The shear stress assumption in Navier Stokes

Question: Viscous fluid mechanics starts with the assumption that shear stress is
linearly related to velocity by $$\tau = \mu\frac{du}{dy},$$ but it later
turns out that some forces in high speed situations go as the square of the
speed. This is really a reference request: are there critical discussions about
the correctness of the linear assumption in view of experiments? This might be in
every good textbook but I just haven't seen it. I'm framing this as an experimental
topic because of the limited understanding (http://www.claymath.org) of Navier Stokes
in a strict mathematical sense.
Answer: First, the difficulty of the Clay problem is not because of any limitation on our understanding of where the Navier Stokes equation comes from, or how to solve it. In practical situations, you just solve it on a computer. The Clay problem is asking whether the numerical solution converges to a differentiable continuum limit at small lattice spacings, and the reason this is difficult is because it might not be true. There might special configurations of small vortices which produce even smaller vortices in a scale invariant way, which shrink to zero size with negligible energy in finite time. This is heuristically unlikely based on our current turbulence models, but it might be possible in special initial conditions. The existence (or nonexistence) of shrinking size blowing-up velocity solutions would have little bearing on practical simulations of fluid flow, where the difficulties come from the turbulent scales, not the microscopic blow ups, if they even exist.
The linear approximation for momentum flow in the Navier Stokes equation is only for regions where the velocity field is locally linear, for small enough regions where the velocity is described by its value and its first derivative. The reason to believe this is both experimental and theoretical.
If you place a sheet of water between two plates to make a thin lubrication layer, and slide the two layers against each other, the flow in the middle region stays laminar as long as the layer is thin. The profile of the velocity is linear, and the momentum flow from one plate to the other is the friction force you measure between the plates. This friction force is linear in the plate velocity for all realistic velocity gradients acheivable in an every-day fluid.
Since the plate friction is proportional to the velocity, the flow of momentum (the internal stress) is to counter the local velocity gradient, and it is proportional to the velocity gradient. This is just saying that velocity moves down a velocity gradient, or that velocity diffuses (component by component). Diffusion is a smoothing process, it takes functions with sharp corners and local blowups to smoother functions. In k-space, it shrinks high-k modes by a factor of $exp(-Dk^2)$, so that unless the k-modes are exponentially growing to begin with, they end up smooth.
If there were blow-ups in Navier Stokes, then inside the very high velocity microscopic blow-up region you would expect that the next order term would become important, so that some higher derivative operator would kick in to smooth out the blow up. It is likely that the ordinary viscosity term is enough to smooth out the blow ups by itself.
The approximate velocity-squared forces for fast moving objects are caused by complicated turbulent flow in the bulk fluid. The reason the force goes as the velocity squared is that if you move twice as fast you are displacing twice as much fluid to twice as fast a speed. In the linear Stokes regime, you are dragging a big clump of fluid with you as you move, with a power-law decay of the velocity away from your position, and this, plus the fundamental linear momentum flow, make a friction proportional to velocity. But at high speeds, the moving body leaves behind a turbulent disturbed wake which is comparable to its geometric cross-section.
You can view the mismatch between the growth rate of the linear-in-velocity Stokes friction and the quadratic-in-velocity inertial friction as the fundamental reason there is turbulence in the first place. The linear flow is not very efficient at getting momentum from one place to another in the fluid, so that some small advection speeds up the momentum flow from the body to the bulk fluid, and leads to more advection, which amplifies until the steady state turbulent profile is set up. In the turbulent profile, the velocity gradients are much larger than in the laminar flow, so that the momentum removed from the body is much larger, and grows as the velocity squared times the cross section.
But this phenomenological law does not change the fact that the momentum flow locally in the fluid is still proportional to the velocity gradient.
The theoretical reason to expect a force linear in the velocity is a Taylor expansion. The friction is zero at zero velocity, and grows from there. You can in principle compute the linear viscosity term in statistical mechanics using a Kubo-type formula, and the length scale is the mean-free path, which is the atomic scale for a fluid like water. This means that each successive gradient term is suppressed by the ratio of the scale to the atomic scale, so the next order Taylor coefficient, the quadratic in velocity force, will kick in when the velocity gradient is comparable to 1/mean-free-time (the mean free path over the mean thermal velocity), which is smaller than typical macroscopic scales by a lot, so this term is theoretically negligible unless you have appreciable velocity gradients at the mean-free-path scale. For air, the mean free path is many dozens of time larger than the atomic scale, and the continuum approximation breaks down sooner.
Comment: I understood that the question is about the domain of linear shear (laminar flow) and the experimental "foundations" of it. This combination of strange "theorist"-like asking and and lack of most simple knowlege on experimental physics seems to be common today.
Comment: @Georg: The question was asking to reconcile the known v^2 friction at high speeds with the assumption that the friction is linear in the laminar (or small scale) regime. The high speed v^2 friction is not a nonlinearity in the fundamental description, but a consequence of turbulence.
Comment: This is the core question : ""are there critical discussions about the correctness of the linear assumption in view of experiments?""
Comment: @Georg: You can give order of magnitude estimates to quadratic-in-velocity corrections, and these are suppressed by the smallness of the atomic scale. Gradient expansions are macroscopically correct, and to get them to be wrong you need complex polymers, or large mean free path, or some other macroscopically relevant complication.


How can I determine if a pump bottle's nozzle can be locked?

Question: A friend in Japan handed me the Pharma-Act Japanese hair conditioner bottle beneath by Kumano Comestics. I don't know a word of Japanese. Thus please see the question in the title.
Answer: An interesting variant is when you have a bottle that is already in use and want to find out if its nozzle can be locked (because, say, you forgot, or somebody else put the bottle to first use).
In this case, unscrew the whole cap, point the nozzle back into the bottle, press down on the pump mechanism and try to lock the nozzle by turning it right. If it stays down, it is obviously lockable.
The advantage of this hack is, you did not spill and waste any content of the bottle, as you poured it back into its own bottle. The same technique is useful when packing a pump bottle for traveling.
Answer: If the pump is completely down when you buy it, and you need to 'unscrew' the nozzle prior to first use, it has a screw thread and should be lockable (by doing the reverse).


How do I run two different conky sessions at the same time?

Question: I want to run Conky lua (link to conky lua on Gnome-look.org) and Conky: Metro Clock (link to conky: metro clock on devianart.com) at the same time. I want to have them both on the start up applications. I know how to install them individualy, but I am having a rough time finding out how to get them simultaneously. I'm running Ubuntu 12.04, unity desktop environment.
Answer: Conky without parameters looks for a file named ".conkyrc" in your home folder. To use another config file you have to use the "-c" or "--config" parameter. This way you can start conky twice or how many times you want to, with each conky session using another config-file.
The included .sh-file in the archive on deviantart starts conky without parameters, so ".conkyrc" in your home-folder will be used. => The first session
If you put the "conkyrc"-file from the archive on Gnome-look in your home-folder, you can use the command "conky -c ~/conkyrc". => The second session.
Both instances of Conky should be running, independently from each other, each with its own config-file.
Comment: Where do I find the .sh file or its equivalent? Kubuntu 19.10. Maybe it works differently now?
Answer: Simply add a second conky command for running a second instance of conky in the .sh file provided in the package.
That is just add another conky -c ~/.conkyrc under the command wrritten in .sh file


Should I replace the Timing belt in 2004 Kia

Question: I own a 2004 Kia Spectra.  It only has 47,000 miles on it (I don't drive much)  When I had the transmission fluid changed they said the 3 outer belts needed to be replaced due to dry rot. I am in Texas so guess this is possible.  When I called to make the appt to take it in, the guy said I needed 'for maintenance' to have the timing belt replaced, as well as coolent, spark plugs and the three belts.  Thus far I've not had any problems with my car and do regular oil changes, etc.  What is your opinion please?  I have friends that are saying there is No need.  I'm an elderly female and don't want to be taken advantage of.
Thank you in advance.
Answer: Even if the timing belt is "probably" okay, you have to weight the costs and benefits of not replacing it. From what I can tell, this car has an interference engine, meaning if the timing belt breaks, the cylinders will smash into the valves. As long as they don't break off and also tear up the cylinders/combustion chamber, this is a difficult but doable job for someone with car repair experience to do themselves, but if you need to take it to a professional to repair, you're looking at a repair bill possibly exceeding the value of the car.
As such, if there's any good indication the timing belt needs replacement, I'd just do it. If the shop is asking too much, shop around for a cheaper price or get a book and do it yourself (admittedly it sounds like this option is not for the OP) -- but remember, if you mess up the timing when installing the new one, then in a best-case you're just going to get horrible engine performance and smoking, and in a worst case you'll smash the valves just like if the timing belt broke.
Comment: Thank you for taking the time to respond. I'd rather be safe than sorry.
Answer: Suggested maintenance on a timing belt is 60k miles.
Comment: You need to consider the age of the belt too. Suggested maintenance is either by mileage or by time. Timing belts need to be replaced at least every 4 years on a low mileage vehicle.
Comment: Paul, you should be aware that this isn't a standard open-ended discussion forum.  Also, while you are correct that 60K miles is the recommended distance for replacement, there is also a recommended replacement interval of 48 months.


How to tell if specific twisted pair cable is of good quality?

