Presta or shrader valve for mtb?

Question: Is it better to use presta or shrader valve for tubeless conversion on a mtbike, 29" 2.20 tire, and why?
Comment: Go with what the rim is
Comment: Both are good. Only consideration is rim width. Narrow rims will use presta . Well in extreme cases if high pressure tires then presta.
Answer: The debate of Presta or Schrader is mostly religious and often based on poorly misunderstood facts or historical differences.
Rim Width. In the 21st century, the problem is less about the strength of the rim and more to do with fitting the tire beads and valve into a narrow rim. If the rim is wide enough to accept a Schrader and mount the tire, its strong enough.  As we are discussing MTB's – the last time I ran a rim too narrow for Schrader on a MTB was 1993. In the last decade, MTB rim widths are getting wider so it makes no difference what valve you run.
Weight. Presta are marginally lighter; only you know why you care so much.
Length. Although long Schrader valves are available, if you have deep rims you are more likely to find Presta in long valves.
Pressure. It's commonly stated that Presta is better at pressure – Schrader are used to contain pressures of over 600bar. They are used in MTB shocks (up to 250 PSI ) and your car's air conditioning unit (200PSI, expected no gas loss for a decade), so I think that myth is busted.
Inflation. Presta had the edge way back before the days of plastic heads and computer engineering. In 2017, if your pump does not have a head that successfully inflates Schrader every time, buy  a pump worth more than $1.
Robustness. From personal experience and anecdotally from internet discussions, Presta is prone to having the end broken off by some combination of one or more of clumsy novice and cheap pump. I have not done in the last couple of decades...  Schrader really do require the caps to keep rubbish out of the core (although plenty don't and seem to last OK).
Reliability. Schrader are used in every car tire in the world. Presta are probably at least as reliable, nuf said.
Valve Length. Although tubes with long Schrader valves are now available, it's more common for rims requiring long valves to be Presta.
So why use one or other? I mostly run Presta because many people still think  Schrader is a mark of a BSO, and although I ride a 10 year bike, I would not be seen dead on a BSO.
For tubeless though, I think the balance swings to Schrader – bigger hole for putting the slime into the tire and for initial fast inflation to seat the bead.
Comment: I am not sure I completely agree with the last line.  I've never had issues getting unicorn juice into a tire through a presta valve.  I've also never noticed differences in inflation rates between valve type, but I suppose that may be a thing.  Otherwise, well listed.
Answer: This is a good appraisal, but I would add one thing, which is especially for the traveller. If you're on a long journey, say, multi-month, maybe even multi-country, there is distinct possibility that you might lose, damage or break your pump. Travelling lightly, your unlikely to have a spare one. Therefore, I would recommend Schrader valves because, pretty much anywhere in the world you can find a pump that can inflate your tyres. This is, of course, because all cars, motorbikes, trucks and buses use Schrader and often have a pump, too. That also means that practically every gas station and auto workshop has a compressed air pump with a nozzle for Schraders (though this requires extreme care to use, and is not suitable for high pressure tyres. Mountain bikes should be OK.). So, for expeditions, I'd say use Schraders on a mountain bike. Or carry an adaptor to be able to use car pumps.
Comment: I would argue that presta valves are much easier to deal with in roadside repairs -- less trouble keeping the chuck on, etc.  And the stop nut is quite useful when pumping up -- much easier to get the chuck on all the way.
Comment: Daniel R Hicks: I'd say the exact opposite. Presta valves are easily bent or even broken. Of course it's also easy to replace them.
Comment: +1 for mentioning the petrol stations' pumps compatibility.

Presta or shrader valve for mtb?


Does this count as breaking the first precept?

Question: Let's say that my face was itchy but just before I scratch it, I just remember that there might be face mites living on my face and I might be killing them. And as I'm scratching my face, I keep thinking that I might be killing the face mites. My original intention was to scratch my face but if I think I might be killing the face mites but continue to scratch my face because it's itchy then does that count as breaking the first precept?
Comment: interesting also in that itching could be illusory in some contexts, and that when the mterial was written down, & presently, lots of persons weren't/ aren't aware of such mites, &, whether the relief or illusion of relief from the illusion would be recieved as pleasant/ desirous, etc; and whether each thusly demised mite or if demise of a flock of mites would be just one infraction, or a flock of infractions, then extrapolating as for the rest of the question; general cleanliness; and the inconsistent level of consideration re the situation might also be of interest in the context of the q
Comment: if Asker could please indicate if asking only re Therevada texts, or for all contexts of Buddhisms answers, could be helpful for replying
Answer: The first precept is:

Panatipata veramani sikkhapadam samadiyami.
I undertake the precept to refrain from destroying living creatures.

Conditions that Break the Precept
(By Ñanavara Thera)
The five factors of the first precept are:
<code>* pano -- presence of a living being.
* panasannita -- one knows that it is a living being.
* vadhacittam -- the intention to kill.
* upakkamo -- the effort to kill.
* tena maranam -- the resulting death of that being.
</code>
All factors must be fulfilled in order to break the first precept.
If you can answer yes to all of them then the first precept is broken. If you can answer no to one of them the precept is not broken.
Comment: This is a great answer. It's clear, doesn't digress and has a reference!
Comment: Thank you Erik:)
Answer: Aside of possible akusala actions (which is as soon as mental approve or wishing death, factor of intention), good householder, there are those factors to prove whether a full offence or not:

Object: living being, (In Vinaya a wording of "even small as a egg of a bed-bug" is found, here also perception of living being matters, of course)
Intention: knowingly, consciously, deliberately, and purposefully wanting to cause that beings death. "Knowingly" also includes the factor of —
Perception: perceiving the object as a living being.
Effort: whatever one does with the purpose of causing that being to die.
Result: The life-faculty of the being is cut as the result of one's act.

So the matter of the perception isn't that easy here (from seeing it, till thinking it might be). Acting in doubt, of course is another wrongdoing, but an other.
Also wheter alive and whether death on one action may make it difficult to know.
Don't forget for example that one "knowns" that there are being under the soil where stepping on, or doesn't he actually?
Most importand, how ever, is intention. It's actally an not easy topic and my person currently struggle with sand fleas, which actually need to be removed as well. Since month heavy disease because of those tiny beings. Knowing things in detail often makes things not easier. For many years not known that this "throns" are parts of living beings.
In any case, may one always be most patient and try all to cause most less harm => Sila, Metta.
Enduring just pains is of cause a very needed practice to cut off certain fermentaions arising on it. Circumstances, how ever, are always different and proves can be at east only by oneself if honest and discerning enoug.
[Note that this isn't given for stacks, exchange, other world-binding tade but for escape from this wheel, a gift for such]


Why are the Istari only allowed to inspire?

Question: It its known that the Istari were sent to Middle Earth to aid in the battle against Sauron, being only allowed to inspire and no direct actions could be taken.
But, why is this? Could this rule be broken? Did Eru himself create the rule?
Comment: I think they could also expire.
Comment: @DavRob60 don't be so sure, they couldn't keep Gandalf down could they?
Answer: The problem that the immortals had discovered, from their previous interventions, was that they didn't work. They tended to wreak havoc across the land, but only served to change the current power structure, not remove it. By intervening directly, they could not change the nature of people, and there was a danger that they would be relied on to always save the world in time of need.
The Istari were another approach, of trying to provide guidance and influence, so that the residents of Middle Earth could do their own work, and could be slowly but gently guided towards the right way, without any direct intervention. This means that they would win the victories primarily on their own, and would not rely on or expect help from over the sea.
In many respect it worked. The last battle was a combination of many forces, all of whom needed to separately see their need to fight, but who could all fight for one cause and one purpose, which was Middle Earth itself.
Answer: The "rule" was more of a charge, an ideal that they were supposed to hold to.
The reason behind this is stated in the Wikipedia:

They had tried direct intervention in the destiny of the Elves in the Years of the Trees by leading the Eldar into the West, but this resulted in many bloody wars and confrontations. In the struggle against the Dark Lord, they hoped instead to help Men achieve their own destiny. Thus Gandalf and the other wizards were meant to use their great wisdom to persuade Men to courses of action which would achieve Men's own goals, rather than trying to dominate them.

As far as the rule being able to be broken, that is exactly what Saruman did:

Saruman failed in this when he tried to set himself up as a commander in opposition to Sauron, but Gandalf remained faithful to his charge.
Answer: I know that in the beginning of the Silmarillion, Eru created the Ainur and subsequently through the Music of the Ainur, layed a plan for the world that was to be created. In my interpretation (and i'm sure others as well) it was Melkor diverting from the song and doing his own individual will that was the starting point for all the strife in the world that came afterwards.
I would argue that the Valar and maybe Eru himself, try to "stick to the plan" as much as possible. In this specific example, I would say that because the war against Sauron was not in the original music/plan the Valar try to keep their distance and manipulate the situation instead of directly getting involved and diverting even further from the original song.
Comment: The Music didn't predict the history of the Third Age (at least not in a form that anyone but Eru understood). The quasi-non-intervention of the Ainur was [decided after the mess that intervention had made of the First Age](http://scifi.stackexchange.com/a/18908). The [downfall of Númenor](http://en.wikipedia.org/wiki/Akallab%C3%AAth) was the Valar's last direct intervention.
Answer: The canon knowledge of the restrictions upon the Istari largely come from the relevant passages in Unfinished Tales.  Manwe asked

Who would go? For they must be mighty, peers of Sauron, but must forgo might, and clothe themselves in flesh so as to treat on equality and win the trust of Elves and Men. But this would imperil them, dimming their wisdom and knowledge, and confusing them with fears, cares, and weariness coming from the flesh.

