 50% of their genes, but in individual cases it may be 60% or 45% or even 0% (though this would be exceedingly unlikely). - Lindert (talk) 22:45, 8 January 2015 (UTC)

(ec) Usually this is addressed with a twin study. Twins who are genetically identical are compared to those who share 50% of their genes, and the difference is considered. Of course, it might not be foolproof (what if the smart twin was sneaking in and takes tests for the dumb one, affecting how they are later treated, or if the teachers have a visually driven bias) but it's a pretty good way to look at it. Wnt (talk) 22:49, 8 January 2015 (UTC)

(ec) Siblings have 50% genetic overlap, the same as the overlap between a parent and child. That isn't enough to determine whether differences are due to environment or genes. However, identical twins have 100% genetic overlap, so the differences between them are much easier to interpret. The most powerful test of environment-versus-genes is to compare identical twins reared apart with identical twins reared together. It's hard to find good data, because the number of identical twins reared apart is quite small, but the data that exists is extremely useful. Looie496 (talk) 22:51, 8 January 2015 (UTC)

Because the exact proportion of "shared" genes in siblings can theoretically vary between 0% and 100% (though with an average of 50%) it is possible to have more "shared" genes with other people than with your sibling. (Note that these percentages refer to the genes that can vary between two people (i.e. your parents), and not to genes that you "must" have in order to be human rather than some other species). Your question about the factors determining intelligence is not yet resolved, though scientists are hard at work on it. Geneticists talk of the heritability of different traits. As you will see from our article on the heritability of IQ the best estimate of the heritability of intelligence seems to be about 50%. That is, about half of the variability of intelligence among the human population at large can be explained in terms of a genetic component. However, this figure refers to the population as a whole, and, because we don't know the proportion of genes you share with your sibling, we can't reach any conclusions about the relative impacts of nature and nurture in your particular case. RomanSpa (talk) 00:41, 9 January 2015 (UTC)

I saw my doppelganger at a train station a few years back. Looked nothing like my brother. Can't speak for the genes beneath the skin, but my resemblance was uncanny. InedibleHulk (talk) 00:48, 9 January 2015 (UTC)

• Looie's answer is the correct one in general, but both male and female children bear their mother's mitochondrial DNA, so are slightly closer to each other and their mother than their father in that respect. There's also the question of the Y chromosome. Since the Y carries fewer genes than the X, again, children are closer to their mother, but two brothers, each carrying the same Y, are thereby closer to each other and their father than to a sister not carrying the identical Y. Ex chromosomes cross over, so the exes inherited by sisters are not necessarily identical, while the wyes carried by brothers are.μηδείς (talk) 01:33, 9 January 2015 (UTC)

Sisters do have identical X-chromosomes, since their father only has X-chromosome. Both sisters necessarily inherit all of the genetic material in their father's single X-chromosome (unless some genes could cross over between the father's X and Y). Of course the X-chromosomes they inherit from their mother could be quite different, but in that respect they are no different from two brothers either. - Lindert (talk) 09:59, 9 January 2015 (UTC)

There actually is a portion of the Y chromosome, the pseudoautosomal region, that can recombine with the corresponding portion of the X chromosome. It only comes to about 5% of the Y chromosome, though. Looie496 (talk) 16:31, 9 January 2015 (UTC)

One thing to clarify: First, let's ignore things that can alter the 50% from each parent (the things Medeis mentions, but also genetic disorders like XXY or XYY, etc), and focus on the simplified 50/50 model of genetic inheritance from each parent for alleles in nuclear DNA. Now, a parent is guaranteed to share 50% of your alleles ( not genes, let's be precise here :), and cannot share any more or less unless you get into edge cases. We don't know exactly which alleles are shared, in part due to recombination. In contrast, a sibling shares 50% as an expected value. So the situation is this - if you want a "safe bet" for most shared alleles, pick a parent, you'll never be far off from 50%. If you want to potentially maximize the shared alleles, pick a sibling, who will share more than 50% of alleles roughly half the time. As Lindert points out above, a sibling has the potential to share much more than 50% with you, but can also be lower, and this makes for some interesting differences, depending on our motive for finding a highly similar person. SemanticMantis (talk) 16:41, 9 January 2015 (UTC)