explain xkcd - User contributions [en]

1244: Six Words

2016-12-29T18:47:17Z

Calculus: /* Explanation */

{{comic
| number = 1244
| date = July 29, 2013
| title = Six Words
| image = six words.png
| titletext = Ahem. We are STRICTLY an Orbiter shop.
}}

==Explanation==
The six words are "It works in ''Kerbal Space Program''".

The {{w|Kuiper Belt}} is a region of the outer Solar System beyond the orbit of Neptune consisting of numerous small icy bodies, including the dwarf planets {{w|Pluto}} and {{w|Eris (dwarf planet)|Eris}}. An {{w|Oberth effect|Oberth maneuver}} is a spaceflight maneuver, specifically an engine burn performed during the flyby of a celestial body. The point of this is to optimize usable energy, because rocket burns are more effective to perform at high speeds than at low speeds. The more massive the body and the lower the altitude, the higher the flyby speed will be, and the greater the performance gain due to the Oberth effect. The theoretical way to use rocket fuel most efficiently is therefore to execute the burn during a flyby of the most massive celestial body available, as close as possible.

[[Cueball]] is proposing to implement an "Oberth Kuiper Maneuver", and the proposal diagram shows the spacecraft using {{w|Gravity assist|gravity assists}} to travel first towards (presumably) {{w|Venus}} for a first boost, then towards Jupiter for another swing by, aiming it back towards the {{w|Sun}}, the most massive Solar System body, to perform an Oberth maneuver at the point of closest approach, as indicated by a small dot along the spacecraft trajectory. It is possible that the diagram might only be a simplified outline of a more complex flight plan. For example, the trajectory from Earth to Venus would require two separate burns in deep space. The first burn would occur immediately after escaping Earth's sphere of influence. The second burn would occur about halfway to Venus. Alternatively, Cueball may have gotten it wrong, or [[Randall]] may simply not have concerned himself with such things for the purpose of a webcomic sketch.

An Oberth maneuver in the close vicinity of the Sun, while theoretically possible and extraordinarily effective at the speeds the spacecraft would reach, would however be very difficult to carry out in real life, because the neighborhood of the Sun is an extremely hostile environment and the spacecraft could be incinerated during a too-close flyby (citation needed).

Cueball's argument for why the maneuver will work in real life is that it works in ''{{w|Kerbal Space Program}}'' (KSP), a sandbox spaceflight simulator. While KSP does take into account quite a bit of the physics of space flight, it is (necessarily) simplistic in its modeling of orbital dynamics. KSP does not make any attempt to deal with the {{w|N-body problem}} (for which there is no complete solution), or even simpler 2-body mechanics. Instead celestial bodies are "on rails" and have fixed {{w|Sphere of influence (astrodynamics)|Spheres of Influence}} (SOI); a craft within a particular SOI feels the gravitational influence of only that body, and transitions are abrupt when a craft crosses the edge of an SOI. This means there are no {{w|Lagrangian point|Lagrangian points}}, {{w|Lissajous orbit|Lissajous orbits}}, or, critically for this context, {{w|Orbital perturbation analysis (spacecraft)|orbital perturbations}} by distant bodies or other factors. Therefore, orbits modeled using KSP would poorly reflect the actual orbital behavior of a probe traveling through the solar system on a multi-year mission involving multiple fly-by maneuvers. KSP does however have a big {{w|modding}} community and one of its members has created a mod called [https://github.com/mockingbirdnest/Principia Principia] that implements an approximation of N-body physics by using {{w|numerical integration}}.

The humor in referencing KSP in the comic lies in using a simple game program to simulate complex space missions which in reality take a great number of experts to plan and implement. Fly-by maneuvers, used to minimize the fuel needed to reach a destination, need to be very carefully timed - often to within seconds - so the use of ''Kerbal Space Program'' to simulate them isn't a good enough argument for NASA to agree to implement the proposal, and implies simplistic thinking on the part of the proposer; therefore one should not say it at NASA.

The title text refers to ''{{w|Orbiter (simulator)|Orbiter}}'', which is another spaceflight simulator program. The title text suggests that the argument doesn't work for NASA, not because it's not scientifically sound, but because NASA relies on the ''Orbiter'' simulator rather than the ''Kerbal'' simulator (although the proposed maneuver would appear to work in both).

