Initial Experimental Results
The four flash units were each covered with a
filter of a different color. This was done to give each image of the
towel a distinct signature. The sequence of colors was red, green,
blue, and yellow. For the first set of trials, the time
interval between flashes was selected between 300 and 400
microseconds. (1 microsecond = 0.000001 s) This gave a total time
span from 1st to 4th image of about a millisecond (0.001 s). The
experimenters found that this interval did span the region of
interest, that is, the region where the towel tip was flipping over.
However, getting the timing and placement of the snap just right was
very difficult. After a few weeks of attempts, the outlook for
success was gloomy. The next three clips show some of the
problems that were encountered and lessons learned in the process.
(The file sizes average only 50 kB.)
The movement of the towel is in the plane of
the page, and the tip is moving downward on the page. The towel
flips from the right side to the left side. The images and clips are
one-fourth of the recorded size.
VFW
QT
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Play either the Video for
Windows (VFW) or QuickTime (QT) version of the clip.
You'll see the image of the towel tip flash by on a single
frame. You'll also hear a sharp cracking sound,
indicating a good snap. Unfortunately, the video was
overexposed and the towel tip frayed, making quantitative
analysis of this clip difficult. |
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VFW
QT
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The towel tip hasn't reached
its greatest extent, where the speed is expected to be
highest. Play the clip to listen for the sound. It
doesn't have the sharp crack that the first clip does.
This wasn't a good snap. In fact, this was typical of
most of the towel flips. |
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VFW
QT
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This was a very good snap,
producing a sharp cracking sound. Note the great
increase in speed from the 3rd to the 4th flash. This is
evidence of the tremendous acceleration that the tip must
undergo if it is to reach supersonic speeds. The
problem with this clip was that the tip probably hadn't reached
its greatest speed. A clip was needed that showed the tip
flipping from one side to the other of the towel body. |
Near the end of September, after a month of
disappointing results, the students finally recorded a snap that
gave good evidence that the tip went supersonic. The clip below
shows two images on either side of the towel. (The image order has
been changed to red, green, yellow, blue.)
VFW
QT |
The tip reached its greatest
speed between the green and yellow images when the tip flipped
from one side to the other. A meter stick held in the
plane of the towel's motion was videotaped immediately
afterward and used for distance calibration.
Between the middle two images, the tip was found to have
traveled through an arc of 0.148 m. The time between
images had been set at 0.000402 seconds. Therefore, the
average speed of the tip in this time interval was 368 m/s.
Taking into account uncertainties in measurement (in
particular, distance), the uncertainty in the speed is 20 m/s.
At the lower limit, then, the speed is just above the speed of
sound, 345 m/s at the lab's temperature. |
| Click
here for a full-size frame that can be analyzed |
Go to Verification
of the Initial Results
|
