Projects in High-Speed Photography
Verification of the Initial Results
The students' initial results were encouraging. However, their one good recorded snap was too close to the speed of sound to provide unequivocal support for their hypothesis. Therefore, they set out to make a repeat measurement and hopefully one at higher speed. They tried a number of changes in the experiment, including tripling the number of flash units in order to span a larger time interval, recording on film with a still camera to increase image resolution, and taping a marker near the towel tip to improve the accuracy of distance measurements. The latter technique was abandoned, as the extra mass of the tape may have had the effect of slowing down the towel.
|The students shot several rolls of film and obtained many sharp images from good snaps. However, none of their measurements yielded speeds close to the speed of sound. In this phase of the experiment, the greatest measured speed was 270 m/s (about 3/4ths the speed of sound) for the photo to the right. By Thanksgiving, 3 weeks before the end of the semester, the students had not been able to record a second supersonic snap. The previous 2 months of experimentation seemed to be a failure.|
Several possible reasons were considered.
- The plane of the towel's motion may have been tilted away from the film plane of the camera. This would have the effect of shortening the distance measurements and giving smaller measured speeds than actual.
- The time interval in which the speed of sound was exceeded may have been much smaller than the time interval between flashes (typically, 0.0003 s). This made sense, because the tip underwent tremendous acceleration going into the flip. Perhaps the speed of sound was exceeded only during the short interval when the tip was flipping from one side to the other.
- The towel had been shortening due to fraying of the tip and shedding of fibers. It had become more and more difficult to make the towel crack.
There wasn't much that could be done about item 1. Any tilt would likely be small and, by chance, some of the snaps would be nearly parallel to the film plane. However, it was possible to reduce the time interval between flashes. The students chose to decrease the time interval to 0.000098 s (98 microseconds). They also made a new, longer towel from a piece of a cotton bed sheet. Finally, they returned to video recording. This technique was faster than using regular film, and time was running short.