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If you rub a finger over a variety of wet, smooth, solid surfaces (like glass, plastic, metal, etc.) you hear a squeaking sound. How is that squeaking sound produced?

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Your finger first sticks to the glass, then slips, then sticks again, then slips and so on. Each time it sticks it exerts a force on the glass. If those forces occur at a steady rate that matches the glass's resonant frequency, it causes the glass to vibrate at that frequency with enough amplitude to be audible. The same principle applies to violin strings rubbed with a bow, squeaky door hinges, fingernails on a chalkboard, etc.

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  • $\begingroup$ @RichardHDowney-I can even feel it now when I rub my finger slowly on a glass snowball filled with water. $\endgroup$ – descheleschilder Mar 31 '17 at 7:06

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