Timing: Measuring High Speeds
With a high-speed clock such as the one described previously, one can measure very high speeds. The photo below shows the elements of a setup to measure the speed of a balloon rip. The clock is centered above the balloon. Two sound triggers placed at opposite ends of the balloon were used to trigger two flash units. The sound from the pop took longer to reach the trigger on the right, thus delaying the second flash discharge.
The time delay would approximately be the difference in time for sound to reach the two triggers from the puncture site. (The delay also depends on the response time of the sound trigger circuitry. This may not be the same for the two triggers.)
The two flash units captured two superimposed, V-shaped images of the moving rip. One could estimate the speed of the rip in relation to the speed of sound simply by comparing the distance traveled by the vertex of the rip to the difference in distances from the puncture site to each of the triggers. The result is about 5/7 of the speed of sound or 250 m/s.
A more accurate measurement can be obtained using the angle swept out by the hand of the clock. This method of measuring the time interval is described on the Timing page. The distance traveled by the rip vertex can be measured in relation to a known distance. The diameter of the clock disc can serve this purpose as long as the disc is the same distance from the camera as the rip.
If you want to do the calculations, here's what you need to know: frequency of disc = 51.7 rotations per second, diameter of disc = 0.105 m. An enlargement of the clock face, superimposed on the balloon at the same scale, is provided for better measurement accuracy.