explain xkcd - User contributions [en]

Talk:1366: Train

2023-08-09T06:34:33Z

141.101.76.11: Edit of comment

I just did an explanation from scratch for the first time, please could you tell me how I could improve it? Thanks :) [[User:Cheeselord99|Cheeselord99]] ([[User talk:Cheeselord99|talk]]) 07:02, 9 May 2014 (UTC)

Am I the only one who gets [[Inflation]] when going to xkcd.com (without the www)? This comic shows at www.xkcd.com and m.xkcd.com however. [[Special:Contributions/108.162.221.5|108.162.221.5]] 07:11, 9 May 2014 (UTC)

:I created an account. The 108.162.221.5 ip address today is me, along with 108.162.221.53 today. [[User:Mikemk|Mikemk]] ([[User talk:Mikemk|talk]]) 07:16, 9 May 2014 (UTC)
:I thought today's comic was late. http://www.xkcd.com/1366/ kept on displaying "Web-page not available" (browser thing, not server-thing), then I checked here. So. Oh, http://www.xkcd.com/ also... Hmmm... That's not right. Oh, "Ping request could not find host www.xkcd.com. Please check the name and try again." DNS errors? Only those trying via cached details get anything? Things are not working for xkcd.com or m.xkcd.com either. So, DNS poisoning or human error of some kind? Not the place to discuss this, I know, sorry... [[Special:Contributions/141.101.89.211|141.101.89.211]] 10:05, 9 May 2014 (UTC)

:Explanation is good, but there are certainly related comics or maybe what-if ... I've found [http://what-if.xkcd.com/58/ Orbital Speed], but I think there were something mentioning how fast sun goes relatively to galaxy ... -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 10:14, 9 May 2014 (UTC)
::Found two related comics - any other? [[User:Condor70|Condor70]] ([[User talk:Condor70|talk]]) 11:33, 9 May 2014 (UTC)

It sounds like the dark matter engine in Futurama:http://futurama.wikia.com/wiki/Dark_matter_engine {{unsigned ip|108.162.215.77}}
:That's just what I was thinking! --[[Special:Contributions/173.245.55.66|173.245.55.66]] 17:53, 12 May 2014 (UTC)
I think the last paragraph, considering the situation from the point of view of multiple trains, is not relevant. The whole concept of what makes this idea funny and interesting is that you MUST view the situation from the point of view of a single train (or elevator). --[[User:RenniePet|RenniePet]] ([[User talk:RenniePet|talk]]) 13:24, 9 May 2014 (UTC)
: Second-last paragraph - my comment was written at the same time as another paragraph was added. --[[User:RenniePet|RenniePet]] ([[User talk:RenniePet|talk]]) 13:26, 9 May 2014 (UTC)

I do not understand what the last paragraph is suggesting as it seems to violate the 3rd Newtonian law of motion.

The last paragraph is not correct, the Earth would also experience an acceleration (albeit a small one).--[[User:Sturmonium|Sturmonium]] ([[User talk:Sturmonium|talk]]) 13:54, 9 May 2014 (UTC)

This line:
"The logic of the comic also fails when taking acceleration into account. Whether the train or earth is moving can be determined by which one experiences a force due to acceleration or deacceleration when the train starts."
is incorrect, according to the principle of General Relativity. You cannot experimentally distinguish between your own acceleration against a fixed universe, and your position remaining fixed against an accelerating universe. This applies for rotation as well; if you fix the reference frame of the train rider, the acceleration of the universe creates gravity waves that cause any rider on the train to experience what feels like an acceleration. Therefore, the logic of the comic is indeed correct, even for accelerating trains. I will correct this edit.--[[User:JB Gnome|JB Gnome]] ([[User talk:JB Gnome|talk]]) 14:12, 9 May 2014 (UTC)
: But the comic doesn't say that the train accelerates the universe: rather, it just accelerates the Earth. Does that make a difference? [[Special:Contributions/141.101.89.225|141.101.89.225]] 14:34, 9 May 2014 (UTC)

Does anyone have an idea where "train guy" is heading? He's saying "almost", like he's almost there but wasn't sure if there was something more. Maybe he's timing when he needs to jump off the train? [[Special:Contributions/173.245.55.62|173.245.55.62]] 14:58, 9 May 2014 (UTC)Pat

I see this comic as a nice ab absurdo for the many people who think the sun rotates around us, and further to those who claim the earth has 6 thousand years etc... [[Special:Contributions/108.162.242.117|108.162.242.117]] 18:12, 9 May 2014 (UTC)

