explain xkcd - User contributions [en]

Talk:3090: Sail Physics

2025-05-17T23:40:16Z

172.71.142.188: Solar sails, magnetic sails, sailing upwind with solar

After the last step, the sailors would then need to ground the boat to avoid being pushed in a circle, wouldn't they? [[User:Sophon|Sophon]] ([[User talk:Sophon|talk]]) 20:47, 16 May 2025 (UTC)

Note that for eastward wind, the boat will be propelled upwards, while the opposite is true for westward winds. This provides a basis for the functioning of airships and planes (Helicopters are more complicated, and additionally rely on their own magnetic fields) [[Special:Contributions/162.158.217.45|162.158.217.45]] 21:21, 16 May 2025 (UTC)
: Hence why you should always touch an earthing rod before approaching a helicopter, to avoid the magnetism pulling you into their rotors. [[User:Kev|Kev]] ([[User talk:Kev|talk]]) 03:11, 17 May 2025 (UTC)

Is this actually wrong? Wouldn't it still be ''a'' force on a sailboat, even if it's not the strongest? [[User:Smurfton|Smurfton]] ([[User talk:Smurfton|talk]]) 22:20, 16 May 2025 (UTC)

: I added some explaination on direction and magnitude of the lorentz force, maybe that will help - sga {{unsigned ip|172.68.234.227|22:33, 16 May 2025 (UTC)}}

The explanation states that of the four forces, only the electromagnetic force operates at the macro level. This is incorrect, as gravity is also directly observable by humans. There should also probably be a link to https://en.wikipedia.org/wiki/Airfoil to provide an explanation for how sails actually allow a boat to sail upwind. I recommend removing the remark about the poles potentially flipping in the future, as this is irrelevant. [[Special:Contributions/172.68.55.124|172.68.55.124]] 23:52, 16 May 2025 (UTC)

: What i meant was, for 2 objects at scales of humans =, maybe did not prase it well. In this case, it is the wind and the sail. Wind does not have a "mass" (the atoms most certainly do, but) we essentially have a pressure force, or momentum of wind, where instead of using the energy of atoms (and hence the mass) as given by kinetic theory is not used (that is random (as given by boltzman maxwell statistics)) and uniform (in the sense that for any direction, number of particles going against and towards is equal) and what we have is just pressure applied by a effective "group velocity" of the wind atoms. The gravity interaction between wind and boat, or the local waves and boat is negligible, and planetary gravity is not considered because that is not relavant for in plane motion. the pole fillping was added just for future proofing the article. I am sorry for the puns. I have rewwritten some parts, and reduced the part about pole flipping, and also added the average case scenario for the force, hope it is better now. - sga {{unsigned ip|172.70.143.75|02:37+, 17 May 2025}}
::That is one huge rambling paragraph, if it's (mostly) yours. I'm no stranger to ''writing'' huge rambling paragraphs, myself, but I gave up only a little way in on trying to make it read better. Grammatically, prosaically and with relevence.
::May I suggest that each 'frame' is treated to its own (shorter) paragraph, explaining what effect it tries to convey, what logic it individually tries to follow, but where it fails and what actual forces dominate a true example. (e.g. the hull-shape, including keel, helping convert roughly lateral sideways forces into forward ones against the water; those lateral ones having already been a conversion of largely head-on winds in the first place, thus two "almost up to 90 degree" redirections of force allow ''very nearly'' a 180-degree reversal of wind-blown movement. Feel free to discuss the comparisons and differences between 'flappy sail', though blown taught by the air, and an 'upright aircraft wing' solid design. ...See, told you I could ramble, but someone can surely do better at segmenting and summarising the basics of this.) [[Special:Contributions/172.71.178.32|172.71.178.32]] 08:32, 17 May 2025 (UTC)

This is super embarrassing to admit, but I came here to verify whether this was a serious thing or not. I had no idea how a sailboat sails against the wind. [[User:Catgofire|Catgofire]] ([[User talk:Catgofire|talk]]) 23:58, 16 May 2025 (UTC)
:You aren't alone - I think I was an adult before I understood tacking in the sailboat sense of the word. [[Special:Contributions/162.158.174.127|162.158.174.127]] 02:45, 17 May 2025 (UTC)
: I'm wanting to add in some wisdom about "science-y" explanations that appear to be sensible but are completely wrong, segueing into how generative language models appear to be far more reliable than they are. However this margin is too narrow [[User:Kev|Kev]] ([[User talk:Kev|talk]]) 03:09, 17 May 2025 (UTC)