Question: How to decide if wiring in office / server room qualifies for replacement ?
Comment: There are a number of ways. For a sysadmin, you usually just get a cable tester, check the link. Other than that you should show what grade of cable to use in different applications. As a cabler, you may have some signal analysis tools to measure attenuation, signal/noise ratio etc.
Answer: Qualifies for replacement? You mean you have an old or inherited installation and you want to know if it's worth ripping out and replacing everything?
If the cables are fraying, splayed, jacket worn, if the blue cables are white (stretched) in places, if examining the cables entails wiping off rat or bird crap...chances are it needs to be replaced.
If the cable isn't at least cat5, it probably should be replaced.
If the network is running slow and it's not because you're maxing out your available bandwidth or it's a problem at the switches/routers, you should replace it.
If you have drops that aren't used or taped over because it's a quirky port or sometimes doesn't work or just plain doesn't work, you should re-run it.
If everything is running satisfactory with no "quirky" behavior, unexplained network drops, unexplainable speed quirks, I'd be inclined to leave it.
Get a tester and test every connection for proper speed and termination, using a quality tester. Beg or borrow one if you can't get one (although you should have one in your available tool kit.)
It also depends on the size of your installation. Are you talking about rewiring a building, or a small business? Are you a candidate for getting rid of your wiring or running just a few new lines and replacing others with wireless? Are you already planning on replacing your hubs with actual switches or managed switches, so that leaving ancient wiring in place would be kind of silly for the money you're pouring into the new equipment you're purchasing?
So there's a number of factors. If things are running well and there aren't any speed issues that's a reason to leave some things as they are, or re-run select cables. If you're seeing a lot of gremlins and your tester is showing a shaking head saying, "Uh-oh...sumthin' ain't right here..." that's a good indication you may need to start running wires. Make sure you do it properly with the correct plenum cabling or hire someone to come in and do it for you with cat6 or 5e, either of which should last awhile if properly done, and make sure everything is correctly labeled and tested. No corner cutting, no "I'll fix this later." Do it right so the next guy isn't having to ask the same questions you are, and make some maps of how things are run and laid out.
Answer: Get a network cable tester such as a Fluke, test your cables.
Answer: Well, there are types of cable for one... Cat3, Cat5, Cat5e, Cat6. To make the rating, it must be of a certain quality. Cat5e is typically good enough for most applications. There is also stranded, and solid copper as well.
Past that the only real way to tell is with one of those nice cable testers (not a continuity tester, a real one). Some NIC cards in some servers have the capability of testing a cable I believe, maybe Broadcom? Perhaps tell us a little bit more of what you're after here.
Comment: +1 for "Cat5e is typically good enough"


What are the criteria to decide whether a photo is "technically correct" or not?

Question: Here and there, I hear people saying a photo is "technically correct". How can I tell whether a photo is technically correct or not?
Comment: Remember that for any given image, the criteria listed below might be very important, or might not apply at all.  For example, this photo http://farm1.static.flickr.com/18/70458366_7a26052091.jpg is blurry, front focussed, has fairly large distortion to its perspective from the angle, and lacks any significant horizontal or vertical reference.  It's still technically proficient in many other ways, when combined with its subjective qualities, more than enough to still be a successful photo.
Answer: A technically correct photo should:

be sharp rather than blurry

be focused properly rather than on some random AF point
be properly exposed
have correct color balance
not have too much noise
Comment: A well-chosen subject for the notion of a technically correct photo! :)
Comment: More people should be posting visual examples like these. +1
Comment: Fantastic example.. :)
Comment: The focus criterium could be extended as "much more important, SOMETHING should be in good focus" ;)
Answer: For start, one should be aware that technical correctness is no substitute for artistic vision. Here are some technical criteria in no particular order:

The exposure is correct, shadows are not lost, highlights are not clipped
The parts that need to be in focus, are in focus
There is no motion blur (caused by camera shake)
The photo does not have a color cast (probably because of incorrect white balance)
The colors are correct (what you usually see, not oversaturated or changed)
The noise/grain is not dominant
There are no sensor dust spots
There are no scratches, dust spots, interference patterns in case of scanned images
There are no obvious digital artefacts (sharpening halos, banding, compression artefacts)
Post-processing is not too obvious (think of overcooked HDR)
There is no obvious distortion, chromatic aberration, lens flare, vignetting
There is enough depth of field (important parts of your subject are not out of focus)
There is enough contrast (the photo is not flat nor overdone)
Orientation is correct, vertical objects are vertical and not leaning because of bad shot angle, horizontal objects are horizontal (usually horizon)
Skin tones are correct (in case of portraits/people shots)
The retouching marks/feel are not noticeable (think of visible cut edges in cases where different parts of image are from different photos or "Playboy skin" where all the detail is lost)

Any of these can be part of your concept or meant to create the mood, then they can be discarded.
Whether you're following this or not depends largely on the purpose - standards are higher when you sell your images or large prints, enter contests, etc and lower when you only do them for family album.
Comment: +1 for the artistic vision statement. After all, technically correct isn't the same as saying a photo 'says something' to the viewer. In fact, in some ways, when somebody comments that a photo is technically correct, they're pretty much saying it didn't touch them.
Comment: Might want to add something about appropriate framing (to expand on the bit about orientation), something about appropriate lighting, and something about posing.  Might also think about amending the "colors are correct" point to allow for black+white, like "colors, unless using black+white, are correct ..."
Comment: Perfectly said Karel.
Comment: @thomasrutter - feel free to edit my answer :)
Answer: The essentials of a "Technically Correct" image would be:

Sharp
Correct Exposure
Correct Focus (Auto-focus should take care of this)

For the most part, if you use good equipment in auto mode and avoid things like camera shake, your camera will take care of the technically correct part. As long as you know how to avoid the big issues, you can take a picture that is technically correct, the hard part is taking one that tells a story.


Looking for dream sequence references

Question: Hello I am in early pre prod for a fantasy type film where a girl jumps from reality to dream world, the dream world is a "golden desert" type, I was just wondering what films I could look at that have dream sequences with interesting uses of sound?
Answer: I'd strongly recommend looking into Twin Peaks (both the show and movie...whatever you can get your hands on). The film heavily revolves around a series of dream sequences which have outstanding sound design (no surprise since it is a lynch film!).
Trainspotting has a much darker dream sequence, when the protagonist (Ewen Mcgregor) is going through withdrawal at his parents home. Different, but still great sound design.
Cheers!
Comment: Great answer.  The sound design (among EVERYTHING else) in Twin Peaks was amazing.
Answer: The Cell
http://www.youtube.com/watch?v=-eCD5Nuoyi4
Answer: Some of these are Dream Sequences, some are Drug Sequences and some are Flashback/Forward scenes. They're all pretty interchangeable when it comes to techniques to portray a alternate sort of reality in my opinion.
Inception (probably painfully obvious), The Imaginarium of Dr. Parnassus, The Matrix, Memento, Dark City, Jacobs Ladder, Enter The Void.
That's all off the top of my head that no one else has mentioned yet. If any more come to mind I'll modify my post.
Answer: The Conversation has a fantastic dream sequence. It is also a great film for sound!
Answer: Ink is a cool one to! Its about a group that protects you in your dreams.
And of course  the guy that tries to kill you in your dreams.
Answer: I can think of a few films where there are differences in reality between scenes:
Scott Pilgrim has some interesting dream bits,
Vanilla Sky would be a really good one as thats all about dreams.
Answer: The most recent Nightmare on Elmstreet film.

Looking for dream sequence references


Centralized Instant Messaging Logging

Question: Is there a centralized place/software where I can store all my chat logs?  I switch computers a lot, and want one place to sign into to do all my chatting.  I currently love Gmail because it can log all my AIM conversations as well, but I also chat through Facebook and other IM services and want them logged.  I also used Meebo for a while, but there's no way I can export and save these logs.
Answer: Try using Trillian. It will let you login to mutliple chat clients, including g-chat, aim, facebook, twitter, etc. and gives you the option to log your chats. Not sure if the logs are exportable, but they are searchable.
Answer: Most multi-protocol desktop IM clients such as Adium, Pidgin, Trillian, Digsby, etc. keep local logs for all of your accounts, and most have an interface to search those logs. While this works well when you do most of your chatting from one computer, it's not server-based, and is not a perfect solution if you move computers a lot.
Facebook does not keep chat logs, or rather, does not make them available to you, so really the only way to log FB chats is with a 3rd party client.
Comment: @warren hmm, I got one too... maybe someone was displeased that the answers on an ancient and now-closed question didn't provide a contemporary solution? ¯\\_(ツ)_/¯
Comment: You could point the logging directory to a cloud storage service like Box
Comment: @warren thank you for making that insightful comment 7+ years later on an answer about messaging clients that almost nobody even uses anymore.
Comment: you're welcome. I had this question pop into my feed after getting a downvote on my answer overnight.
Answer: I use Adium, which will do the job, too (on Mac OS X).
For Windows, I use Pidgin.
They're both all-in-one chat applications with logging functionality.
Answer: I recently starting using http://imo.im
It is web-based, multi-protocol chat handler that does video, audio, and text chatting via: MSN, Yahoo, AIM, gTalk, MySpace, Skype, and Facebook.
It save the chat history for all the clients in a easy to use interface.
Answer: I have been very happy with Digsby, it archives conversations that could be saved or exported elsewhere.
Answer: Try bitlbee if you have a ssh login with an irc client. It enables you to chat with your friends on MSN, ICQ, wherever and hanging around on IRC.
Also, you can use it locally with your graphical IRC client which proves the flexibility of this smart program, allowing you to keep your logs in a single place.

