explain xkcd - User contributions [en]

3090: Sail Physics

2025-05-17T22:51:09Z

172.71.150.33: /* Explanation */ simplify, reapply some small fixes

{{comic
| number = 3090
| date = May 16, 2025
| title = Sail Physics
| image = sail_physics_2x.png
| imagesize = 699x263px
| noexpand = true
| titletext = Turning in other directions can be accomplished by using a magnetized centerboard and ocean currents, since a current flowing through a magnetic field induces a Laplace force.
}}

==Explanation==
{{incomplete|This page was created by a wind-blown electron. Don't remove this notice too soon.}}

This comic starts off looking like a straightforward explanation of how sailboats can travel upwind — a topic that continues to spark debate and refinement in physics circles. However, it quickly veers into a completely fictional and incorrect theory involving triboelectric charging and the Lorentz force, rather than referencing real mechanisms like {{w|airfoil|aerodynamic lift}}.

This humor works at another level — most interaction of physical things at macro scale (humans and boat sized objects) are electromagnetic in nature. So one unaware of sailing mechanics may start to explain the situation with electromagnetism, and could come to this line of thinking, but it is wrong. If we are to consider this, we find that either no force is appearing in the direction shown, or very little.

The first panel is a fairly accurate diagram used to explain the reasons why a boat can sail into the wind (see below), but gets no further into it that setting up the scenario.

The second panel portrays the {{w|triboelectric effect}}, which is transfer of static charge through the motion between two 'objects', which in turn depends on effective interaction surface area. It shows charge being accumulated by the wind stripping electrons from the sail of the boat, leaving the vessel positively charged. Among other problems, the charge that can be acquired is typically very small.

The third panel shows the boat being blown sideways by the wind, which is what one would expect under usual circumstances but is not in the desired upwind direction. This motion of a charged body through the {{w|Earth's magnetic field}}, however, results in a {{w|Lorentz force}}. Depending upon the relative directions of initial motion and the magnetic field, this ''could'' generate a perpendicular force, as indicated, assuming the entire premise was even as promised.

The final panel demonstrates this force as diverting the downwind (and sideways) track of the boat in a forward direction. As well as the various other problems that exist with the whole scenario, this is quite contrary to promise of allowing the boat to sail upwind, as the originally indicated wind direction and the finally indicated path is, if anything, still slightly downwind in nature.

The title text invokes further {{w|technobabble}} to suggests using a magnetised {{w|centreboard|retractable keel}} to adjust the nature of the forces. It conflates {{w|ocean current}}s (the global flow of water) and {{w|electric current}}s (the movement of charged particles). Perhaps from the supposed ability to move the magnet through the charge, as opposed to the other way round. It invokes the "Laplace force", which is just a {{w|Lorentz force#Force on a current-carrying wire|technical variation}} of the Lorentzian one.

=== The More Correct Explanation ===
[[File:Forces on sails for three points of sail.jpg|thumb|An indication of the forces on a sailboat in various directions relative to the wind direction. VT is the true velocity of the wind, VA is the apparent velocity, as seen by the moving boat. Various forces (F) arise from the way the wind hits the sail, which translate into the forward velocity of the boat, VB.]] The actual manner of how a boat is able to sail into the wind relies upon the way the wind hits the boat's angled (and curved) sail, producing forces that are divided between those directly in the direction the boat and perpendicular to it. Sideways forces encounter resistance from the water, leaving a net motion forward, a direction through the water by which a boat hull is designed to more easily pass. This allows a boat to sail at an angle into the wind (though not directly into it), with the right use of sails. The same effect also allows you to travel faster ''with'' the wind, at a slight angle away from its direction, than if you just ran exactly in its direction; using the sail square on will limit you to going no faster (and usually significantly slower than) the wind that you are relying upon to push you, whereas an angled sail and boat track can convert the forces into greater speed than even the following wind.

Your speed of sailing perpendicular to the wind tends to be greater than that which you can achieve heading at any angle into the wind, but this is no use if you wish to sail to a destination directly in the direction the wind is coming from. Aiming at an angle into the wind and {{w|Tacking (sailing)|tacking}} (briefly use your existing speed to turn directly across the wind), lets you combine sets of aiming off to slightly one side of the wind and doing the same slightly to the other, as required to reach your destination. The expert sailor can choose the {{w|point of sail}} to the wind that makes for the fastest journey time, combining the possible speed and the necessary amount of additional distance. Similarly, turns ('{{w|jibe}}s') across the wind allow a more optimal passage to a directly downwind destination than running straight with it.

Supposing that the comic physics ''had'' been more capable of doing what it suggests, tacking/gibing could also be important concepts. With two sails made of different materials, one could unfurl that which is able to accumulate a positive charge (by losing electrons) or else another that accumulates a negative charge (by 'borrowing' electrons from the air). In this way, you could account for how the effective direction (and {{w|Magnetic declination|declination}}) of the magnetic field would be different for any given location, wind direction and intended destination and ''perhaps'' eventually make progress in whichever direction the vessel is required to go.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

[Panel 1]

How sailboats use physics to sail upwind:

[A schematic boat with a sail is shown (top-down view). Winds shown with directional arrows, pointing in the direction of the sail (going towards 4:30 on a clock face).]

[Panel 2]

1. Wind passing over the sail strips away electrons via the triboelectric effect.

[Schematic similar to panel 1, but with charged ions shown across both sides of the sail, representing the triboelectric effect.]

[Panel 3]

2. The positively charged boat is blown downwind; its movement in Earth's magnetic field produces a Lorentz force.

[The same schematic, except a force vector is shown in the direction of the wind, and a perpendicular force vector (along 1:30 on a clock face) is shown with a dashed arrow.]

[Panel 4]

3. The Lorentz force acts perpendicular to the direction of motion, redirecting the boat upwind.

[A net force vector is shown perpendicular to the downstream vector.]

{{comic discussion}}<noinclude>

Talk:3090: Sail Physics

2025-05-17T20:28:22Z

172.71.150.33: electromagnetic force - how Pauli exclusion principle plays into this

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)