I've been really annoyed with ExplainXKCD in the last few months ever since the initial posting has always been LLM generated. It requires more brain power to make sense of AI slop and edit it, than to contribute to a blank page. [[Special:Contributions/162.158.162.103|162.158.162.103]] {{unsigned ip|162.158.162.103|15:44, 17 May 2025|...yes, probably signed with just three tildes, by accident, but the intended message is the same...}}
:I don't think that LLM has been used for the most troublesome bits. LLMs can 'hallucinate', but tend (unless ''specifically'' asked) to make a lot more grammatical sense if you don't look too much further. [[Special:Contributions/162.158.33.240|162.158.33.240]] 18:29, 17 May 2025 (UTC)

Any chance we can add an explanation of how it *actually* works? [[Special:Contributions/162.158.216.174|162.158.216.174]] 10:03, 17 May 2025 (UTC)
:Through judicious angling of sail, wind (from any direction other than fully head-on) is deflected(/uses 'wing-effect') to create a force, trying to push the boat, that might be mostly sideways but also a bit forward. Because of the shape of the hull, any sideways force is resisted by the water, reinforcing the remaining forward component which the hull is far more ready to take advantage of. Enough sail (and enough stability to resist rolling) gives a large amount of movement towards, but not ''exactly'' towards, the wind. [[Special:Contributions/172.69.224.72|172.69.224.72]] 10:41, 17 May 2025 (UTC)

The joke is that the most commonly used explanation for why flow over a foil generates lift - particles going one way have a longer way to travel than the other, which generates a difference in speed and therefore a pressure differential - is wrong. {{unsigned ip|172.69.109.91|10:36, 17 May 2025}}
:What is wrong with the explanation which you say is wrong? What is the more correct explanation? [[Special:Contributions/172.71.150.33|172.71.150.33]] 20:28, 17 May 2025 (UTC)

The picture seems to show an axis of rotation (the mast) for the sail being on the end of the sail. Is that correct for a certain class of sailing vessel?~~ {{unsigned ip|162.158.146.128|15:57, 17 May 2025}}

Currently the explanation says "most interaction of physical things at macro scale (humans and boat sized objects) are electromagnetic in nature" I have certainly read that, and have seen examples of electromagnetic interactions between atoms. However, I also encounter explanations that describe interactions in terms of Pauli exclusion principle (see for instance {{w|Contact force}}). This makes me question the view presented in the first sentence. Since my physics is a bit rusty I haven't tried to fix it, but I think it may need clarification. [[Special:Contributions/172.71.150.33|172.71.150.33]] 20:28, 17 May 2025 (UTC)

I split up the example calculating Lorentz force on a boat. It still needs some work (I was just untangling it so I could see what it said).
The paragraph about one coulomb of charge I left as is - it needs untangling, so be bold.
In the example - somebody should recheck the math (I just copied what there, but in changing units to be more familiar, like km/h, I might have introduced errors). I also changed the field strength to the right order of magnitude for Earth's surface, and multiplied the wind speed by 10 to compensate.
As best I can figure the numbers for the example may have been chosen to get a force of 1 Newton. (I can't see any other reason for the ludicrous wind speed of thousands of km/h.) Might be better example to use a reasonable wind speed (e.g. dial it back to hurricane force) and a reasonable charge (something like what you could accumulate with an automobile, or when you zap yourself after getting out of a car seat) wind up with an even more negligible force. Then we could dispense with the paragraph explaining why 1 Coulomb is silly. [[Special:Contributions/172.71.151.93|172.71.151.93]] 22:15, 17 May 2025 (UTC)

:If anybody wants to restore parts of it or play with it, the version with the calculation of Lorentz forces is here [https://www.explainxkcd.com/wiki/index.php?title=3090:_Sail_Physics&oldid=378099] I thought it somewhat interesting just as physics problem to show what the effect was.
:I got curious about how much charge is involved when one zaps oneself on a car seat. A lighting bolt is a few coulombs. [[Special:Contributions/172.71.142.188|172.71.142.188]] 23:40, 17 May 2025 (UTC)

