On the ocean beach-why elliptic pebbles do not become spherical

Among pebbles strewn across a sandy ocean beach, one can find relatively many with a nearly perfect elliptical (ellipsoidal) shape, and one wonders how this shape was attained and whether, during abrasion, the pebbles would remain elliptical or eventually become spherical. Mainly the latter question was addressed in a previous publication which identified frictional sliding and rotation of an elliptic pebble as main abrasion processes in the surf waves. In particular, it was predicted that the ellipticity epsilon={1-b2/a21/2 (a>b, principal ellipse axes) converges to a common equilibrium value for elliptic-like pebbles. Unfortunately, the derivation was based on an invalid force expression and a dimensionally unsuitable curvature. In this paper, not only force and curvature but also the contact duration with the sand surface during rotations is taken into account by fairly simple physical arguments, and it is shown that elliptic pebbles neither approach the same ellipticity and nor become more spherical nor more disk-like but rather that the ellipticity epsilon increases.

Automated summary

This paper addresses the issue of shape change in elliptical pebbles during abrasion, taking into account factors such as contact duration and forces. The study shows that elliptical pebbles do not become more spherical or disk-like, but rather the ellipticity increases.