Wikipedia:Reference desk/Archives/Science/2017 July 31

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July 31

What elements or molecules does the Earth have more than half the non-sun solar system's quantity?

(From a discussion about Steven Universe.) Excluding the Sun, what elements and molecules does the Earth have more than half the Solar System's supply? I imagine some of this might boil down to how much we know about the cores of the Gas Giants... The elements that the Earth seems to have a lot of relative to at least the inner rocky bodies are Iron and maybe some of the other metals. For Molecules, I would expect most organic molecules (complexity beyond glucose), but I'm unsure on Water, Carbon Dioxide and Oxygen itself.— Preceding unsigned comment added by Naraht (talk • contribs)

The Earth may have a higher percentage of water and some other elements and molecules than any other planet, but the vast size of the gas giants likely means they have more total. Complex organic molecules, like proteins, may meet your criteria, as they seem to require life to be generated in quantity. StuRat (talk) 01:09, 31 July 2017 (UTC)

Certainly Neptune should have more water! According to our article its mantle is a water-ammonia ocean (but too hot for Earthlike life - maybe Planet Nine is a better temperature), and that's 15 Earth masses, even if you don't count what it says may be ionic water deeper down. Wnt (talk) 01:22, 31 July 2017 (UTC)

• Technetium and the transuranium elements only exist on the Earth. If Europa has life, it may have more of many organic elements than are found on the Earth, but that's rank speculation. μηδείς (talk) 01:16, 31 July 2017 (UTC)

There are stars with technetium. Not a lot of technetium but it does not only exist on Earth. Sagittarian Milky Way (talk) 01:41, 31 July 2017 (UTC)

The OP excluded the sun. μηδείς (talk) 03:21, 31 July 2017 (UTC)

• Also promethium, same situation as technetium. Nyttend (talk) 02:45, 31 July 2017 (UTC)

• I don't have a source but think it likely that Earth has the preponderance of the solar system's total of DNA, Rubisco, ATP and other molecules closely related to life processes. Shock Brigade Harvester Boris (talk) 02:49, 31 July 2017 (UTC)
• O₂ is a likely candidate. It's too reactive to survive long in most places. It only exists on Earth on sizable amounts because of plants. No other planet has had a Great Oxygenation Event--Jayron32 02:56, 31 July 2017 (UTC)

Question can not be answered because our system is not explored to that degree yet. Infact we even still dont even know what our homeplanet earth is made of. The deepest Kola Superdeep Borehole reaches just 12 km deep. That is 0.2% of the over 6000 km from surface to the middle. --Kharon (talk) 03:45, 31 July 2017 (UTC)

Not really. We have pretty good evidence that Earth's core is predominantly iron and nickel, and this fits with what theories of planetary formation predict. You can say until we can look at it with our own eyes we can't know for sure, but at that very high level of skepticism pretty much everything is unknowable. You can't see individual atoms, or photons, so we can't be sure they exist, so I guess lasers, semiconductors, nuclear physics, etc. don't exist. For that matter, how do you know you're not just a brain in a jar? --47.138.161.183 (talk) 04:18, 31 July 2017 (UTC)

Well, "predominantly" excludes a lot of material. Do we know there isn't a lump of platinum down there the size of Long Island? Do we know there's not one in the core of Jupiter the size of North America? This is a very guessy question. Wnt (talk) 12:25, 31 July 2017 (UTC)

It's actually not very guessy at all. We have reliable means of analyzing the basic make-up of the interior of the earth, and a large chunk of platinum would definitely have shown up. Unless, again, you ascribe to the "If I haven't directly observed it visually myself, I can't believe it" school of thought, and I don't know that that level of solipsism requires any response other than to summarily ignore it. --Jayron32 13:06, 31 July 2017 (UTC)

@Jayron32: This recent reference on "high resolution" seismology talks about a linear north-south anomaly across the center of the United States. [1] I will admit I did not access the paper, but my impression had been that this was about the resolution they were getting. Are you sure they could resolve core features down to the size of Long Island? Because I'd like to see a map like that! Wnt (talk) 18:01, 31 July 2017 (UTC)