Humorously, in real life [http://news.cnet.com/8301-1023_3-57495117-93/mars-curiosity-rover-team-prefers-macs-to-pcs/ a group of NASA team members "love" to play ''Kerbal''].

==Transcript==
:[Caption above the frame:]
:The six words you ''never'' say at NASA:

:[A diagram shows a possible trajectory path for a space probe starting at Earth and involving two slingshots around two other planets, and finally the Sun. There is a title above and a label beneath the diagram. The diagram is being presented by Cueball in front of three other Cueball-like guys. Behind Cueball Ponytail appears to be taken by surprise by his six words, and holds her hand to her mouth.]
:Title: Proposal:
:Label: Oberth Kuiper Maneuver

:Cueball: And besides—
:Cueball: It works in Kerbal Space Program.

==Trivia==
*{{w|NASA}} is planning a mission to the sun at a closest distance of only 8.5 solar radii. The {{w|Solar Probe Plus}} will use seven Venus flybys to reach its [http://solarprobe.jhuapl.edu/index.php first close approach]. The maneuvers will last nearly seven years, before the real mission does start.

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Multiple Cueballs]]
[[Category:Physics]]
[[Category:Space probes]]
[[Category:Space]]
[[Category:Video games]]

1092: Michael Phelps

2016-12-06T22:35:55Z

Calculus: /* Explanation */

{{comic
| number = 1092
| date = August 8, 2012
| title = Michael Phelps
| image = michael phelps.png
| titletext = [shortly] ...he ate ALL of it!?
}}

==Explanation==
{{w|Michael Phelps}} is an American {{w|Olympics|Olympic}} swimmer, who could easily be considered the best swimmer worldwide: he is the most decorated Olympic athlete of all time, with 28 medals, 23 of them gold (won in the 2004, 2008, 2012 and 2016 summer Olympics). He was most dominant in the 2008 Beijing Olympics where he won gold in all of the eight events in which he competed, the record for a single games.

[[Cueball]] and [[Megan]] find that the Olympic medalist is in Megan's pool. He refuses to leave, and is too fast to be caught. Cueball brings in boxes of Jello Mix to fill the pool with, thereby solidifying the pool and trapping Phelps or forcing him to leave.

However, according to the title text, after having waited the time necessary for the water to solidify, Cueball realizes that Phelps has eaten all of the resulting Jello. This may be a reference to Phelps being used to eating impressive food quantities (about 12,000 calories daily), to keep up with his strenuous exercise regimen;[http://www.michaelphelps.net/michael-phelps-diet/] or it may be a reference to pictures of Phelps smoking from a bong that arose after the 2008 Olympics in Beijing, China, as Marijuana use is often associated with an increased appetite. Otherwise, the text may simply be a reference to Phelps being capable of achieving super-human feats, such as devouring an entire pool full of Jello.

==Transcript==
:[Megan and Cueball standing outside their en-Phelps-ified swimming pool.]
:Cueball: Why is Michael Phelps in your backyard pool?
:Megan: I don't know. He's been there all day. ''Go home, Michael!''
:Michael Phelps: Woo! 18 gold medals!

:[Megan and Cueball break out a pair of pool nets and unsuccessfully try to snag Phelps.]
:Cueball: Can you get him?
:Megan: He's so '''''fast'''''!
:Phelps: Ha hah! Can't catch me!
:''Splash splash''

:[Cueball heads off to fetch something.]

:[Cueball returns with a hand truck full of Jello mix.]
:Phelps: Oh crap.

{{Comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Comics featuring real people]]

219: Blanket Fort

2016-12-06T15:43:18Z

Calculus: /* Explanation */

{{comic
| number = 219
| date = February 5, 2007
| title = Blanket Fort
| image = blanket_fort.png
| titletext = Also, we have a fort out in the woods where we stashed that hooker's body.
}}

==Explanation==
In an apparent continuation of comic [[150: Grownups]], [[Megan]] is showing off a {{w|blanket fort}} to her friend [[Ponytail]]. Ponytail answers that Megan's childishness stems from a fear of growing up.

Megan responds that she's fine with growing up. She sees her behavior as a mature realization that some of the things she enjoyed as a child are still enjoyable. When pressed for evidence that she has in any way become an "adult", it is revealed that her boyfriend is inside the blanket fort. He has not spoken because he fears to be found by them when "not exactly dressed," thus implying "adult" activities had been going on in the fort.