From that it is obvious that they shall have restrictions, and they were imposed by Manwe or higher, i.e. potentially Eru.
This rule could definitely be broken - Saruman did not forgo might, nor treat on equality with Elves and Men.  And given such, he failed the task set upon him and the other Istari.
As to why the rule was set, we can only speculate.  The reasoning for this is not explicit in Tolkien's writings, so we cannot know whether it was:

to avoid temptation (imagine Saruman with his full powers as a Maiar),
to avoid collateral damage with Maiar pitting themselves directly against Maiar, or
to avoid merely setting up another Maiar in place of Sauron, should they have submitted to the temptation of the Ring, where most would have.

In my opinion it was most likely the latter given what we saw of Saruman and the mention of the Blue Wizards being the cause of dark magic sects, but it is purely speculation.
Comment: This is the only answer that actually Shows Your Work by quoting a primary source.


One time form for Joomla Users?

Question: I want to place a custom html/php form for my registered Joomla users but I want it to be submitted by the user only once, i mean if a user has submitted the form , the next time he'll open that form page, he should not see the form again? Instead, he should see a custom message (like You have already submitted the form or bla bla bla....) How to achieve this?
Answer: When you say "custom", I hope you mean that you are going to stick to Joomla methods and make this a module or component ;)
Best way you can do this is by using some of the methods you have been already using. And by that I mean getting the user object.
So, when a user submits the form, not only does it submit the data they have inputted, but it also submits their username, using <code>$user->username</code>. Moving forward, we now supposedly have their username stored in the database along with the data they submitted.
You now need to write a database query getting the username from the table and match it against the current logged in user. Here is a small example:
<code>$db = JFactory::getDbo();
$user = JFactory::getUser();

$query = $db->getQuery(true);
$query->select($db->quoteName('username'));
      ->from($db->quoteName('#__myform'));
      ->where($db->quoteName('username') . ' = '. $db->quote($user->username));
$db->setQuery($query);

$result = $db->loadResult();

if($result) {
    // display custom message
}
else {
    // display form
}
</code>
I think it would be an idea to get the form working first and then to start integrating the user check.
Hope this helps
Comment: I am sorry, i couldn't understand your answer properly, i mean the data of the form gets deposited into my mailbox when the submit button is pressed & a thankyou message is displayed to the user. How will there usernames get stored in the database?
Comment: You will need to do the coding for that. Its the only reliable method to check whether the user has filled in the form before or not
Comment: I don't know how to code this out! Can you provide me the script which i will paste in the PHP file which gets executed on the submitting the form. The script should store the username in the database, May be after then your above answer will help out
Comment: I'm not going to do all your work for you. You should start writing the form module and post the another question when you run into a issue with your current code. You initial question has already been answered twice. Each one being different so it's not up to you to choose which method you want to go along with and start ;)
Comment: http://joomla.stackexchange.com/questions/1027/inserting-data-into-joomla-database [Inserting data into Joomla database]
Answer: I am not sure what extension you are using to generate the form. But if you have the possibility to change it to "Chronoforms" you can do this for this extension as suggested here:
http://www.chronoengine.com/forums/posts/t9535/p9535/user-submit-only-once.html
NOTE: Even if you do not use chronoforms, you can still use the underlying method as it uses standard functions and framework of Joomla.
This is exactly what you are looking for, but the catch is you have to use chronoforms, which by the way are the best free form extension for Joomla anyway, in my opinion. The support is excellent, well maintained. I use it for my site too.
Answer: Use B2J Contact to generate custom fields in a form. It's a free Joomla form creator component provided by Bang2Joom Team. Nice and easy backend! Friendly support!
Just try it!
Download here: http://www.bang2joom.com/joomla-components/b2j-contact
Comment: And does this extension have what the OP requires? Explain your answer in more detail please

One time form for Joomla Users?


Downtown Chicago car free bike ride?

Question: I'll be traveling to Chicago for the weekend and would like to take a casual ride through the city.  A friend of mine will also be joining who isn't comfortable riding on busy city streets.  Are there any bike friendly trails or paths in or around downtown?  A 5-15 mile route would be ideal.  Any recommendations on where to rent a hybrid/cruiser bicycle would be awesome too.
Comment: Dorothy's suggestion of the Lakefront Trail is perfect, especially that it's less crowded at this time of year.  To add to that, depending on where you're staying, check out the 606.  Many streets in Chicago have excellent bike lanes.  Google Maps' bike layer is up-to-date and accurate. Also, this seems to be out of your distance range but do consider the Chicago Perimeter Ride which is this weekend!  https://www.facebook.com/events/958416154285424/  I'm flying back to the city for it. It's a chill informal ride, traffic is sparse for most of it.  Lots of fun. ~100 miles if you go all the way.
Comment: Also, the Perimeter Ride goes down the Lakefront Trail on Saturday Morning, so if you want to kick off the ride with us, you're welcome to do so.  Starts at the Buckingham Fountain in Grant Park, around 8:30 AM.  (Honestly, probably end up leaving around 9:00 AM.)
Comment: Here's some more rides over the weekend:  http://www.thechainlink.org/events
Answer: The fantastic Lakefront Trail would be a great choice, a paved path that extends for 18 miles along the shoreline of Lake Michigan. It runs from Ardmore on the North Side to 71st  on the South Side.  It's a very lively path (pedestrians have the right of way) and a great way to 'see' the city.

Run, walk, bike or rollerblade with the blue waters of Lake Michigan to one side and gorgeous, green parks to the other. There's so much to soak in in addition to the skylines views — pass by peaceful boat harbors, serene nature sanctuaries, bustling boardwalks and more. Along the trail are parks, beaches and gardens; recreation areas including playgrounds, soccer fields, volleyball nets and tennis courts; and amenities like washrooms, drinking fountains and concession stands.

As for bike rentals, there are lots of options, such as Bike and Roll Chicago , Lakeshore Bike, Divvy, Wheel Fun, Bobby's Bike Hike, just to name a few. You'll have to check to see which one(s) might have the hybrid/cruiser bicycle you'd like.
Happy trails to you!
Answer: I'll add that many streets in downtown Chicago have a bike lane, and for the most part your car driving neighbors are used to and wary of bikes. There have been some recent incidents in the news, but the sheer volume of bikers in Chicago proves it's quite safe. Happy cycling.

Downtown Chicago car free bike ride?


Why is the episode called "Shut up and Dance"?

Question: The third episode of season 3 of Black Mirror is called "Shut up and Dance". Can anyone please explain why?
Answer: My opinion is because the 'controller' is acting as a puppeteer of sorts. No matter what the people he/she says makes no difference. Its like the controller is saying shut up and do as I say. When I say give a guy a cake, you give the guy a cake. When I say dance, you dance.
Answer: I think it is because the person/people who are telling Kenny and the others what to do are somewhat like puppeteers. Whatever they tell them to do, they obey. If they tell them to shut up and dance, that's exactly what they will do.
Comment: Saying an opinion is not a good answer. If you have something to back up your assertions, then add them. You can modify and add them.


I have developed the habit of suppressing my voice when singing, how to resolve it?

Question: Recently, when I tried to sing, all my friends said that they could not hear me. But when I talk they can hear me well. I want to know how to resolve this.
Comment: One thing you can do is to try and sing more from your chest and navel region than from your throat or lips. Try to observe from which region you feel the most vibrations when you sing. Opening your mouth wide open when singing also helps (usually, beginners feel that their mouth is sufficiently open when they sing whereas in fact it is not).
Answer: If I were in your situation, I'd record myself so I can listen what others hear when I sing. Pretty horrible at first, but very useful. You'd be surprised how much it's actually there that you totally miss by listening only as you perform rather than after you've done it and you're able to focus solely on listening. I used to do it all the time (playing, not singing) and it really worked for me.
Comment: Recording is a very universal and efficient technique for understanding issues with your performance. It would be good if a person who downvoted this explained what they dislike about the answer.
Answer: I'm not much of a singer, and I can relate to your issue. One thing that's made a huge difference for me is diaphragmatic support. Essentially breathing into your lower abdomen - sticking your gut out instead of puffing out your chest - to support your volume. There are lots of Youtube videos demonstrating and describing this much better than I can.
When I feel like I'm not projecting and/or I'm straining, I stick out my belly and try to relax my chest and throat. Not only does this help me project better, but I can hold notes longer, too.
Comment: If I've suggested something harmful or counterproductive, please let me know.
Answer: First of all do you really know how to sing?  Have you had voice lessons?
Singing correctly is NOT easy.  There is no way that a properly trained voice is quieter than speaking, unless the singer is really singing pianissimo.
Also, your description is so sparse it may not be possible to help you.
One thing I'd mention is that what you describe sounds like an example of your voice not having "support" from your diaphragm.  It takes practice to learn how to use your diaphragm correctly, to control your breathing while singing.  Untrained people who try to sing sometime hold their breath while singing.  I did this too before having lessons with a real vocal coach.  You take deep breath and think you have enough to sing but fail to push it out steadily.
The other thing is not having the correct resonance in your head.  The sound should really resonate up in the sinus cavity.  If it doesn't it will sound very muffled and hitting notes on the high end of your register will be painful, throaty.
Recording yourself or trying to emulate others will not fix these two issues.  It takes coaching and practice to get these right but when you get it you should be able to sing at any volume you want with a full rich tone.
Lastly, it is possible that if you know what you are doing you are just nervous.  What situation are you singing in where your friends say that they cannot hear you?  A chorus, karaoke, band practice?  Help us help you with more info.


