Spinning eggs--which end will rise?

Abstract

We examine the spinning behavior of egg-shaped axisymmetric bodies whose cross sections are described by several oval curves similar to real eggs with thin and fat ends. We use the gyroscopic balance condition of Moffatt and Shimomura and analyze the slip velocity of the bodies at the point of contact as a function of $\theta$, the angle between the axis of symmetry and the vertical axis, and find the existence of the critical angle $\theta_c$. When the bodies are spun with an initial angle $\theta_{\rm initial}>\theta_c$, $\theta$ will increase to $\pi$, implying that the body will spin at the thin end. Alternatively, if $\theta_{\rm initial}<\theta_c$, then $\theta$ will decrease. For some oval curves, $\theta$ will reduce to 0 and the corresponding bodies will spin at the fat end. For other oval curves, a fixed point at $\theta_f$ is predicted, where $0 <\theta_f< \theta_c$. Then the bodies will spin not at the fat end, but at a new stable point with $\theta_f$. The empirical fact that eggs more often spin at the fat than at the thin end is explained.