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Description of Experimental Method The final goal for the project was to determine the deceleration of a projectile through the given media. The project group was comprised of Jihad Nance, Henry Lancaster and George Davis. We decided to shoot apple slices and derive the deceleration as the BB passed through them. Our BBs were fired from a Crossman 2100 Classic BB gun. We shot refrigerated granny smith green apples that were cut into slices approximately 1 cm in thickness. Our first step was to get our supplies. Supply List: Apple Two Plus computer Multiplexer Intervalometer program Sound trigger Interface box 4 Vivitar 283 flash units Outers Varminter Rifle Rest Small cardboard box with a notch in it to hold a 1 cm thick apple slice BB trap Several apples Black drapes to put behind and under the set Crossman 2100 Classic BB gun with BBs String 2 ring stands
(Description of Experimental Method Continued): Our original method involved hanging the apple slices (using our two ring stands) to be shot. We decided against this method because it was time consuming and we could not keep the apple slices still while they were hanging. We discovered this during our projects preliminary testing, prior to our photo shoot. Our next step was to synchonize the timing of the flashes with the flight of the BB. This took some time, but we were eventually able to pinpoint the BBs position by narrowing down the time between when we knew the BB was in the gun’s barrel, and when it had passed completely by the stage. We then slowly closed this gap of time until we achieved a delay prior to the first flash where we could see the BB between the barrel and the trap. The amount of delay needed to do this with out set up was 1.002 milliseconds. During this period we were not taking pictures, but getting visuals by turning out the lights and looking for a mid-air BB as illuminated by the flash. We eventually saw the BB between the barrel and the trap and began taking pictures. We then added a second flash unit with a small interval of time between it and the first. Now a second image of the BB could be seen in the same picture because of the interval of time we put we added. We used the Intervalometer Program to put in the delay. Once we got two images of the BB in one photo, we were able to get in three and finally four images. The amount of time between each flash that would show four stages of our BB in a single image was .499 milliseconds. After we got several good photos of four stage of the BB in one frame, we decided to put our apple slices in the picture so that we could document impact photos. The stage for the apple was made of the small cardboard box with the notch in it big enough to hold a one centimeter thick apple slice. The apple faced the gun upright, stuck in the notch of the cardboard box. We shot several apple slices in this fashion, taking pictures with the Fujifilm S602 camera. Our first several shots were not quite perfect, with one image of the BB cut out of the frame or one BB still inside the apple and therefore not visible. After working a few minor details out such as moving the gun forward or backward a little or moving the stage for the apple forward or backward, we were able to get a picture of the apple slice with two images of the BB in front of it and two images of the BB after the apple. Once we had captured this first picture, we made sure nothing was moved or changed at all in the project so that we could get consecutive pictures with two BB images before and two BB images after the slice. These photos in hand, we had everything needed for our final calculations.
Labeled Diagrams of Representative Equipment Setup Side view:
Labeled from left to right: Computer Intervalometer Interface Box Enunciator Box Sound Trigger BB-Gun Vivitar Flash Unit Stage/Subject Bullet Trap Camera
Description of Triggering, Timing, and Imaging Methods We used our computer to control the time at which the flash units would be set off, allowing for a regular time interval to ease our calculations of acceleration. The Apple Two Plus running the program Multiplexer Flasher was our way of establishing a consistent, regular interval at which our flash units would be triggered. Several pieces of hardware were required in order to interface our flash units to the Apple Two Plus. We had the flash units connected by four RCA cables to the first of our custom hardware. Our flash units were connected to a box with 16 input areas, and this was the box that received the signal from the computer to send an electrical impulse to the flash units. This was connected to two other pieces of hardware, one was the annunciator, and the other was the Apple Two Plus. The annunciator was connected to the sound trigger that would detect a spike in noise and relay it back to the annunciator, which would then send the signal into the interface box, which in turn would pass the signal into the Apple Two Plus through a custom hardware adapter plugged into the game port. We allowed the Apple Two Plus to control every aspect of our timing. We set the resistance on both the flash units and the sound trigger to zero so that there would be no delay except what we input into the computer. The Multiplexer Flasher delay program was written in assembly to minimize the time it would take for the computer to execute the code when called. This program checked to see if the signal from the annunciator was high, meaning there had been a sound spike from the sound trigger, and if this was true it began the delay routine. This first added a delay between the time when the sound was detected and the time when the first flash unit was fired, and then proceeded to set off the remaining flash units at another interval defined by the user. We determined that the best delays were a .948ms initial delay and .499ms intervals. This allowed us to get several solid photos of four images of the BB, which was what we needed in order to perform our data analysis. Throughout this experiment, the settings on the camera that remained constant were the 1 second shutter speed, ISO rating 400, no zoom, and 6mp resolution. The f-stop and the aperture were changed as needed to accommodate the varying amount of light reflected off the subject. We finally determined that f-8.0 provided best light for the stage, and were able to get a great deal of interesting and technically high quality photos. The camera work was handled by Henry, who would set off the shutter at the correct time during the countdown to firing, and managed to get away with only a few missed shots.
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