Title text:

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.

Transcript:

[Four panels show a schematic sail boat, seen from above, to indicate how it can sail into the wind. In the first panel the boat is heading straight up in the panel. The sail is fixed at the bow and describes a slight curve going to the right of the boat and then curving to the left, ending close to the stern. The rudder can be seen behind the boat. Five arrows, pointing towards 4:30 on a clock face, are drawn at the top left part of the boat, indicating the direction of the wind. There is a frame above the drawing of the boat with text. And then the arrows are labeled, and small lines going to the sail and the hull of the boat connects with two more labels:]
How sailboats use physics to sail upwind:
Wind
Boat
Sail

[In the second panel the boat is drawn similar to panel 1, but the wind arrow have been changed to showing how the wind now blows past the sail on either side. This is done with two lines of three arrows that goes on either side of the sail, and the second and third arrow bends to follow the curve of the sail. Charged ions are shown across both sides of the sail with positive on the left side of the sail, (over the hull of the boat) and negative on the right side, over the sea to the right and behind the boat. The positive charges are small + signs in circles and the negative minus signs in circles. Above the drawing there is the following text:]
1. Wind passing over the sail strips away electrons via the triboelectric effect.

[In the third panel the boat has turned towards right and has been moved closer to the bottom of the panel (this could be to acomodate more text above though). The entire hull is now covered in positive charges. A large broad dashed vector is shown going in the direction of the wind. The arrow is not over the boat but on either side of it, with the arrow head ending right of the stern of the boat. Two thin arrows are shown above the end of the force vector. A short solid arrow, that points along the same direction as the large arrow. And then a dashed arrow is drawn perpendicular to the first of these thin arrows (pointing along 1:30 on a clock face. Above the drawing there is the following text:]
2. The positively charged boat is blown downwind; its movement in Earth’s magnetic field produces a Lorentz force.

[In the fourth panel the boat has turned even more towards right and is back to the same height in the panel as the first two panels. The entire hull is still covered in positive charges. The broad dashed vector is still shown, but after starting in the wind direction it can be seen to turn slightly upwards before reaching the boat. And then when it comes out the other side of the boat it points in the direction of the bow of the boat, the arrow ending in front and a bit to the right of the boat. The two thin arrows from before are now shown to the left of the boat , with the short solid arrow pointing along the same direction as the start of the large arrow. And then a the dashed arrow drawn perpendicular to the first of these thin arrows pointing in the direction the boat is sailing. Above the drawing there is the following text:]
3. The Lorentz force acts perpendicular to the direction of motion, redirecting the boat upwind.

Source: https://xkcd.com/3090/

explainxkcd for #3090

  • who@feddit.orgEnglish
    2·
    5 hours ago

    If you upload an image, the URL field is populated with the URL of the uploaded image, so there’s not really multiple fields like it appears.

    Sure enough; I just discovered this for myself when running some tests. I also noticed that Lemmy offers an Alt Text field, which it looks like the bot is already populating.

    Test 1 : The URL field and the image attachment field were used; the latter overwrites whatever is placed in the former. I guess this might be modeled after Reddit.

    Test 2: A direct link to the image at xkcd.com was placed in the URL field, and the source link placed at the top of the body. Result: This is similar to the bot’s current format, including the flaw that a desktop browser with strict privacy settings won’t show the comic image as part of the post when the thumbnail is clicked, because it’s an off-site image. Having the source link at the top of the body does at least make it a little more convenient to click through to xkcd.com’s single-page view.

    Test 3: Only the image attachment field was used; the URL field was left blank. Result: This allows a desktop browser to show the comic image in-line when the thumbnail is clicked even with strict browser privacy settings, if the post is being viewed on the Lemmy instance where the post was made, because then it’s not an off-site image. Unfortunately, it’s still an off-site image when viewed on other Lemmy instances. The source link was again placed at the top of the body.

    (Side note: I used m.xkcd.com links instead of plain xkcd.com links in these tests, just to see how the mobile site looks in different browsers. In practice, either ought to work.)

    Conclusion: I don’t have one just yet. It would be helpful if we could direct all Lemmy instances to make their own local copy of a post’s attached image, to avoid the off-site image problem. Unfortunately, I don’t think there is a way to do this, and I suspect it would be too burdensome for some small instances.

    Having the source link at the top of the post body is helpful, at least.