Does a high MTB seat post risk frame breaking

Question: On a full suspension MTB, does a high seat post (extending from frame by 25cm, 10" or more) contribute to frame weakness, should one look to have a bike with a tall seat tube, or are MTB's designed like this and shouldn't break?
Comment: I often wonder about some of the bikes I see, especially BSOs with long seat posts.  But I don't recall seeing any reports of seat post/tube failures.
Answer: There is usually a minimum insertion marker on the seat post to indicate the minimum amount of the seat post which should remain in the seat tube.  The frame manufacturer might also specify a minimum insertion depth for the frame.  It is not recommended to run the seat post inserted any less than the greater of these two this as it can put the frame under stress and you risk cracking the frame at the top of the seat tube (I've seen this happen first hand).
Having said that with a 400mm seat post I think you'd be able to have 250mm extending from the frame and the 150mm remaining in the seat tube would be sufficient.  You'd have to check the seatpost/frame recommendations but if I remember correctly the Rockshox Reverb dropper posts have an 80mm minimum insertion depth, so 150mm is certainly not extreme.
Comment: But if you weigh 150kg there will be a slight problem, of course! ;)
Comment: +1: Obviously Riding style and terrain make a difference - if on paved road it won't matter as much as extreme downhill or rock gardens.
Answer: Mountain bikes in general are going to have a more seat post out compared to a road bike as that is the frame design.  Mountain bike have a sloped top tube and road bike flat or slightly sloped.
If you are using the bicycle as designed then it should not be a problem.
Don't buy a small frame if you are 6 foot and put a monster seat post on it.
Mountain bike also have bigger diameter seat tubes - typically 31.6 versus 27.2.
As stated by chored there is a minimum insertion distance.
Answer: There are also a few rules of the thumb to know how much of the seatpost to leave in the frame. If the limit line is not visible on the seatpost, te safe way is to keep 1/3 of its length in the frame. Although it's more than enough for most models (it's around 10-15cm depending on the length of your seatpost), some more expensive ones tend to have thinner walls towards the bottom, which is why going below minimum is a risk.
Also the seatpost should be inserted so that the bottom of the seatpost is below the top tube of a frame (if a given frame has a physical top tube), so the top tube acta against the loads on the seatpost.


How to engage low-level students?

Question: I'm a recent graduate in my first year of teaching. I teach secondary students, and I have found that some low-level students are not listening in class. They seem to have given up and stopped trying. I feel it's hard to make a difference or help them because they don't seem to care. How can I engage these students?
Comment: I think that one thing that would be helpful is to edit your post to have one concrete example of a class and giving up behavior.  This one is so vague as to be impossible to give assistance on.
Comment: Welcome to the site.  If you edit the question you may get better answers. Clarify that you are a recent graduate who is teaching secondary students (not a graduate teacher).  Also let us know what you've tried and how wide a gap there is in the class.
Comment: When you say you are a graduate teacher do you mean you teach post-graduate (masters) level university courses? If that's the case, unless you are losing a significant (50+%) part of the class the problem is your student's and not yours. At the graduate level IMO you really should be teaching to the average or above average, the students are there to learn not to be coddled. I'm assuming this is the US too in much of Europe this applies at least from undergrad and for the better schools from high school.
Comment: @DRF I meant this is my first year teaching, I teach secondary students.
Comment: To piggyback on Amy B's comment, it would help to know what class you're teaching, and what levels the students are at. For example, if you're teaching HS algebra and you've got some students who can't add fractions, other students who can do fractional arithmetic just fine but can't handle the concept that $x$ can stand for anything, and a small number of students who are breezing through and really should be in a higher class, that would be helpful information to have. You can [edit] your question to add this info.
Comment: Are you a teacher who's actually trying to figure out how to engage students or are you writing this question on behalf of other people who are teachers? If so, what are you struggling to get the students to learn? I'm going to write one answer. However, I may also have a second potential useful answer to write that's different that I will only write after I get an answer to this comment and even then, I may still feel that I cannot think of a satisfactory second answer.
Answer: There isn't enough information to answer the question, but I'll speculate, based on what often happens in the world of math education.
People who enjoy & teach math are often shell-shocked at how many people consider math to be a meaningless, tear-filled, horrible subject in which effort is pointless. My guess is that most students have given up on trying because 287 adults before you have told them to try harder, then given them rote or inappropriately difficult work... in which trying doesn't make a difference. See my recent post on fractions for examples of how hard work combines with rote gibberish to demoralize and confuse.
I suspect many of your students view you as the 288th adult who's giving them more rote gibberish and empty pep talks. Some likely signs of this: your lessons are very heavy on symbols and students (rightly?) don't believe you when you say they should pay attention and try harder.
Their response: The bare minimum effort to: (a) get you off their backs and (b) avoid summer school or repeating the course.
Consider doing gentle but probing "math biography" interviews with some of your students. Tell them you need their input to become a better teacher and want to begin by understanding when they stopped trying. Ask them when math became a painful subject for them. In kindergarten, math is for many children the favorite academic subject; among the general population, about 37% of people specifically use the word "hate" to describe how they feel about math. [Many others use other negative words to describe their feelings.]
Expect a lot of stories such as:

"I loved math until grade 4. Then my teacher didn't really teach us division or fractions and never tested us on anything. Sometimes we'd go weeks at a time without a math class, months without a test. I've been terribly confused in most math classes since then. I noticed that studying really hard didn't help, so I just don't try hard any more because it's not fun." Note that this is likely true: given terrible confusion regarding grade-4-level division, does it matter if you study really hard to master proportional reasoning and ratios? Of course not. Hard work didn't matter then even though adults falsely told them it would. Why are you different from those adults?
"My teacher made the class do all these timed exercises. I was the slowest one in the class no matter how hard I tried and that's when I realized I was stupid." Notice that maybe the student just needed to learn, say, how $6+6=\_\_\_$ and $6+7=\_\_\_$ are related, then they could have stopped counting by ones to do all calculations. But if you don't know such relationships, then, well, math = counting by ones, which is slow and makes people feel stupid. It's a hopelessly weak foundation for future math. Are you trying to build on their hopelessly weak foundations?
"The teacher would make me come to the front of the class and tell me to do math I couldn't do and that was humiliating. I hate math. So just tell me how to pass this class so I don't have to go to summer school." How often do your students feel great about what they've learned and accomplished in your class?
"Math is just all these steps. I just memorize those steps. I'm the memorizing type. I'm not interested in understanding because I don't think there is anything to understand. Just tell me the steps and I'll do them." Are you sure you're really testing for understanding? Are you really convincing your students that math is understandable?
Comment: Now that I'm old enough, I think I can figure out that a satisfactory answer to the question how are "6 + 6 = ___ and 6 + 7 = ___ related?" is "I guess you're talking about the two problems themselves, rather than the numbers that are the answers to the problems. Actually the pair of questions is related to the pair of answers. The two questions have the property that they differ by 1 only in the second operand and the two answers have the property that they differ by 1." I gained that ability eventually and probably a lot else school was trying to teach. If so, there was no need to waste
Comment: material on trying to push me to learn abstract stuff like how to explain things earlier. Maybe not everyone eventually gains that ability to a high enough extent but still I think it's better not to push it on those students. They start struggling and adopting a linear thinking approach. When the teacher teaches what takes longer to teach with an understanding than the amount of time spent in school, the students probably end up struggling and learning even less.
Comment: The share of students who cannot relate "6+6=___" to "6+7=___", when presented visually and/or with manipulatives, is miniscule. Many students will lack the *words* to tell you how they're related, but they can typically show you with counters or apply the same concept to 25+25=50,  25+26=___, 25+24=___, etc.
Answer: In my opinion a very important tool for the teacher is a lesson plan for every lesson. I was a math tutor in a math center for 8 years and I conducted multiple worshops in a classroom setting with over 20 students at a time. Techniques that worked for me were: inviting students to ask questions during the lesson , polite responses to questions , eye contact with the students , doing a worked out example on the board and then writing a similar problem beside it and asking the students to solve it at their desk while I walked around and looked at their work and gave helpful hints if the student seemed stuck. Also I would have students work in groups of 4 or 5 so they could exchange ideas with their peers. You have to mix it up a little so some students don't get bored.
Hope this helps :)
Answer: The answer isn't the same for every educator or every group (or every combination thereof). I find that the key is to consider myself constantly trying to find this answer. In other words, from day 1 to the final second of your time with these students, you are trying to find the "holy grail," so to speak. ...or at least get closer...
In other words, you are trying to reach every student, every small group of students (as they exist), and the larger group (class as a whole). Therefore, I try to connect with each individual and each group as authentically as possible (a little fake-it-til-you-make-it can help here) If it feels forced, it is... but don't give up.
If a strict lesson plan is authentic to you, then do that, but make sure you PAY ATTENTION TO EXACTLY WHO YOU ARE TEACHING. Connect with them.
As you manage to understand them better, you may also find ways to articulate the lessons differently, ultimately framing them in a more relatable way. A very simple example: you find a certain cross-section of students are soccer players, so all of your momentum lessons (physics application here!) have to do with soccer balls. Just be careful you don't get too hokey ;-) ...you don't want the students rolling their eyes too hard. But thy really do appreciate the effort if you're being sincere and totally authentic.

How to engage low-level students?


Does the age of the universe take into account General Relativity / Special Relativity?