Centralized Instant Messaging Logging


Can I go to Israel with a Jordanian visa on my UK passport?

Question: I travelled to Jordan on business with my UK passport and it was stamped on arrival. Will I have challenges later going to Israel or if I can go to Israel with the Jordanian stamp, will I have challenges returning to Jordan at a later date?
Comment: If you decide it's too much hassle, this is a legitimate reason to get a second passport.
Comment: @Jigga At least at Ben Gurion you get entry slips nowadays, rather than stamps
Comment: i hear the Israeli officials can stamp a piece of paper instead of stamping your passport on request too..
Comment: Yes, you can enter Israel. They will look carefully through your possessions and passport. You'll get queried on each and every Arab country you've visited and what you did there. Be prepared to see a lot of security at the border, you'll be in the Palestinian territories once through the border at Allenby bridge.
Answer: Yes, you can enter Israel or Jordan after visiting the other without problems.  The two signed a peace treaty in 1994 and have permitted travel to each other ever since.
Of course, you may be questioned in Israel about what you were doing in Jordan, but if you went there for legitimate business reasons, you'll be fine.
Answer: Yes, you can enter Israel, but be prepared for the possibility of getting grilled hardcore at Israeli immigration for having been to Jordan (they can be even worse than the US at times - I experienced it even though I am Swedish and have never been to an Arab country).
Not saying it will happen, but it could. Bring as much documentation as possible proving the purpose of your Jordan visit
Entering Jordan after Israel is 100% fine, and not just due to the fact that Israel doesn't stamp passports anymore (at Ben Gurion at least).
Comment: @pnuts: the suggestion to bring documentation, and the fact that Israel doesn't stamp passports?
Answer: You can enter Israel directly from Jordan. There are three open border crossings and I myself crossed into Eilat, Israel from Aqaba, Jordan at the Yitzhak Rabin crossing between the two sister cities. Obviously, being a European, I had a Jordanian visa, entry and exit stamp in my passport. If the direct crossing is possible, entering Israel at a later date is possible, too.
When having entered Jordan, about three-quarters of the group I was with had been to Israel three years prior. Nobody had any issues entering Jordan whatsoever. Furthermore, our guide — a professor of theology at the university of Innsbruck — semi-regularly visits both these countries on a single passport.
We (i.e. mainly the guide, but a little cross-checking was performed) were asked quite a few questions about what we were doing in Jordan. I'll imagine, if you turn up at Tel Aviv airport immigration at a later date than just after exiting Jordan, they will have more. It'll probably raise a questioning flag for them but not necessarily a red one.
Israel does not stamp passports a lot anymore. Tel Aviv airport seems to be fully equipped for only supplying you pieces of paper instead of passport stamps. When I crossed the border into Israel at Eilat, I got a stamp but I have since read here that others (who did the trip at an earlier date) received a paper instead. I'm pretty sure you can specifically request a paper rather than a stamp at each port of entry.


Can juices fulfill requirement of water ?

Question: As our body needs more water and I drink more juices than water. So, I want to ask that can juices fulfill requirement of water ?  ?
Comment: what do you men by "powered juice",
the best juice is the one that you make at home with your juicer, and usually it's without any added water
Answer: The problem with getting your daily requirement of water solely from juices, is that most juices have significant amounts of sugar in them. Let's say your favorite juice is orange juice. Orange juice is about 88.3% water so in order to get your recommended 64 fl. oz. (1.892 L) of water you'd have to consume 72 fl. oz. (2.129 L) of juice. Orange juice has 112 Calories in 1 cup (248 g), so you'd have to drink nine cups which would be 1,008 Calories. Add that to whatever food you eat and you'd have a whole helluva lot of calories. Unless you led a very active live style, you'd gain a lot of weight. But as far as water goes, your body really doesn't care where the H2O comes from. If you got it from juice or plain water, you'd more or less remain just as hydrated. (I say "more or less" because variations in sodium content of one juice to another would affect how much water your body retains.)Edit: Contents of orange juice came from this Wikipedia article.Edit 2: For those who consider Wikipedia an unreliable source: this site also has similar data.
Comment: Wikipedia is not considered a reliable source, but a quick google search should give you some good results.
Comment: Reference was added. I did not include this in my original answer, as I thought references concerning the contents of orange juice were pretty much readily available.
Comment: @Narusan, what!? I see participants in many Stack Exchange communities using Wikipedia articles as reference sources. It's not as if the contents of orange juice are a big esoteric secret.
Comment: @Narusan, nope.
Comment: Actually, there has been a [Meta Discussion](https://health.meta.stackexchange.com/questions/184/is-wikipedia-a-reliable-source) about Wikipedia, and it concluded that Wikipedia is not suitable to support the main claims of your answer but it's okay to link to a Wikipedia article so that other users understand terminology you are using. I think that this supports my position. The answers there also say that if there is **any** source for the claim, cite it. Only if there is no other source, then you can turn to Wikipedia.
Comment: I used Wikipedia to cite nutritional information, not medical. The two things are different.


Scrum job title

Question: I know that in Scrum there is only three role, Product Owner, Scrum Master and Development Team. As a sign of change we must be important, not only understand these roles, but also start to change the job title to an homogenic form as Developer, Scrum Master and Product Owner. Maybe this is pointless, but we try to set an identity to the team, currently there is a mixture of titles and departments for the members of the same Development Team and we wan to change it. So how does an organization work this thing?  A restructure of this is part of implement scrum or this don't care?
 Thanks for your advise
Comment: Polling questions (ie. "how does your organization...") are off-topic on PMSE. Consider rewording to focus the question on a problem you are having.
Comment: I believe your questions is predicated on an assumption that may not be true. I see two questions here? 1) how should we change our job titles and 2) does scrum, with its limited roles, imply that there should be equally limited job titles.
Comment: I don't understand the question - what is it you want to know? If you're merely asking for job titles in other organizations, how will you use that information?  How would someone else use that information? PM:SE is for practical problem in project management. Perhaps I'm missing something but I'm not sure how knowing the job titles used at another organization would assist me to close projects or generate value in my organization.
Comment: I lookin for information if part of impleneting Scrum is normal take in count this kind of trivial things. i agree wich the title not is important, because it dont bring any value per se. But motivation and commintment some time start with this minimal changes, because it gave authority to the members of the organization. This is important for that reason, for the motivation it can bring, there are other way to increase the motiviantion, my question is about if someone already try this or how generally management the job title inside scrum
Answer: Roles and job titles are distinct entities. You don't need to change job titles at all, if the jobs have not changed.
For example, it's perfectly normal to have job titles of "Java Developer" or "C++ Developer". It's also normal to have pay grade and experience titles like "Senior Java Developer". This is what you do in the company.
For Scrum, no matter what title you have, both a "Junior Java Developer" and a "Senior C++ developer" are a "Developer". Their seniority has no influence on how they act on the team in regard to the Scrum process. A Senior's estimation is not worth more than a Junior's estimation. A Junior's opinion on the retrospective is not worth less than a Senior's.
So job titles can stay as they were, as long as all adhere to Scrum principles and those titles match what they actually do. Obviously if you have a PM that now is the Scrum Master, this person may want to get his title changed. But because it's a different job, not because Scrum dictates job titles.
Comment: Scrum definitely changes the way people work together and you should make sure this actually happens. No more "I'm more senior than you" or "But I'm the architect". But that's behavior, you can leave the job titles as they are as long as the behavior changes.
Comment: thanks for your questions, actually i think this is a trivial thing, but in some blogs i has read than is change the terminology is important as sign of we are changing the way we work, i totally agree in the fact of job title is not important, so as part of scrum it not have sence reorganize this thing?
Comment: Small point, you mean you're a "Development Team member". Scrum members can (and typically do) come from more disciplines than software development.

Scrum job title


Higher resolution on Macbook air - simulated?

Question: When I connect my MBA to Apple TV it adjusts the MBA resolution to something else which makes everything smaller but gives me more screen real-estate. I don't want to do this often but sometimes its nice when an app window is large and not scalable.
Is there a way to do this without connecting to ATV?
Preferably a script to run that makes the adjustment and then one to set it back?
Thanks,
Answer: SwitchResX provides functionality for this.
When you have installed it, open it from System Preferences, go to the Color LCD tab at the side, and go to the second tab (I believe). From there you should be able to set 1920x1080 without being connected to your TV.


Urgent Help Needed - Formatting Content on Site Pages

Question: I'm building new site pages on a team site, in 2010.  I'm new to SharePoint and am having great difficulty formatting the content on the pages. I've tried various techniques suggested in several of my user guides, but no luck. Are there any resources that provide a site page as an example, with details on page layout or an editable template?
Answer: First, and I don't mean this to be rude but from experience -- people will overlook anything marked urgent. Especially if it's followed by something trivial like formatting content.
Could you please describe what's going on in more detail? We don't have your user guides at hand, so it's tough for us to paint a picture of what steps you're taking.
What it sounds like to me, is that you need to place a Rich Text Content Editor web part on your page and you should be able to access a ribbon similar to Word 2010 that will let you pick your styles. But I'm assuming here. I don't know what you're referring to by "content". In SharePoint "content" can range from items from a list, to a site collection, to a paragraph on a page. Simply put, we need more information before we can help you out.


