Indiana Jones proved just how useful a good bullwhip can be, both as a tool and as a weapon, but people are still surprised when neuropsychologist Jessica Cail tells them that one of her favourite hobbies is practising whip-cracking. She talks about this peculiar sideline in the latest instalment of the NOVA video series, Secret Life of Scientists. Cail studies the psychopharmacology of addiction at Pepperine University. But in her secret life, she's a badarse, whip-cracking stuntwoman. She's even appeared on Nickelodeon's Supah Ninja series as a villain wielding not one, but two, bullwhips. She landed the role in part because her husband gives whip-cracking lessons, but also because — as Cail points out — it's not a skill you can pick up in a day to prepare for an audition. She's been wielding a whip for 12 years now, giving her the equivalent of a "doctorate in bullwhips". It's a handy skill to have: "If there's ever a zombie apocalypse, everyone's gonna want me on their team."
There's some pretty interesting physics behind that seemingly simple crack of the whip. Scientists have known for over 100 years that it's basically a sonic boom that occurs when one part of the whip starts moving faster than the speed of sound. A 1958 study demonstrated this by analysing 1927 high-speed shadow photography of a whip in motion, and more recent studies determined that the tip of a cracking whip moves as fast as twice the speed of sound.
But one thing still puzzled physicists. The crack didn't occur at the same time the tip hit the speed of sound, thereby producing a shock wave. In 2002, the University of Arizona's Alain Goriely published a paper in Physical Review Letters showing that the sonic boom isn't created by the tip, but a loop travelling along the length of the whip. When the loop hits the speed of sound, that's what creates the sonic boom, which we experience as a loud crack.
The kind of motion — technically known as filament dynamics — can also be seen in casting a fly-fishing line and the motion of a sperm's tail — and possibly the long tail of an apatosaurus. Goriely was inspired to investigate the phenomenon after watching traditional Hungarian dancing with cracking whips while attending a conference in Hungary. "The first step was to buy a whip — everybody should try," he told the American Physical Society.
Cail would probably agree.