Rubber Bands: What REALLY happens when you shoot a rubber band?
Have you ever gotten into a rubber band fight? The one experience I remember was back in college.
I was a DJ at the college radio station WWSU (a big 10-watt station that only had about a 10-mile listening radius!). The studio was located in the Student Activities Center, that was sometimes closed late at night during certain holidays. Because the radio station had to be on the air as many hours as possible to maintain our FCC license, the DJs had access to the building even when it was closed. I had a classical music show from 10:00 PM to midnight and would often play 20-minute segments from vinyl records (anybody remember those?), so I had time to work on homework or go to the vending machine in between making announcements and changing records on the turntable.
Late one evening during my show, I was in the sound studio, and George, another DJ, was in the back of the station pulling records for his radio show the next day. I was deep into an engineering homework problem when a rubber band smacked me on the back of my head. I immediately jumped up, scrounged some rubber bands from the desk, ran out of the studio, and gave chase! George and I ended up racing all over the deserted two-floor building, charging up and down the stairwells, sneaking up on each other, shooting around corners in the hallways and… mostly missing. Fortunately, I was able to make it back to the studio in time to breathlessly introduce the next musical piece and avoid the dreaded DEAD-AIR during my show! Later, we tried to be meticulous in searching the building to clean up the evidence of our escapade, but I’m sure the janitor ended up increasing the size of his rubber band collection.
I remember having sore thumbs afterwards because my poor shooting skills got my thumbs smacked a lot. Had I read, Shooting Rubber Bands: Two Self-Similar Retractions for a Stretched Elastic Wedge, written by mechanical engineers Alexandros Oratis and James Bird, I might have avoided that humility. At Boston University they used slow motion cameras to catch the details of shooting rubber bands. Here’s a nice video put together by Science News that summarizes their work:
Here's some Geeky Rubber Band Physics:
When you pull back on a rubber band, you are using the energy stored in your muscles, and converting it to potential energy in the rubber band. When you release the rubber band, the potential energy in the rubber band converts to kinetic energy, and the rubber band speeds through the air.
For the REALLY GEEKY person, here are the equations:
This is the formula for Potential Energy:
This is the formula for Kinetic Energy:
In an IDEAL world, all the potential energy is converted to kinetic energy. In the REAL world, we deal with messy details such as rubber bands not exactly following Hooke’s Law (that’s how the spring constant (k) is calculated for a particular spring), the rubber giving off and absorbing heat as a form of energy, and the air slowing down the rubber band.
IN SUMMARY: To avoid (or minimize) thumb injuries during a rubber band fight:
1 – Don’t pull the rubber band back too tightly. More tension equals more speed, and there’s not enough time to get the thumb out of the way.
2 – Use wide rubber bands -- they are better for quick thumb removal.
Do you have a funny rubber band story? Or a favorite rubber band gadget? Let us know what toys or gadgets you want to see featured here.