Projects in High-Speed Photography
Design of the Experiment
The three student experimenters hypothesized that the tip of a snapped towel did exceed the speed of sound. In order to test this hypothesis, they had to work within certain constraints.
- They didn't have a budget, so they would have to use available equipment and improvise as needed. This meant that using a high-speed motion picture camera was not a possibility. However, they could use high-speed photographic equipment that had been developed for experiments by former students.
- They didn't know any towel flipping marksmen, so they would have to develop that skill (seriously).
- They would have to complete the project in a semester (3½ months).
The students decided on the experimental setup shown above (view looking down). A bank of 4 flash units, controlled by a computer,1 would be used to obtain multiple-image photographs from videotape.2 The triggering event would be the passage of the tip of the towel through a light beam. The computer would detect when the light to the photodetector was blocked and would then discharge the flash units in a timed sequence.
Why not use a sound trigger? The response of a sound trigger would lag that of a photogate. The difference might be only a millisecond, but that could be all the time it took for the towel to make its flip. The event would be over before the first flash discharged. (In fact, this is what the students found in some initial testing.)
The odds against performing a successful experiment were high for the following reasons:
- The tip of the towel had to be targeted for a very small region defined by the intersection of the narrow light beam and the field of view of the camera.
- The towel had to reach its greatest extent just after it reached the target region. (In experimentation, it was found that the time window was less than a millisecond.)
- The towel had to be snapped with sufficient force and with the right technique to produce the cracking sound that would signal, if the hypothesis were correct, that the towel tip had exceeded Mach 1.
- The towel had to be snapped in near total darkness, since the camera would be running continuously.
A video camera recorded images directly to a freeze-frame VCR. This method of recording was chosen for two reasons:
- The ratio of failed to successful snaps was expected to be quite high for the reasons listed above. Videotape would be an inexpensive medium for recording.
- With a freeze-frame VCR, images could be played back immediately. Estimates of the speed of the tip could be made from the TV display.
As far as the students knew, no one had attempted such an experiment before with a towel. Spence volunteered to be the towel snapper. As you will see in the pages that follow, he deserves the title of "world's most accomplished towel snapper".