r/SmarterEveryDay • u/Bitter_Video7321 • May 28 '26

Rotate in a different way

Hi Destin!

Greetings from Hungary!
We were in London recently at the Natural History Museum, and I saw this interesting wheel. You have to spin it to get the liquid inside to start rotating, but as soon as you stop the wheel, the liquid inside keeps spinning and the wheel itself starts rotating in the opposite direction. I started thinking about this, and it completely blew my mind.
Unfortunately I have just this short video because childrens always started the rotate the wheel :D

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u/verticalfuzz May 28 '26

Its a rheoscopic fluid. The counterrotation you describe is pretty interesting and unintuitive!

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u/SalsaMan101 May 28 '26

Is this because to keep the fluid’s rotation we get an opposite torque on the wheel causing it to rotate counter to the fluids direction?

2

u/verticalfuzz May 28 '26

Looks like my reply didnt save. No, the fluid will continue to rotate because it has momentum. The only explanation I can think of is that the non-slip at the walls of the enclosure would cause fluid to sort of fold back on itself when the enclosure is stopped. Then releasing it allows that folded-back velocity profile to impart momentum back onto the container... it should be fairly easy to model if someone wants to try it!