| 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. |
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.)
VFW
format
QuickTime format |
|
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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