HEADLINE: When we get the Right Vibes
BYLINE: By Kathy Wollard . Kathy Wollard is a free-lance writer.
CAN A HIGH-PITCHED NOISE from a singer really break glass? asks Eric Walker, a student in Brooklyn.
We talk about "being on the same wavelength" with a friend, or about ideas being "in sync." These are all ways of talking about resonance - a kind of vibration that sets up an answering vibration inside something else. In this case, your friend's ideas and feelings resonate with yours. A simple look passing between you "says it all." You get each other's jokes. His ideas echo yours.
Resonance is everywhere in nature. Everything in the universe, from atoms to skyscrapers, is moving a little, vibrating. And so nearly everything has its own natural frequency - a certain number of vibrations in a certain amount of time. Apply a force vibrating in the same way, and you can make that natural frequency more powerful than it was.
Here's how. Have you ever swung on a swing, and noticed that if you pump your legs in just the right rhythm, the swing will go higher and higher? That's an example of resonance. If you get it just right, your rhythm of pumping will exactly match the natural rhythm, or frequency, of the swing. The swing will echo the pumping of your legs.
Something similar happens when a singer's voice shatters an empty wine glass. When the singer's voice hits just the right note, it can match the frequency of a piece of crystal. As the glass begins to resonate with the vocal frequency, it vibrates more strongly. If the singer holds the special note long enough, the glass vibrates more and more violently, eventually shattering.
Bigger things than glass can get broken when an object resonates. On July 1, 1940, a new suspension bridge across the Puget Sound in Washington officially opened to cars. The bridge seemed well-built. But four months later, a disaster occurred that would change the way bridges were built forever.
It was a windy day on the Tacoma Narrows Bridge. As the wind blew, the bridge began to sway. All bridges (and tall buildings) sway a bit in the wind. But the huge Tacoma Bridge soon began swinging back and forth. A photographer captured the bridge buckling on film. Finally, as the bridge swung back and forth like a pendulum, the main span tore loose from the cables holding it up. The roadway broke into pieces, plunging into the water. (Fortunately, there were no fatalities.)
What had happened? Bridges have natural frequencies, just like glasses. By accident, the wind had blown with a fluctuating force that was in resonance with the frequency of the bridge (just as you applied a fluctuating force to the swing by pumping your legs). This set the bridge swinging back and forth, the swinging steadily increasing until the bridge ripped itself apart. (This is also why soldiers wisely stop marching in rhythm when they venture to cross a bridge.)
Even icebergs fall victim to the force of resonance. Ocean waves, rhythmically lapping against a big berg, can increase the ice's small natural vibration until it splits apart.
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