explain xkcd - User contributions [en]

Talk:1047: Approximations

2013-09-08T00:51:25Z

67.188.195.182:

They're actually quite accurate. I've used these in calculations, and they seem to give close enough answers. '''[[User:Davidy22|Davidy22]]'''[[User talk:Davidy22|<tt>[talk]</tt>]] 14:03, 8 January 2013 (UTC)

I only see a use for the liters in a gallon one. The rest are for trolling or simple amusement. The cosine identity bit our math team in the butt at a competition. It was painful. --[[User:Quicksilver|Quicksilver]] ([[User talk:Quicksilver|talk]]) 05:27, 17 August 2013 (UTC)

Annoyingly this explanation does not cover 42 properly, it does not say that Douglas Adams got the number 42 from Lewis Carroll, who is more relevant to the page because he was a mathematician named Charles Lutwidge Dodgson. He was obsessed with the number forty-two. The original plate illustrations of Alice in Wonderland drawn by him numbered forty-two. Rule Forty-Two in Alice in Wonderland is "All persons more than a mile high to leave the court", There is also a Code of Honour in the preface of The Hunting of the Snark, an extremely long poem written by him when he was 42 years old, in which rule forty-two is "No one shall speak to the Man at the Helm". The queens in Alice Through the Looking Glass the White Queen announces her age as "one hundred and one, five months and a day", which - if the best possible date is assumed for the action of Through the Looking-Glass - gives a total of 37,044 days. With the further (textually unconfirmed) assumption that both Queens were born on the same day their combined age becomes 74,088 days, which is 42 x 42 x 42. --[[Special:Contributions/139.216.242.254|139.216.242.254]] 02:43, 29 August 2013 (UTC)

:: This explanation covers 42 adequately, and would probably be made slightly worse if such information were added. The very widely known cultural reference is to Adams's interpretation, not Dodgson's original obsession. Adding it would be akin to introducing the MPLM into the explanation for the hijacking of Renaissance artists' names by the TMNT. I definitely concede that it does not cover 42 exhaustively, but I think it can be considered complete and in working order without such an addition. If it really irks you, be bold and add it! --[[User:Quicksilver|Quicksilver]] ([[User talk:Quicksilver|talk]]) 00:37, 30 August 2013 (UTC)

"sqrt(2) is not even algebraic in the quotient field of Z[pi]" is not correct. Q is part of the quotient field of Z[pi] and sqrt(2) is algebraic of it. The needed facts are that pi is not algebraic, but the formula implies it is in Q(sqrt(2)). --DrMath 06:47, 7 September 2013 (UTC)

13/15 is a better approximation to sqrt(3)/2 than is e/pi. Continued fraction approximations are great! --DrMath 07:23, 7 September 2013 (UTC)

How could he forget 1 gallon ≈ 0.1337 ft³?! [[Special:Contributions/67.188.195.182|67.188.195.182]] 00:51, 8 September 2013 (UTC)

Talk:237: Keyboards are Disgusting

2013-09-05T22:59:41Z

67.188.195.182:

This needs to be added to Category:Comics with color but I have no idea how. 17:01, 22 June 2013 (UTC)
:I can't see any color, am I blind?--[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 19:44, 22 June 2013 (UTC)
::Dgbrt, look at the last panel. [[Special:Contributions/131.191.68.103|131.191.68.103]] 08:10, 5 July 2013 (UTC)
:::Uhhh, the dust... so I'm blind ;) --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 10:31, 5 July 2013 (UTC)
::::Yeah, the dust is very slightly coloured (dark blues, greens, and browns). Unfortunately, I don't know how to add it either.[[Special:Contributions/67.188.195.182|67.188.195.182]] 22:58, 5 September 2013 (UTC)

I tried this trick with my new Netbook. The effect is actually pretty impressive (especially with the € on-key). [[Special:Contributions/67.188.195.182|67.188.195.182]] 22:58, 5 September 2013 (UTC)

Talk:237: Keyboards are Disgusting

2013-09-05T22:58:30Z

67.188.195.182:

This needs to be added to Category:Comics with color but I have no idea how. 17:01, 22 June 2013 (UTC)
:I can't see any color, am I blind?--[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 19:44, 22 June 2013 (UTC)
::Dgbrt, look at the last panel. [[Special:Contributions/131.191.68.103|131.191.68.103]] 08:10, 5 July 2013 (UTC)
:::Uhhh, the dust... so I'm blind ;) --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 10:31, 5 July 2013 (UTC)
::::What he means, I think, is that the just is very slightly coloured (dark blues, greens, and browns). [[Special:Contributions/67.188.195.182|67.188.195.182]] 22:58, 5 September 2013 (UTC)

I tried this trick with my new Netbook. The effect is actually pretty impressive (especially with the € on-key). [[Special:Contributions/67.188.195.182|67.188.195.182]] 22:58, 5 September 2013 (UTC)

Talk:237: Keyboards are Disgusting

2013-09-05T22:57:45Z

67.188.195.182:

This needs to be added to Category:Comics with color but I have no idea how. 17:01, 22 June 2013 (UTC)
:I can't see any color, am I blind?--[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 19:44, 22 June 2013 (UTC)
::Dgbrt, look at the last panel. [[Special:Contributions/131.191.68.103|131.191.68.103]] 08:10, 5 July 2013 (UTC)
:::Uhhh, the dust... so I'm blind ;) --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 10:31, 5 July 2013 (UTC)
::::What he means, I think, is that the just is very slightly coloured (dark blues, greens, and browns).

I tried this trick with my new Netbook. The effect is actually pretty impressive (especially with the € on-key).

Talk:172: Skateboarding is Not a Crime

2013-09-05T22:28:30Z

67.188.195.182:

How would the second locker be opened by its user? Would s/he and the fourth locker's owner break the law as well as the third's? [[Special:Contributions/67.188.195.182|67.188.195.182]] 22:28, 5 September 2013 (UTC)

Talk:172: Skateboarding is Not a Crime

2013-09-05T22:28:02Z

67.188.195.182: Created page with "How would the second locker be opened by its user? Would s/he and the fourth locker's owner break the law as well as the third's?"

How would the second locker be opened by its user? Would s/he and the fourth locker's owner break the law as well as the third's?

Talk:765: Dilution

2013-09-05T21:32:05Z

67.188.195.182:

Technically, however, homeopathy states that diluted semen should act as a contraceptive. To get pregnant, they would have to dilute a birth-control pill or something.

Can we conclude that the woman is Megan? [[Special:Contributions/67.188.195.182|67.188.195.182]] 21:32, 5 September 2013 (UTC)

Talk:765: Dilution

2013-09-05T21:31:12Z

67.188.195.182:

759: 3x9

2013-08-31T23:06:04Z

67.188.195.182: /* Explanation */

{{comic
| number = 759
| date = June 28, 2010
| title = 3x9
| image = 3x9.png
| titletext = Handy exam trick: when you know the answer but not the correct derivation, derive blindly forward from the givens and backward from the answer, and join the chains once the equations start looking similar. Sometimes the graders don't notice the seam.
}}

==Explanation==
In college courses with a very large number of students (picture the huge, tiered, amphitheater-style lecture halls shown in any movie or TV show about college), teaching assistants are employed to help the professors grade student work. In math and science cources, students are expected to solve the problems and show their work as supporting evidence. Due to the high volume of work to grade, whether it's being done by the professor or a TA, the grader will get lazy and look for correct answers and the existence of work without checking that the work is accurate.

27 is the correct answer to 3 * 9, 3√81, ''and'' 81 ÷ 3.

==Transcript==
:[A problem is given on an arithmetic test: "4) 3x9=?". In handwriting, the student's work follows. The student has accurately reformatted the question as 3 times the square root of 81, which visually resembles the long division problem of 3 divided into 81, and then solved the latter to get 27 — the correct answer to both.]

{{comic discussion}}
[[Category:Math]]

803: Airfoil

2013-08-23T13:15:02Z

67.188.195.182: /* Transcript */

{{comic
| number = 803
| date = October 8, 2010
| title = Airfoil
| image = airfoil.png
| titletext = This is a fun explanation to prepare your kids for; it's common and totally wrong. Good lines include "why does the air have to travel on both sides at the same time?" and "I saw the Wright brothers plane and those wings were curved the same on the top and bottom!"
}}

==Explanation==
This comic is about the common teaching that an {{w|airfoil}} works because the air on top of the wing must travel faster to "keep up" with the air flowing across the bottom of the wing. The theory goes that, because the air on top of the wing is traveling faster, it must, as a result of {{w|Bernoulli's Principle}}, create an area of lower pressure above the wing; this causes {{w|lift (force)|lift}} (that is, the wing rises) because the higher pressure below the wing (symbolized thick "up" arrow arrow) pushes it up more than the low pressure above the wing. As it turns, out this is, to put it mildly, a vast oversimplification of how lift is truly created.

