Cracking a bullwhip is a sonic boom

Yesterday when I was reading something about acoustics, I saw the author wrote that bullwhip cracking is because of the sudden change of polarity when, as can be seen as an open-end tube, the sound energy comes from one end and reaches another and is reflected in a different way compared to a close-end tube. The difference of reflection in a close-end tube and an open-end tube is that the polarity changes in an open-end tube but not in a close-end tube. In an open-end tube, when the sound energy reaches the open-tube, it gets reflected back, only the polarity is changed, e.g., the peak becomes the valley, etc.

This polarity change actually comes from a huge energy release. In bullwhip cracking, this released energy makes the end of the whip moving at a speed faster than that of the sound (which is 344 m/s at 70F at sea level), and creates a sonic boom. The speech of the popper (the tip of the whip) can be as fast as 700 mph or appropriately 312 m/s. At some conditions, this speed can be faster than that of sound.

Here is an interesting article in American Scientist Online about calculation of the speed of the bullwhip.

A sonic boom is caused when an object, e.g., a jet plane, goes through the air at a speed faster than the sound and hence the sound waves in frond and behind the object cannot spread as they usually do when the speed if slow, but forced together and compressed and merged into a single shock wave.

This is a beautiful picture of a sonic boom created by a fighter plane photographed on July 27, 2005, and published by national geographic.


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