If we find such anomalies, it is useful to come up with explanations. We don't, however, start with the possible existence of an anomaly (which, by definition, is an unexpected deviation), no matter how unexpected, and then throw up our hands and say that all knowledge is suspect. That sort of anti-scientific thinking is why we have stupid people in politics confusing us about climate change and evolution and all that bullshit, and it's best not to feed it. --Jayron32 18:12, 31 July 2017 (UTC)

@Jayron32: I'm not following you. The point of this subthread is simply to illustrate that our knowledge of planetary cores is limited. We are in no position to say which has more of what kind of trace elements than the other, as far as I know. We have a rough idea of the positions of discontinuities within the Earth that allow some very interesting modelling of general features of the planet. The anomaly simply illustrates the resolution of our map; it isn't really "unexpected" any more than taking a picture of Pluto and finding something other than a featureless billiard ball is "unexpected". There's terrain down there, we just don't know that it is. Wnt (talk) 18:29, 31 July 2017 (UTC)

Perhaps a better illustration would be ringwoodite; it was recently reported that there is an ocean's worth of water held within this mineral in the Earth's mantle. [2] If we can suddenly find out there is (even a non-navigable) ocean of water hidden away in the mantle, a place more prone to investigation via sampling in diamond inclusions, a substance that one might have expected to find on Earth, then what other things might be hidden away that we don't have access to and don't know to look for? Wnt (talk) 18:34, 31 July 2017 (UTC)

Inner core#Composition says it is unlikely to have significant amounts of anything denser than iron and inner core#Dynamics mentions features on the inner core boundary 1km in size. Some solar system bodies' moments of inertia haven't been measured yet, only estimated, so even thought Jupiter having a much higher platinum:iron ratio than Earth would be weird and need to be explained maybe that isn't ruled out yet. Sagittarian Milky Way (talk) 19:36, 31 July 2017 (UTC)

I don't think there's anything "unscientific" in acknowledging that in this particular area the error-bars are large and overlapping, especially for the elements in celestial objects, making it impossible to pick a "winner". 19:03, 31 July 2017 (UTC)

Nitpick: Earth's oxygen comes from algae and cyanobacteria as well as plants. As that article discusses, in modern science "plants" refers to the green plants, and excludes prokaryotes. Cyanobacteria were responsible for the Great Oxygenation Event, and the plastids that plants and some other organisms rely on for photosynthesis are themselves descended from cyanobacteria. No offense meant; I just think this stuff is cool. --47.138.161.183 (talk) 06:43, 31 July 2017 (UTC)

The transactinides should count whenever the folks at Dubna (or at one of the other facilities) turn on their accelerators. ^_^ Double sharp (talk) 05:22, 1 August 2017 (UTC)

So far

OP here. From what we seem to have posted so far...

• The man made elements: Technicium, Promethium, and more or less everything beyond Uranium, since there are man-made
• O₂), possibly, but not the atom (too much in CO₂)
• DNA, ATP, etc.

Naraht (talk) 11:49, 1 August 2017 (UTC)

Well-portioned food supply

Why do dogs and cats get well-portioned food in cans and bags based on size, physical activity level, pregnancy state, and age and humans have to make and portion their own food? Why can't humans get well-portioned meals based on the size of the human, age, pregnancy state, and physical activity level? The only difference is the quantity. Some humans just eat more because of specific conditions. Since dog food may be rice, meat, and veggies, can humans just eat wet dog food and adjust based on the human's size, physical activity level, pregnancy state, and age? If dog food is nutritious for dogs, then is it also nutritious for humans? 50.4.236.254 (talk) 01:47, 31 July 2017 (UTC)

Dog food may be lower quality but humans can eat it. It does usually contain fiber, ash(minerals) and "crude" versions of protein and fat. So if you like to eat chicken feet, tendons, cartilage and can accept that no veterinary will have check the source of your food for signs of worms, germs, fungi, insects and fermentation (age), go try if you like the taste aswell. --Kharon (talk) 03:27, 31 July 2017 (UTC)

