TacomaNarrowsImagine pushing a child on a swing set. When, exactly, do you push? Intuitively, most of us wait for the swing to be at its closest, then give it a shove as it starts to fall away. At any other time we’d either be inviting a collision or chasing after the child.  Amplifying the swinging motion is easiest at a specific, recurring point. This is called resonance.
On a swing set, resonance provides an efficient way to move someone higher and higher. In other contexts, it’s a major safety threat. The Tacoma Narrows bridge, shown at left, famously collapsed in 1940 when gusting winds pushed it at its resonant frequency. The bridge thrashed up and down so violently that it eventually disintegrated. (Image credit: Barney Elliott.)
Musical instruments resonate, too, although this is harder to see. We hear oscillations of different speeds as individual pitches. Press a piano key, and a hammer strikes a wire, which vibrates at its characteristic frequency, producing the same note every time. Trapping a guitar string against a fretboard changes its resonant frequency, so the same string can generate many pitches.  
We can use this science to make a wine glass sing! A wet finger run along the edge of the glass alternates sticking and slipping as it slides across the surface. Under just the right pressure, the stick-slip pattern matches the glass’s resonant frequency. This periodic push oscillates the glass, which in turn vibrates the air nearby, creating a sound wave. The faster the glass moves back and forth, the higher the pitch we hear.
Adding water to the glass is a bit like applying a brake to the vibration. In order to move, the glass has to push the water out of the way. This slows down the oscillation, resulting in a lower pitch.
Line up several glasses filled with different levels of water for a glass harp with easily repeatable notes. Pick one up and tilt it as you play to change its pitch like a trombone! Alternately, try partially submerging an empty glass in a large container of water, and see what happens when you adjust its depth. Can you think of any other ways to tune a musical glass?
Written By: Caela Barry

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