Wikipedia:Reference desk/Archives/Science/2014 January 12

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January 12

Tooth enamel

If some bit of tooth enamel has been scratched away, is it possible for it to grow back or is it made of a non-living substance? NOTE: This is not a question asking for medical advice. — Melab±1 ☎ 02:30, 12 January 2014 (UTC)

Not surprisingly, we have a Tooth enamel article, which says "Remineralisation of teeth can repair damage to the tooth to a certain degree..." The linked article, Remineralisation of teeth, doesn't help much. HiLo48 (talk) 02:39, 12 January 2014 (UTC)

Heh. The linked to article reads like an advertisement. — Melab±1 ☎ 03:35, 12 January 2014 (UTC)

I am hardly an expert on the subject, but I found this source from the Mayo Clinic [[1]] that says that tooth enamel is inanimate and cannot grow back. Cogito-Ergo-Sum (14) (talk) 03:08, 12 January 2014 (UTC)

It say here "Because enamel has no living regenerative cells, the body cannot repair chipped or cracked enamel on its own under normal circumstances. However, research is now giving us a peak [sic] into the fact that tooth enamel can, in fact, be repaired – given the right environment". Richerman (talk) 14:36, 12 January 2014 (UTC)

Rodents solve the problem by having some teeth which grow continuously. This does cause other problems, though, like the need to constantly wear them down to prevent them from getting too long (similar to our hair and fingernails). It would be wonderful if we had teeth like that, then we could just go to the dentist to have them shortened, whenever they got too long.

Sharks produce an assembly line of teeth, and the old ones fall out. We could also just grow a new set of teeth every decade, that would solve the problem, too, except that we would often look funny with several missing teeth. StuRat (talk) 17:15, 12 January 2014 (UTC)

But if missing (adult) teeth were a normal and frequent circumstance, we wouldn't consider it funny looking. {The [gap-toothed] poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 14:04, 13 January 2014 (UTC)

Well, it's common enough in the elderly, yet we consider it funny looking unless they cover them with dentures. StuRat (talk) 03:01, 14 January 2014 (UTC)

Speed of an electron's rotation

Hi, My friends and I were having a conversation about physics and the topic of the speed of an electron's rotation came up. He argued that an electron orbits the nucleus of the atom faster than the speed of light; I said that such a speed was physically impossible. Which of us was right, and if I was, how quickly does the electron orbit? Thanks!

Cogito-Ergo-Sum (14) (talk) 03:03, 12 January 2014 (UTC)

You are correct that an electron traveling faster than the speed of light would be physically impossible, and there is no exception for an electron in an atomic orbital. However, electrons in inner orbitals of heavy atoms do attain speeds that are a significant fraction of the speed of light; see Relativistic quantum chemistry for details. Red Act (talk) 03:44, 12 January 2014 (UTC)

A fun aspect of this is that the fine-structure constant is the dimensionless ratio between the speed of an electron "orbiting" hydrogen in the Bohr model and the speed of light, about 1/137, which means that roughly around 137 (the article gives an unsourced refined estimate to be 173) you hit the "end of the periodic table". Though apparently it doesn't really end... I doubt you can banish a stray electron all that far from a positive ion even if it can't wrap its little head around it, but I don't know the specifics. Wnt (talk) 04:54, 12 January 2014 (UTC)

Beyond that limit the binding energy exceeds 2 m c^2, which means that the 1s shell will be filled spontaneously by pair creation, leading to an electron moving into that shell and a positron appearing at infinity. Count Iblis (talk) 10:42, 12 January 2014 (UTC)

Actually I was thinking of neutral atoms, and I'm sure you meant to say the positron appears next to the electron but being next to a +172 charge discovers a strong desire to be elsewhere. :) Wnt (talk) 19:40, 12 January 2014 (UTC)

Thank you very much! Cogito-Ergo-Sum (14) (talk) 05:11, 14 January 2014 (UTC)

Why does BETA stands for?

I see beta HCG test, and I know what is HCG stands for, but I don't know the meaning of BETA, what does it mean? Why is it called BETA? (the econd letter of greece) I would like to understand it. Thank you.194.114.146.227 (talk) 09:08, 12 January 2014 (UTC)

See Human chorionic gonadotropin. In general, if you purify out some kind of protein complex from a cell under gentle conditions, you may get several polypeptides that are tightly stuck together. Put them in harsh detergents and separate them (SDS-PAGE, for example) and they can be resolved into multiple bands, which you might call alpha, beta, etc. You then might find out that you've seen alpha somewhere before and know it under a different protein name, or that it matches a sequence named by sequence homology already, etc., so you tend to come up with multiple names, which is why you can have alpha and beta subunits each with their own names in addition. Wnt (talk) 09:41, 12 January 2014 (UTC)

