Wikipedia:Reference desk/Archives/Science/2018 December 12

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December 12

Science problem: Statistical solutions vs non-statistical solutions

What does it mean when an equation has a statistical solution? Like: Statistical solutions of hyperbolic conservation laws Or: Statistical solution Navier Stokes equation Wouldn't any solution be somehow a statistical solution? Wouldn't any solution be algebraic too? Could a science problem have an algebraic solution but no statistical solution, or the other way round?--Doroletho (talk) 17:57, 12 December 2018 (UTC)

Can you give a link to a specific article where you read this so we can have some context? --Jayron32 18:04, 12 December 2018 (UTC)

For example: [1] or [2]. But the question is in general. What makes a solution 'statistical'? What non-statistical solutions are there? Can statistical and non-statistical approaches to a problem differ? Or one approach, as long as it's not a blunder, will always confirm the other?--Doroletho (talk) 18:48, 12 December 2018 (UTC)

I think that means statistical in the sense of statistical mechanics. E.g. the ideal gas law is statistical. It starts with the idea of gas molecules flying in all different directions and colliding randomly. It then estimates statistical quantities like the mean kinetic energy of the molecules (i.e. the temperature of the gas) and the distribution of the energy (the Maxwell-Boltzmann distribution) without predicting the energy of individual molecules. Even in purely classical mechanics it's not possible to solve such a system exactly, but only statistically. On the other hand, if you can solve a system exactly (e.g. a simpler system involving a few billiard balls) then you can compute the statistics from the solution. 173.228.123.166 (talk) 21:22, 12 December 2018 (UTC)

In what sense is a Newtonian fluid elastic?

The first sentence of Speed of sound says, "The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium." But Elasticity (physics) implies that Newtonian fluids are not elastic: "Elasticity is not exhibited only by solids; non-Newtonian fluids, such as viscoelastic fluids, will also exhibit elasticity in certain conditions..."

Yet the concept of sound compression waves seems to apply just fine in Newtonian fluid models such as the ideal gas model. Are there different senses of "elastic" at work here? For example, are ideal gases elastic media despite not being elastic substances? --Allen (talk) 20:24, 12 December 2018 (UTC)

I thought elastic just meant kinetic energy is conserved after a collision. In that sense, gases are elastic, while viscous liquids are less so. 173.228.123.166 (talk) 21:17, 12 December 2018 (UTC)

Thanks. I can see how that's true of elasticity of the collisions of the constituent particles. But I can't see how that relates to elasticity of the material as a whole, which according to Elasticity (physics) is "the ability of a body to resist a distorting influence and to return to its original size and shape when that influence or force is removed." So rubber would be elastic, but not a gas (at least not in any sense that's obvious to me). --Allen (talk) 22:49, 12 December 2018 (UTC)

A guess at answering my own question: From Elastic modulus, I see that bulk modulus (which of course gases have) is one type of elastic modulus. So perhaps when people say things like what I quoted before from Elasticity (physics) (statements that imply that Newtonian fluids are not elastic), it's understood that they're only talking about Young's modulus and shear modulus. But when they refer to sound traveling through elastic media, it's understood that any elastic modulus counts. On the other hand, it seems weird to say "elastic media" in a context in which no media could possibly not be elastic. --Allen (talk) 22:49, 12 December 2018 (UTC)

Sound waves in solids can be either P-waves (compressional waves) or S-waves (shear waves), but only the P-waves can pass through a fluid, where it moves as a pressure wave. Shear wave propagation requires elastic rigidity. Mikenorton (talk) 00:15, 14 December 2018 (UTC)

Very handy for looking at the waves from earthquakes or underground nukes as they travel through the earth and figuring out which parts are liquid and which parts are solid. --Guy Macon (talk) 02:28, 14 December 2018 (UTC)

Indeed, that's how we know that the outer core is a liquid. Here's an explanation of the longitudinal pressure waves that carry sound through air - nothing elastic there. Mikenorton (talk) 21:40, 14 December 2018 (UTC)

Thanks! So should the first sentence of Speed of sound be changed? Or is even air an "elastic medium" in the appropriate sense? --Allen (talk) 17:24, 15 December 2018 (UTC)

In my understanding, it should. A gas is not an elastic medium and only non-Newtonian liquids have any elasticity. The key thing is that any change in shape of an elastic material should be recoverable - to quote from our article "elasticity .... is the ability of a body to resist a distorting influence and to return to its original size and shape when that influence or force is removed". Mikenorton (talk) 18:48, 15 December 2018 (UTC)

fatty acid beta oxidation

Hi all, I'm trying to understand beta oxidation of fatty acids, I've watched a few youtube vids and looked at the wiki article on beta hydroxylation but I'm still a bit uncertain! Please note, I'm trying to learn this from a human biochemistry perspective, not any other organism. From my understanding, beta oxidation cleaves 2 carbons from the fatty acid per cycle, until it is completely used up. My understanding is that acyl-coA dehydrogenase carries out the first step of beta oxidation through the process of dehydrogenation. My question is this: If only 2 carbons are removed at a time, why is it necessary for the human body to have short chain, medium chain and long chain acyl-CoA dehydrogenases? Surely it doesn't matter how long the chain is if the body is only removing 2 carbons at a time? There must be a reason why but I don't understand it. I also don't understand if there's 3 or 4 acyl-coA dehydrogenases... Any help would be much appreciated! RichYPE (talk) 20:52, 12 December 2018 (UTC)

Courtesy links: Acyl CoA dehydrogenase, beta oxidation. It looks like mammals have three acyl CoA dehydrogenases: short, medium, and long; very long chain fatty acids are first broken down in peroxisomes into medium-chain fatty acids, which are then sent to beta oxidation.

