Physicists Explain the Shape of Perfect Ponytails Every
schoolboy has, at some point, contemplated the fountain-like
springiness of the ponytail attached to the girl sitting in
front of him. Five centuries ago, even Leonardo da Vinci mused
about the fluid quality of hair, which streams from the head
like water. Now, ponytails have been subjected to the fine-
toothed comb of science. In a paper published Feb. 13 in the
journal Physical Review Letters, physicists in the United
Kingdom derived a "ponytail shape equation" — a mathematical
formula that accounts for the stiffness of the hair in a given
ponytail, its weight and its average curliness in order to
predict that ponytail's shape. The "fourth order non-linear
differential equation," as study co-author Robin Ball, a
physicist at the University of Warwick, described the ponytail
formula, "is a three-way balance of bending, gravity and
curliness." As gravity pulls a bundle of hair downward,
collisions between the individual hairs cause the tail to swell
outwa