Question: It is generally accepted that the age of the universe is approximately 12-15 billion years old based on the speed of the expansion of the universe. Since everything is moving very fast away from us, doesn't relativistic effects from high speeds come into play here? If something is moving quite fast, time slows down, affecting how "old" it is. Is all this taken into account when calculating the age of the universe? I don't know if I'm making any sense to anyone else or not.
Comment: @userLTK That's not completely accurate. When we observe phenomena that have been redshifted, like the duration of supernovae and reverberation mapping of AGN, the stretching of the total signal also spreads out the arrival time of its parts, giving an apparent time dilation. Gravitational redshift, a related effect, and how it affects time's flow is also necessary for getting GPS right.
Comment: This was probably answered below, but the expansion of space doesn't cause the time dilation of relativity.  It causes red shift, but not time dilation.
Comment: @SeanLake  You're right on apparent time dilation and I'm not sure if they can be told apart in any practical way, so you make a good point.
Answer: This is a confusing question - your title mentions GR, but of course the age of the universe is entirely derived as a result of using GR to solve for the dynamics of the expanding universe.
The text of your question is talking about time dilation and the effects of special relativity (a subset of GR). Here there is a point to be addressed. It is not a good way to think about the explansion of the universe in terms of things receding from each other at ever increasing velocities. It is better to think about space expanding. It is also not possible to apply special relativity except in local inertial frames.
It can and has been shown that the redshift-distance relationship cannot be simply interpreted as the doppler shift due to things moving away from us at ever increasing speeds - since this of course predicts a maximum recession velocity that approaches the speed of light. In fact, beyond redshifts of $z=1.5$, the "recession velocity" in all viable models exceeds the speed of light. What actually happens is that light emitted at some earlier epoch travels across expanding space to get to us and this expansion increases the wavelength of the light.
Essential reading: Davis & Lineweaver (2003).
Comment: @iMerchant that doesn't change the fact that SR is not of any importance here. I reckon you're not going to get a better answer than Rob's.
Comment: I updated question to say GR/SR instead of just GR.
Comment: It's all a bit above my head, but here's another article, @iMerchant, which I have linked to before on similar questions:  https://arxiv.org/abs/0707.0380v1 .  For example: http://physics.stackexchange.com/a/223697/10319
Answer: Rob Jeffries gives a good response to this question, but I wanted to go through the basic outline of how the age of the universe is calculated, just so you can see how it works more or less. Be warned though, I'm only giving you the highlights and you'll either have to accept what I'm saying or fill in the blanks yourself.
The Friedmann Equation
As with anything in physics, when you want to analyze something, you need an equation to represent it. Think of the Friedmann Equation as the "equation of the universe". It's kind of crazy (to me) to believe, but yes there is an equation that represents the entire universe. The Friedmann equation is directly derived from the GR field equations, however it is quite possible to derive them from Newtonian mechanics (and a little hand waving). A good source for the Newtonian derivation can be found in Ryden's Indroduction to Cosmology, which is a good undergraduate level cosmology textbook.
There are many forms of the Friedmann equation1 and it can be represented numerous ways (especially depending on the universe model), but I think a convenient form is the following:
$$\bigg(\frac{\dot{a}}{a}\bigg)^2 = H_0^2 \Big(\Omega_{m,0}a^{-3} + \Omega_{r,0}a^{-4} + \Omega_{\Lambda,0} + \Omega_{k,0}a^{-2}\Big) \equiv H_0^2E^2(a)$$
The terms in this equation are defined as follows.

$a(t)$ The size of the universe at time $t$, compared to its size now.
$H_0$ The value of Hubble's constant for the current time. Nominally taken to be $\sim68\:\mathrm{km/s/Mpc}$.
$\Omega$ The "density" of the various types of matter/energy in our universe. The subscript $0$ indicates that it is the density for the universe at the present time. For these, $m$ represents matter (both normal and dark), $r$ represents radiation (i.e., light), $\Lambda$ represents the cosmological constant (i.e., vacuum energy), and $k$ is the curvature of our universe (i.e. energy in the space-time fabric itself).

This equation may seem scary, but effectively it is telling you is how the size of the universe, has evolved over time based on the current content of the universe. If you know precisely how much matter, radiation, and dark energy you have in the universe right now (or more precisely the density), you can calculate the history of the universe!
1Technically there is more than one equation, but we'll look at an important form of one of them.
The Age of the Universe
Remember, $a$ is a function of time in the equation above. This means, with a little bit of rearranging and integrating, we can calculate the age of the universe.
$$t=\frac{1}{H_0} \int_{0}^{1} \Big(\Omega_{m,0}a^{-1} + \Omega_{r,0}a^{-2} + \Omega_{\Lambda,0}a^2 + \Omega_{k,0}\Big)^{-1/2} da$$
To calculate the age of the universe, $t$, you have the non-trivial task of measuring good values for the constants and then doing the integral above.
As a trivial case, you can assume we live in a flat, matter-only universe - no light, no dark energy, no curvature. In that case $\Omega_{m,0}=1$ and $\Omega_{r,0}=\Omega_{\Lambda,0}=\Omega_{k,0}=0$. You'll wind up with $t=2/(3H_0)$ which amounts to ~10 billion years old. This type of universe is known as an Einstein-de Sitter Universe and is obviously not the universe we live in.
A Note on Dark Energy
Technically the equations above are a bit more complicated because I haven't accounted for dark energy. I won't go into the specifics, but one can choose to add components to the equation above which include the dark energy equation of state parameter. Generally this parameter is denoted by $w$. The concepts are more or less the same, the equations just become uglier.
Conclusion
With all of the above, I think I can really address your question.

Does the age of the universe take into account GR/SR?

In short, yes. The Friedmann equation, as I said above, is derived directly from the GR equations. Therefore, we're using an equation to represent the universe which directly results from assuming GR is an accurate theory of gravity in our universe.

Since everything is moving very fast away from us, doesn't relativistic effects from high speeds come into play here? If something is moving quite fast, time slows down, affecting how "old" it is. Is all this taken into account when calculating the age of the universe?

Yes and no. As I said, the age of the universe is ultimately derived from GR so anything GR/SR related is accounted for. However, it seems to me that your understanding of how the universe's age is calculated is somewhat simplistic (and there's nothing wrong with that - its a complicated process!). In calculating the age of the universe, we actually don't care about how fast some far-flung galaxy may be moving away from us. We don't even look at those galaxies. What we really look at is the Cosmic Microwave Background (CMB) Radiation. By looking at this radiation, we're able to work out the $\Omega$ parameters more or less (and the dark energy parameter $w$) without every having to concern ourselves with recessional galaxy speeds. Hopefully you can see that the concepts of receding galaxies and time dilation are moot points when it comes to determining the age of the universe.
Comment: @JoeBlow What you're asking is a fundamentally different question, and most of my answer was trying to show that these types of considerations aren't relevant to calculating the age of the universe. My answer is essentially that "it doesn't affect the age of the universe". What's more the twin example is markedly different from two galaxies receding due to space expansion. In the twin case, they're moving *through* space. In the galaxy case, *space is moving*, but as far a SR is concerned, the galaxies don't move through space wrt each other (aside from peculiar motion).
Comment: "What you're asking is a fundamentally different question" Fair enough, you're quite right. I should ask it separately.  Later:  I did ask a new related question!  :)
Comment: +1 for including those cool looking equations that I have no idea how u type them out on here. It looks really impressive and almost artistic. As for what they actually mean, my eyes glaze over like a dog trying to comprehend blowing bubbles or something.
Comment: @iMerchant Thanks! I hope that even if the equations seem daunting, the point behind them was not lost on you. To make the equations, surround your text with the $ character and use Latex, a typesetting language designed for creating nicely formatted text/equations as you see above. There are plenty of tutorials online to learn Latex from. Try playing around with Latex [here](http://arachnoid.com/latex/).
Comment: @zephyr - I'm allergic to Latex. :-p But I now know how you do it. Very cool.


Was Tibet ruled by Yuan Dynasty empires?

Question: Did Yuan dynasty empires ever ruled Tibet as they claim or was it always an independent state as claimed by The Tibetan Govt in Exile?
Comment: i have changed the question as my perspective was from Yuan dynasty rules for Tibet.
Comment: Welcome to History:Stack Exchange. Thank you for your question; please consider revising it to be more in line with our community expectations. Like many other stacks, we expect questions to [provide evidence of prior research](https://interpersonal.meta.stackexchange.com/a/3227/1599). That helps us to understand the question, and avoids our repeating work you've already done.  Our [help], and [other stacks](https://interpersonal.meta.stackexchange.com/questions/3224/how-do-i-write-a-good-question) provide additional resources to assist with revisions.
Answer: There is an ancient relationship with Tibet, but it is with the Mongols, not with China.
During the Yuan dynasty, Tibet was part of the Mongol Yuan empire, but not part of Yuan China. They were two entirely separate administrative units, that happened to share one ruler. It is true that the office administering Tibet was located in Beijing, but it was an independent office reporting directly to the Yuan (Mongolian) leadership. The wiki link compares it to the India Office in London during the heyday of the British Empire.
During this period the Yuan (Mongolian) rulers of China converted to Tibetan Buddhism. Slowly this faith edged out their traditional native religion and became the dominant religion among the Mongol people throughout east Asia. (Even today, most Mongolians are Tibetan Buddhist.) So Mongolians developed a relationship with Tibet that is somewhat comparable to what happened between Europeans and the Papal State during the Middle Ages. Whoever happened to be the biggest military power in the Mongol World would seek to legitimate themselves as Khan by being the "patron" of Tibet.
On the ground in Tibet the military forces enforcing Yuan rule were Mongolian, not Chinese. When the Yuan dynasty fell in China, so did Yuan rule in Tibet. For the next four centuries, to the extent there was foreign influence in Tibet, it was Mongolian. During the latter half of this period, the Dalai Lamas were effectively clients of the Mongols.
The first non-Mongolian outside rulers of Tibet were the (Manchurian) Qing starting in 1720. However, they also converted to Tibetian Buhdisim and attempted to portray themselves as just the latest powerful foreign patrons of the native religion.
The first instance I can find of Tibet being ruled for a significant period of time as just another Chinese province by ethnic Chinese who were not by and large Tibetan Buddhist was after the PRC invasion in 1950.
Answer: There is an article on Tibet on Wikipedia.

