←back to thread

Mathematicians have found a hidden 'reset button' for undoing rotation

(www.newscientist.com)
184 points mikhael | 3 comments | 16 Oct 25 15:47 UTC | HN request time: 0s | source
Show context
v7n ◴[21 Oct 25 19:12 UTC] No.45660268[source]▶
I was immediately reminded of the anti-twist mechanism, perhaps unrelated but "reset rotation, twice/half" comes up there as well.

https://en.wikipedia.org/wiki/Anti-twister_mechanism

replies(4): >>45660536 #>>45661316 #>>45661783 #>>45666297 #
amelius ◴[22 Oct 25 08:32 UTC] No.45666297[source]▶
Huh, looking just at the link at the top of the box, and forgetting the remainder of the links, this cannot work. I tried it with a flat cable. If you rotate it like that, it becomes twisted.
replies(2): >>45666450 #>>45667328 #
crooked-v ◴[22 Oct 25 08:57 UTC] No.45666450[source]▶
There's some Youtube videos out there of people who have built practical versions that work, like this one (with flat cables, even): https://www.youtube.com/watch?v=1x_oQv_qj_U
replies(1): >>45666511 #
1. amelius ◴[22 Oct 25 09:05 UTC] No.45666511{3}[source]▶
Sure, but the animation of the wiki page is wrong. The cable that ends at the bottom of the picture is fixed there, while the other end twists. That will result in a twisted cable.

(update: I was wrong, not the wiki page)

replies(1): >>45666670 #
2. Luc ◴[22 Oct 25 09:27 UTC] No.45666670[source]▶
I tried it and it works. The animation uses belts that are very flexible. With a real belt I needed to give it a shake to make it untwist itself, but it does work.

It is indeed easy to twist the belt until you have the hang of it.

I think the animation is a bit deceptive because even with elastic bands you'd have to provide some way for the correct untwisting to occur. In the animation it happens 'automagically'.

replies(1): >>45666789 #
3. amelius ◴[22 Oct 25 09:45 UTC] No.45666789[source]▶
Yes, indeed, it can work, I can see it now. But I wonder if/how you can make it practical.