Extreme apologies to an(other) IP editor who may have made several possibly great improvements to the article. I had ''so'' many problems with what was already there that I did a massive rewrite and set things up so differently that I'm not sure that (at a very long glance, but maybe not as long as it diserves) much of that effort is really worth feeding back in. Or even capable of being. Obviously, it's up to the rest of the you (including the person/people I overrode) to make your own judgement about that. I will also go back in to carefully check what I may have desecrated. [[Special:Contributions/162.158.74.68|162.158.74.68]] 22:37, 17 May 2025 (UTC)

:No problem. I remade a few tweaks. The calculation is linked above is anybody wants to restore part of it. [[Special:Contributions/172.71.142.188|172.71.142.188]] 23:40, 17 May 2025 (UTC)

Talking of multiple sails (as the explanation does, at least right now), I'm reminded of a children's SF-based book I read when I was... well, a child. It was set on a (mostly) waterworld, as I recall, that had ''multiple'' suns (in orbit around ''it''..? ...already we can be sure it was probably not the hardest of hard-SF settings, of course). The boats/skiffs/whatever had multiple sails to move around. Multiple ''solar sails''. Depending upon which ones were deployed (kite-surfing-like), they'd get pushed (and you'd get pulled) by the ''sun that they were for''. So if you wanted to go <- thataway, deploy one particular sail, or thataway -> deploy another instead. As if solar sails even work that way. (Or even would work that way ''as effectively'' as a kite might in the... I presume there was an atmosphere... if there wasn't, then that'd explain the need for no-air sail-like solution, but raise significant other questions ...though clearly could not raise kites.) I may have misremembered some of the details, even perhaps some of the 'wrongness', but... I definitely remember I had to suspend quite a lot of disbelief (don't ask me from which sun it is hung!) when I read that. [[Special:Contributions/172.68.205.187|172.68.205.187]] 23:25, 17 May 2025 (UTC)

I was just looking at some things about solar sails and similar. This comic got me curious about:
Whether one could use a solar sail to sail "upwind"? -- you can - thanks to gravity and orbital mechanics. e.g., Sail in direction of your orbit - shifts apogee out, perigee in.
Whether you could build a {{w|magnetic sail))? -- yes - doesn't work quite like the one in the comic (sun provides wind of particles, sail is magnet to redirect them).
There is also a version of solar sail using electric fields to redirect charged particles.
One question I haven't found anything about is do solar sails (conventional ones, not electric) accumulate charge, and what effects that might have.
I just mention here in case anybody thinks way makes sense in comic explanation. [[Special:Contributions/172.71.142.188|172.71.142.188]] 23:40, 17 May 2025 (UTC)

2882: Net Rotations

2024-01-18T06:48:38Z

172.71.142.188:

{{comic
| number = 2882
| date = January 17, 2024
| title = Net Rotations
| image = net_rotations_2x.png
| imagesize = 318x477px
| noexpand = true
| titletext = For decades I've been working off the accumulated rotation from one long afternoon on a merry-go-round when I was eight.
}}

==Explanation==
{{incomplete|Created by a DIZZY ROBOT - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

This comic may refer to a thing that some people with {{w|OCD}} do, which is to spin around to get rid of "net rotations," hence the title of this comic. [[Cueball]] (perhaps representing [[Randall]]?) takes this one step beyond the typical person with OCD - he calculates the net rotations each day and spins around at the end of the day to cancel this out. In this case, he would be spinning left 17 rotations to return to zero.

Alternatively, this could be referring to the Einstien-Cartan theory, which is an extension of Einstein's general relativity. The theory suggests a coupling between the intrinsic spin of elementary particles (fermions) and the torsion of spacetime, and this comic appears to humorously extrapolate this idea to even supermolecular structures like a human, telling readers to "cancel out your accumulated turns at the end of each day to avoid worldline torsion", where in reality, it is highly unlikely the spin on such a large scale would cause any torsion in anyone's worldline, or their path traced by a particle or observer in spacetime.

A mobile device with position/orientation sensing would be able to keep track of one's net rotations, eliminating the need for calculations. One would need only to do one's spinning while monitoring the device to see when it returned to zero.