Emerging with control over most of mainland China after the Chinese Civil War, the People's Republic of China incorporated Tibet in 1950 and negotiated the Seventeen Point Agreement with the newly enthroned 14th Dalai Lama's government, affirming the People's Republic of China's sovereignty but granting the area autonomy.

The reasons could be:

In 747, the hold of Tibet was loosened by the campaign of general Gao Xianzhi, who tried to re-open the direct communications between Central Asia and Kashmir. By 750 the Tibetans had lost almost all of their central Asian possessions to the Chinese. (...)
The Qing dynasty placed Amdo under their control in 1724, and incorporated eastern Kham into neighbouring Chinese provinces in 1728. (...)

There is also another article on Wikipedia, which - I think - answers your question:

The position of the People's Republic of China (PRC), which has ruled mainland China since 1949, as well as the official position of the Republic of China (ROC), which ruled mainland China before 1949 and currently controls Taiwan,[23] is that Tibet has been an indivisible part of China de jure since the Yuan Dynasty of Mongol-ruled China in the 13th century,[24] comparable to other states such as the Kingdom of Dali and the Tangut Empire that were also incorporated into China at the time.
The PRC contends that, according to international law and the Succession of states theory,[25] all subsequent Chinese governments have succeeded the Yuan Dynasty in exercising de jure sovereignty over Tibet, with the PRC having succeeded the ROC as the legitimate government of all China.[26][27]
Comment: I **really** wouldn't trust wikipedia over this matter…
Comment: @Lohoris The answer was in fact given to show it took me 10 minutes to find *anything* about the subject.
Comment: @Voitcus did you consider that a long search time? or a short one?
Comment: I think if the OP would "waste" an hour of searching, they would find all necessary information
Comment: This is wrong for ROC, if you look at the link.  (The Republic of China (ROC) claimed that "Tibet was placed under the sovereignty of China" when the Qing dynasty (1644–1912) ended the brief Nepalese rule (1788-1792) from parts of Tibet in c. 1793.).


If longlines-mode is removed in new emacs, how can I soft-wrap the line around the column instead of window edge?

Question: I'm now trying to switch from Vim to Spacemacs. It seems that Longlines Mode is the way to go in Emacs to soft-wrap the line around the column http://emacswiki.org/emacs/LineWrap. However it seems that this mode has been removed since Emacs 24.4 http://www.emacswiki.org/emacs/LongLines, in favor of Visual Line Mode, which soft-wraps the text around the window edge... Which is not exactly what I want. How am I able to get the functionality of longlines mode back? I see a longlines.el at http://www.emacswiki.org/emacs/longlines.el, should I just manually download the file and add it to Spacemacs configuration layer?
Answer: No. To emulate the behaviour of longlines-mode, use visual-line-mode in combination with the visual-fill-column package.
Comment: Thanks. This seems to be the package to go to. However now I'm unable to install it in Spacemacs... Guess I'll turn to somewhere else for help on package intallation.
Answer: It has not been removed (yet).  It has been marked obsolete, which among other things implies that it is not autoloaded any more.
(require 'longlines)
(longlines-mode 1)

still works as before.
Comment: That's true. I guess I misunderstood the meaning of "obselete" here. Though isn't it quite weird that `longlines-mode` is able to softwrap by `fill column` while `visual-line-mode` is only able to soft wrap around the window, while the later is considered as a replacement for the previous? It actually looks like a loss of important functionality. `visual-fill-column` package messed up my `popwin` buffer so I couldn't really use it.


Can a Mexican Citizen who does not have a Mexican passport use a CBX with a boarding pass to fly from the USA to Mexico?

Question: I am a Mexican citizen with a valid US-issued border crossing visa (CBX). I do not have a Mexican passport but I do have valid ID to prove I am a Mexican citizen.
I will have a valid boarding pass for a future flight from Tijuana to Veracruz, Mexico. Can I use the CBX for boarding from the US side, or do I need to go to Tijuana through the normal border crossing and then go to the airport?
Comment: Is your "border crossing visa" in a form of a border crossing card issued by the US government? Then yes, you can use it. See https://www.tsa.gov/travel/security-screening/identification - a border crossing card is a valid document.
Comment: @GeorgeY. My understanding is that the relevant list here is what the Mexican immigration and airport authorities require. TSA isn't involved at all here, as far as I know, assuming that the plan is to use the Cross Border Express to fly out of Tijuana.
Comment: As I read the question, you want to walk across the Cross Border Xpress from San Diego, showing your US-issued border crossing card, boarding pass, and Mexican ID, and then board a domestic flight from TIJ to Veracruz. Do I have that right?
Comment: Are you inside the US? If not why do you need to cross the bridge?
Answer: CBX (assuming you are referring to the Cross Border Express ticket) is NOT a visa, it is simply a ticket that allows you to use the sky walk that connects the USA check in area to Tijuana Airport.
To cross the skywalk either direction (into USA or into Mexico) you need your passport, boarding pass, CBX pass and any necessary visa.  The boarding pass needs to be for a flight departing within 24 hours or having arrived no more than 4 hours earlier.
Theoretically you should be able to check in on the USA side. Your flight is a domestic Mexico flight, so technically your Mexico ID should suffice for ID, since it will be Mexican authorities who do any security screening for the actual flight not TSA.
But then there is the issue of a passport being required to use the sky walk.  You might trying emailing firstname.lastname@example.com for clarification on your specific case.
Might not hurt to look into to ironing out your passport issue in advance, getting it renewed or replaced, etc.


How do you avoid caching during development?

Question: Is there a simple way to prevent all caching when testing the appearance of changes to the site? I use WP Super Cache.  I can delete its cache using the option provided, delete the cache for my browser, and still some changes to CSS or widgets do not refresh. I try other work-arounds like switching browsers or computers, but there must be a more stream-lined workflow where I can ensure I'm viewing the changes I made and not some cached earlier format?  What's the best solution for this?
Comment: It seems there are also [several plugins](http://wordpress.org/extend/plugins/css-cache-buster) just to do this for CSS.  Is it really necessary?  Do those plugins do something that clearing browser cache does not?
Comment: In my case it turned out I had to clear the cache created by the DNS provider (cloudflare). Thanks all for the suggestions below though.
Comment: I use Chrome browser; Its incognito window comes handy, when I come across browser level cache issues during development.
Comment: Hope this plugin will help you: https://wordpress.org/plugins/prevent-browser-caching/
Answer: Add the filemtime() of your stylesheet as version parameter. Lets say, your default stylesheet is in css/default.css and css/default.min.css (not style.css). When we register a stylesheet on wp_loaded (not init), we can pass a version as fourth parameter. That will be the last modified time and therefore change every time we change the file.
$min    = WP_DEBUG ? '': '.min';
$file   = "/css/default$min.css";
$url    = get_template_directory_uri() . $file;
$path   = get_template_directory() . $file;
$handle = get_stylesheet() . '-default';

// Overridden?
if ( is_child_theme() && is_readable( get_stylesheet_directory() . $file ) )
{
    $url  = get_stylesheet_directory_uri() . $file;
    $path = get_stylesheet_directory()     . $file;
}

$modified = filemtime( $path );

add_action( 'wp_loaded', function() use ( $handle, $url, $modified ) {
    wp_register_style( $handle, $url, [], $modified );
});

add_action( 'wp_enqueue_scripts', function() use ( $handle ) {
    wp_enqueue_style( $handle );
});

If you are using Node.js and Grunt, I strongly recommend Browsersync. It will watch your files and update them instantly whenever they change. It can also synchronize the scrolling position, form submissions and more across multiple open browsers. Very cool.
Comment: Many thanks.  It appears this is also the approach of the above plugin.  It did not solve the problem in my case because my DNS provider (CloudFlare) was caching the css file, so I had to clear the cache there -- guess there is no simple way around that.  In general though, I think this is the best answer to this kind of issue so I'll mark accepted.  Thanks again.
Comment: Why don't you use your local stylesheet during development?
Comment: Some cloud caching services will hard cache your file for 8+hours, so you either have to request them to implement a better service for versioning (some do this), change services,  or stop using them.
Comment: @cboettig CloudFlare has a dev mode setting which will stop caching for a 3 hour window. It then automatically resumes caching after 3 hours.
Answer: After searching for a simple solution many times i decided to find something that works!
so... after thinking about it i found a great way to override caching while developing new websites... (and its easy).
What we need is to tell wp this is a new CSS version like this...
Before changes:
wp_enqueue_style( 'maincss', get_template_directory_uri() . '/css/style.css', array(), false, 'all' );

After changes:
wp_enqueue_style( 'maincss', get_template_directory_uri() . '/css/style.css?v='.time(), array(), false, 'all' );

This is what we added:
?v='.time()

