When the researchers turned their high-speed camera on a rubber band being released, they discovered, to their surprise, that the shape of the back of the rubber band was round (top clip), and stayed round as it traveled forward. This shape differed from that of a thinner elastic string pulled to form a larger angle (bottom). Video credit: Alexandros Oratis & James Bird.
The mundane act of shooting a rubber band exhibits physical phenomena that have researchers intrigued. Using a high-speed camera, researchers analyzed the physics of a stretched rubber band that’s then released, gleaning new insights that could some day be useful in a range of areas, from molecular devices to satellite design. “Part of the allure of this is that something as mundane as firing an elastic band can teach us new physics,” says James Bird, a mechanical engineer at Boston University.
In a paper published in Physical Review Letters, Bird and mechanical engineer Alexandros Oratis, also at Boston University, found that right after release, the back end of a rubber band has a rounded shape—not the angular, trapezoidal shape that was previously assumed. That simple observation was a revelation for researchers.
Because no one had examined a stretched-and-released rubber band that closely before, the assumption was that the physics would be similar to that of a plucked guitar string. When you pull a string back and release, Oratis explains, the string takes on a trapezoidal shape that then travels forward. In this case, the main factors that come into play are inertia and how much the string is stretched.
But when the researchers turned their high-speed camera on a rubber band being released, they discovered that the shape of the back of the rubber band was round, and it stayed round as it traveled forward. The roundness, according to the researchers, was due to the fact that a rubber band has a certain thickness, and that the internal angle it forms when pulled back is relatively small.
The rubber band’s thickness, as well as its elastic stiffness and how much it’s curved, determine how much energy is needed to bend a rubber band. In this case, the researchers found, this bending energy is more important in determining the band’s shape than stretching and inertia, which is most important in the case of a plucked guitar string. The dynamics of shooting a rubber band turn out to be fundamentally different, Bird says.
“This is something around us in everyday life, but we never thought to look at it properly. And it has relatively unexpected and surprising behavior,” says Alain Goriely, an applied mathematician at the University of Oxford in the UK. “What they show—the surprising part—is that a lot of the process, if you look closely, is not in extension, but in bending.”
“What I think is most impressive about what they’ve done,” says Kari Dalnoki-Veress, a physicist at McMaster University in Canada, “is they’ve managed to understand the entire behavior of how this stretched elastic band collapses and how that gets turned into kinetic energy.”
But offering more than just trivial physics of an office implement, the newly discovered behaviors, Bird says, could be important for a wide range of design considerations—from musical instruments and slingshot rides at amusement parks, to the dynamics of viscous fluids and applications in engineering and biomedicine. For example, researchers have recently used strands of DNA as tiny slingshots to release drugs when a particular antibody is present, thus targeting the drug to a specific area of infection; the insights from the new study could be relevant for these kinds of molecular devices. Goriely adds that the bending of a rubber band could even apply to the winding and unwinding of space tethers used to connect satellites or to generate power and propulsion.
But for the researchers, it’s not just about potential applications. “I get excited whenever I see beautiful, pure curiosity-driven research, and I think this is a perfect example of that,” Dalnoki-Veress says. “Who would’ve thought you could take an elastic rubber band and fire it and pull out some beautiful science?”