I don't know how to read the labels. What does "crude" mean? If dog food is cooked, then will heat destroy the bugs? Some humans make homemade dog food for dogs. Will homemade dog food or homemade vegetarian dog food be healthier? I just want to know if humans and dogs can share EXACTLY the same food at every meal. If the dog gets an apple slice as a treat, then the human will also get an apple slice. 50.4.236.254 (talk) 03:41, 31 July 2017 (UTC)

Dogs are classical Carnivore so they are only used to eat other animals and may have developed an dependency that way. See Carnivore#Obligate_carnivores. I doubt you can feed them only with vegetable food tho they can digest that. They can be feed with a mixture aka you can substitute some percentage of their usual meal from animal sources with plant sources. --Kharon (talk) 04:03, 31 July 2017 (UTC)

Actually, dogs are omnivores; it's cats that're obligate carnivores. Dogs can easily be fed vegetarian diets. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 23:45, 31 July 2017 (UTC)

Some human foods are definitely bad for dogs [3] [4]. WP even has an article about one. Bazza (talk) 11:19, 31 July 2017 (UTC)

Actually, we have two. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 23:46, 31 July 2017 (UTC)

@Whoop whoop pull up: The ability to eat starchy foods is part of the key domestication locus that differentiates dogs from wolves: [5] The chromosomal region, which also includes giving dogs Williams syndrome to make them pathologically friendly, was selected with some ridiculous p-value like 10^-38, I mean, I've never seen a p-value like that before. There's practically no variation left in the dog genome around that area. Wnt (talk) 16:12, 1 August 2017 (UTC)

Moist pet foods are usually in cans, while dry pet foods are usually in large bags. And the size and number of whatever you give the pet will vary based on those factors you state. The "one size fits all" that you're postulating is incorrect. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:56, 31 July 2017 (UTC)

There are all sorts of human foods that come in pre-prepaired, pre-defined portions. Everything from TV dinners to baby foods. You may be interested in meal replacements like Soylent, which at least claim to be scientific. ApLundell (talk) 13:14, 31 July 2017 (UTC)

Humans could have such a 'nutritionally' balanced diet out-of-a-can, but who would want to eat it long term? Ensure (for example) produces liquid foods for people with health problems and whilst it tastes OK and is sating, I was glade to be able to eat normal food again. Second point is: that whilst modern propriety dog and cat foods contain mostly cheaper vegetable proteans, the fortification with essential vitamins does not turn it into the animals natural diet. Therefore, these modern alternative appear to be the cause of early health problems in obligate carnivores, such as kidney failure etc. So, traditional home prepared dog and cat food is preferable. If one can find a local wholesaler of raw horse meat and buy a bulk purchase to place into the freezer until it is required – it works out cheaper anyhow. This is because you have cut out the middle men namely the pet food manufacturer, the distributor and the shop owner – and of-course one later saves on those expensive veterinary bill for the resulting health issues that these modern pseudo diets appear to encourage... Aspro (talk) 17:37, 31 July 2017 (UTC)

Nobody mentioned MREs, yet? (Please don't). 2606:A000:4C0C:E200:E14F:DD6F:CFA5:F319 (talk) 20:49, 31 July 2017 (UTC)

