explain xkcd - User contributions [en]

Talk:2509: Useful Geometry Formulas

2021-09-01T15:57:01Z

172.69.55.131: The cone is just an approximation!

Area formulas are for 2D object as seen instead of surface of a projected 3D object. Sebastian --[[Special:Contributions/162.158.89.200|162.158.89.200]] 02:36, 31 August 2021 (UTC)

The "decorative stripes and dotted lines" are the parts of the diagrams that are intended to indicate the third dimension. The conceit of the comic is that these are superfluous. [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 02:56, 31 August 2021 (UTC)

Ca someone explain how the last one works? [[User:GcGYSF(asterisk)P(vertical line)e|GcGYSF(asterisk)P(vertical line)e]] ([[User talk:GcGYSF(asterisk)P(vertical line)e|talk]]) 04:28, 31 August 2021 (UTC)

: ''bh'' is the area of the front face. The top face is a parallelogram with sides ''d'' and ''b'', with an angle of ''θ'' between them, so its area is ''d b sin(θ)''. The right face is a parallelogram with sides ''d'' and ''h'', with an angle of ''90º - θ'' between them, so its area is ''h d sin(90º - θ) = h d cos(θ)''. So the area of the whole picture is ''bh + d b sin(θ) + d h cos(θ)''.
: --[[Special:Contributions/172.68.24.165|172.68.24.165]] 04:46, 31 August 2021 (UTC)

:: In case you don't know the area of a parallelogram by heart, you can read d b sin(θ) as b * d sin(θ), where d sin(θ) is the height of the parallelogram; if you cut the right corner of the parallelogram off and add it on the left, you get a rectangle where the bottom side is b and the height is that d sin(θ), so it works out. The other parallelogram's area is h * d cos(θ), with the same reasoning. [[Special:Contributions/162.158.90.241|162.158.90.241]] 05:00, 31 August 2021 (UTC)

Funnily enough, both this comic and [[2506]] are about projection. [[User:CRLF|CRLF]] ([[User talk:CRLF|talk]]) 05:11, 31 August 2021 (UTC)

: I had considered working that into the explanation, but that needs to account for the fact that the indicated measurements (e.g. the angle θ) have to be read in 2D, not in 3D and projected. But it would be correct to say that the 2D shapes are projections of simple 3D objects. [[Special:Contributions/162.158.90.149|162.158.90.149]] 05:23, 31 August 2021 (UTC)

: Between this, [[2506]], and all the ones about Mercator and other map projections ... "projection" is a very large word in Randall's brain's word cloud. [[Special:Contributions/172.69.63.8|172.69.63.8]] 15:29, 31 August 2021 (UTC)

:: Feels to me like every comic since 2500 could be tagged "projection" in one sense of the word or another. --[[Special:Contributions/172.69.69.225|172.69.69.225]] 21:55, 31 August 2021 (UTC)

;Does the bottom-left formula have a mistake?</s>
It seems like the bottom-left formula should be ''A''=''d''(''πr''+''h'') rather than ''A''=''d''(''πr''/2+''h''), because there are two half-ellipses that add up to a complete ellipse. Am I missing something? (This doesn't ''seem'' like an extra joke, does it?) [[Special:Contributions/162.158.106.179|162.158.106.179]] 05:28, 31 August 2021 (UTC)
: No, it's correct. ''d'' is all of the major axis, not just half, so we have to divide that by ''2''. [[Special:Contributions/162.158.92.83|162.158.92.83]] 05:51, 31 August 2021 (UTC)
:: Oh, right; good call! [[Special:Contributions/162.158.106.179|162.158.106.179]] 06:49, 31 August 2021 (UTC)

;Does the top-right formula have a mistake?
I think it should be in brackets, the top triangle area needs the ''1/2'' also, so it should be: ''A''=''1/2(πab + bh)''
:No, it's correct. The bottom is a half ellipse, with area ''1/2 π a b'', and the top is a triangle with base ''2 b'' and height ''h'', so its area is ''1/2 2b h = bh''. The total area is ''1/2 π a b + b h''.
--[[Special:Contributions/172.68.25.144|172.68.25.144]] 06:49, 31 August 2021 (UTC)