In the next panel, a student asks a particularly insightful question: Why, if the theory is true, can planes fly upside down? (If the simple airfoil theory is all that permits planes to stay up in the air, then flying upside down should reverse the pressures — pushing the plane down and causing it to crash.) The teacher thinks about it and clearly has no answer.

The final set of panels posit three potential responses from the teacher, upon realizing her theory has been disproved. In the "right" one, the teacher realizes that perhaps the model she's been using to explain how an airfoil is wrong (or, at a minimum, too simple). She is curious about it and suggests that this is an area for further exploration, and encourages additional study — in effect, rewarding the student for his insight. In the "wrong" panel, the teacher, out of apparent embarrassment, avoids the question entirely, saying simply that it's complicated (and implying that such questions are outside the student's understanding). In the "very wrong" panel, not only does the teacher avoid answering the question, she attempts to distract them (or even punish them for asking such an insightful question) by telling the kids that Santa Claus isn't real and that Santa Claus is really their parents — something that would obviously distress them (in addition to distracting them from the question they've asked) and constitute harsh punishment for pointing out the teacher's ignorance.

The alt-text suggests additional reasons for re-thinking the common theory as to how airfoils create lift. It points out that (1) it is absurd to believe the air has to get across the airfoil's two sides in the same amount of time, and (2) the Wright brothers plane's wings were curved the same amount on both sides of the airfoil, meaning that the distance that the air needs to travel to get across the wing is not the dispositive factor in creating lift.

The strip is correct in noting that lift is a far more complicated process than the simple theory posited by the teacher. While the role of Bernoulli's Principle (that is, the difference in pressures) cannot be entirely discounted, the theory here is vastly too simple. As an initial matter, as suggested by the alt-text, there is no reason that the air on top of the wing should be compelled to "keep up" with the air on the bottom of the wing. Indeed, as demonstrated by the illustration below, in the time that the air below the wing travels across, the air on top of the wing has not only traveled the length of the entire top of the wing (a distance that may be farther than the distance under the wing, due to its shape), but often additional distance.

[[File:Karman trefftz.gif]]

Lift may be more usefully described as resulting from the deflection of air, although this explanation still does not explain how symmetrical wings will work (at least, absent effects caused by a change in the "angle of attack") nor how a plane may fly upside down. The Wikipedia article on {{w|lift (force)|lift}} provides a more detailed explanation. It in fact gives an explanation as to these two issues. It explains that with zero angle of attack, a symmetrical wing will not generate lift (though it is possible that other factors may generate other slight upward force, such as updrafts, the shape of the plane, and the angle of the engine relative to the wings. It also explains that an asymmetrical (or "cambered") wing may adjust angle of attack to compensate and still generate lift.

==Transcript==
:Handling a student who challenges your expertise with an insightful question:
:[There's a picture of the cross section of an airfoil, with an arrow above and below, pointing from right to left. Layered on top of these arrows, pointing up and down at the cross section, are a larger arrow below and a smaller arrow above.]
:(This panel just contains text, and has a speech curlique hanging towards the person in the next panel.)
:Teacher: So, kids, the air above the wing travels a longer distance, so it has to go faster to keep up. Faster air exerts less pressure, so :the wing is lifted upward.

:Student: But then why can planes fly upside down?

:(The teacher is standing, pondering the question. Three arrows point out of this panel, leading to each of the next three panels which are :arranged vertically.)

:(This is a label at the top of the panel, not a character speaking.)
:Right:
:(This is the character speaking.)
:Teacher: Wow, good question! Maybe this picture is simplified -- or wrong! We should learn more.

:Wrong:
:Teacher: It's... complicated.
:Teacher: And we need to move on.

:Very wrong:
(Fists are clenched.)
:Teacher: Santa Claus is your parents.

{{comic discussion}}