for the claim that raw food diet results in few health problems. And when I call for references, I'm asking for at a minimum a published peer reviewed article. Not some crap from 'dogsnaturallymagazine.com' or 'Mercola', nor even the opinion of some random expert (even a vet). There is [6] for example, but it wasn't actually comparing pet food vs raw food (it was after all part funded by pet food companies) and it only looks at issues like microbial taxa and apparent digestibility and faecal health rather than long term health outcomes. People like to cite [7] but from what I can tell the study was never actually published in peer review form, and frankly from reading that variant, it's not surprising since there's no way that will ever survive peer review in any decent journal. (I'm not commenting specifically on the study, but the comments made.) Another commonly cited study is by Dr. Kollath, of the Karolinska Hospital in Sweden, but if that study was ever published I've never seen a copy. The reality is, as human research has shown. linking specific diets to long term health outcomes is quite difficult. And while dogs have shorter life spans and you can probably get ethical approval to do some sort of randomised control study (probably not double blind though), actually carrying out such a study is still quite expensive. A lot of the hype around certain dog diets are based on overly simplistic and very poorly supported theories, "gut feelings" and personal experiences with very few animals and full of confirmation bias etc (my dog got kidney problems at 10 years old so clearly there was something wrong with their diet). This is not the sort of crap that should be welcome on the science reference desk. Nil Einne (talk) 03:46, 1 August 2017 (UTC)

A citation would certainly be welcome but unlikely to materialize since peer reviewed articles cost money to perform and no food manufacture would allow publication of anything that casts doubt on its products, thus creating a field-day for those that want to use Argument from ignorance. Dietary connection to maladies are not that hard to come by in humans The western diet and lifestyle and diseases of civilization Quote: “ On a final note, this chronic, low-grade metabolic acidosis is exacerbated in elderly people who experience a decline in glomerular filtration rate and hence have a decreasedrenal acid excretion capacity.” So, one presumes from that, that dogs and cats are on the other end of the spectrum and suffer low-grade metabolic alkalosis. Also, companies that insure pet owners against high veterinary bills have been increasing their premiums. Being statistically driven (and in competition with each other) they appear to be reacting to the shortening healthy life expectancy of a pets. So what we really need, is a peered reviewed article that provides evidence that modern pet foods ether maintains or improves former health and life expectancy rather than the other way around – if you get my point. Aspro (talk) 15:02, 1 August 2017 (UTC)

Thanks for proving you're talking nonsense, you clearly don't understand how science works if you think food manufacturers have control over what's published about their food other than via libel laws. You mentioned something about 'fund' but of course while it's true funding can be difficult to obtain it's not impossible for someone dedicated to performing quality research. More to the point, funding can bias research in numerous ways as well documented, it's only in extreme cases, that the funder has veto power over the publication of research. But in any case, I already acknowledged funding was a reason why you'd find limited research, but since this is the science reference desk, you can't use this as an excuse to talk nonsense. If there is no actual evidence for your claims, they either should not be presented, or they should at most be presented with a great deal of caution emphasising you're basically talking out of your arse, which you did not do.

insure pet owners against high veterinary bills have been increasing their premiums

Highly flawed statement for several reasons. Firstly there could be several reasons including increasing vet costs for the same procedures and the availability of newer techniques and procedures that cost more but are again unrelated to different health of the animals. Then there could be an increases usage of vets for more trivial problems and a greater willingness of owners to use vets for various health problems (especially on co-pay situations). More to the point what is your time frame? The widespread availability of Pet insurance generally post dates modern pet foods. If you're thinking in a 30-40 year time frame, there's a very good chance raw meat diets have increase in that time period, as with other higher meat etc diets based on half baked theories. In other words, the vet bill evidence if anything is actually against a raw meat diet. In reality for the other reasons cited, it's useless as evidence but this does demonstrate why your claims are so flawed.

peered reviewed article that provides evidence that modern pet foods ether maintains or improves former health and life expectancy rather than the other way around

Again you've demonstrated you have no idea how science works. I never said that modern pet food improves former health or life expectancy. If someone does make the claim then yes, they should provide evidence. As for "maintains", well again this is a flawed statement. If you're arguing that pet food is just as good as raw meat then yes you need evidence. However if you're choosing between raw meat and pet food, then you need actual evidence one of them is better. Even more so if you're saying you should choose raw meat. If you have no actual evidence, then you can't say you have any reason to choose one over the other, and so the claim should not be presented on the science reference desk since science doesn't work by gut feelings, nor by highly flawed "theories" lacking any evidence.