If Santa can time travel, why does he need elves to make toys?

Question: Santa is a time traveller. Notice that when going back in time, he steals the presents which he delivers going forward. So, what do the elves do?
Comment: I think the elves must ensure that all burger alarms are deactivated and owners should be sleeping, also to clean up the crime scene afterward...
Answer: Just because you can, doesn't mean you do.
Making, delivering, and stealing a lot of toys is hard work. Even if you have (theoretically) infinite time, you may still want some help. In addition, some things you can't do on your own, or you may not have the skill or expertise to make or pull off.
In the case of the time-travelling santa, these elves could have the jobs of:

Literally and Figuratively Guiding Santas (all of them): The Fat Man must have exceptional memory, or he needs help to remember which presents go where, and more importantly, where the santa/anti-santas pairs have been. Also, the psychological stress of being both Santa, anti-Santa, and the Santa for next year will likely require therapy. Perhaps these elves are really experts in psychology as well. They keep Santa working as he should. The Christmas Cycle Must Continue!
Hide Santa's Powers: They maintain the image of a single santa, trying to hide his amazing powers. They claim to make all of his toys, only so that people don't wizen up to santa's real method. After all, just the ability to visit every house (undetected) in the world and take/leave things would like to be abused by very many, usually naughty people. This does not include the time shenanigans people could do with such knowledge. We shouldn't also ignore the dangers of simply being such a well-known celebrity. (Just because santa can travel forward/backward in time and teleport/phase through things doesn't mean he is immune to surprises!)
Combining, gathering, and sorting intel during the year: Yes, I know Santa knows when you're sleeping, he knows when you're awake, he knows when you've been bad or good. (So be good for goodness' sake!) However, there are a lot of people and usually one santa. Even if santa knows this through supernatural means, it would only be natural that he needs support to maintain the lists, remembering who did what, what they want, and so forth. It could be that these elves store and deliver information for santa as he needs it. (Alternatively, the elves are how he knows so much about so many people, putting government intelligence agencies to shame.)
Comment: I think no. 1 is the biggest point here! The logistics of that many Santae (yeah, I used *that* plural) will be brutal. You'll need elves to organise the elves that are doing the organising of Santae organising elves. And then the same again, but backwards!
Answer: Nothing better than a good coverup the elves are actually intelligent bioengineers we believe there is "one" Santa and there is however clones my friends clones. Big brother is always watching and guess what so is your own personal Santa! Hmmm makes me wonder if the santas in shopping malls are on strike???


How persistent are "continue this discussion in chat" rooms?

Question: Occasionally a colloquy on a post will prompt a commenter to move the discussion to chat, as recently happened to me. The chat room thereby created: is it persistent, or does it expire after some time?
Answer: Rooms with any sort of meaningful content exist forever.
As outlined in the chat FAQ, "A room is considered worth retaining if it has more than 15 messages by at least 2 users." Any such room is available for everyone (including non site members) to find on this list of RPG.SE chat rooms, for as long as the Stack exists. Also they're seeded to Google.
The original Question/Answer page doesn't automatically get a link to that chat room, because anything productive to improving the main site content should be applied via edits to the questions and answers. The chat is there to reduce the clutter so questions and answers are easily found, not to add to the reading someone needs to do to get a good answer.
Comment: ahh... thanks. I'd been poking around in the comments portion of the site's FAQ, not the chat FAQ.
Comment: As an aside, is the *discussion moved to chat* link in the comment the sole indicator that a chat exists about a question or answer?
Comment: @HeyICanChan Added a line about how they can be found and why they aren't automagically linked on the main site.


Who is elder in Charlie and Alan?

Question: In the TV series Two & a Half Men, Charlie and Alan are brothers, but there is no indication as to who is the elder. Even Wikipedia doesn't say anything about this.
I have watched only season 1.
So may be it is revealed in a later season.
Who is the elder?
Comment: Don't remember exact quote now. But charlie is elder and it has been confirmed multiple time in various dialogues in show
Answer: Quoting from Two and Half Men Wiki, in Season 7, episode 11 - "Warning, It's Dirty"

Evelyn reminds Charlie how forty years earlier, he had asked for a
  little brother for Christmas, which in contrast puts in doubt whether
  Alan was unplanned or not.

IMO, this proves that Charlie was the elder brother.
Answer: Charlie is the older brother. There are jokes in a number of episodes that establish this.Also, the Two and a Half Men wiki does confirm this:Dr. Alan Jerome "Al" Harper, DC, is a character from the CBS sitcom Two and a Half Men. Portrayed by Jon Cryer, Alan is the hapless and helpless biological father of Jake Harper, roommate and best friend of Walden Schmidt and the surviving younger brother of Charlie Harper.The wiki lists the years of birth: Charlie: 1967, Alan: 1969.
Answer: According to the following quote from s08e08, Alan is 2 years younger than Charlie (the parts in italic I added for context, those are not part of the actual transcript):

Alan says: You're gonna leave me alone with her?
Charlie replies: Hey, I was alone with her for two years before you were born.

Source: transcript by springfieldspringfield.
Answer: In season 11, episode 13("Bite Me, Supreme Court"), during Evelyn's marriage with Matt, Evelyn says that this will be her first marriage without his sons. Someone says, what about her first ever marriage to which she replies, Charlie was in her stomach during the first marriage. Hence Charlie is older.
Answer: In episode 3 season 1 Charlie reffers to Alan as my baby brother looking at the cab driver while travelling in the cab after Alan fails to confront his wife and she chases him off with a golf club.
Answer: Season 4 episode 8: Alan at the psychiatrist says:

"my brother is older with 2 years"
Comment: can you add some source to this?

Who is elder in Charlie and Alan?


Restore partition from merged partition

Question: I accidently merged my BOOTCAMP and Mac OSX Partition via diskutil.
Is there any way to recover my data from the Bootcamp partition?
Answer: You'd have to do some very serious data forensics on the partition and even then, you're not guaranteed to get all your data back, as some of it will likely have been overwritten already. If it's important, you're probably best off paying for a professional data recovery company to get the data back. If it's not important, just reinstall bootcamp from scratch.
You could try to do this yourself, though you'd risk making things worse, so don't do this unless you don't really care about losing all the data. Basically, you would need to re-shrink your OSX partition to its original size, create the bootcamp partition in the exact same position and size (e.g. "80GB" is not good enough - this needs to be precise down to the sector) as before without initialising/formatting it (just adding the partition table entry - NOT via Disk Utility). Then, you can run an NTFS data rescue utility on this partition and see what it finds. You probably don't know the exact size and start of the old bootcamp partition, so you either need to see if it's recorded in any log or backed up files, otherwise you'll have to scan the drive for the NTFS header, if it's still there. Again, there are tools that can automate this.
I can't overstate how easy this is to mess up though, so unless you're prepared to lose the whole lot forever, let a professional handle it.

Restore partition from merged partition


Could any life from Earth survive in Enceladus' oceans?

Question: Enceladus, the sixth-largest moon of Saturn, is probably unique in being the only body in our solar system besides Earth (and possibly Europa) to have significant amounts of liquid water. Most of this water is contained in vast oceans underneath a thick layer of ice on the surface, and is kept warm by the planet's geologic activity. As such, it has been proposed as a potential place that could harbor life. Irregardless of that debate (of whether there is life on Enceladus), are there any Earth-based lifeforms known that could hypothetically survive in Enceladus's oceans?
Comment: @JackBNimble: Europa is ***much larger*** than Enceladus (Europa's mean radius is about 1560 km, where Enceladus' is 252 km.)
Comment: Although smaller, a related ice moon with suspected water is [Europa](http://en.wikipedia.org/wiki/Europa_(moon)).
Answer: Probably the closest thing to that environment on Earth would be Lake Vostok, which is 4 kilometers below the antarctic surface. Very little, if any, light gets down there, and it's really really cold, but still liquid.
Scientists recently found bacteria living there, so it stands to reason that bacteria would survive on Enceladus, but the really amazing thing is that the bacteria are ones that go with fish and crustaceans and mollusks, which would indicate that there are actually molluscs, crustaceans and even fish living in Lake Vostok!
From the BBC article:

A large number of bacterial sequences, reports the team, were from
  "animal commensals, mutualists and pathogens… including those
  associated with annelids, sea anemones, brachiopods, tardigrades and
  fish."