Explanation:
We are basically adding a dynamic version number to the css file which forces the browser to load the new css every time we update it.
Don't forget to remove it after your done developing otherwise your caching won't work for this file and it would load for returning users again and again.
This technique works for css & js files - hope this helps ;)
Comment: Pretty nice, but better still, just use time() in the 4th parameter which is version. Which would give you: `wp_enqueue_style( 'maincss', get_template_directory_uri() . '/css/style.css', array(), time(), 'all' );
`
Comment: Using time as the 4th parameter might cuase chrome to hold on to that version. This is especially true if your caching everything else while you work on a live site. This way you are submiting a "new file" to chrome which he would use instead of trying to keeps that "old" version for an amount of time (as set for other resources).
Comment: Nope, works fine in chrome, I was using it yesterday.
Comment: that means you havent set cache / expiration time in your htaccess or via a plugin. But, whatever works for you ;)
Answer: This might seem overly simple, but how about just disabling caching until you're done with the development portion of your site? It's more than simple to turn on and off.
Comment: +1 - I use "Web Developer" toolbar for Firefox, allows you to very quickly disable cache (among other things)
Comment: Thanks - is it only necessary to disable the browser's cache?  Will WP superCache still result in providing cached content? Even my DNS service caches, so I am uncertain if I need to disable every cache (WP, browser, DNS, etc) separately.
Comment: Sorta depends, you'll have to play around with it and figure out what you need to do for your configuration.
Comment: Im with him, just disable it, thats what I do.
Answer: I know that this question has had an answer accepted, but I think that that answer is still too complicated for the problem at hand, and may actually be incorrect depending on the user (no offense though), so I thought I'd still share how I bypass caching when I do my dev (not just with Wordpress).
Most modern browsers have something called incognito mode. In this mode, nothing in your computer is cached, so every refresh is a fresh slate download off the server. In Internet Explorer you press Ctrl + Shift + P. In Firefox and Chrome, you press Ctrl + Shift + N.
If your browser doesn't have incognito mode, you can normally force a hard reload by pressing Ctrl + F5 for IE, or Ctrl + Shift + R on Firefox and Chrome.
As for your question regarding the CSS files (and essentially, all your asset files, like images and Javascript files), those aren't cached in any way by WP Super Cache. Your settings and/or use of this plugin does not affect how those files are served. What's caching those files are your browser, and that's the reason why you do a hard reload.
What the plugin does is it evaluates how Wordpress builds your HTML files (via PHP), and stores a copy, so that the next time someone requests the same post, page, or whatever, it serves the copy, and won't have to reevaluate the PHP-generated HTML again, and therefore save some computing time, loading your pages that much quicker. (I hope that's clear.)
The problem with that is, if you're slapping on a timestamp on your CSS files' URL via a PHP function, that is a PHP evaluation to HTML, and that will be cached by WP Super Cache. Every request to the same post will have the same timestamp because users are being served a copy of the original timestamp evaluation. (Correct me if I'm wrong.)
The correct way to bypass WP Super Cache's caching is to set the option Don't cache for known users to true in the plugin's setting page.
Finally (and this is a personal preference, as I'm a real stickler when it comes to coding), resorting to the use of incognito or forced hard reloads won't require you to add unnecessary markup on your HTML pages. Of course, adding a timestamp only adds about 13 bytes per static file per request, but hey, like I said, I'm a stickler for this kind of stuff. It's still 13 bytes unnecessary.
Comment: Thanks for the useful tips. Unfortunately none of the answers here are correct in my case, since my solution required clearing the cache of the Domain Network Service provider, cloudflare, but many were still good information.
Answer: What worked for me running Wordpress on a Bitnami stack under Windows was to edit my php.ini file and restart my server.
In the [opcache] section change the opcache.revalidate_freq to 1
opcache.revalidate_freq=1

I then restarted my servers and my pages now immediately show changes made to my plugins.
Answer: Gosh, many ways to answer this one! First and foremost, you asked about two different things: WP Super Cache and CSS files. These are cached differently, in different places, so it's important to recognize where your problem is.
If WP Super Cache, you can define the constant DONOTCACHEPAGE in your functions.php during development to prevent WP Super Cache from caching anything. Don't forget to remove this on launch though!
define('DONOTCACHEPAGE', true);
Each site also has a unique key to append to the URL to load a fresh version of the page, which you can find in the "Advanced" tab, I believe.
Breaking it down to an even better solution, you should consider setting up a development environment and a production environment, where your development environment doesn't have WP Super Cache enabled (again, assuming that's your problem).
If your issue is with CSS/JS files, then see the answer by toscho and subsequent comment by m0r7if3r above.
Answer: HyperCache disables caching when you're logged in as an admin. Not sure whether WP Super Cache has the same functionality.
Comment: There is an option for that, yes.
Answer: If you're using Chrome (which I highly suggest), open Inspector, click the settings icon in the lower right corner, and under "Network" select "Disable Cache."
Answer: As said for wp super cache, but for general WP caching in wp-config.php
Change to this:
define( 'WP_CACHE', false );

Reference: codex.wordpress.org
Comment: This was actually the only solution to the problem. Thanks!
Answer: You can use this snippet https://gist.github.com/jhayiwg/92bae4330aeb738a98022d7ab63ce9b1
It will generate new version of your active theme css and js every time you load the page
Comment: Please, post the answer here, including all the relevant code. Use external sites only for reference. Your post should provide a full answer without the need to go to other site; if the third party site goes down, your answer will be unuseful..


Applying for 2 jobs with the same employer

Question: I am looking at two opportunities with the same employer, either one I would be very good at.  Should I apply to both?  Or should I pick only one?  I worry that applying to both makes me look like I'm wallpapering the internet with my resume, whereas if I apply to just one, I worry about missing out on the other opportunity if my qualifications come up short.
Answer: Apply for both.  You never know which one will be the one they want to interview you for. They may have different hiring managers who also have no idea you are applying elsewhere in the organisation or if it is the same person, he may be happy to know you are interested in both because he may have lot of good candidates for one and nobody but you that is good for the other.
If it gets to interviewing though, let them know you have applied for both jobs.  If both managers are interested, then they might interview you in the same session and they will have a chance to consult together to determine the best fit if both want to hire you. You wouldn't want to start a new job with a manager mad at you because the other guy stole you away from him and he didn't even know the other guy was looking at you.
Comment: +1 there's nothing wrong with applying for both, especially if it's just one person. It gives them the option to put you in the position they think you'd be better for. In fact, a lot of companies I've applied to in the past allow you to select all the positions you'd be interested in, rather than applying for one specific position.
Comment: +1. Even if you are applying for two different positions and you are qualified for both of them, go for it.  Just like a company is evaluating you if you are a fit for the positions, you are also evaluating the two positions as well.
Comment: Both jobs are under the same assistant director.  One of the jobs is more of an administrative/strategy managerial focus, while the other is more of a project-management/software dev position.
Comment: @HGLEM : And what about if HR mail's contain his name and is the same on the two offers?
Answer: Talk to their HR person.  Ask them how, based on the size of the company, industry they are in and culture, whether it would make sense to do this.
Personally I would recommend you pick 1 and go for it because:
Imagine the conversations otherwise -

yeah, since you asked (e.g. found out), I also applied for position Y because, er, ...
I'm excited about this position.  What about the other one?  well I'm excited about that too...
Well, I didn't get position x with Bob, but now I'm applying for this one because, er...

As you can see, while there certainly may be clever answers to the above, the overall impression I'm trying to give is that this is just not a place you want to be in !
Comment: Talking to their HR rep is a smart idea, I think I will do that.  Thanks!
Comment: I disagree with picking just one, but I can't agree more that speaking to HR is a great idea.
Comment: I doubt anyone would view wanting any of multiple similar positions at the same company negatively. If the positions were vastly different, it might be viewed negatively, but it might also be view positively (in that you're interested in multiple things the company does). I don't really get why you're recommending telling the HR person you want to apply to multiple positions, but then recommend only applying to one - whatever you say to the HR person could very well end up in your application (HR is not your friend, especially if you don't even work there yet).


Can I apply polyurethane to my hardwood floors in sections?

Question: I have a hardwood floor I've recently finished staining. I'd like to apply polyurethane but I don't think I can cordon off the entire area for the duration. Is it alright to apply the sealer in sections?
Comment: If you can block off the entire area for four hours you can try waterbased urethane it dries quicker than the solvent based type. It dries quicker when the weather is cooler and drier then in humid weather. You can put down a coat,go to bed then light sand and second coat in the morning.
Answer: Surface finishes are notoriously difficult to get smooth when the process is interrupted.  The glossier the finish, the harder to have sections blend.
If you are talking about preliminary coats, especially if they will get a light sanding between coats, this is probably ok. For the finish coat, I would strive hard to do it all in one shot.
If you simply cannot, try to find a transition area that will be less noticable, such as an area covered by rugs or furniture.  You also might try cutting the transition from one section/session to another along a long board line, but this is hard with poly.
Comment: If it is absolutely necessary to do it in two sessions, run some blue painter's tape exactly on a joint in the floor. Be sure to pull the tape immediately after putting down the urethane so it has no chance to start setting up.
Comment: Thanks for the explanation. I tried looking around online but didn't really find anyone addressing the problem itself. I'll probably end up goin the extra effort to make sure I don't screw it up. =D
Answer: I highly recommend NOT seperating the coats. You will see a visible line where you stop and start again and the sheen won't exactly match. Definitely go with a water-based polyurethane as Mike suggested for a quicker dry time. I know it's exciting to want to complete the project but don't be afraid to be patient with your floors. They're a lifetime investment and worth the care and additional wait.


Swagger behind Nginx instead of Zuul

Question: How does Zuul make it swagger on http://foo-service:8080 works correctly for external access from http://public.com/foo-service ?
I have a task to replace Zuul with nginx (It's not my decision, I can't change it). I rewrite in nginx http://public.com/foo-service/names-controller/swagger-ui.html to http://foo-service:8080/names-controller/swagger-ui.html , but in swagger response I get broken links - without /foo-service/ that is I get http://public.com/names-controller... instead of http://public.com/foo-service/names-controller/....
With Zuul it was working fine out of the box - magic.
Is there any http request header Zuul is setting that is used by swagger?
How does Zuul make it Swagger works fine?
How could it be done in Nginx?
Answer: Hip Hip ...
<code>proxy_set_header X-Forwarded-Prefix /foo-service/; </code>
helps.