Of course for the reasons I and you yourself mentioned earlier about funding, there is actually a fair amount of research about pet foods and even some comparing pet foods and raw meat diets. Most of this is about short term health outcomes rather than long term. And most of it is against raw meat diets. (Good evidence for this is reading stuff promoting raw ,eat and seeing how quick they often are to dismiss the research. Some of them then offering the even more questionable paper and alleged Swedish research I presented above. At least the NZ paper seems to be decent research but as said it's very limited.) I'm not going to presented these because as I said, I'm explicitly not claiming that either raw meat or pet foods are better, I'm solely questioning an unsupported claim made on the science reference desk. In fact I don't believe the evidence is strong enough either way even if what evidence does exist often comes down in favour of pet foods but bearing in mind the earlier mention risks arising from funding.

Edit: Also I forgot to mention but this statement also seems to be assuming a largely raw meat diet, which is what you presented, is what dogs were normally eating before modern pet food. In reality a lot of evidence suggests this often wasn't the case. Until very recently, meat was often expensive and for plenty of cultures it was the norm to consume nearly all the animal. So there often wasn't that much to give the dog. So it wasn't uncommon for dogs to get a fair amount of plant derived food. In fact in some cases e.g. some Polynesian Dogs, they seem to have subsisted nearly completely on a vegetarian diet. In more recent times, meat became more readily available so people could more easily give their dog meat, but still cost, convenience and the simple fact this predated halfbaked theories means even if there was an increase in their meat, many dogs would still have been feed a substantial proportion of plant derived food, particularly cooking and table scraps.

So if you're going to claim raw meat is better, you need to actually compare a raw meat diet to pet food, rather than compare undefined historic diets which often had a substantial portion of non meat to pet food. Now if you're going to claim historic diets were better than modern pet food, that's a different point but you need to accurately explain what that diet is, and it probably wasn't mostly raw horse for most dogs. In reality of course, the diets were highly varied between cultures and individual dogs. (Our Dog food article discusses this in limited detail.) It's largely a moot point since you don't have any real evidence health outcomes were better historically but still I felt this was important enough to mention.

P.S. I'm not really sure the relevance of a single paper about humans to this discussion. As I already said, research with humans has shown how difficult dietary research actually is. It's also shown the obvious, a great deal of caution should be applied in linking specific short term consequences like glomerular filtration rate and renal acid excretion capacity to longer term health outcomes. This doesn't mean looking into such factors are useless, simply that in most cases it's a far cry finding some link between them and some dietary component and concluding negative long term health outcomes.

Notably, while there is a fair amount of research suggesting certain aspects of the highly "Westernised" especially American fast food heavy diet is harmful, the research comparing older post First agricultural revolution diets with alleged hunter gather/paleolithic ones is a lot more mixed. As we've demonstrated before on the RD, a lot of the paleolithic diet claims are similar halfbaked "theories" with little evidence based on a flawed understanding of humans, evolution and what paleolithic diets likely really were like e.g. I'm fairly sure I've presented this ref before [8]. (Well for obvious reasons, the paleolithic claims are generally even more flawed than the pet food ones.) There is evidence lifespans appeared to have decreased after the first agricultural revolution, but even if true the precise reasons for this and what those reasons mean to us over 10k years later are a lot less clear.

Nil Einne (talk) 04:52, 2 August 2017 (UTC)

Wow, what a diatribe of prolixity. Let's stick to accepted understanding of scientific progress. If a pharmaceutical company or food manufacturer wants to claim anything, then it it up to them to provided peer reviewed articles – to prove old school is wrong. Complete silence from them in this respect. Also, if the funder pays for the research then they can forbid publication – and they often do. As for my understanding of science, there is an old saying “don’t teach your grandmother to suck eggs”. Having said that, I would welcome a peer reviewed article to suggest that I err and which shows that modern proprietary pet food are equal or better – can you come up with one... ? Aspro (talk) 11:54, 2 August 2017 (UTC)

