Rattleback with and "without" friction @animations

Uploaded March 2022 | Updated September 2026, 2 weeks ago

I find it hard to understand how the "Rattleback" (or "Celtic stone") does not violate conservation of angular momentum. I'm not a sceptic though - I'm aware that my mind is very limited.

Much is available on YT and elsewhere on the net regarding this little gadget. Put simply and non-explanatory, its cross-sectional profile is slightly asymmetric, which, together with friction makes spin-flip possible if initially spun in clockwise rotation direction. Curiously, the opposite does not apply if fired off in a counter-clockwise direction. If no or little friction, the stone lacks "grip" and won't change rotation direction. This is demonstrated in the last part where I've applied a thin layer of olive oil on the table top to reduce friction.

Camera: Nikon D850, Nikkor 20mm 1.8 ED (in poor light conditions, as seen)

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