Projects in High-Speed Photography
Experiment -- Part II
The setup for this series of photographs was identical to the first with the exception that the rear nail was sheared off level with the surface of the board. The same rubber band was used and was stretched initially by the same amount. The photo below shows the band being held in the clamp (at the left) before release.
For comparison, the photo below shows the rubber band in its relaxed state. This and the previous photo have the same scale.
A limitation of the experiment was that each photograph was of a different firing of the band. (It would have been preferable to take a high-speed motion picture of a single firing but the equipment for this was not available.) In order to draw conclusions based on a series of images, it was necessary to know how reproducible the results were. This was tested by taking more than one photograph of the band under as nearly identical conditions as possible.
The two images below are representative of the reproducibility that was obtained. Both photos were taken at a time delay of 4.00 ms. Note the similarity in shape and size, even down to the bumps and wiggles on the band. Note also that the right end of the band is just starting to break contact with the nail. (In Part I of the experiment, this event occurred about a quarter of a millisecond earlier. That may be due to a small adjustment that was made in the position of the photogate in Part II.)
A better comparison of the two images is shown in the overlay below. The photographs were digitally merged, the two nails being used as registration points. One may wonder why the upper side of the band has a different shape than the lower. This was probably due to the experimenter's bias in the way the band was initially stretched and placed in the jaws of the clamp. The bias was toward stretching the upper side slightly more than the lower. This apparently made the upper side retract faster.
The complete photographic results are shown on the next page.