Could you explain precisely what part of the responses to your last 2 questions Wikipedia:Reference desk/Archives/Science/2017 March 26#Can food be made to be suitable for both dogs and humans? and Wikipedia:Reference desk/Archives/Science/2017 May 18#Homogenous packaged,_pre-cooked food (I'm fairly sure this is you too) didn't satisfactorily answer this question? Maybe throw in Wikipedia:Reference desk/Archives/Science/2017 July 2#Can dogs control their appetites? even if it was mostly on a different issue just to make sure you've fulfilled your appetite of dog food and humans. Nil Einne (talk) 04:10, 2 August 2017 (UTC)

You might like Eating your own dog food about the president of a pet food company eating some of its dog food a shareholders meetings to show it was okay :) But yes I agree going on with repeated stupid questions here is just annoying. Dmcq (talk) 13:59, 2 August 2017 (UTC)

Oh missed Wikipedia:Reference desk/Archives/Science/2017 July 2#How can dogs eat white rice? Nil Einne (talk) 16:52, 2 August 2017 (UTC)

Alpha= stronger?

Is the alpha male or female always the physically stronger? — Preceding unsigned comment added by B8-tome (talk • contribs) 14:36, 31 July 2017 (UTC)

There's an article Alpha (ethology).

Short answer : No. Animals achieve dominance through different means. (For example, contrary to popular belief, wolf packs are usually organized by family structure. Only packs made of non-relatives (ie: in zoos or other unusual conditions.) have the fighting for alpha dominance pattern pop-culture would lead us to believe. ApLundell (talk) 14:44, 31 July 2017 (UTC)

Yes, that last bit is important, especially as the "alpha male" terminology has crept in to some less savory parts of human culture. For further reference on the myth of the alpha wolf, see [9], where Mech, who is largely responsible for the pop-cultural misunderstanding, tries to set the record straight, and gives plenty of good further references. SemanticMantis (talk) 16:34, 31 July 2017 (UTC)

Self-proclaimed human "alpha males" generally just make me wonder when they'll be ready for beta testing. Iapetus (talk) 09:55, 1 August 2017 (UTC)

lol. May I add that they are for sure Beta blocker ready for Beta decay? Because, you know, real boss just command without shouting "i am the boss", while a guy shouting "I am alpha" surely isn't, being approval seeker instead. That was my 2 cents Gem fr (talk) 15:14, 1 August 2017 (UTC)

Feynman Lectures. Exercises. Exercise 9-16 JPG. Lecture 9. Archive

. .

I think I understand why in ex. 9-16 (question a) m/k seconds = 1 second' , but t = (m/k) t' . The first equation is wrong. We should write

$(m/k){\text{ second }}=1{\text{ second' }}\cdot ?{\text{ }}{\tfrac {\text{second}}{\text{second'}}}$

therefore $?=m/k$

and $t'\cdot ?=t$.

Username160611000000 (talk) 17:01, 31 July 2017 (UTC)

I really wish you'd start writing out your questions in a standalone format. Nobody is eager to chase through a bunch of links trying to figure out the question and your effort at an answer.

The question was typed in previous article July_26. Someone deleted the article, despite the fact that on 30.07.2017 I asked a new question. The question was :

• • • Can't solve a) question. It seems that Feynman is expecting a formula $x'(t')=-v_{o}(e^{-t'}-1)$. So we must have next relating formulas ${\text{sec'}}={\tfrac {k}{m}}{\text{sec}}$ and ${\text{meter'}}={\tfrac {m}{k}}{\text{meter}}$.
      But combining $x'(t')=-v_{o}(e^{-t'}-1)$ and $x(t)=-{\tfrac {m}{k}}v_{o}(e^{-{\tfrac {k}{m}}t}-1)$ we can write $x={\tfrac {m}{k}}x'$. It does not agree with ${\text{meter'}}={\tfrac {m}{k}}{\text{meter}}$. Why?
      