Also discovered: eukaryotes and archaea.
Comment: It may be even more hospitable than that when you factor in the presence of volcanic vents, which are believed to be present on Enceladus: http://www.astronomy.com/news/2017/04/enceladus-sea-floor-has-hydrothermal-vents
Answer: Depending on exactly what temperature the water on Enceladus is, the answer is yes. Bacteria are an obvious choice. But there is a higher lifeform which can almost certainly survive:
Tardigrades (Water bears) can survive for days at −200 °C - longer at higher temperatures, and can cope with extremely acid or alkaline environments. They can even survive in outer space.
The probable water temperature on Enceladus is much higher than this, and while there will be dissolved salts, this should not be a problem to the hardy tardigrade.
Comment: One must also consider food sources. Given the unlikelihood of photosynthesis being practical, I would have *guessed* that something like hydrogen-sulfide 'eating' bacteria to be a better bet.
Comment: Which is why I mentioned them first:-)
Answer: A recent and open access paper in Nature Biological methane production under putative Enceladus-like conditions describes an in-depth analysis of sources of energy in Enceladus' ocean and suggests that molecular hydrogen as H2 may be present and "eatable" by some organisms currently doing similar things here on Earth, e.g. Methanogenic forms of the Archaea domain single-celled microorganisms. From the introduction:

The most prominent potential source of H2 in Enceladus' interior may be oxidation of native and ferrous iron in the course of serpentinization of olivine in the chondritic core. Olivine hydrolysis at low temperatures is a key process for sustaining chemolithoautotrophic life on Earth9 and if H2 is produced in significant amounts on Enceladus, then it could also serve as a substrate for biological CH4 production.

You can read more about serpenization in the answers to the question What is serpentinization, in the context of disappearance of surface water on Mars?
Abstract:

The detection of silica-rich dust particles, as an indication for ongoing hydrothermal activity, and the presence of water and organic molecules in the plume of Enceladus, have made Saturn's icy moon a hot spot in the search for potential extraterrestrial life. Methanogenic archaea are among the organisms that could potentially thrive under the predicted conditions on Enceladus, considering that both molecular hydrogen (H2) and methane (CH4) have been detected in the plume. Here we show that a methanogenic archaeon, Methanothermococcus okinawensis, can produce CH4 under physicochemical conditions extrapolated for Enceladus. Up to 72% carbon dioxide to CH4 conversion is reached at 50 bar in the presence of potential inhibitors. Furthermore, kinetic and thermodynamic computations of low-temperature serpentinization indicate that there may be sufficient H2 gas production to serve as a substrate for CH4 production on Enceladus. We conclude that some of the CH4 detected in the plume of Enceladus might, in principle, be produced by methanogens.
Answer: There actually just was a scishow video about this, where they said, that a specific kind of bacteria can survive in a simulated Enceladus ocean in the lab.


What is the difference between application server and web server?

Question: What is the difference between application server and web server?
Answer: Most of the times these terms Web Server and Application server are used interchangeably.
Following are some of the key differences in features of Web Server and Application Server:

Web Server is designed to serve HTTP Content. App Server can also serve HTTP Content but is not limited to just HTTP. It can be provided other protocol support such as RMI/RPC
Web Server is mostly designed to serve static content, though most Web Servers have plugins to support scripting languages like Perl, PHP, ASP, JSP etc. through which these servers can generate dynamic HTTP content.
Most of the application servers have Web Server as integral part of them, that means App Server can do whatever Web Server is capable of. Additionally App Server have components and features to support Application level services such as Connection Pooling, Object Pooling, Transaction Support, Messaging services etc.
As web servers are well suited for static content and app servers for dynamic content, most of the production environments have web server acting as reverse proxy to app server. That means while servicing a page request, static contents (such as images/Static HTML) are served by web server that interprets the request. Using some kind of filtering technique (mostly extension of requested resource) web server identifies dynamic content request and transparently forwards to app server

Example of such configuration is Apache Tomcat HTTP Server and Oracle (formerly BEA) WebLogic Server. Apache Tomcat HTTP Server is Web Server and Oracle WebLogic is Application Server.
In some cases the servers are tightly integrated such as IIS and .NET Runtime. IIS is web server. When equipped with .NET runtime environment, IIS is capable of providing application services.
Comment: "Apache Tomcat HTTP Server is Web Server and Oracle WebLogic is Application Server."  So first of all, Apache Tomcat and Apache HTTP server are 2 different products.  And that's not really an accurate statement.   Apache Tomcat is an application server.  Sure, it can serve web pages as well, but it's an application server for deploying Java. I realize a lot use the term "web server" loosely.  But it just confuses people.
Comment: Apache Tomcat is not a web server, it is an application server that runs Java servelets. Apache HTTP server is a web server. There is no server called Apache Tomcat HTTP server.
Comment: JBoss(now WildFly) too is a renowned example of Application server :D
Comment: Nice explanation, since we can use application server instead of web server what are the advantages of having a web server and application server both for a single application? And performance wise what is the best option?
Comment: -1 for confusing Apache Tomcat and Apache HTTPD.  https://stackoverflow.com/questions/30632/difference-between-the-apache-http-server-and-apache-tomcat
Comment: Good explanation,... if application server can act as web server, but why these web servers still exist? due to they being light weight to handle static web pages?
Comment: coming from an IIS background, I never knew that app & web servers are two different servers
Comment: Very nice explanation! People use one more term "component web server" which is synonymous to "Web Server" in this post. Static content served by "Component web servers" can be easily spread across CDNs (Content delivery network). Also "Application server" is used interchangeably with "Application web server".
Comment: _application servers have Web Server as integral part of them_ Thanks for that !
Comment: Glassfish, JBoss, Tomcat, Apache Geronimo are some of the Application servers
Answer: This is a detailed answer with some scenarios to clearly understand the difference and similarity, and how both can work in conjunction.
Application Server is a term that sometimes is mixed with a web server. While a web server handles mainly HTTP protocols, the application server deals with several different protocols, including, but not limited, to HTTP.
The Web server's main job is to display the site content and the application server is in charge of the logic, the interaction between the user and the displayed content. The application server is working in conjunction with the web server, where one displays and the other one interacts.
The information traveling back and forth between the server and its client is not restricted to simple display markup, but to interaction between the two.
In most cases, the server creates this interaction through a component API, such as J2EE (Java 2 Platform), EJB (Enterprise JavaBean) and other different application software models.

An example:
The best way to understand the difference between the scenarios where an application server works with the web server versus a scenario where there isn't an application server is through an online store.
Scenario 1: Web server without an application server
you have an online store with only a web server and no application server. The site will provide a display where you can choose a product from. When you submit a query, the site performs a lookup and returns an HTML result back to its client. The web server sends your query directly to the database server (be patient, I will explain this one in our next nugget) and waits for a response. Once received, the web server formulates the response into an HTML file and sends it to your web browser. This back and forth communication between the server and database server happens every time a query is run.
Scenario 2: Web server with an application server
if the query you want to run has already been done previously and no data has changed since then, the server will generate the results without having to send the request to the database server. This allows a real-time query where a second client can access the same info and receive real time, reliable information without sending another duplicate query to the database server. The server basically acts as an intermediate between the database server and the web server. This allows the information pulled to be reusable while in the first scenario, since this info is embedded in a particular and "customized" HTML page, this is not a reusable process. A second client will have to request the info again and receive another HTML embedded page with the info requested -highly inefficient. Not to mention that this type of server is very flexible due to its ability to manage its own resources, including security, transaction processing, messaging and resource pooling.
To support such a variety of complex tasks this server must have a built in redundancy, great processing power and high amount of RAM to handle all the data it's pulling in real time.
Comment: That's not accurate/confusing, even for web applications (i.e. the term application-server applies to none-web apps). Considering web only: A web server includes software (apache, nginx) to handle web requests (http). An application server contains/exposes the application (e.g. php code). They might be the same machine, they might not - for example it'd be considered normal to have nginx on one machine (web server) forwarding requests to php-fpm on a different machine (application server) which doesn't itself have any http access (only exposing the port for php-fpm itself).
Comment: After re-reading the examples given, I don't see any real clarity here - the descriptions relate mostly to caching. What aught to be clear is that a webserver is software, an application is software. if they are deployed on the same machine the machine can be referred to however you like. If they are on different machines it would be normal to refer to the one running the webserver _as_ the webserver, and the one running the app as the appserver. You'd normally split things according to load and load balancing. Overall I find this answer doesn't add anything useful.
Comment: @AD7six My answer is meant to complement the other answers i.e others answers already mean what you have asked its just an extension to that.
Comment: @AD7six A Web server exclusively handles HTTP requests, whereas an application server handles business logic to application programs through any number of protocols including HTTP.
Comment: My point there is the application server **may** handle http requests, it's by no means a requirement. `the application server deals with several different protocols, including, but not limited, to HTTP` <- that says it definitely does handle http requests - that's not accurate.
Comment: Your examples are confusing. Also, your answer doesn't seem to complement the other answers as you have only mentioned that it's a detailed answer with difference and similarity between the web server and application server. AD7six has got a point and explained clearly. Examples konjam edit pannunga.
Comment: When a request comes into the webserver (nginx) then how does it know whether request for a particular file (say page1.php or image1.png) is to be sent to the app server (page1.php) or whether to the directly return the file (image1.png)?
Answer: Both terms are very generic, one containing the other one and vice versa in some cases.

Web server: serves content to the web using http protocol.
Application server: hosts and exposes business logic and processes.

