A general continuum theory for the distribution of hairs in a bundle isdeveloped, treating individual fibers as elastic filaments with randomintrinsic curvatures. Applying this formalism to the iconic problem of theponytail, the combined effects of bending elasticity, gravity, andorientational disorder are recast as a differential equation for the envelopeof the bundle, in which the compressibility enters through an 'equation ofstate'. From this, we identify the balance of forces in various regions of theponytail, extract a remarkably simple equation of state from laboratorymeasurements of human ponytails, and relate the pressure to the measured randomcurvatures of individual hairs.
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