      And also I am not sure should we convert $v_{o}$ and $k$ to different units. Username160611000000 (talk) 10:40, 30 July 2017 (UTC)
    • By plotting graphs of $f_{1}(t)=-{\tfrac {m}{k}}v_{o}(e^{-{\tfrac {k}{m}}t}-1)$ and $f_{2}(t)=-v_{o}(e^{-t}-1)$ with arbitrary values of m,k,v_o png, I found that for t = m/k sec , 2m/k sec etc. the function f₁ possesses values (m/k) f₂(1) , (m/k) f₂(2).
      Therefore, it is possible to call m/k sec as 1 sec' and (m/k)f₂(1) meters as f₂(1) meters' or (m/k) meters = 1 meter'. Then, by calculating the pairs (x' ; t'), and having performed the inverse conversion, we get the correct pairs (x ; t ).
      
      I still can't understand next: e.g. in power ${-{\tfrac {k}{m}}t{\text{sec}}}$ to eliminate the factor ${\tfrac {k}{m}}$ we should use a formula ${{\tfrac {k}{m}}{\text{sec}}}={\text{sec'}}$...Username160611000000 (talk) 16:03, 30 July 2017 (UTC)

Your question, [10], involves a particle with mass m moving at v=v0 at time t=0 at position x=0 and subject to a friction force, I think, F=-kv. You are asked to produce a formula for x by "numerical integration" and calculate the time at which half the velocity is lost and the total distance travelled. Part (a), somewhat mysteriously, asks you to adjust the scale of x and t so that the equation of action has "simple numerical coefficients".

I'm not sure if the "numerical integration" is a call for writing a computer program (though Feynman himself lectured a long time ago) or references some kind of general numerical approximation. It seems very simple to say that the acceleration dv/dt = -kv/m, so v = A e^{-k/m t} + C. The velocity is 0 at large t so C is 0. The velocity is v0 at t=0 so A = v0. And v is dx/dt so x = -mv₀/k e^{-k/m t} + C2; to make x=0 at t=0, C2 = mv₀/k. Thus x = mv₀/k (1 - e^{-k/m t}). At t = infinity x = mv₀/k -- the mass of the initial particle times the initial velocity (momentum) divided by the coefficient of friction. At "half-velocity" e^{-k/m t} = 1/2, so (taking ln of both sides) t = m/k ln 2. Now to check this a little, k = F/v, so it has units of (kg m/s^2) / (m/s) = kg/s. So m/k does indeed have units of time and k/m multiplied by time is dimensionless. I'm a little unclear on what the equation of action actually is that we'd be trying to optimize; I'm wondering if they want time in units of m/k ln 2 for example. Wnt (talk) 19:07, 31 July 2017 (UTC)

Feynman asks to find equation of motion as x = f(t). The logarithmic scale for time is interesting, but in this case we would not have found the correct equation of motion. And according ex. 9-4 JPG I think scale must be linear . Username160611000000 (talk) 03:45, 1 August 2017 (UTC)

Forget the ln 2 thing (which was still a linear scale, but it's not actually a useful one I don't think). I didn't appreciate that a, b, c are all things supposed to be helpful for doing the main set up for an (unnecessary) numerical integration. I agree the idea is to simplify the problem (I suppose it is pre computer) by getting rid of the k/m and m/k factors, so we simplify x = -mv₀/k e^{-k/m t} to x = - I v₀ e^{t / I}, where I = m/k (in seconds). At this point you're supposed to make I go away entirely, creating a mathematical equation without valid units, but I'll put that off because this is confusing enough already. With a sort of Doctor Seuss logic, we can say I = 1 second' if a second' is m/k seconds, and that certainly does the exponential; the problem is that in the coefficient we have second' * m/second. The least sane way to work that out is to say that there is a meter' = I meters and so the velocity v0 in meter'/second' is the same as m/second and therefore x can be returned in meter'. I would not want to play around with the units like this in a modern day calculation since it all seems very confusing; stuff like this is what gets a Space Shuttle flying into the Sun. But I now have that second' = m/k seconds and meter' = m/k meters ... I hope that's right... Wnt (talk) 11:40, 1 August 2017 (UTC)

confusing -- Yes, it works only when m and k are magnitudes without units, and only for one specific value of k.

second' = m/k seconds and meter' = m/k meters -- This was the stumbling block in my original question. We can't write "=" sign, we should say 1 second' is equivalent m/k seconds. Like with e.g. kg and lb. We should write 1 lb = (2.2 lb/kg) • 0.45 kg , where (2.2 lb/kg) should be obtained by direct comparison of the two standards.