Swagger behind Nginx instead of Zuul


Why is it trivial that $\left(1+\frac{2\ln3}{3}\right)^{-3/2}\leq\frac{2}{3}$?

Question: Can someone tell me why $$\left(1+\dfrac{2\ln3}{3}\right)^{-3/2}\leq\dfrac{2}{3}$$ is trivial because for me its not and I will need to do the calculation to see it.
Comment: That's not trivial for me as well.
Comment: It's trivial. The left hand side is 0.438556..., which is obviously smaller than 2/3.
Answer: I imagine something along the lines of this, where each step is a gross simplification
$$ \left(1+\dfrac{2\ln3}{3}\right)^{-3/2}
\leq  \left(1+\dfrac{2}{3}\right)^{-3/2}
=  \left(\dfrac{5}{3}\right)^{-3/2}
= \left(\dfrac{3}{5}\right)^{3/2}
\leq \frac{3}{5}
\leq
\dfrac{2}{3}$$
Comment: @JoelRondeau but x^(-3/2) is a decreasing function.
Comment: Thank you. Of course. Will leave that here so no one else makes the same mistake.
Comment: Your first step is incorrect. 2 ln 3 > 2.
Answer: It's not trivial if you have to prove this "with bare hands"; see Hurkyl's answer.
But it's trivial if you have a pocket calculator at your disposal. So I'm interpreting the phrase as follows: The author was on his way to heavier and more important things, and he didn't want to interrupt his argument with a proof of this little fact. Depending on circumstances and the envisaged audience he could have made a "Lemma" out of it.
Comment: +1. "Trivial" does not mean "obvious" or "quickly proven". It means that, whatever amount of work is needed for the proof, it is uninteresting grunt work. Of course, being obvious makes a proof also trivial, but people forget that not everything trivial is also obvious.
Answer: Taking reciprocals of both sides gives the equivalent statement
$$
\left(1+\frac{2\ln3}{3}\right)^{3/2}\ge\frac{3}{2},
$$
which is fairly obvious once you realize that $\ln3>1.$  The latter follows from the fact that $e<3,$ which is not hard to see.
Answer: We can see everything is positive, so just take reciprocal and square both sides:
$\left(1+ \frac{2 \ln 3}{3}\right) ^ {\frac{3}{2}} \ge \frac{3}{2}$
$\left(1+ \frac{2 \ln 3}{3}\right) ^ 3 \ge \frac{9}{4}$
Bring down the three inside the bracket (first step of expansion), since everything is positive the extra terms will be positive:
$1 + 2 \ln 3 + \text{stuff} \ge 1+ \frac{5}{4}$
Which is clearly true since $\ln 3 > 1$
Answer: You have that
\begin{align*}
\left(1+\frac{2\ln3}{3}\right)^{-\frac{3}{2}} &= e^{ -\frac{3}{2}\ln\left(1+\frac{2}{3}\ln3\right) } \\
&=e^{ \frac{3}{2}\ln\frac{1}{1+\frac{2}{3}\ln3} } \\
&\leq e^{ \frac{3}{2}\ln\left( 1-\frac{1}{2}\cdot\frac{2}{3}\ln3 \right) } \\
&\leq e^{ -\frac{3}{2}\frac{1}{3}\ln 3 } = e^{ -\frac{1}{2}\ln 3 } = \frac{1}{\sqrt{3}} \\
&\leq \frac{2}{3}
\end{align*}
where was used the "fact" that $\frac{1}{1+\frac{2}{3}\ln3}\leq 1-\frac{1}{2}\cdot\frac{2}{3}\ln3$ (proving it is not hard) and $\ln(1+x)\leq x$ (for $x>-1$).


Why is FIFA against adding instant replay to the game?

Question: Is there any evidence or reason why FIFA is so against adding instant replay to soccer? I find myself seeing many unlucky or just plain wrong decisions made by refs that could be corrected with instant replay.
Comment: I dont agree with the accepted answer below.  One red card, one penalty and even one offside decide a game and a tournament. Challenging (like in tennis) several per match wont break the fast paced game. Lack of technology in some places shuold not apply to tournaments. Uncertainty could be ok but not for tournaments.
=== The reason ladies and gentlemen is: CORRUPTION.  There is a lot of money in betting , airing and even hosting matches; enough to keep many mouths shout. There have already been too many scandals
Comment: Sepp Blatter and Platini were both against technology of any kind (hence no goal line technology in the champions league) but now they have been deposed, trials of video replays are due to take place in some competitions this season.
Comment: Things change and many answers to this question have become obsolete as FIFA used instant replays in many competitions. See my [answer](https://sports.stackexchange.com/a/19915/12991).
Answer: There are a few fundamental considerations:

Uncertainty is a part of the game. The referee and linesmen are the arbiters of the action. Instant replay would limit their authority.
Football is global. Many places do not have access to the technology necessary to implement instant replay.
Football is a fast-paced game with few opportunities for stoppage. Instant replay would unnecessarily slow the pace.

In 2008, FiFA President Sepp Blatter said:

"Let it be as it is and let's leave [football] with errors. The television companies will have the right to say [the referee] was right or wrong, but still the referee makes the decision – a man, not a machine."

Blatter and others at FIFA have repeatedly argued against using instant replay. But lately, FIFA has begun to come around on the idea. At FIFA's Congress in São Paulo prior to the 2014 World Cup, Blatter proposed allowing managers 2 challenges per game, much like the NFL handles challenges today.
Although it's not quite "instant replay," in 2012 FIFA approved two different goal-line technologies to improve referee accuracy: "Hawk-Eye" -- which uses visual triangulation to determine ball location -- and "GoalRef" -- which uses a microchip in the ball and magnetic sensors within goalposts. (Many say FIFA approved the technologies in response to Frank Lampard's snubbed "goal" against Germany in the 2010 World Cup.) In 2013, FIFA announced it would use GoalControl, a German competitor to Hawk-Eye, at the Confederations Cup, basically a rehearsal for the World Cup. After a successful run at that tournament, FIFA announced it would implement GoalControl at the 2014 World Cup. The Guardian provides a pretty helpful FAQ on the technology.
FIFA may choose to implement further changes to approved goal-line technology and referee rules when it meets again after the World Cup.
Comment: ... The beauty of football is that multi million players from EPL are playing the same game with the same equipment as any other football player in the world and I think Fifa is trying to keep it as it is.
Comment: Wow, talk about putting up or shutting up! Good on you, Sam. I reviewed your edits and while they were substantial, it stayed very true to the spirit of the answer I wrote two years ago, and approved the edit. Thank you!
Comment: I agree that it mostly has to do with time, and the interest in keeping the game moving.
Comment: The second point is one that I hadn't even considered, but it makes total sense. Right on!
Comment: You can counter both points.  1. Errors are made everywhere, in all sports.  And as long as a man makes the final decision (and even if a machine did... computers *can* err), you can still have errors.  2. Sure, sounds good.  But then again, FIFA has quite expensive standards for acceptance *into* FIFA.  Stadiums have to have floodlights (at a certain level of competition).  Not cheap.  So?  And even in the developed nations, not all leagues are equal.  Some have (FIFA or national body mandated) great fields, some have a "Bolzplatz".  Same thing: top leagues require video reply, lower don't.
Comment: I can accept your first point, though I don't think is right to allow some kind of mistakes that can decide a tournament for example. Your second point reminds me to tennis, where they use in some tournaments the Hawk-eye ( http://www.hawkeyeinnovations.co.uk/?page_id=1011 ) although it's not available in many places.
Comment: @JürgenA.Erhard There could be problem with having videos replays only for some (elite) leagues. One scenario that has been discussed is in an international tournament. This would skew the match in favour of the more developed nations. Take for example a match between France and Senegal. Most of the French players would be exposed to video replay technology and they would be better placed to use them compared to players from Senegal. (I am assuming that it works similar to how it is tennis where each player has a set of reviews which he can make use of)...
Comment: @rest_day: Interesting point.  But the multi-million players don't really play the same game, they have very different resources at their disposal to begin with.  As I said in my answer, there *are* differences due to, of course, money available.  And from what I know/understand from other instant-replay team(!) sports, it isn't like tennis.  Tried to find out details, but that seems hard.  Hmm, time for a question? :D
Comment: Read my answer at the bottom, FIFA announced on using goal line technology from 2013!
Comment: Basically it's because Football is such a huge and old sport run by old white men with so many different interests it's difficult for any change to happen.  The idea that a kick-around in the park is played in the same way as a top league game is laughable.  I've only ever played once with a ref and never with assistant refs.  If technology can be used to assist referees make correct decisions quickly, bring it on.
Comment: If we had instant replays in fifa, the game would take an entire day.
Comment: This answer can use an update. For one, I'm not sure the main point is correct: that "mistakes are part of the game." If so, please add a citation. Also, this answer omits the assertion that instant replay would limit the authority of the referee. Also, goal line technology is now implemented.
Comment: This answer (and all answers) is creative commons licensed and can be edited by anyone at any time. If you feel you have improvements or updates that could be made to it, feel free to make them.
Comment: goal-line technology is approved and is in use in this world cup 2014
Comment: Absolutely, Sam brought the article current yesterday, importantly noting "After a successful run at that tournament, FIFA announced it would implement GoalControl at the 2014 World Cup."
Answer: corsiKa touched on time, but I wanted to add what I think is the primary point.  Perhaps not for FIFA, but personally.
Soccer is a game of motion, timing, and momentum.  It is not a game of plays like American football.  Scoring and penalties especially are relatively uncommon, while throws and even free kicks and crosses are mostly taken pretty quickly while players jostle for position.
Interrupting this flow ruins the game.  Ever played on top of a hill with people who kick too hard?  Same thing.  You spend too much time standing around while the ball's being fetched.
I'd certainly be fine with goal-line technology as it's come of age and is reliable and quick, though regardless I still think the real solution is better training for officials.  Most errors could be easily corrected by properly trained linesmen. Refs as well, but mostly linesmen. There's no excuse for not being level enough with the play to call offsides and goals properly.
Comment: @ChrisH: in terms of eysesight and cognition it's pretty well *impossible* to call offside correctly. It's hard to pay attention to the ball and the player at the same time when they might be separated by 90 degrees in your field of view, even more if you aren't level with the player. Worse, the mind plays tricks in respect of what appears to be simultaneous when the events are widely separated. So, lo and behold, assistant refs get "easy" ones wrong all the time. The issue is whether it would improve the game to correct them with technology, they *cannot* be eliminated.
Comment: Well, I would agree, except that hockey now has instant video replay, and its flow is very similar to soccer. I think the main thing is that the clock never stops in soccer except for halftime, so the refs are expected to resolve any stoppages quickly. If instant replay were added, we'd see 10 and 20 minutes of extra time as refs had delayed play to review video.
Comment: I don't think the flow of hockey is remotely similar to that of soccer.
Comment: I've stood by the side of a pitch while an amateur 11-a-side was being played.  It's really really hard to call off-sides correctly (well, who knows if you're correct as there was no video replay).  So I'd be cautious on blaming the officials.
Answer: There is another problem with applying instant replays beyond just interrupting the flow of the game. It all stems from the nature of football in sometimes lacking breaks in play for quite some time. So how will this cause a problem?
Well, let us say that the referee is allowed to use a radio link to the video referee. A long ball is played up to player X, a striker who breaks away into a one to one against the keeper, but who may possibly have been offside.  At present the referee (aided by his assistant) makes an instant decision. And if we see he gets it wrong (once we have watched the replay from five angles), we all moan at him.
But, even if video replay was allowed, the referee would not stop play to use it, since if player X was onside then the ref has just stopped a possible goal scoring move anyway. He needs to wait for a break in play.
So the referee lets the move continue:  If X scores then the video can be checked and a goal awarded or disallowed. Even if it results in a goal kick or corner then we have a natural break to check the replay.
But what if when play is allowed to continue, X shoots; the goalie pushes the ball out; it is played around by the players of X's team for half a minute or so (still too close to goal to just stop play); and then X's team mate Y dashes into the penalty area with the ball at his feet and is viciously fouled by defender Z... a clear sending off offence. The ball however runs on into the goal.
At which point the replay referee sends down a message that the original ball to X was offside!
What does the poor ref (aided by modern technology) do now?

