Can the motion of a cracking whip be described as the interference of two waves? I was watching a whip crack in slow motion and I noticed that the motion of the whip could be described using two different circular descriptions.
1) the user circles the whip around over his head, creating a rotation with a period.
2) next, he jerks the whip, causing a different and faster movement of the whip.
the new movement appears to travel down the whip (which is already moving) until it reaches the tip.  At this point, the tip creates a loud crack, which is associated with a supersonic movement.
In my mind, the two different motions cause an visual effect similar to a (constructive?) interference pattern.  Can the cracking of a whip be described as the interference of two waves?  If so, then what is going on? 
 A: In a sense yes, because if you disturb a moving object you can always express its motion as the sum of the gross motion and the disturbance.
But I think this interference is not important to the nature of the crack. The reason the tip of the whip reaches such high speeds is that as the wave travels down the whip, it is concentrated into a smaller mass of whip material. Momentum is mostly conserved in the wave, so a relatively low-velocity movement of the heavy handle results in much higher velocities in the lightweight tip.
There's nothing special about the circular motion at the tip (as opposed to the same circular motion in any other part of the whip) which causes constructive interference at that point specifically. So any interference is not inherent to the crack. I suppose the circular motion somehow helps the whip-operator to actually generate the necessary waveform in the whip, but I don't know exactly how. The fact the whip is in this circular motion means it's under tension even though the end is free. I speculate this is what allows the wave to propagate, rather than dispersing its energy making net displacements to the whip, as happens when you try to put a wave into a slack rope.
There is a fairly famous related problem. Take a very flexible string. Fold it in half, with the two ends upwards and the middle hanging down. Now release one end so that it falls directly downwards, while holding the other end. The normal assumptions of abstract mechanics problems say that the same thing will happen to the string as happens to the waveform in the cracking whip: the part of the string above the falling end "should" be motionless, and this represents more and more of the string as the end falls, and therefore the falling end itself "should" move extremely fast in order to conserve kinetic energy and momentum. The difficulty is to figure out in this model what internal force in the string (if any) is capable of making that happen. Of course it doesn't actually happen in practice: rather, the top part of the string does go into motion.
