#physics riddle
My parents have a garden ornament that contains a "vertical spiral thingy" that can freely[^] rotate. When wine blows, it sometimes rotates clockwise and sometimes counterclockwise. What gives? When it has rotated by 180 deg it should be in the same position as if the wind was blowing in the opposite direction (the cage around it nonwithstanding), so I'd expect it to always rotate in the same direction, or to oscillate.
Some videos: https://photos.app.goo.gl/CQWvBBggW4jXQPKZ9
[^] there might be some weird hysteresis there, because it's basically a wire that slips in a hole
@timorl it is mounted on top only, so it seems unlikely that the axis is not vertical
bonifartius 𒂼𒄄
@robryk i think what plays part is that it's three spiral thingies offset 120° and that the top and bottom of the spiral thingies are the same but pointing in opposite directions.
@bonifartius how? I would understand if it rotated always in the same direction. I don't understand what changes to cause it to sometimes rotate in one and sometimes in other direction.
@robryk if you turn one of the arms 180°, the other two are not at the same position but offset by 60° each. the S shape provides the same drag for the top and bottom, but in different directions. so wind from the same direction can push on either the top or bottom part.
@bonifartius but how does that explain different directions of rotation? From what I understand you are saying that the torque from wind will vary between positive and negative over one rotation. I would expect that to cause oscillations.
@robryk i cant really draw it properly. imagone you look at it with one wing being to the left, one to the right and one in the back.
the top half of the shape will on one side point towards you and on the other side away from you, the reverse is true for the bottom half.
i think it's something which cleverly exploits chaos :)
@robryk but it's just a guess, i'm sure there are people with better knowledge about these things on here :)
@robryk Hm, maybe an obvious thought, but perhaps it isn’t perfectly vertical?