I would not want to play around with the units like this in a modern day calculation -- In modern day we still deal with foot, pound etc. I like SI system, although it's no less artificial than fps units. For a good physicist, units should be child's play. Thank you for help. Username160611000000 (talk) 13:04, 1 August 2017 (UTC)

By "playing around with the units" I meant inventing "second'" and "meter'" and trying to remember what they mean.

I think this is what you're saying above, but to be clear, the main confusion I see is something we've both done: saying "A second' is m/k seconds". In English that works: if x = 3 ft then you can talk about "x feet away". But that's not a multiplication! So the math formula is that 1 second' = m/k ... and that's all. And alas, I think I even fouled it up above, because -- meter' most definitely is not = m/k meters, because m/k is in seconds. We need a conversion factor 1 second'/second = (m/k)/ 1 second, in fact. So 1 meter' = m/k meter/second. I think. Hope the Kremlin doesn't complain about that Space Shuttle roof ornament... Wnt (talk) 15:58, 1 August 2017 (UTC)

What % of the food we eat is converted to energy, and what % left as waste?

I heard it to a small extent depends on the person, their metabolism.

But a bigger extent is to the type of food.

Meats - 96% digested. Fruits/vegetables - 25% digested.

And so and so forth. Thanks. 12.204.64.35 (talk) 23:20, 31 July 2017 (UTC).

Efficiency of food conversion Greglocock (talk) 01:52, 1 August 2017 (UTC)

And Feed conversion ratio, with much more ref. Careful, however: high conversion ratio are obtained with animal feed designed for this purpose (maybe full of antibiotics and thing like that), not "natural" food; and with animals that die young, for the purpose of "wasting" as few food as possible in just keeping them alive.

However, articles, although these interesting, do not really answer the question, which is quite different

So

• 100% of the food we assimilate is converted to energy,
• 100% of the indigestible (Dietary fiber...) are left as waste, even though that part contains energy that other animals (herbivores, detritivores) could use, and probably WILL use after we defecate,
• A variable part of the food we could assimilate, but do not, is left in feces.

The latter article provide a reference stating that ~50% of the energy of food remains in them (this include the energy of the aformentioned indigestible). But this obviously depends on the diet, the health, etc.

Gem fr (talk) 13:45, 1 August 2017 (UTC)

Original person posting here, I asked this question before in reddit microbiology, and 2 people suggested it's more like 10%. Which means 90% of the food we eat is defecated out. Think about it. If you eat 10 pounds of food, do you defecate 5 pounds or 9 pounds? So I think only 10% of the food we eat is actually converted to energy, rest left as mass. 12.130.157.65 (talk) 10:16, 2 August 2017 (UTC).

Certainly not 10%, as this is rather the feed conversion ratio, which is obviously much lower (because lots of energy is used just to stay alive).

You may also look at human power: ~80W, that's 80*24*3600/4180 = ~ 1600 calories, the equivalent of 400g of sugar or starch. You daily eat in the ~1 kg range, not in the tens of kg range that would required if only 10% were used, do you ?

Gem fr (talk) 14:20, 3 August 2017 (UTC)

And if meat is more digested than vegetables, then that means our feces is primarily vegetables than meat. 12.130.157.65 (talk) 10:17, 2 August 2017 (UTC).

Indeed. That's quite easy to verifies yourself ;-) . But rice, potatoes, and other starch-rich food can also be digested very completely, too. Gem fr (talk) 14:20, 3 August 2017 (UTC)