I don't have a citation, but some traditional Polynesian navigation works using this view. Their "maps" are made of a grid of bush materials where intersections are stars or islands (possibly with a pebble tied on to represent the island). They consider the map and the earth to be connected, and you don't move along the map - the map moves. So you don't go to another island, you bring it to you. At night you move the stars to the right place, and during the day you paddle the sea and land so they are in the right place and direction.--[[User:DivePeak|DivePeak]] ([[User talk:DivePeak|talk]]) 21:15, 9 May 2014 (UTC)

A train rotating the Earth is NOT physically equivalent to a train traversing the Earth. It would be true for a flat Earth, but rotation is absolute, see Newton's bucket argument. [http://en.wikipedia.org/wiki/Bucket_argument] --[[User:Gleyshon|Gleyshon]]

I am only a layperson, but as i read it the explanation confuses coordinate systems for the description of motion with physical explanation. The reasons why the train cannot be seen as accelerating the earth/universe are so numerous (even ignoring the other objects moving in different ways) it's just for subjective fun that i'll point out a few: The rest mass of the earth is not relative, so neither is the force needed to accelerate it -- force which the train cannot create by a factor of kajillions (roughly ;^). And if it did, the area would turn into plasma. Sticking to the comic, there isn't enough friction/rigidity/etc for the wheels/rails to "grab the earth". (Notice these are local observations unaffected by any relativity.) If somehow the earth could be accelerated in this manner, the rest of the universe wouldn't go along with it. And if it did, astronomical objects wouldn't change apparent position until their light reached us. Finally, rotation of massive bodies creates {{w|Frame-dragging}} (the twisting of spacetime) which is locally observable. So: the point of the comic isn't that the view presented is counter-intuitively valid (with awkward math); rather the comic is funny because of how badly this view fails. But i don't think i'm the person to rewrite the explanation... [[User:Noobgeek|Noobgeek]] ([[User talk:Noobgeek|talk]]) 16:59, 15 May 2014 (UTC)
:That physics are easy to understand:
:*At the train you just see the Earth is moving.
:*On Earth surface you just see that the train is moving.
:Just get into the train without any knowledge about the size or mass of the Earth, and you just see the surface beyond you is moving. I will try to enhance the explain on this matter. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 21:39, 15 May 2014 (UTC)
::Hi there! You're right that you can often exclude information and be left with various "relativities". But (to give only one argument) the comic explicitly describes the entire planet being grabbed and rotated to a new orientation. Mountains and all :^) [[User:Noobgeek|Noobgeek]] ([[User talk:Noobgeek|talk]]) 15:07, 16 May 2014 (UTC)

It's fun to play with the physics, but the explanation is just wrong re the comic expressing a valid perspective in physics, most directly for the reason supplied by the title text: Multiple trains and elevators are moving relative to each other, so their mode of operation can't be to stay still and shift the earth. (The point of the title text might be to make the false-physics joke extra clear.) I'm going to rewrite the explanation if no one else does -- but this will be my first big edit, so before i do, to be a polite noob, i'm requesting objections. [[User:Noobgeek|Noobgeek]] ([[User talk:Noobgeek|talk]]) 19:58, 23 May 2014 (UTC)

hey cool, i live in oxford [[User:Beanie|Beanie]] [[User talk:Beanie|talk]] 11:24, 1 July 2021 (UTC)

Having read this proposed explanation and those of other comics, it seems that a lot of them (including this one) appear to omit explaining their humour, which is very often derived from their completely absurd propositions. Other commenters have alluded to this too. Lay people don't understand physics to this depth, or physics may appear boring to them due to their ignorance of it. But it doesn't follow that Randall, being an expert in physics, is either boring or doesn't have a sense of humour. Shouldn't these comic strip explanations attempt to explain the ''absurdity'' of their proposals first as opposed to the physics of them? That way, we get to know Randall's wit a little better and can better appreciate his past and future work. This comic strip, to me, represents a very novel take on the classic ''Tail wagging the Dog'' scenario and I came to this wiki hoping to get a bit more insight into Randall's interest in these kind of scenarios. I have been disappointed this time, but I hope that might change. [[Special:Contributions/141.101.76.164|141.101.76.164]] 06:30, 9 August 2023 (UTC)

Talk:2509: Useful Geometry Formulas

2021-08-31T16:31:08Z

141.101.76.11: Show the 3D shapes? ;-)

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)

;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 SOCAHTOA... ;) [[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)

Talk:2509: Useful Geometry Formulas

2021-08-31T11:56:55Z

141.101.76.11: /* 3D formulae for reference: */