Rapunzel, Rapunzel, let down your golden ponytail?
This is Sandra Tsing Loh with the Loh Down on Science, saying, ponytail??? Now there's a version I haven't heard before.
But I bet English physicists Raymond Goldstein and Robin Ball would love it. They're the brains behind the Ponytail Shape Equation--a way to predict what shape a ponytail will take.
Uh ... couldn't you just put the hair up and see? Sure, but where's the fun in that?
Besides, physicists like hairy problems.
So Goldstein and Ball began with hair switches--commercial ponytails. Taking a random sample of hairs from each switch, Goldstein and Ball measured the hairs' elasticity, density, and waviness. In the process, they discovered an important effect, the switches' "swelling pressure," or fluffiness. It comes from hairs of random waviness being forced together.
Using their results, they devised the Ponytail Shape Equation. It includes a ratio they termed
"the Rapunzel number" because it involves length.
When they used the equation to predict the shapes of switches they'd trimmed, it worked like a
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