The glass that rings

Rub a wet finger around the rim of a wine glass and it sings. Add water and the note drops. This is a complete physics demonstration using equipment from a kitchen, and the explanation is the same one that applies to every musical instrument.

The finger does not slide smoothly. It grips and slips, hundreds of times a second, and each grip pushes the glass very slightly out of shape. The glass springs back, and because the pushes arrive at the rate the glass naturally wants to vibrate, the movement builds instead of cancelling. This is resonance: energy added in time with a natural rhythm accumulates.

The pitch depends on how fast the glass can spring back, and adding water slows it, because now the glass must move water as well as itself. More mass, slower vibration, lower note. The same relationship explains why a thick guitar string sounds lower than a thin one and why a large drum sounds deeper than a small one.

What makes the demonstration worth doing rather than reading is the moment of finding the right speed. Too fast and nothing happens; too slow and nothing happens; at one particular speed the note appears suddenly and loudly, and the suddenness is the point. Resonance is not a gradual effect that grows as you approach the right rate. It is close to nothing, and then it is everything.