Method

Setup

           We placed a piece of red, frozen Jell-O on a stand that was positioned on the countertop. To create this stand, we covered a mouse pad that sat on top of two stacked bricks with a black cloth. We then placed a spray painted black wooden tray on top of the covered stand to create a surface that would not “weather” or change by supporting the Jell-O. Readings of the dimensions of the Jell-O that was placed on this stand were taken for each new piece used; however, each piece was approximately 9 cm x 4 cm x 4 cm. A Crosman 760 air rifle was aimed on its horizontal stand at the Jell-O, and a pellet trap was placed approximately 0.5 meters past the Jell-O to catch the .177 caliber BBs. The rifle was 1 meter from the Jell-O (see figures 1 and 2).

Figure 1. Side view of setup

Figure 2. Overhead view of setup

            We were interested in capturing four images of the pellet, two prior to its penetration of the Jell-O, and two after leaving the Jell-O so that we could measure the deceleration of the BB. To capture these images, we set up four Vivitar 283 flashes, all aimed at the subject 0.40 meters away. To obtain the correct delay time for each of the four flashes, we used a computer intervalometer (figure 3). [3] We used a Toshiba 1600 for our computer. A sound trigger was placed on the wall, 0.74 meters from the rifle. The trigger was attached to the wall behind the gun to avoid the vibrations that the gun produced when fired. With preliminary testing, we found that these vibrations interfered with the timing of the sound trigger, causing our pictures to be inconsistent. The signal from the sound trigger traveled to an input box which connected to the parallel port interface in the computer. This sent the interval and delay information that we entered into the computer prior to each picture to the digital output. The digital output sent the information to each of the optoisolators, each of which connected to a single flash unit, and therefore set off each flash unit at the appropriate time. [4]

Figure 3. Diagram of computer intervalometer

Timing, Triggering, and Imaging

            The timing for each picture was determined for each picture in advance, with our delays ranging from 7.55 ms to 9.00 ms and our time intervals between the flashes ranging from 0.70 ms to 0.90 ms. The timing had to be adjusted throughout our photo shoots because the Jell-O would defrost as we took the pictures, causing the pellet to penetrate it more easily and us to need less time between the flashes. Also, we found the gun to be inconsistent in speed. As the barrel heated, we found that we needed to increase the delay to the flash units.

            We used a sound trigger as the triggering method for our pictures. The sound of the rifle being fired set off the trigger, which sent the signal to our computer.

            We used the open-shutter technique to take our pictures. We used a shutter speed of 1 second, an aperture of f/8, and a film speed of ISO 400. We used a manual focus to prevent the Minolta Dimage Z1’s auto focus feature from causing the camera to go out of focus when the lights were turned off.

Experimental Method

            We prepared our frozen Jell-O according to the directions provided on the back of the box. The pan was approximately 4 cm deep. We placed the pan containing the Jell-O into the refrigerator, where we allowed the Jell-O to solidify for approximately 24 hours. We then placed the Jell-O into the freezer, where it remained throughout the duration of our experiment, except for the times when it was removed to be used in a photo shoot.

            To take our pictures, we first made sure that all of our electronic equipment (computer intervalometer with the set delays, the flash units, and the piezoelectric sound trigger) was on and functioning. We placed one piece of frozen Jell-O on a piece of wood, which was spray painted black, and then put the wood on the stand. The rifle was loaded with a BB and pumped ten times.  It was then put on the rifle stand and lined up with the Jell-O and the pellet trap. At the same time, the camera was turned on, and was adjusted to our determined f-stop, zoom, and shutter speed. When the photographer and shooter were both ready, the safety was taken off of the rifle, the input was enabled on the computer intervalometer, and the lights were turned off. The photographer said, “taking picture,” and triggered the camera. The shooter listened for the shutter lag and then fired the rifle.  This triggered our computer interface, which set off the four flashes, each at the appropriate time. The lights were turned back on, and the photo was analyzed to make adjustments for the next picture.