I think that the main point is that the web server exposes everything through the http protocol, while the application server is not restricted to it.
That said, in many scenarios you will find that the web server is being used to create the front-end of the application server, that is, it exposes a set of web pages that allow the user to interact with the business rules found into the application server.
Answer: Web server
Run python -m 'SimpleHTTPServer' and go to http://localhost:8080. What you see is a web server at its workings. The server simply serves files over HTTP stored on your computer. The key point is that all this is done on top of the HTTP protocol. There also exist FTP servers for example which do exactly the same thing (serving stored files) but on top of a different protocol.
Application server
Say we have a tiny application like below (snippet from Flask).
@app.route('/')
def homepage():
    return 'My homepage'

@app.route('/about')
def about():
    return 'My name is John'

The small example program maps the URL / to the function homepage() and the /about to the function about().
To run this code we need an application server (e.g. Gunicorn) - a program or module that can listen for requests from a client and using our code, return something dynamically. In the example we simply return some very bad HTML.
What's the business logic all the other people talk about? Well, since a URL maps to somewhere specifically in our codebase, we are hypothetically showing some logic about how our program works.

Recapping
web server - serves files stored somewhere (most commonly .css, .html, .js). Common web servers are Apache, Nginx or even Python's SimpleHTTPServer.
application server - serves files generated on the fly. Essentially most web servers have some sort of plugins or even come with built-in functionality to do that. There exist also strict application servers like Gunicorn (Python), Unicorn (Ruby), uWSGI (Python), etc.
Notice that you can actually build a web server with the code of the application server. This is done in some cases during development where you do not want to have a gazillion of different servers running on your computer.
Comment: Uff.. Finally, thanks for giving a Python perspective. As language agnostic as this topic may seem, it is not. Someone who has never used EJB will not understand the Java oriented answers clearly.
Comment: @aderchox that would turn Apache into an application server. So not sure what you mean that it's not accurate. Nginx can be extended with Lua, so you can make it even make coffee. Doe that mean that Nginx is also a coffee maker?
Comment: This is the best and most succinct answer. I was wondering whether a web server could be considered a subset of an application server. Right now I'm thinking of it as a web server is like a getter method and an application server is like a factory method (where the URL is a constructor argument :D)
Comment: Thanks. "To run this code we need an application server", could you specify what the application server is for running the flask program?
Comment: This is a near perfect answer
Comment: Not quite accurate I believe. Apache for instance could be set to serve dynamically as well by adding a handler for running Python code(Also fastcgi could be used for not spawning distinct new processes per every request, but that's not relevant here). I believe the most important thing and the most succinct description about an "application server" is the fact that __it has MORE things than just HTTP handling stuff__. Full stop.
Comment: When a request comes to the web server (say 1 request is for web page and another request is for a plain html file, and another request is for an image). Now how does the web server (nginx) know whether to send the request (/pagename) to the app server (gunicorn) or whether to server the request (/image1.png and /file.html) by by-passing the app server?
Comment: @variable a server doesn't "know" anything. It receives a request, and if it can handle it (right permissions, valid URL etc.) then it will. The setup of the whole infrastructure has to be configured as such so that the right server gets the right request. That's where middleware, routing, devops, etc. come in. It's up to you and/or the devops
Comment: I've been reading a lot of documentation and struggled to get a clear picture. This simple code example helped me to connect the dots at last. Thank you! +1
Answer: As Rutesh and jmservera pointed out, the distinction is a fuzzy one.  Historically, they were different, but through the 90's these two previously distinct categories blended features and effectively merged.  At this point is is probably best to imagine that the "App Server" product category is a strict superset of the "web server" category.
Some history.  In early days of the Mosaic browser and hyperlinked content, there evolved this thing called a "web server" that served web page content and images over HTTP. Most of the content was static, and the HTTP 1.0 protocol was just a way to ship files around.  Quickly the "web server" category evolved to include CGI capability - effectively launching a process on each web request to generate dynamic content.  HTTP also matured and the products became more sophisticated, with caching, security, and management features. As the technology matured, we got company-specific Java-based server-side technology from Kiva and NetDynamics, which eventually all merged into JSP.  Microsoft added ASP, I think in 1996, to Windows NT 4.0. The static web server had learned some new tricks, so that it was an effective "app server" for many scenarios.
In a parallel category, the app server had evolved and existed for a long time. companies delivered products for Unix like Tuxedo, TopEnd, Encina that were philosophically derived from Mainframe application management and monitoring environments like IMS and CICS. Microsoft's offering was Microsoft Transaction Server (MTS), which later evolved into COM+. Most of these products specified "closed" product-specific communications protocols to interconnect "fat" clients to servers.  (For Encina, the comms protocol was DCE RPC; for MTS it was DCOM; etc.) In 1995/96, these traditional app server products began to embed basic HTTP communication capability, at first via gateways. And the lines began to blur.
Web servers got more and more mature with respect to handling higher loads, more concurrency, and better features. App servers delivered more and more HTTP-based communication capability.
At this point the line between "app server" and "web server" is a fuzzy one.   But people continue to use the terms differently, as a matter of emphasis. When someone says "web server" you often think HTTP-centric, web UI, oriented apps. When someone says "App server" you may think "heavier loads, enterprise features, transactions and queuing, multi-channel communication (HTTP + more).  But often it is the same product that serves both sets of workload requirements.

WebSphere, IBM's "app server" has its own bundled web server.
WebLogic, another traditional app server, likewise.
Windows, which is Microsoft's App Server (in addition to being its File&Print Server, Media Server, etc.), bundles IIS.
Comment: "new tricks" implies, running server side logic. Scripting logic like ASP or others. The original "web servers" just returned static content, from the filesystem. We've come a long way from that, now.
Comment: Very clear answer. But can you elaborate a little more about the 'new tricks' that allowed a web server to perform as an application server.
Comment: I read about "enterprise features" a lot. Can you please elaborate on that?
Answer: As many have said before, web servers handle HTTP petitions, while application servers handle petitions for distributed components.
So, maybe the easiest way to understand the difference is to compare the two products in regards to programming environment they offer.
Web Server -> Programming Environment
IIS :                             ASP (.NET)
Tomcat :                          Servlet
Jetty :                            Servlet
Apache :                          Php, CGI
Application Servers -> Programming Environment
MTS :                                COM+
WAS :                               EJB
JBoss :                              EJB
WebLogic Application Server :        EJB
The crucial difference is that application servers support some distributed component technology, providing features like remote invocation and distributed transactions, like EJB in Java world or COM+ on Microsoft platform. Http server often support some more simple programming environments, often scripting, like ASP (.NET) in case of Microsoft or Servlet--based, including JSP and many other in case of Java or PHP and CGI in case of Apache.
Other capabilities like load-balancing, clustering, session-failover, connection pooling etc. that used to be in the realm of application servers, are becoming available on web servers as well directly or through some third party products.
Finally, it is worth noting that the picture is further distorted with "lightweight containers" like Spring Framework, that often supplement the purpose of application servers in more simple manner and without the application server infrastructure. And since distribution aspect in applications is moving from distributed component towards service paradigm and SOA architecture, there is less and less space left for traditional application servers.
Comment: Can any of the application servers you listed be used as a http web servers like apache http?
Answer: In short,
The web server is a server that serves static web pages to users via HTTP requests.
The application server is a server that hosts the business logic for a system.
It often hosts both long-running/batch processes and/or interop services not meant for human consumption (REST/JSON services, SOAP, RPC, etc).
Comment: What does the term 'host's the business logic' mean? How is it performed?
Comment: who is bringing all these confusing terms everywhere, with ambiguous definitions everywhere?
Comment: Is the business logic exposed to the client through web services?
Comment: It can be served via web-services, or it can be served by any other interface (TCP, MQ, flat files on a share (I don't recommend the last)).
Comment: This can be misleading. The application server doesn't host anything. Your code hosts the business logic and the application server works as the glue between that and the web pages that users request.
Answer: The main difference between Web server and application server is that web server is meant to serve static pages e.g. HTML and CSS, while Application Server is responsible for generating dynamic content by executing server side code e.g. JSP, Servlet or EJB.
Which one should i use?
Once you know the difference between web and application server and web containers, it's easy to figure out when to use them.
You need a web server like Apache HTTPD if you are serving static web pages. If you have a Java application with just JSP and Servlet to generate dynamic content then you need web containers like Tomcat or Jetty. While, if you have Java EE application using EJB, distributed transaction, messaging and other fancy features than you need a full fledged application server like JBoss, WebSphere or Oracle's WebLogic.
Web container is a part of Web Server and the Web Server is a part of Application Server.