;3D formulae for reference:
''4πr^2''

''πb(a+√(b^2+h^2))'' if a=b

''πr(2r+h)''

''2(bd+bh+dh)''

[[Special:Contributions/162.158.107.80|162.158.107.80]] 09:54, 31 August 2021 (UTC)
:It would be clarifying to add these to the comic, but of course they are flagrantly wrong. [[User:Baffo32|Baffo32]] ([[User talk:Baffo32|talk]]) 09:57, 31 August 2021 (UTC)
::Surely ripe for a table, in place of much of the longhand paragraph spiel (which could be kept, but simpler for just the narrative but otherwise non-technical details)... "Shape (2D)", "Area", "Pretended Shape (3D)", "Surface Area", "Volume", ¿"Notes"? (Not sure about specific Notes, some things could/should be said below the formulae/descriptions in the relevent cell to which that matters, in special cases where necessary, which might be better than a Notes either empty or jammed up with all the combined row-specific corollaries, etc, that I can imagine.) Anyway, an idea. [[Special:Contributions/141.101.76.11|141.101.76.11]] 11:56, 31 August 2021 (UTC)
:::I think the formulas are correct. Those given should be from the text book, not for those with ellipse bases. Someone has put a lot of work into giving these complicated formulas for the cone and cylinder. But I think that is overkill. I have added to the explanation the simple versions before, and would suggest deleting the complicated, which was never the intention of either text book or Randall! ;-)--[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 12:36, 31 August 2021 (UTC)

add an extra edited image that is the comic without dotted lines to make it easier to see the 2d shapes? [[Special:Contributions/172.69.71.177|172.69.71.177]] 12:46, 31 August 2021 (UTC)Bampf
:And an animated GIF of the 3D solid objects rotating to show their real shapes. At different speeds. If you have the time. :-) Robert Carnegie rja.carnegie@gmail.com [[Special:Contributions/141.101.76.11|141.101.76.11]] 16:31, 31 August 2021 (UTC)

Please do check my (additional) changes to the bottom-right item (hexagon-cum-prism) in both main and transcript texts. As hinted in my edit notes, cos-theta is important because the skewed tetrahedron (rhomboid, whether in plan or the true area of the 'fake' perspective) is not d*b in area. The fact that without the theta it would look like a standard oblique orthographic projection with entirely right-angled corners is perhaps part of the (intended?) confusion, although we can probably assume that all unmarked (and, of course, uncongruent/uncomplimentary) angles are 90° so that it isn't a full on parallelepiped with an additional phi-angle on an adjacent face and a complicated third dependent-angle somewhere upon the remaining face-plane. As such, I put in the cosine element to both the 3d surface formula (it only affects the bd-shape, the both of them) and the 3d volume (from this shape, extrudes without further adjustment straight up the h-axis), but I ''always'' have to second guess if I've done this simple bit of trig right, it seems, even though I should know better and just trust to SOHCAHTOA... ;) [[Special:Contributions/162.158.158.146|162.158.158.146]] 13:24, 31 August 2021 (UTC)
:(Case in point: I thought I'd added cosines, and I'd put sines ''anyway'', when fussing about copying the clipboarded theta-character into the right place! Re-read, seen, corrected(?) this myself. Unless I thought I was was wrong; but I was wrong, I was right!) [[Special:Contributions/162.158.155.145|162.158.155.145]] 13:33, 31 August 2021 (UTC)

I believe both of those prism formulas should use sine theta. If theta is ninety degrees, then sine theta will be 1 (thus reducing to the rectangular case), whereas cosine of 90 degrees is zero.[[User:Tovodeverett|Tovodeverett]] ([[User talk:Tovodeverett|talk]]) 15:19, 31 August 2021 (UTC)
:You're right (me again, from just above), I was rushed and ''had'' been right first time, I realised while I was off-grid and it was nagging away at the back of my head. I'm better on paper (or when I can sanity-test real code, but for some reason tapping it in like this just screws my mind up, taking away/inverting my technical ability and reason. (I blame the microwaves emitting from my tablet... pass the tinfoil hat!) [[Special:Contributions/162.158.158.178|162.158.158.178]] 16:29, 31 August 2021 (UTC)