Foam rollers and floors

When using a foam roller on a floor, the part where the cylinder touches the floor becomes incredibly slippery especially with socks. Why is that? The last floor I tested it on was plastic and grainy, but I've heard it happens on other types of floors. Th4n3r (talk) 18:13, 12 January 2014 (UTC)

Not exactly answering your question, but foam rollers are best used with non-slip mats.--Shantavira|^{feed me} 18:26, 12 January 2014 (UTC)

Probably due to a small contact area with the floor. First off, the cylindrical shape means, if everything was perfectly rigid, it would only contact the floor on a line segment, with no surface area at all. Of course, everything isn't perfectly rigid, so you do get a narrow strip of contact area. However, within that area, there are bumps and such, so that only a small portion of that is in contact. Then when you get down to the molecular level, even less of the two surfaces in is contact. So, all this makes slipping more likely, since the friction needed to prevent it only happens over a very limited area.

As for the socks, maybe some fluff from the socks sticks to the roller, and acts as a mechanical lubricant, much in the same way as armpit hairs do, to reduce friction further. There the hairs roll between two surfaces, in a manner similar to how the Egyptians used logs under pyramid stones to roll them along the ground. StuRat (talk) 21:14, 12 January 2014 (UTC)

Acceleration and speed bumps

When going over a speed bump in a car, does acceleration (positive or negative) matter at all? Both in terms of potential damage to the vehicle as well as the perceived bump for passengers. Or will only the velocity have an effect? Basically I'm wondering if you're suddenly surprised by a speed bump if it's better to decelerate quickly or to go over it at a constant speed. 68.0.144.214 (talk) 21:15, 12 January 2014 (UTC)

In a rear-wheel-drive vehicle, accel causes the car to pitch up a bit, so you get less weight on the front wheels and more on the rear. So, in that case, you might want to give it a little gas right before you hit the front wheels, then brake. Of course, the amount is probably not significant enough to be worth the wasted gas. In front-wheel-drive, I expect it to make even less difference.

What I would recommend, though, is that you protect yourself. Biting the tongue can happen if you are eating or talking, so pause either of those activities. StuRat (talk) 21:24, 12 January 2014 (UTC)

Personal experience shows that a steady rate is less jolting; presumably due to the additional force added from acceleration/deceleration (break before bump).~:71.20.250.51 (talk) 23:13, 12 January 2014 (UTC)

Yes brake before the bump, to avoid breaking on the bump. StuRat (talk) 23:55, 12 January 2014 (UTC)

Note incidentally that the pitch-up on acceleration and pitch-down on braking is independent of which wheels are involved, up to the point where those wheels lose traction. It's simply an equal and opposite reaction to the torque applied to the wheels by the engine or brakes. --50.100.193.107 (talk) 05:23, 13 January 2014 (UTC)

I disagree. The drive wheels act as the fulcrum around which the car rotates, and if the drive wheels are in front, the only part of the car to pitch-up would be that in front of the wheels, which wouldn't matter here. No matter how powerful the engine is, a front-wheel-drive vehicle won't be able to do a wheelie without the assistance of ramps. StuRat (talk) 15:47, 13 January 2014 (UTC)

This is a complex question. For example:

1. When braking, the brakes are tending to stop the wheels from rotating - which means that they'll resist not only horizontal motion - but also any vertical component caused by the speed bump.
2. Once you're across the peak of the bump, the amount of pressure on the tire drops - perhaps even to zero - so you'll lose traction. If there is power or brake force going to the wheel between the peak of the speed bump and when it impacts the ground again, then the wheels will have changed speed and that means that its rotation rate won't match the speed of the road. That would result in a jerk of some kind that wouldn't be there if the car was just rolling over the bump.
3. Braking will cause the car to nose down a little bit - which means that the mass of the body is accelerating downwards, compressing the springs - which would make it much harder for the springs to compress to take up the vertical motion due to the speed bump....so braking hard seems like a bad idea. Accelerating will raise the nose of the car a little bit - and should have the opposite effect - so for that reason, maybe you get less of a bump. However, that's just for the front wheels. The suspension on the back wheels will be doing the reverse of that. So accelerating until the front wheels are over the top of the bump, then braking until the back wheels hit it would seem to be a good strategy.
4. Another issue is the natural resonant frequency of the car suspension. Suppose you drive super-slowly over the bump...the suspension won't compress at all because the vertical acceleration will be very low and the car body will follow the shape of the speed bump exactly. On the other hand, if you drive over the bump too fast then the dampening effect of the shock absorbers will prevent the suspension from reacting fast enough to allow the springs to compress - and again, the body of the car will track the shape of the speed bump quite closely. However, at some speed between those two extremes, there will be enough vertical acceleration of the wheels to cause the springs to compress but not so much as to cause the shocks to prevent that - and the body will not rise up by the same height as the speed bump. What speed that is will depend critically on the kind of car you have.

