| 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.
Go to Final
Experimental Results |
|
