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Projects in High-Speed Photography

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. Click on the images to view QuickTime videos.

 

snap1253.jpg (5163 bytes) The video shows 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.
snap0200.jpg (3789 bytes) 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.
snap0229.jpg (3442 bytes) 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.)

 

snap1253.jpg (5163 bytes) 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

 


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