Unconvinced by the cone! The equation shown, is correct for an isosceles triangle with a half-ellipse on its base. But that shape has 'corners' where the sides meet that half-ellipse. In a 3D projected view of an actual cone, the sides will meet the base ellipse at a tangent, meaning that it is more than a half-ellipse. But I suppose it's close enough as an approximation...[[Special:Contributions/172.69.55.131|172.69.55.131]] 15:57, 1 September 2021 (UTC)

Talk:2504: Fissile Raspberry Isotopes

2021-08-27T11:59:33Z

172.69.55.131:

This joke is like a visual pun, a raspberry fruit looks sorta like a nuclear model, and so it behaves the same (ie can go supercritical). [[Special:Contributions/172.69.35.72|172.69.35.72]] 20:38, 18 August 2021 (UTC)
:Along with an actual pun: pi in "pi meson" sounds like pie in "raspberry pie". [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 20:43, 18 August 2021 (UTC)
::If it isn't also an intentional tertiary reference to the Raspberry Pi computer board, I'll eat my [https://thepihut.com/collections/raspberry-pi-hats hat]! [[Special:Contributions/141.101.98.109|141.101.98.109]] 21:22, 18 August 2021 (UTC)
:::I'd say you would have to eat it. Cannot see what this comic has to do with a computer board, just because it is named after a raspberry pie. This joke is obviously about the berries looking like nuclear cores, and pie mesons. Not about anything with a computer. So take some salt an eat (or swallow one the hats in your link, along with a camel :p ) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 10:21, 19 August 2021 (UTC)
::::I interpreted 'tertiary' to mean that randall was [https://en.m.wikipedia.org/wiki/Priming_(psychology) 'primed'] to talk about raspberry pies due to his exposure to the board, and similarly for readers finding it interesting and humorous. [[User:Baffo32|Baffo32]] ([[User talk:Baffo32|talk]]) 08:28, 20 August 2021 (UTC)

Though raspberries resemble the common ''depiction'' of nuclei, perhaps we need to explain that in reality, nuclei are rather different..? [[User:BunsenH|BunsenH]] ([[User talk:BunsenH|talk]]) 03:41, 19 August 2021 (UTC)
:I think anyone reading xkcd and this page, will figure it out via the links ;-) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 10:21, 19 August 2021 (UTC)
::I tend to forget that nuclei aren't little raspberries made of nucleons, even though I used to be a fusion researcher. In fact I'm sure I was only ever half-aware they weren't (I didn't study the actual nuclear physics, ok!)... so +1 from me in favour of adding a bit about the 'real' nature of nuclei, that would be interesting. --[[User:192·168·0·1|192·168·0·1]] ([[User talk:192·168·0·1|talk]]) 09:39, 20 August 2021 (UTC)
I would suggest referring to the raspberry parts as 'drupelets' rather than 'ovaries'.[[Special:Contributions/172.70.114.173|172.70.114.173]] 12:58, 19 August 2021 (UTC)
:I love eating the juiciest and sweetest of fruit ovaries, raspberries and strawberries are my favorite but I also enjoy apples and grapes --[[User:Lackadaisical|Lackadaisical]] ([[User talk:Lackadaisical|talk]]) 17:29, 19 August 2021 (UTC)
::I looked this up briefly and I _think_ that the ovaries are what develop into the drupelets, here, not certain, judging by phrasing on wikipedia. So I changed it. Don't eat the ovaries, eat the part that's designed for eating. [[User:Baffo32|Baffo32]] ([[User talk:Baffo32|talk]]) 08:37, 20 August 2021 (UTC)
:::Fine but you forgot the transcript. I have changed it there so drupelet is the word used. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 15:57, 20 August 2021 (UTC)