Web Server is composed of web container, while Application Server is composed of web container as well as EJB container.
Comment: "Web Server is composed of web container": as per https://youtu.be/ATObcDPLa40 this video, it is false
Answer: A Web server exclusively handles HTTP/HTTPS requests. It serves content to the web using HTTP/HTTPS protocol.
An application server serves business logic to application programs through any number of protocols, possibly including HTTP.  The application program can use this logic just as it would call a method on an object.  In most cases, the server exposes this business logic through a component API, such as the EJB (Enterprise JavaBean) component model found on Java EE (Java Platform, Enterprise Edition) application servers.
The main point is that the web server exposes everything through the http protocol, while the application server is not restricted to it.
An application server thus offers much more services than an web server which typically include:

A (proprietary or not) API
Load balancing, fail over...
Object life cycle management
State management (session)
Resource management (e.g. connection pools to database)

Most of the application servers have Web Server as integral part of them, that means App Server can do whatever Web Server is capable of.  Additionally App Server have components and features to support Application level services such as Connection Pooling, Object Pooling, Transaction Support, Messaging services etc.
An application server can (but doesn't always) run on a web server to execute program logic, the results of which can then be delivered by the web server. That's one example of a web server/application server scenario.
A good example in the Microsoft world is the Internet Information Server / SharePoint Server relationship. IIS is a web server; SharePoint is an application server. SharePoint sits "on top" of IIS, executes specific logic, and serves the results via IIS.
In the Java world, there's a similar scenario with Apache and Tomcat, for example.
As web servers are well suited for static content and app servers for dynamic content, most of the production environments have web server acting as reverse proxy to app server. That means while service a page request, static contents such as images/Static html is served by web server that interprets the request. Using some kind of filtering technique (mostly extension of requested resource) web server identifies dynamic content request and transparently forwards to app server.
Example of such configuration is Apache HTTP Server and BEA WebLogic Server. Apache HTTP Server is Web Server and BEA WebLogic is Application Server.
In some cases, the servers are tightly integrated such as IIS and .NET Runtime. IIS is web server. when equipped with .NET runtime environment IIS is capable of providing application services

Web Server                               Programming Environment
Apache                                   PHP, CGI
IIS (Internet Information Server)        ASP (.NET)
Tomcat                                   Servlet
Jetty                                    Servlet

Application Server                       Programming Environment
WAS (IBM's WebSphere Application Server) EJB
WebLogic Application Server (Oracle's)   EJB
JBoss AS                                 EJB
MTS                                      COM+

Comment: Has some mentions of other stuff, But lot seems like plagiarism to me. Like the list at the end, as if copied from Dan's post. And "... reverse proxy to app server..." Also part using  HTTP Server and BEA WebLogic Server as examples at the end pretty much the same thing Rutesh Makhijani wrote.
Answer: The border between these two are getting ever so thinner.
Application servers expose business logic to clients. That means application servers comprise of a set of methods (not exclusively though, can even be a networked computer allowing many to run software on it) to perform business logic. So it will simply output the desired results, not HTML content. (similar to a method call). So it is not strictly HTTP based.
But web servers pass HTML content to web browsers (Strictly HTTP based). Web servers were capable of handling only the static web resources, but the emergence of server side scripting allowed web servers to handle dynamic contents as well. Where a web server takes in the request and directs it to relevant scripts (PHP, JSP, CGI scripts, etc.) to CREATE HTML content to be sent to the client. Once the content is received, web server will send the HTML page to the client.
However, nowadays both these servers are used together.
Where web server takes the request and then calls a script to create the HTML content. Then, the script will again call an application server LOGIC (e.g. Retrieve transaction details) to fill the HTML content.
So both the servers are used effectively.
Therefore ....
We can safely say that nowadays, in most of the cases, web servers are used as a subset of application servers. BUT theoretically it is NOT the case.
I have read many articles about this topic and found this article quite handy.
Answer: In Java terms, there's one more: web container (or more strictly, servlet container). It's, say, in between web server and application server.
A web container in Java terms is an application server that basically only implements the JSP/Servlet part of Java EE and lacks several core parts of Java EE, such as EJB support. An example is Apache Tomcat.
Answer: An application server is typically designed and deployed to facilitate longer running processes that will also be more resource intensive.
A web server is used for short bursts that are not resource intensive, generally.  This is mostly to facilitate serving up web based traffic.
Answer: A web server runs the HTTP protocol to serve web pages. An application server can (but doesn't always) run on a web server to execute program logic, the results of which can then be delivered by the web server. That's one example of a web server/application server scenario.
A good example in the Microsoft world is the Internet Information Server / SharePoint Server relationship. IIS is a web server; SharePoint is an application server. SharePoint sits "on top" of IIS, executes specific logic, and serves the results via IIS.
In the Java world, there's a similar scenario with Apache and Tomcat, for example.
Answer: An application server is a machine (an executable process running on some machine, actually) that "listens" (on any channel, using any protocol), for requests from clients for whatever service it provides, and then does something based on those requests.  (may or may not involve a respose to the client)
A Web server is process running on a machine that "listens" specifically on TCP/IP Channel using one of the "internet" protocols, (http, https, ftp, etc..)  and does whatever it does based on those incoming requests... Generally, (as origianly defined), it fetched/generated and returned an html web page to the client, either fetched from a static html file on the server, or constructed dynamically based on parameters in the incoming client request.
Comment: can u please give examples for bath cases.
Comment: Can you please provide examples of both? Thanks.
Answer: On a first hand, a web server serves web content (HTML and static content) over the HTTP protocol. On the other hand, an application server is a container upon which you can build and expose business logic and processes to client applications through various protocols including HTTP in a n-tier architecture.
An application server thus offers much more services than an web server which typically include:

A (proprietary or not) API
Object life cycle management,
State management (session),
Resource management (e.g. connection pools to database),
Load balancing, fail over...

AFAIK, ATG Dynamo was one of the very first application server in late 90's (according to the definition above). In early 2000, it was the reign of some proprietary application servers like ColdFusion (CFML AS), BroadVision (Server-side JavaScript AS), etc. But none really survived the Java application server era.
Answer: Biggest difference is a Web Server handles HTTP requests, while an Application server will execute business logic on any number of protocols.
Answer: Actually Apache is a web server and Tomcat is an application server. When as HTTP request comes to web server. Then static contents send back to browser by web server. Is there and logic do to done, then that request send to the application server. after processing the logic then response send to web server and send to the client.
Answer: Basic understanding :
In client server architecture

Server :> Which serves the requests.
Client :> Which consumes service.

Web server & Application server are both software applications which act as servers to their clients.
They got their names based on their place of utilization.
Web server :> serve web content
           :> Like Html components
           :> Like Javascript components
           :> Other web components like images,resource files
           :> Supports mainly web protocols like http,https.
           :> Supports web Request & Response formats.

Usage --
      we require low processing rates,

      regular processing practices involves.

Eg: All flat servers generally available ready-made  which serves only web
   based content.

Application server :> Serve application content/component data(Business data).
                   :> These are special kind which are custom written
                      designed/engineered for specific
                      purpose.some times fully unique in
                      their way and stands out of the crowd.

                   :> As these serves different types of data/response contents
                   :> So we can utilize these services for mobile client,web
                      clients,intranet clients.
                   :> Usually application servers are services offered on different
                      protocols.
                   :> Supports different Request& Response formats.

Usage --
      we require multi point processing,

      specialized processing techniques involves like for AI.

Eg: Google maps servers, Google search servers,Google docs servers,Microsoft 365
  servers,Microsoft computer vision servers for AI.

We can assume them as tiers/Hierarchies in 4-tier/n-tier architecture.
 So they can provide
                    load balancing,
                    multiple security levels,
                    multiple active points,
                    even they can provide different request processing environments.

Please follow this link for standard architecture analogies:
https://learn.microsoft.com/en-us/previous-versions/msp-n-p/ee658120(v%3dpandp.10)
Answer: All of the above is just over-complicating something very simple. An application server contains a web server, an application server just has a couple more additions/extensions to it than standard web servers. If you look at TomEE as an example:
CDI - Apache OpenWebBeans
EJB - Apache OpenEJB
JPA - Apache OpenJPA
JSF - Apache MyFaces
JSP - Apache Tomcat
JSTL - Apache Tomcat
JTA - Apache Geronimo Transaction
Servlet - Apache Tomcat
Javamail - Apache Geronimo JavaMail
Bean Validation - Apache BVal

You will see that Tomcat (Web container/server) is just another tool in the app servers arsenal. You can get JPA and the other tech in the web server as well if you want, but the application servers just package all of these things for your convenience. To be fully classified as an app server you essentially need to comply with a list of tools set forth by some standard.
Answer: There is not necessarily a clear dividing line.  Nowadays, many programs combine elements of both - serving http requests (web server) and handling business logic (app server)
Answer: IMO, it's mostly about separating concerns.
From a purely technical point of view, you can do everything (web content + business logic) in a single web server. If you'd do that, then the information would be embedded inside requested the HTML content. What would be the impact?
For example, imagine you have 2 different apps which renders entirely different HTML content on the browser. If you would separate the business logic into an app-server than you could provide different web-servers looking up the same data in the app-server via scripts. However, If you wouldn't separate the logic and keep it in the web-server, whenever you change your business model, you would end up changing it in every single web-server you have which would take more time, be less reliable and error-prone.
Answer: Application server and web server both are used to host web application. Web Server is deal with web container on the other hand Application Server is deal with web container as well as EJB (Enterprise JavaBean) container or COM+ container for Microsoft dot Net.
Web Server is designed to serve HTTP static Content like HTML, images etc. and for the dynamic content have plugins to support scripting languages like Perl, PHP, ASP, JSP etc and it is limited to HTTP protocol. Below servers can generate dynamic HTTP content.
Web Server's Programming Environment:
IIS : ASP (.NET)
Apache Tomcat: Servlet
Jetty: Servlet
Apache: Php, CGI
Application Server can do whatever Web Server is capable and listens using any protocol as well as App Server have components and features to support Application level services such as Connection Pooling, Object Pooling, Transaction Support, Messaging services etc.
Application Server's Programming Environment:
MTS: COM+
WAS: EJB
JBoss: EJB
WebLogic Application Server: EJB
Answer: While there may be overlaps between the two (some web servers may even be used as application servers) the biggest difference IMHO is in the processing model and the session management:
In Web server processing model, the focus is on handling requests; the notion of "session" is pretty much virtual. That  is to say that "session" is simulated by transferring the representation of state between client and server (hence REST) and/or serializing it to an external persistent storage (SQL Server, Memcached etc).
In Application server the session is usually more explicit and often takes form of an object living in memory of the application server for the entire duration of the "session".
Answer: From https://en.wikipedia.org/wiki/Web_server
A web server is a computer system that processes requests via HTTP, the basic network protocol used to distribute information on the World Wide Web. The term can refer to the entire system, or specifically to the software that accepts and supervises the HTTP requests.
From https://en.wikipedia.org/wiki/Application_server#Application_Server_definition
An application server runs behind a web Server (e.g. Apache or Microsoft Internet Information Services (IIS)) and (almost always) in front of an SQL database (e.g. PostgreSQL, MySQL, or Oracle).
Web applications are computer code which run atop application servers and are written in the language(s) the application server supports and call the runtime libraries and components the application server offers.
Answer:
web server: for every URL, it returns a file. That's all it does. The file is static content, meaning, it is stored somewhere in the server, before you make your request. Most popular web servers are apache http and nginx.
application server: for every URL, it runs some code, written in some language, generates a response, and returns it. The response doesn't exist in advance, it is generated for your particular request, that is, it is dynamic content. Application servers are different for each language. Some popular examples are tomcat/jetty for java, uwsgi/gunicorn for python.