The title text provides another example of this: They built a fort in the woods, another type of fort that a kid would love to have, but then used it to hide a body. This implies that they killed someone, which is not usually something associated with children (citation needed). A hooker is also more formally known as a {{w|prostitute}} which is also not usually something associated with children.

==Transcript==
:[Megan and Ponytail are talking with each other. There is a fort made of cushions and blankets on the left.]
:Megan: Like my fort? It uses every blanket and cushion in the apartment.

:Ponytail: Okay, no offense, but this is like that ball pit you made -- Cute, but don't you worry you're clinging to childhood games because you're afraid of change?

:Megan: No. I'm happy to grow up. But I won't pretend fun things aren't still fun out of fear of looking silly.
:Ponytail: But you're 24 and building blanket forts. How have you changed? What's adult about that?

:Megan: Well, there's my boyfriend curled up in the back.
:Ponytail: ...Ah.
:Boyfriend [from inside the fort]: Excuse my shyness. I'm not exactly dressed.

{{comic discussion}}
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Megan]]
[[Category:Sex]]

1184: Circumference Formula

2016-12-06T12:20:12Z

Calculus: /* Explanation */

{{comic
| number = 1184
| date = March 11, 2013
| title = Circumference Formula
| image = circumference_formula.png
| titletext = Assume r' refers to the radius of Earth Prime, and r'<nowiki/>' means radius in inches.
}}

==Explanation==
The {{w|circumference}} C of a {{w|circle}} is 2{{w|pi|π}}''r'', where ''r'' is the {{w|radius}} of the circle. [[Randall]] then makes a {{w|Note (typography)|footnote}} about ''r'', using 2. This creates a typographical ambiguity, since a superscript 2 can also be an exponent (as in ''x''2). The comical purpose of this ambiguity is that the formula initially makes an appearance of a mistake and confusion with the formula for the ''{{w|area}}'' of the circle: A = π''r'' 2. If and only if the reader realizes that the superscript text is a reference to a footnote will they understand that the author has in fact supplied the correct formula.

The title text makes a related joke. Randall has used r' (r-{{w|Prime (symbol)|prime}}) and r" (r-prime-prime, typically pronounced as ''r double prime''). The title text can be explained thusly:

Like many symbols, prime has widely differing meanings depending on context. In mathematics prime is often employed to distinguish corresponding components in analogous systems. For example, in a description of a basic physical system, if the velocity of an object is denoted with the variable ''v'', the velocity of that object at time=0 may be denoted with ''v′''. Playing off this use of prime, Randall has selected the radius of {{w|Earth Prime}}, a concept used in speculative fiction with parallel universes and multiple Earths. Earth Prime is our Earth (or at least the Earth from which the protagonists originate).

However, other disciplines use prime to mean other things. In timekeeping and navigations ' denotes minutes (fractions of hours or degrees, respectively) and " denotes seconds (fractions of minutes). In the United States and some other places not using meters to measure distance, ' denotes feet and " denotes {{w|inches}}.

The suggestion of using r' or r" does not cause any mathematical confusion, but using the former to denote the radius of a specific object and the latter to denote a radius using a specific unit of measurement would be highly esoteric. Furthermore, r' and r" can be used in calculus as a method of denoting, respectively, a first derivative and a second derivative. For someone attempting to use the formula and some derivative representing a circle's radius, trouble could result quite easily.

2 These are not intended to indicate the presence of a footn... oh, never mind.

==Transcript==
:Circumference of a circle:
:2πr2
:2The circle's radius

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

Talk:1714: Volcano Types

2016-12-06T12:06:05Z

Calculus:

Actual antlions, not to be confused with the antlions from Half-Life 2. [[Special:Contributions/173.245.52.62|173.245.52.62]] 15:08, 1 August 2016 (UTC)

