Why does rubbing a wet finger around a wine glass make it sing?
Why does dragging a moistened finger around the rim of a wine glass produce a sustained musical tone, and why does adding water to the glass change the pitch?
History and references
Settled in 1975 by Stick-slip friction driving the glass's natural vibrational modes (qualitative mechanism long known; quantified in detail by later acoustics studies, e.g. Leatherman, Dunlap & Widenhorn, 2019).
The finger alternately sticks to and slips along the glass rim due to friction (aided by moisture, which regulates the friction coefficient), and each slip event injects a small kick of energy into the glass's natural bending vibration modes, which is why the tone locks onto the glass's resonant frequency rather than the finger's speed. This stick-slip mechanism has been understood qualitatively since at least the mid-20th century (echoing Chladni's and Rayleigh's studies of vibrating shells) and was Walker's explanation in the book; later work, including Leatherman, Dunlap, and Widenhorn's 2019 AJP paper, mapped the full frequency spectrum and confirmed the mode structure quantitatively.
A dinner-table party trick is literally the same stick-slip physics that makes a violin bow sing.
Explanations that circulated
- The sound is caused by air being blown across the rim like a bottle
- It is simple sympathetic resonance with the room, unrelated to friction
References
- The Fourier spectrum of a singing wine glass, M. Leatherman, K. Dunlap, R. Widenhorn, American Journal of Physics, 2019
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