Almost every page you visit uses both. The static content (eg, images, videos) is served by the web server, and the rest (the parts that are different between you and other users) are generated by the application server.
Comment: Apache with mod_cgi, mod_perl, mod_php, etc. is an application server, then? What about .NET Core's Kestrel, which is specific to .NET, runs in the process and provides HTTP server capabilities to the application?
Answer: IBM makes a really nice comparison between the two:

By strict definition, a web server is a common subset of an application server.
A web server delivers static web content—e.g., HTML pages, files, images, video—primarily in response to hypertext transfer protocol (HTTP) requests from a web browser.
An application server typically can deliver web content too, but its primary job is to enable interaction between end-user clients and server-side application code—the code representing what is often called business logic—to generate and deliver dynamic content, such as transaction results, decision support, or real-time analytics. The client for an application server can be the application's own end-user UI, a web browser, or a mobile app, and the client-server interaction can occur via any number of communication protocols.
In practice, however, the line between web servers and application servers has become fuzzier, particularly as the web browser has emerged as the application client of choice and as user expectations of web applications and web application performance have grown.
Most web servers support plug-ins for scripting languages (e.g., ASP, JSP, PHP, Perl) that enable the web server to generate dynamic content based on server-side logic. And an increasing number of application servers not only incorporate web server capabilities, but use HTTP as their primary protocol and support other protocols (e.g., CGI and CGI variants) for interfacing with web servers. They also allow web applications to leverage services like reverse proxy, clustering, redundancy, and load balancing—services that improve performance and reliability and allow developers to focus less on infrastructure and more on coding.
To make matters more confusing, many web servers and some application servers are referred to, or refer to themselves, as web application servers.
The bottom line is that today's most popular web servers and application servers are hybrids of both. Most of the increasingly rich applications you use today feature a combination of static web content and dynamic application content, delivered via a combination of web server and application server technologies.
Answer: Drawing my conclusion from posts of awesome SO evangelists above, I conclude that both are software that resides on actual metal servers(physical servers). Both work hand in hand to get service to the end-user. Then, depending on the setup, we can choose what to call it, which means how to describe it depends on the actual setup in use. If they both reside on the same bare metal(physical server), we call it web and application server...simple! If on separate machines, obviously, we have two distinct bare metals(physical servers) at our hands. Then we can label these servers according to their function: one web server and the other application server.
Answer: It depends on the specific architecture.  Some application servers may use web protocols natively (XML/RPC/SOAP over HTTP), so there is little technical difference.  Typically a web server is user-facing, serving a variety of content over HTTP/HTTPS, while an application server is not user-facing and may use non-standard or non-routable protocols.  Of course with RIA/AJAX, the difference could be further clouded, serving only non-HTML content (JSON/XML) to clients pumping particular remote access services.


How much of earth's molecular oxygen in the atmosphere is due to plants?

Question: I know plants form a critical part of the earth's biosphere, including molecular oxygen production. Molecular oxygen makes up ~20% of our atmosphere.  How much of the earth's molecular oxygen in the atmosphere is due to plants? Is there a minimal level of plant life necessary to maintain levels of oxygen so that land mammals can live? Also, are land plants mostly important or are underwater plants a significant source of atmospheric oxygen?
Comment: The answer to the question is none. Earth's oxygen comes from the stars mostly. Now, it is clear that you are asking about Earth's oxygen in a molecular form. I think the question needs a good edit to clarify
Answer: The oxygen in the atmosphere was produced by cyanobacteria during the Great Oyxgenation Event, around 2.3 billion years ago. Cyanobacteria produced & still, produces, to a lesser extent, oxygen by photosynthesis. Plants did not exist during the Great Oxygenation Event, but these day plants replenish atmospheric oxygen by removing carbon dioxide by photosysnthesis, like cyanobacteria. According to David Biello, The Origin of Oxygen in the Earth's Atmosphere the production of oxygen by cyanobacteria during the Great Oxygenation Event took one billion years.
During the Carboniferous period, about 350 million years ago, the amount of oxygen in the atmosphere increased to 30% because the rate of burial of organic matter prevented it from reacting with atmospheric oxygen, History & Significance of Oxygen in the Atmosphere
According to Wallace S. Broecker our atmospheric oxygen supplies are vast:

Simply put, our atmosphere is endowed with such an enormous reserve of this gas that even if we were to burn all our fossil fuel reserves, all our trees, and all the organic matter stored in soils, we would use up only a few percent of the available $\ce{O2}$. No matter how foolishly we treat our environmental heritage, we simply don't have the capacity to put more than a small dent in our $\ce{O2}$ supply.

From Atmospheric oxygen - Broecker
Comment: Broeker also says, "the Earth's plants produce through photosynthesis an amount of O2 equal to that of the atmosphere in about 2,000 years". Of course they're simultaneously absorbing oxygen, that's why it's called a cycle and why removing them wouldn't mean we asphyxiate in 2000 years. But it's sometimes significant what speed the cycle operates relative to the reservoir. It's not like we're still coasting on a 2.3 billion-year-old, or even 350-million-year-old, store.
Comment: How could Cyanobacteria produce that much oxygen? Why hasn't such production continued?
Comment: I would challenge the idea that the oxygen present in the atmosphere TODAY was produced by cyanobacteria several billion years ago.  Today's oxygen is produced by plants, and cycles through in about 4500 years.  (Per Wikipedia, anyway: https://en.wikipedia.org/wiki/Oxygen_cycle#Capacities_and_fluxes )
Comment: @jamesqf Suppose you are correct. Then what relevance is the Great Oxygenation Event have for today's atmosphere? In other words, that would mean today's plants would be producing all the oxygen, right?
Comment: @Stan Shunpike: Yes.  Or to be more precise, they are driving the Oxygen Cycle (see the Wikipedia link) that keeps the atmospheric O2 concentration at about 20%.  Really simplifying the process, animals breathe O2, which combines with food to produce growth & movement.  The combination produces CO2, which gets converted back to O2 and biomass by plants.
Answer: Yes today's photosynthesis is occurring but so is the oxidation of the carbon based plant material when they decay and consumed.
In order to have a surplus molecular Oxygen quantity the products of photosynthesis cannot undergo oxidation.
Our biggest threat is the diminishing of molecular Oxygen because that is what absorbs UV energy and releases it as light before the UV can reach the Earth's surface and become light and heat where it can cause the most damage with retainment of destructive forces at the earths surface.
Remember that petroleum and coal are the products of photosynthesis and we are consuming them at an exponential rate.
Comment: "Molecular oxygen" is commonly understood to be O₂. The form of oxygen that absorbs UV radiation is ozone, O₃.
Answer: Depends on your definition of "plants", if you simply mean organisms using chloroplastic photosynthesis which is the most basic definition then the answer is almost all of it. If you want to know how much of the oxygen we breath is from land plants like trees and grasses the answer is roughly 55% annually with 45% being from phytoplankton blooms with trace amounts generated by UV radiation acting on water and certain nitrogen oxides in the upper atmosphere.
It's hard to say what a plant minimum for mammalian survival would look like because the availability of atmospheric oxygen would effect the interactions of that oxygen with the biosphere, hydrosphere, and lithosphere. Also which mammals? Mole-rats need significantly less oxygen per kilogram than humans which need less than mice.
Answer: More trees have been planted than ever before with the Arbor Society. Oxygen is the biproduce as plants need carbon dioxide to live(photosynthasis.) Volcanic, platelet fishers and forest fires account for natural carbon dioxide in the atmosphere. Man made fossil fuels are only a small % of carbon dioxide. They do contribute to the % level though.
Comment: Welcome to ES.SE. Unfortunately this doesn't really answer the question asking about the _amount_ of O2 in the atmosphere.  Also, I wouldn't call the 30% of CO2 in the atmosphere attributed to human activities (265 of 860 GtC) a small %.