But car suspension systems are complicated dynamic systems with complex geometry and all sorts of springy and dampening components. Tires will take up some of the bump - depending on what the pressure is inside of them.

I think this is a question that's too difficult to answer without a lot more data. SteveBaker (talk) 19:13, 13 January 2014 (UTC

The director of a place where I once worked decided people drove down the entrance road too quickly so he got speed trenches installed – quite deep ones. The best strategy was to go as fast as possible. Thincat (talk) 19:33, 13 January 2014 (UTC)

Antonym of Derived (phylogenetics)

I've tried to find the closest phylogenetical analogue to the concept of conservativity in historical linguistics. The usual antonym of conservative in this sense is innovative. It seems to me that derived is the analogical concept in biology. While primitive is a possible antonym, it is notoriously prone to misunderstanding.

Which is the best replacement for primitive, then? Basal, ancestral, plesiomorphic? Ideally, the term would be applicable both to individual traits and to entire species.

The concept of living fossils strikes me as the closest analogue to languages which are described as conservative in toto – which is sometimes criticised as an oversimplification, not unlike the metaphor of living fossils: just as species cannot escape changing over the span of many millions of years, languages do not stay the same over the course of the centuries, either, even if the changes are relatively subtle and the overall structure or Bauplan respectively remains remarkably constant, creating the superficial impression that the species or language respectively has not significantly changed at all. It may be more appropriate to say that a language remains conservative only in certain respects, but not in others, and there is no language which remains particularly conservative in truly every respect; similarly, I suspect that species may change very little in some respects, but not in others, and do not stay the same in truly every respect. Conversely, I suspect that even supposedly highly innovative languages and highly derived species retain certain aspects with little modification opposed to their relatives. At least it is my experience that when you look closely, you do find traits that go against the apparent trend. --Florian Blaschke (talk) 21:20, 12 January 2014 (UTC)

• The proper contrasting term is an ancestral trait. "Primitive" doesn't work when you compare things like acquired flightlessness. Basal applies more to clades than traits. μηδείς (talk) 00:32, 13 January 2014 (UTC)

So you'd use ancestral for individual traits, and basal for entire taxons or clades? Then ancestral trait corresponds to conservative trait, derived trait to innovative trait and basal clade to conservative branch/unit?

The analogue of clade (i. e., a monolyphetic taxonomic unit or taxon) in historical linguistics would be genetic unit then. While "language" and "species" are broadly analogous concepts ("mutual intelligibility", as usually understood, and "ability to interbreed" being somewhat comparable criteria), it is true that using only this level was too narrow. "Conservative" can also apply to larger genetic units or branches, comparable to clades being more basal or derived.

The reason for my question was that I'm not completely sure if basal is really what I'm looking for, seeing that Basal (phylogenetics) defines basal clades as outgroups. Outgroups correspond to early splits in language family trees.

For example, Gothic is an outgroup within Germanic, but it was not necessarily a conservative branch of Germanic at the time of attestation, as contemporary Proto-Norse (from what little is attested) appears more similar to Proto-Germanic. Icelandic is a notoriously conservative North Germanic language, but it does not form an outgroup within North Germanic: It diverged from an already differentiated western dialect of Old Norse. (Granted, the dialect differences within Old Norse were minute at the time in question.) Finnish is a conservative Finnic language, but does not form an outgroup within Finnic, so it cannot be described as "basal". Livonian is an outgroup within Finnic, but it is notoriously innovative (although it does have a few archaic traits, these are not obvious at first comparison with its relatives). Tamil is a conservative Dravidian language, but not an outgroup. Maori is conservative within Eastern Polynesian, but not an outgroup: the outgroup is Rapa Nui, which is rather less conservative than Maori. Svan is an innovative Kartvelian language, but an outgroup within Kartvelian. East Semitic is an outgroup within Semitic, but not particularly conservative considering its early attestation, and the traits distinguishing it from the rest of Semitic are mainly innovations.

