THE crack of a whip occurs because the lash鈥檚 flailing tip breaks the sound barrier, producing a sonic boom鈥攄oesn鈥檛 it? Well, not quite. Researchers realised the tip couldn鈥檛 be responsible in 1998, when they unexpectedly found it was already moving at twice the speed of sound when the whip cracks. This left them mystified about what makes the cracking sound.
Now Alain Goriely and Tyler McMillen of the University of Arizona have used their equations of motion for a whip to explain what鈥檚 going on. It鈥檚 the loop travelling along the length of the whip that creates the sonic boom, not the tip itself, they reveal in a paper to be published in Physical Review Letters.