The caption at the bottom of the comic suggests that it is another one of Randall’s [[Category:Tips]] and says that it is healthy and necessary/highly recommended to do this. However, most people don’t, and most people are still ok. In fact, xkcd's own characters are perfectly ok with [[162: Angular Momentum|accumulating net rotations]] and [[2679: Quantified Self|similar topological excesses]].

The offered reason for the necessity to do this is a physics joke: the reference to spacetime and to one's "worldline" has to do with relativity.

Unlikely but could also be influenced by a 1966 novel, The Revolving Boy by Gertrude Friedberg, if Randall has read it, whose protagonist suffered from being out of true depending on the number of turns he was forced to make in his everyday life. No spoiler as to why that was important to the plot.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:[Cueball is standing on one leg in front of a whiteboard with his arms crossed, thinking to himself. There are circular curves around Cueball indicating rotary motion. The whiteboard contains two vertical helix-like curves crossing over each other at multiple points and other notes shown as rows of illegible scribbles, the bottom one of which is circled. There is a thought bubble over Cueball.]
:Cueball (thinking): ...and three lefts for going down the stairwell at work, two rights from cloverleaf interchanges, minus one for the Earth's rotation...
:Cueball (thinking): Okay, that's a net of 17 right.

:[Caption below the comic:]
:Spacetime health tip: Remember to cancel out your accumulated turns at the end of each day to avoid worldline torsion.

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Astronomy]]
[[Category:Physics]]
[[Category:Tips]]

1157: Sick Day

2024-01-18T02:33:51Z

172.71.142.188: /* Explanation */

{{comic
| number = 1157
| date = January 7, 2013
| title = Sick Day
| image = sick day.png
| titletext = Wikipedia path: Virus -> Immune system -> Innate immune system -> Parasites -> List of parasites of humans -> Naegleria fowleri -> Primary amoebic meningoencephalitis -> Deciding I DEFINITELY shouldn't connect an aquarium pump to my sinuses
}}

==Explanation==
This pie chart for the most part reflects the usual experience of being sick — tossing and turning in bed and cleaning up mucus and other bodily fluids from facial orifices— in addition to a few ponderings of a rather more scientific bent.

The "army of critters that patrol my body" would appear to refer to the human {{w|immune system}}, which is made up of various cells and processes that actively fight infections and pathogens.

The punchline appears to be "pondering hooking an aquarium pump to my sinuses," which indicates that [[Randall]]'s sinuses were completely clogged with mucus, which made him wonder whether hooking up an aquarium pump would help clear them out, perhaps akin to a {{w|Nasal irrigation|Neti pot}}. Studies on nasal irrigation, however, have had mixed results, and the practice may not in fact be beneficial.

Randall's {{w|Wikipedia}} path: {{w|Virus}} →‎ {{w|Immune system}} →‎ {{w|Innate immune system}} →‎ {{w|Parasites}} →‎ {{w|List of parasites of humans}} →‎ ''{{w|Naegleria fowleri}}'' →‎ {{w|Primary amoebic meningoencephalitis}}.

''Naegleria fowleri'' is known as the brain-eating amoeba. It is found in warm bodies of stagnant {{w|fresh water}} and causes the disease primary amoebic meningoencephalitis, a rare but highly lethal condition. Although N. fowleri are not commonly found in aquaria, Randall's Wikipedia wanderings force him to conclude that attempting to clear out his sinuses with an aquarium pump is too risky. Since this danger would presumably not be present at all with an unused, sterilized aquarium pump, the comic may be referring to a particular pump currently in use and close at hand.

==Transcript==
:[This comic shows a pie chart with 5 slices, each with a label and a line pointing to these five different sized slices. There is a caption above the chart:]
:Activities while sick:
:[The labels on each slice is given in clockwise order starting from the top. The percentages are estimated from the image and are noted in the square brackets before the transcript:]
:[54%] - Shifting around in bed feeling my skin crawl
:[24%] - Wiping various face holes
:[5%] - Staring at a news site but not reading it
:[14%] - Thinking about how cool it is that I'm partly made of an army of critters that patrol my body ruthlessly dispatching anything they find trying to prey on me.
:[3%] - Pondering hooking an aquarium pump up to my sinuses

{{comic discussion}}
[[Category:Pie charts]]
[[Category:Biology]]
[[Category:Wikipedia]]