Start one of these two pendulums swinging and soon you’ll see the other pendulum start swinging, too. Keep watching and you’ll see the two pendulums take turns, alternately swinging energetically and coming to a near standstill.
Everything vibrates at its own unique rate, or frequency. Bridges, atoms, and playground swings—each wiggles, waves, or swings with a characteristic natural frequency.
In this exhibit, two pendulums are joined together, and there are two natural frequencies. In any system consisting of similar parts joined together—like atoms in a molecule or girders in a bridge—there are as many natural frequencies as there are parts.
Energy added to one pendulum moves back and forth between both pendulums. This happens three times a minute, which is the difference between the two natural frequencies, 47 and 50 swings per minute.
The unfinished aesthetic of this exhibit reflects Exploratorium founder Frank Oppenheimer’s openness to tinkering with exhibit design. Made from hardware store parts and a staff member’s kitchen table, this exhibit went through many stages before it premiered on the museum’s exhibit floor in the early ’80s.
Since then, there’ve been more alterations and iterations, including removal of a motor and an anonymous addition of whimsical feet. The Exploratorium has generated a smaller do-it-yourself version of this exhibit and a full-sized swing-set of coupled pendulums. (click image to enlarge)