Conservativity has nothing to do with the place of a branch in a tree. It only describes the overall impression of resemblance to the proto-stage (MRCA), or amount of change from the proto-stage compared to other descendants. The divergence of the outgroup may in fact have come about through innovations (analogous to derived traits) from the start, and the outgroup can give a more innovative impression than the rest of the family, which may have undergone fewer or less salient innovations (some innovations have important ramifications, many are rather inconsequential). So I don't think basal is a fitting analogue. --Florian Blaschke (talk) 16:20, 13 January 2014 (UTC)

I hope you don't mind, I added a space between your lines to increase readability. I am a little confused now on your focus. Are you looking for an antonym of derived feature, or an antonym of something like a crown group? The point I was trying to make clear is that derived itself needs to be used in a context in order to find its opposite. You have to specify whether you are talking about traits alone, or about organisms or clades. Your points on languages are well taken. You might enjoy Old English and its Closest Relatives if you haven't already. μηδείς (talk) 17:54, 13 January 2014 (UTC)

Marketing and healthy foods psychology

People often think of mcdonalds as unhealthy food but isn't it true that in reality, its no worse than many other fast food chains and restaurants? — Preceding unsigned comment added by Clover345 (talk • contribs) 22:45, 12 January 2014 (UTC)

Point of logic. It's possible for it to be "no worse than" and "unhealthy". HiLo48 (talk) 23:00, 12 January 2014 (UTC)

It can be healthful, but it requires research. There's a book or a series of books called Eat This Not That, which contains optimally healthful suggestions for various eating places. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:09, 12 January 2014 (UTC)

That comes across as a good example of a logical fallacy. Example, lets rephrase: People often think of the AK as firing an unhealthy 7.62 slug but isn't it true that in reality, its no worse than many other rounds from similar caliber rifles?'--Aspro (talk) 23:11, 12 January 2014 (UTC)

I've noticed that the biggest fast food chains, like McDonald's, Burger King, KFC, and Wendy's, offer both healthy and unhealthy foods. They don't seem to much push the healthy foods, though. The healthy food rarely goes on sale, and sometimes isn't even listed on the menu. At my local Wendy's, if I order a Kids Meal, it will come with fries unless I ask for apple slices, and even then there's a good chance I will get fries anyway. Or, if I order unsalted fries, they will have salt on them anyway (possibly a bit less). So I'd like to say "Apple slices with that Kid's Meal please, but when you screw up again and give me fries instead, make them unsalted fries, and when you screw that up, at least put less salt on them, please".

By contrast, the smaller fast food chains often seem to lack any healthy foods at all, and may not even provide nutritional info on their food. I'm guessing they consider their customers to be the niche market of unhealthy eaters, so aren't concerned with having options for everyone. StuRat (talk) 23:54, 12 January 2014 (UTC)

“Ironically, the fast food giant recently ended up with a PR nightmare after suggesting its own employees forgo fast food fare for healthier options like salad and water. As reported by Business Insider:”

"Several excerpts from the posts, which were created from a third-party vendor, warned against the negative effects of fast food, even going so far as labeling a cheeseburger and fries, core items on its menu, as an 'unhealthy choice. articles.mercola.com/sites/articles/archive/2014/01/08/mcdonalds-fast-food.aspx --Aspro (talk) 00:44, 13 January 2014 (UTC)

I've never had a major concern about the healthiness of the food of McD, BK, Wendy's, Subway or similar. The sheer simplicity, ubiquity and cheapness of the food is what makes them a major health hazard. Richard Avery (talk) 07:37, 13 January 2014 (UTC)

Criticism of fast food is the relevant article.--Shantavira|^{feed me} 08:29, 13 January 2014 (UTC)

I disagree. Many fast food items contain more sodium, saturated fat, and trans fats than you should have in an entire day, much less one meal. I suppose if you only ate such a thing once a month, and ate healthy the rest of the time, you might be OK, but very few people do that. StuRat (talk) 15:44, 13 January 2014 (UTC)

Sources please. After a quick scan of McDonalds' nutrition fact sheet (pdf) I find two menu items that contain more than 100% of recommended daily sodium, both of them chicken in amounts somewhat impractical for a single eater. One of them also barely exceeds 100% of recommended saturated fat. These figures are for a 2000 calorie diet, which is pretty low. Sure it is possible to eat too much hamburgers and such but let's not FUD too much on the science reference desk. 88.112.50.121 (talk) 18:52, 13 January 2014 (UTC)

I would have been more impressed if you came back with sources that show fast-food is a healthy diet.--Aspro (talk) 21:55, 13 January 2014 (UTC)

Check out some of the worst foods on this list, which exceed the sodium limit for the day: [2]. And this list seems to have used a 2500 calorie diet for it's percentages, and presumably they are using the company's own info, which tends to underestimate the unhealthiness, by quoting foods without sauces, etc. (Subway is one of the worst in this regard, quoting nutrition stats based on a half sub, with no cheese or sauces, and the most healthy bread.)

Since that list didn't cover trans fats, here's a separate list for that: [3]. StuRat (talk) 03:16, 14 January 2014 (UTC)