The page says "Of course, in real life raspberries don't do that.[citation needed]" - where is one supposed to find a useful citation to state that fields of raspberries ''don't'' explode? [[Special:Contributions/108.162.229.101|108.162.229.101]] 22:18, 19 August 2021 (UTC)
:That’s the point! [[Special:Contributions/108.162.215.45|108.162.215.45]] 02:55, 20 August 2021 (UTC)
::It's a joke, and you are free to remove it if you so judge. Others may disagree. Sometimes raspberry farms have some pretty hard to describe explosive activity when their parts combine in rare chain reactions. [[User:Baffo32|Baffo32]] ([[User talk:Baffo32|talk]]) 08:39, 20 August 2021 (UTC)
:::It's a reference to [https://explainxkcd.com/wiki/index.php/285:_Wikipedian_Protester 'xkcd 285'], a long running joke in the xkcd community, What If? and Randal's other books, and the xkcd merch shop {{unsigned ip|172.69.42.63}}
::::And severely overused. Imho it should only be used when we are actually looking for a citation for someting stated without proof in the explanation. So I generally feel free to remove them as I did here. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 15:54, 20 August 2021 (UTC)
:::::I agree there ''can'' (and has been) overuse, but by your criteria there would be absolutely no 285-backrefetencing at all. In use on The Original Wiki and all the rest there other cite-markers requesting an edit to clarify, expand, use better units, add comic issue and page, etc, and th9se exist (or can be made to exist) here.
::::::I wouldn't suggest every clause of every sentence of every paragraph of every comic's explanation be in-joked, but (with exceptions on a very few rare occasions that will doubtlessly be edited down by a future editor like you, or me, anyway) I see no harm in so labelling up to ''one'' axiomatic statement this way per article (the absolutely most obvious and inarguable and, by editors' aggregate concensus without resorting to an edit-war, humorous-to-so-label statement).
::::::By dint of the humour-decay so described, results in one permanent example fit to tickle the funnybone of all but the most curmudgeonly every 2, 3 or 4 comics, on a rolling average. In every case being absolutely obvious to pretty much everyone that it is there for amusement value (especially amidst dry, technical detail) even to those only just arrived upon these particular digital shores...
::::::IMO, of course, having no authority or desire for authority here. [[Special:Contributions/141.101.77.69|141.101.77.69]] 17:30, 20 August 2021 (UTC)
::::: I'm pretty sure part of the joke here is the edit war it can stimulate, not sure. But clearly it would be overdoing it to formalize including one every single explanation. [[User:Baffo32|Baffo32]] ([[User talk:Baffo32|talk]]) 00:13, 21 August 2021 (UTC)
I feel like there might be more to this joke. When I'm plucking raspberries in the forest, they break in half pretty often, causing me to get juice on my hands, which is pretty annoying.[[Special:Contributions/162.158.90.253|162.158.90.253]] 21:42, 20 August 2021 (UTC)
:And then that juice on your hands breaks another raspberry in half, because you are worried less about keeping your hands clean ... and then you hand your juicy raspberries to a friend, getting their hand juicy from your hand ... The NRC arrive and it's just red, everywhere. [[User:Baffo32|Baffo32]] ([[User talk:Baffo32|talk]]) 00:16, 21 August 2021 (UTC)

If a Raspberry breaks, germs can better feed off the fruit and multiply then decompose nearby fruits.
The title text states blackberries, not raspberries.
Mesons consist of quark and antiquark. Quark is a dairy product used as ingredient in baking pies. [[Special:Contributions/162.158.89.10|162.158.89.10]] 13:33, 21 August 2021 (UTC)

Can't believe that the [https://www.raspberrypi.org/ Raspberry Pi] is not implicated here, but personally I can't see the connection. Can someone? [[User:Asimong|Asimong]] ([[User talk:Asimong|talk]]) 17:33, 23 August 2021 (UTC)
:It's talked about above. Maybe there wasn't room for a microcontroller-based allusion (after helping sparking the base word-association that begot the comic's final target), or maybe it's totally coincidental to the original inspiration and had not enough commonality to suggest bringing it into the fray. I can't believe it did not cross Randall's mind somewhere along the way, though. Perhaps its lack of intricate tying-in is more indicative of it being actively ''not'' shoehorned in. (Every idea I have about how to cameo the Pi seems rather clunky to me, at best. But then I'm not a polymathic parodist of long standing...) [[Special:Contributions/172.69.55.131|172.69.55.131]] 11:59, 27 August 2021 (UTC)