Projects in High-Speed Photography
Experiment -- Part I
Loren Winters, North Carolina School
of Science and Mathematics, Durham, NC
Travis Williams, North Carolina State University, Raleigh, NC
The annotated photograph below provides an overhead view of the experimental setup. Two nails were driven into a two-by-four (painted flat black) a distance of 11.5 cm apart. This distance was about 20% greater than the relaxed length of the rubber band. The band was initially stretched and held in the jaws of a clamp on the left. Colored marks on the band were used to help identify changes in local extension and to make sure the band was positioned the same for each firing. Marks 1 and 5 near the ends were used for positioning. Marks 2, 3 and 4 divided the band into quarters when in the equilibrium state.
The photograph shows the band approximately 1 ms after its release. The pinch that was placed in the band by the clamp is evident even after the band is free of the clamp. The motion of the band is frozen by a flash burst of approximately 30 µs. The flash is triggered by an infrared photogate (not shown) located just beyond the jaws of the clamp. The infrared beam is initially obscured by the rubber band. As soon as the band leaves the beam's path, a computer detects the signal and begins a countdown routine in order to generate the desired time delay before sending the output signal that triggers the flash unit.
The double-image photograph below was used to measure the average speed of the left end of the band 1-2 ms after release. Two flash units were discharged 1.00 ms apart. Red and green filters on the flash units helped to visually separate the images on the photograph. The average speed of the left end of the band during this interval was found to be about 30 m/s.