Disallow the goal for an offence that happened a while ago. (Believe me, players moan if the ref is seconds late with a whistle.)
If he does disallow it (as he should) what about the sending off offence? The defence will argue forcibly that player Z should never have been in the position of having to foul Y since the game ought to have stopped a while ago. And imagine the nastiness from Y's team if Z is let off.  Whichever decision is made will not seem right to one team.
All this will be even more difficult if Y has a broken leg...

This is just a simple example using offside.  The same problems or worse can occur with just about any decision that needs to be verified by another ref using a video screen. This is why IFAB (the law making body of world football) are insisting that any way of checking if the ball has crossed the line for a goal, needs to be instant. Goal line technology will probably arrive: but I think that anything like the above, that needs even a brief time for analysis, falls into huge potential problems if play has to continue.
At least now, right or wrong, an instant decision is made and the game stopped or allowed to continue. Mistakes will often be made, but we all just have to accept that, and get on with the game. So why shouldn't players just accept my imagined scenario too? Because once video technology is accepted for some decisions, then players will expect correct calls at all times... and the free flowing nature of football (unlike cricket, gridiron, rugby which are all played in short bursts of action) is not suited to this technology.
Comment: In cases where the technology isn't available, you simply ignore the rules defining review procedure, and the ref;'s word is final, like it was before review. To say that instant replay shouldn't be allowed because it can't be implemented everywhere is to say that video review shouldn't be allowed in the NFL, or the NBA, or the NHL, or NCAA sports, because high school and adult club teams can't afford to implement it.
Comment: As a counter argument: As a referee, the possible offside of X would be resolved the moment that the goalie saves the shot from X by applying the 'advantage rule'. Everything after that would then be unaffected by the possible offside situation, no need for a review.
Comment: There are calls that are and are not reviewable in pretty much every sport that uses instant replay review. NHL hockey now has booth reviews, and off-sides calls (the closest thing hockey has to onrushing) are one of those things that is still totally at the ref's discretion. Icing is, AFAIK, not reviewable either (and refs can wave off icing if they think a defensive player could have gotten to it before it crossed the goal line). Reviews are used mainly to make sure goals were legal and to assign goals and assists, double-check penalties, etc. Soccer could do much the same.
Answer: Today FIFA approved using Goal-line technology in soccer!

Football's lawmakers have taken the historic step of unanimously
approving goal-line technology systems for use in the sport.
World soccer's global governing body FIFA and the International Football
Association Board (IFAB) made the announcement following a meeting in
Zurich on Thursday.
FIFA intend for goal-line technology to be used at
December's Club World Cup in Japan, and if successful it will also be
implemented at the 2013 African Cup of Nations and the 2014 World Cup
in Brazil.
Two systems, designed by technology companies GoalRef and
Hawk-Eye, have been approved after going through two phases of FIFA
testing.

EDIT:

After a successful implementation of Goal-Line Technology (GLT) at the
FIFA Club World Cup in Japan in December 2012, FIFA has decided to use
GLT at the FIFA Confederations Cup Brazil 2013 and 2014 FIFA World Cup
Brazil™.

You can see demonstration of this technology at the following video
Sources: 1 2
Answer: Unlike in cricket, its really hard to stop the game in between a football game. But the FIFA is working really hard to stop this injustice to certain teams (ex: England's disallowed goal against Germany in WC2010 & Chelsea's ghost goal in the FA cup 2010 semi-finals against Tottenham Hotspurs)
May be a goal-line camera technology will help FIFA in overcoming this issue.
Answer: Actually FIFA has changed some of the rules. Not really an instant replay, but still huge progress. The German company Goal Control has been handed the contract by FIFA to provide goal line technology at the World Cup in Brazil.
There will be 14 cameras – seven for each goal mouth – mounted at or near the roof of the stadium, capturing images at 500 frames per second, installed at all 12 World Cup venues.

Seven cameras will monitor each goal and computers will relay a message to the referee as soon as the ball crosses the goal line. (Source: Reuters)

They will ensure that no controversial goals are allowed or legitimate ones disallowed. The referee will have a special watch which will vibrate with a flash message reading "GOAL" once the ball crosses the line.
The company's boss Dirk Broichhausen also revealed that the system cannot be hacked as it is an "Off-line" system and did not rely on the internet, assuring that the technology is foolproof.
Nearly 2400 tests were carried out to assure that the system was accurate even when cameras were blocked by players, and has minimal margin of error, a plus-minus margin of just 1.5 centimeters.
These measures taken by FIFA have given fans and players hope that the tournament will be the most competitive one yet along with being fair and just.
Source
Answer: There is NO good reason for the failure to implement video plays in professional soccer.  Far from "breaking the flow of the game"  it would actually prevent all those ridiculous delays where the players protest the referees decision. When a video replay will reveal the truth there will be no point in arguing, just as there will be no-point pretending that the merest touch from an opponent carried the force of a sledgehammer, or that a ball hitting the shin somehow caused a facial injury.
We would also get offside decisions correct.  It is almost impossible for a linesman to see the position of the advanced forward at the moment the ball is kicked. IN most situations it is just physically impossible.  It is a miracle that they get borderline cases right as often as they do.
I have watched football all my life and I am sick and fed up of seeing games spoiled by incorrect decisions.  It is not as if bad decisions are rare. They happen in just about every professional game in the English Premier League.  I am also sick and fed up of seeing rich and fortunate grown men behaving like spoiled brats - but that is a different matter.
FIFA likes to present itself as a caring organisation with things like promoting the game in the 3rd world and its RESPECT program, yet youngsters all over the world are being given a terrible example by some of their footballing heroes ... that cheating pays.
The only people that can benefit by ensuring that football games continue to be riddled with awful decisions are the cheating players that try to get unwarranted penalties, or to get their opponents unfairly yellow and red carded, or organisations that want to keep the ability to make sure that we get the "right" result in important matches.  Most fans, and I suspect most players, want to see results decided by good play, within the rules.
Why don't the national organisations simply leave FIFA? The organisation is evidently corrupt, its senior officials are hugely overpaid, and some of them are criminals who have somehow evaded the prosecution that most of us would have faced had we fiddled our financial affairs that badly.
Comment: A lot of unsubstantiated claims here. Can you add some citations to help support your argument?
Answer: FIFA was against instant replays, but not anymore.

Since IFAB (International Football Association Board) approved live experiments with video assistance for clear errors in match changing situations in its Annual General Meeting in 2016 (pdf), FIFA has fully implemented Video Assistant Referee technology in various competitions played under its auspices:

FIFA Club World Cup (starting from 2016 edition)
FIFA Confederations Cup (starting from 2017 edition)
FIFA World Cup (starting from 2018 edition)

Earlier though, FIFA wanted to retain the human element in the game. In 2005, Urs Linsi, FIFA General Secretary said:

Players, coaches and referees all make mistakes. It's part of the
game. It's what I would call the "first match". What you see after the
fact on video simply doesn't come into it; that's the "second match",
if you like. Video evidence is useful for disciplinary sanctions, but
that's all. As we've always emphasised at FIFA, football's human
element must be retained. It mirrors life itself and we have to
protect it.