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Projects in High-Speed Photography

 

Does the Tip of a Snapped Towel Travel Faster Than Sound?

Everyone knows that if a wet towel is flipped in just the right way, it will make a cracking sound.  Most of us also know from experience that being hit by the tip of the towel produces a painful sting.  A bullwhip is designed to intensify both of those effects.  In that case, the cracking sound is known to be created when the tip of the whip exceeds the speed of sound. Three scientists1 at the Naval Research Laboratory in the late fifties measured the speed of the tip of a bullwhip.  They employed an expert whipper snapper to flip the tip in front of a high-speed motion picture camera shooting at 4000 frames per second.  They showed that the tip reaches a speed around 1400 feet per second (speed of sound in air = 1100 f/s).  The cracking sound, in fact, results from a shock wave produced as the tip exceeds the speed of sound in air.  This is a miniature version of the sonic boom created by supersonic jets.

 

towel01snap.jpg (9033 bytes)

A wet neckerchief caught in the act of snapping--The tip throws off water droplets as it flips.

 

Download and play a clip (only 50 kB) of the snapped neckerchief shown above.  (If you have trouble playing this clip, see Players.)

 

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Three seniors--Spence Allen, Nicolas Lee, and Elizabeth Smith--at the North Carolina School of Science and Mathematics embarked on a research project at the beginning of the '92-'93 school year to find out if the tip of a snapped towel exceeded the speed of sound.  A description of their experiment and their results follows.  See reference 2 for the published paper on their results.

 

Go to Design of the experiment

 

1B. Bernstein, D.A. Hall, and H.M. Trent, "On the dynamics of a bull whip," Journal of the Acoustical Society of America, vol. 30, p. 1112 (1958).

2N. Lee, S. Allen, E. Smith, L.M. Winters, "Does the Tip of a Snapped Towel Travel Faster Than Sound?",  The Physics Teacher, vol. 31, p. 376 (1993).

 

 
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