I think pedant's bane actually has them labelled correctly. It's just that the picture is upside down and you've reversed the figure and the ground in your mind. Were it right-side-up, you'd see It's actually a drawing of a teeny pool of underground magma at the bottom of an antlion's trap, spouting up a fountain of lava. {{unsigned ip|199.27.133.107}}
:I can't tell if you're joking or not. A pedant is someone who cares (maybe too much) about being technically correct. The bane of a volcano pedant would be people who mix up lava and magma. This is why the labels are reversed in the figure. [[Special:Contributions/173.245.52.62|173.245.52.62]] 16:27, 1 August 2016 (UTC)
::Exactly. But what's the description going on about? "''Pedant's Bane is therefore impossible by definition but if it were possible, then a pedant correcting someone's description of it would be wrong.''" What is all that rubbish supposed to mean? The explanation is not supposed to require explaining! If the terms refer to the same material but in different situations then they're wrong in the two situations illustrated, and thus should be corrected, especially by a pedant. Where does "impossible by definition" come into it? [[Special:Contributions/198.41.238.32|198.41.238.32]] 22:57, 1 August 2016 (UTC)

Who here was hoping for Harry Potter???? [RedstoneIngot] [[Special:Contributions/173.245.56.60|173.245.56.60]] 20:07, 1 August 2016 (UTC)

Made a very small change, but sarlacc is not supposed to be capitalized (checked both Wikipedia and Wookieepedia). [[User:Spaceside|Spaceside]] ([[User talk:Spaceside|talk]]) 23:02, 1 August 2016 (UTC)

The explanation of "Doot Cone" makes no sense. The "Skull trumpet" Know Your Meme page does not discuss the word "doot" at all, only shows it in some pictures. It doesn't seem to be a significant part of the meme, as it doesn't feature in the animation or its early variants. It just happens to be the first hit when you Google "doot." It makes just as much sense to assume that it's a play on "[https://www.amazon.com/Florida-DOT-Approved-Traffic-Cone/dp/B009RUTKZA DOT cone]," as in the orange cones used by the Department of Transportation (which are sometimes stenciled with "DOT.")
[[Special:Contributions/199.27.128.142|199.27.128.142]] 01:11, 2 August 2016 (UTC)

Re: Doot Cone

"Doot" is actually a significant part of the skull trumpet meme -- meant to represent the sound in the original skull trumpet video. It's strange the KYM page doesn't refer to "doot" specifically. See a relevant reddit thread: https://www.reddit.com/r/OutOfTheLoop/comments/39xnk2/what_is_this_doot_thing_with_the_skeletons/cs7jdsa

However, I'd agree that the evidence is not strong for the doot cone being a reference to this.

[[Special:Contributions/108.162.219.10|108.162.219.10]] 01:36, 2 August 2016 (UTC)
:Before I read the explanation (and the KYM entry) in my imagination the sound was more like that of a foghorn or a ship's horn. I've never heard of that meme before. And even now I feel it hard to imagine a BIG volcano making the comparatively high pitched sound of a trumpet. [[User:Elektrizikekswerk|Elektrizikekswerk]] ([[User talk:Elektrizikekswerk|talk]]) 07:18, 2 August 2016 (UTC)
::Whereas it's perfectly easy to imagine a waffle cone or an inverted volcano :-) [[Special:Contributions/141.101.98.90|141.101.98.90]] 10:12, 3 August 2016 (UTC)
::: Exactly ;) [[User:Elektrizikekswerk|Elektrizikekswerk]] ([[User talk:Elektrizikekswerk|talk]]) 13:04, 3 August 2016 (UTC)

It would look out of place next to the profile drawings, but <big>☻</big> is Kilauea's [https://www.youtube.com/watch?v=e8PuMOsdwm0 Watchmen crater]. [[User:.42|.42]] ([[User talk:.42|talk]]) 12:14, 2 August 2016 (UTC)

For the ghost vent - is this actually a reference to the ghost generators in Gauntlet? Andrew Williams {{unsigned ip|141.101.104.100}}

Does the metasomma volcano remind anyone else of an HNMR spec?[[User:Killerbeez|Killerbeez]] ([[User talk:Killerbeez|talk]]) 21:39, 9 August 2016 (UTC)

Yep. Probably six hydrogens SOMEWHERE near that volcano. [[User: Calculus|Calculus]] ([[User talk: Calculus]]) 12 December 2016

Honestly, I just thought that the "Doot Cone" was just supposed to be a stupid sound. {{unsigned ip|108.162.241.43}}

It's common, at least in my area, for ice cream cones to be served upside down in cups. Randall lives in New England like me, so he'd have encountered this even if it's just local. [[User:Gmcgath|Gmcgath]] ([[User talk:Gmcgath|talk]]) 13:20, 28 October 2016 (UTC)