Rubber Band Stretching

Setup Diagrams

Method

Our original goal was to see how vertically stretching a rubber band affects the horizontal stretch of the rubber band when hit by a BB. To do this we positioned a rubber band vertically, with no stretch for our first trial.  Beginning with a 10.3 cm piece of rubber band, we increased the length of the rubber band by a centimeter each time, and completed two trials per increment. With the width of the rubber band facing the BB gun, we then shot the rubber band with the gun. To record the action we used a high speed video camera, then transferred clips to a camcorder, and finally to the computer for analyzing.  Using the video clip and Tracker software, we determined the maximum horizontal stretch for each trial. We used this data to plot a graph of horizontal stretch vs. vertical stretch to determine if there is any relationship between a rubber band’s vertical stretch and its horizontal stretch.

Equipment:

• Five 10.3 cm x 1.5 cm rubber bands
• 760 PumpMaster 0.177 Caliber Pellet BB Repeater/BBs
• Rifle holder
• Cardboard BB catcher
• Chemistry / Physics Equipment stand
• Clamps for holding rubber band
• 250 W Flood Lamp
• Motion Scope 8000S
• Sony Handycam Vision (DCR-TRV900) Video Camcorder

Equipment Setup

To achieve our goal of determining how a vertically stretched rubber band is stretched horizontally when hit with a BB, we used the Motion Scope 8000S high speed video camera.  We positioned two long tables together to create a long narrow lane for the BB’s to travel down. The BB gun, resting on a rifle holder, faced the clamped rubber band infrastructure and a cardboard BB collector was put at the end of the firing lane. The rubber bands were secured by screw-tight clamps. The top clamp was moved up every time the rubber band’s vertical length increased. These screw-tight clamps were bolted onto a ring stand which was fastened to the table using a C-clamp.

The camera that connects to the Motion Scope 8000S was facing a wall with a dark backdrop perpendicular to the firing lane, approximately a meter from the rubber band infrastructure. Since we were using high speed video we did not need a flash, but instead we used a 250 W floor lamp to provide adequate light for a clear view of the event. The Motion Scope 8000S and our recording devices were stationed on a portable cart for easy control and accessibility. We selected a f/1.8 f-stop with a shutter speed of 10x, which we found to provide enough exposure for a clear view of the event. The frame rate was 2000 frames per second.

Each time we recorded a trial using the Motion Scope 8000S, we would review the clip to see if it was a good shot and if it could be used for analyzing. Shots that went through the rubber band or did not hit the center of the rubber band were discarded. If it was usable we transferred the clip to tape using the camcorder. After completing all of our trials and recording them onto the videotape, we transferred them to the computer to analyze.  

Triggering, Timing and Imaging Methods

While using high speed video to capture the stretching of our rubber bands, we did not have to set up any triggers. Instead, we manually triggered the camera for recording the high speed event. In this experiment timing was crucial in capturing the video clips we wanted. When capturing an event, we would begin recording, then fire the BB gun within the two second recording time of the Motion Scope 8000S. When we recorded to the Motion Scope 8000S, we reviewed each clip thoroughly before transferring it to the video tape.