When talking about biology we need to be careful about using words like "necessary". Evolution is a "blind watchmaker". Biological systems are often not "necessary" or "optimal", just "good enough". Presumably there is some selective advantage in having specific acyl CoA dehydrogenases, or else they wouldn't be conserved. Perhaps a generic enzyme wouldn't bond well to substrates. Or perhaps the different enzymes are localized to different areas of the cell, making them more efficient. Molecules have all kinds of shapes. "Removing two carbons" is a useful model for analyzing the reactions in a schematic way, but remember that this is very reductionist. When analyzing how the enzyme reaction actually proceeds, and how different enzymes interact with different substrates, we have to look at the enzyme kinetics. There might be some research on this, but I'm afraid I'm not familiar with the subject. Maybe someone else is. --47.146.63.87 (talk) 20:29, 13 December 2018 (UTC)

There is a bit on it here, showing overlapping specificities. Quick glance I think they said there's a C-terminal domain that controls substrate specificity, and that this was upheld in [3] later on. Now the thing to bear in mind is that there are fatty acids in a cell that are doing different things, so there is some biological imperative to control which become substrates for degradation under various circumstances, I think. After all, if all fatty acids did exactly the same thing, why would there be more than one compound available to be broken down? Wnt (talk) 22:14, 13 December 2018 (UTC)

Many thanks to both of you for your responses. There must indeed (presumably) be a survival advantage conferred by having a number of different AcylCoA dehdyrogenases. Best wishes RichYPE (talk) 09:23, 14 December 2018 (UTC)

bread vs pasta

Are they pretty much the same thing, nutritionally speaking? I.e. basically mostly wheat flour with about the same carbs, vitamins, protein, etc. per ounce of dry weight? Web search shows a bunch of "health" comparisons but no nutritional ones that I saw immediately. Question came up because of an online discussion of foods to keep around the house in case of natural disasters etc. Thanks. 173.228.123.166 (talk) 21:15, 12 December 2018 (UTC)

Instead of random websites, consider using Nutrition.gov, "a USDA-sponsored website that offers credible information to help you make healthful eating choices."

If you're looking for suggestions about best practices to prepare for natural disasters, consider reading Ready.gov, including their suggested food preparation tips.

In particular, they recommend "dry cereal or granola" - but neither bread nor pasta make the top of the food supplies list. Remember, in a natural disaster, fresh clean water may be unavailable, and if it's really scarce, it's best to save it for drinking (not for boiling noodles). Bread is a fine food, and works well in an emergency, but compared to alternatives, it's harder to stockpile bread in large quantities.

Nimur (talk) 21:29, 12 December 2018 (UTC)

Thanks. The online person had suggested storing stuff like flour in order to bake bread, rather than storing actual bread. That sounded kind of inconvenient compared to pasta because of the long baking time. I'm not trying to do the prepper thing myself but I do have a fair amount of water and canned stuff on hand, just in case. I think it would be ok to re-use the water after cooking pasta. It's also supposedly possible to cook it by soaking it in cold water for 1.5 hours, followed by a quick (1 minute) boil, saving a lot of fuel. I might try that just to see if it works. 173.228.123.166 (talk) 22:05, 12 December 2018 (UTC)

A lot depends on the type and magnitude of disaster that you're preparing for. In many conditions, water and power won't be scarce; but it all depends on what happens and where.

Here's a good free book for residents of the San Francisco Bay area: Putting Down Roots in Earthquake Country. It answers questions like... "Why should I care? Why should I prepare? What should I do?"

Plan for at least one gallon of water, per person, per day, with a three-day supply - and that's just for drinking. Anything else you want to do - like cleaning, bathing, cooking, washing, ... or handling a messy medical emergency, no matter how minor - and you'll want to stockpile a lot more water. This is the single easiest, cheapest, and most useful thing you can do to prepare yourself: even in a mega-disaster like the so-called "Big One" earthquake, you aren't going to starve before help arrives - but you might get thirsty and you'll probably get dirty.

Nimur (talk) 23:06, 12 December 2018 (UTC)

Yeah here in the bay area, quakes are the main concern. I'll look at that book. Thanks. 173.228.123.166 (talk) 01:01, 13 December 2018 (UTC)

Most Italian-style dry pasta is made from durum wheat flour (usually semolina flour) which is higher in protein and contains more iron, vitamin B, selenium, and iron than common wheat. The types of starches are also different, as Durum wheat also has more amylose (30% vs 22-26%) and less amylopectin than common wheat, and amylose takes longer for your body to convert into sugar. That said, there are some rustic breads that are made from durum wheat, and some pastas (incuding most asian wheat noodles) that are made with common wheat. --Ahecht (TALK
PAGE) 19:50, 13 December 2018 (UTC)

The comparison also depends on whether the flour used to make the pasta/bread is wholegrain or refined, and whether the bread is leavened or unleavened - leavened bread provides greater availability of zinc than unleavened.[4] According to this slightly irreverant article, zinc deficiency is linked to irritability and belligerence in males, thus Edward de Bono suggested that conflict in the Middle East be tackled by making zinc-rich yeast extract available there, to supplement the zinc-deficient unleavened bread normally consumed in the region. PaleCloudedWhite (talk) 20:32, 13 December 2018 (UTC)