Why a spinning egg stands up

Settled in 2002 by H. Keith Moffatt & Yutaka Shimomura (Cambridge).

That a spun hard-boiled egg rises onto its end while a raw egg does not is an old kitchen-table curiosity that lacked a rigorous dynamical explanation for decades. In 2002 Keith Moffatt and Yutaka Shimomura at Cambridge showed, using a gyroscopic approximation valid at high spin rates, that slipping friction between the shell and table exerts a torque that drives the spin axis from horizontal to vertical, with the final vertical state being the one that minimizes kinetic energy for a fixed 'Jellett constant.' The explanation also accounts for why raw eggs, whose fluid interior does not spin rigidly with the shell, fail to rise.

A boiled egg spinning on a kitchen table slowly stands itself up on end, driven by nothing more exotic than ordinary sliding friction and a hidden conserved quantity.

Explanations that circulated

• Naive intuition that spinning objects always lower their center of mass to minimize energy predicts the egg should stay lying down, not rise - this misses the crucial role of frictional torque.
• Attributing the rise purely to the egg's asymmetric (prolate) shape, without accounting for slipping friction between shell and table, fails to explain why a raw (fluid-filled) egg does not rise the same way.

References

• Classical dynamics: Spinning eggs - a paradox resolved, H. K. Moffatt, Y. Shimomura, Nature, 2002