I looked this up in the SD archives, but I still don’t believe that my skinny little chicken arms can make anything move that fast. Perhaps I am not grasping the physics involved here. The ‘sonic boom’ seems to occur as the whip is changing direction. Is the tip of the whip really moving at 742 mph when it is snapped? I always thought that the sound came from the sudden straightening of the whip when the direction is changed.
I await the arrival of the SDMB Physics Crew, who are way smarter than I am, but I found you a few online cites:
http://www.rose-hulman.edu/~songsb/whips.htm :
"The crack of a whip is not the whip slapping up against itself, or the ground, nor is it cracking because of the same principle as the “wet towel snap.” The the end of a whip actually goes over the speed of sound. I recently viewed a videotape that mentioned experiments that clocked the speed of a whip to prove it was going supersonic. They used high-speed photography and slow motion / still-frames to clock the speed of that whip at 1400 feet-per-second; or roughly 900 miles per hour. That is almost 200 miles per hour OVER the speed of sound. A whip is able to travel over the sound barrier because its diameter becomes smaller and smaller as one goes from the handle to the tip. . The momentum that someone puts in at the handle is conserved, and therefore the velocity increases down the length of the whip until it is travelling supersonic at the tip. In order to accomplish this, one must create a “loop” to travel down the body of the whip. "
http://earthsky.com/1999/es990902.html :
"Thursday, September 2, 1999
DB: This is Earth and Sky. Philip Urban of Upper Saddle River, New Jersey, asks, “. . . when a whip ‘cracks,’ does it make the noise because the tip of the whip is breaking the sound barrier?”
JB: Yes, it does, Philip. When you snap a whip, you deliver energy to the handle portion of the whip. As that energy passes through the whip, the velocity of the energy increases – because the energy is spread out over a smaller area as the whip gets thinner and less stiff. At its tip, the whip may be moving at nearly twice the speed of sound – and you hear [SFX - whip crack].
DB: So the tip of the whip moves faster than sound – or “breaks the sound barrier.” But why do we hear a noise when any object breaks the sound barrier? The reason is that any moving object creates pressure changes in the air around it. These changes move away from the object at the speed of sound. An object moving more slowly than sound never catches up with those pressure disturbances . . .
JB: But if an object’s velocity exceeds the speed of sound, the air molecules don’t retreat fast enough to get out of the way of the object flying through them. The recoil of the air molecules creates a large amount of energy when the molecules smack back into each other. This movement of air molecules reaches our ear as [SFX - crack]."