Ηow is a melody created when someone strokes a glass of liquid with their wet finger?
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Glass seems pretty rigid, but nothing is perfectly rigid. There is some flex before it actually breaks. If you apply any force whatsoever, you will start that bending. The force in this case is friction. As you move your finger horizontally, friction pulls the lip of the glass with. Your finger eventually slides. It doesn’t take much with a wet finger as slippery as it is, but even the very small amount of friction you get is enough, like I said, to change the shape of the glass.
When your finger does finally slip, the internal forces of the glass, known as elasticity (I know, glass doesn’t seem very elastic right? We’re talking about sub millimeter deformations, though. Tiny tiny amounts) pull the glass back to its resting shape. As you might have learned from stretching and plucking a rubber band, the more tension, the higher the note. That’s because there is more force pulling the rubber band back to its resting shape. More force means faster acceleration. But you also know from plucking the band, that it doesn’t go back and then stop. It overshoots several times, oscillating back and forth. So higher tension force not only means faster acceleration, it also means faster oscillations, higher frequency, higher pitch, and higher note. All the same thing.
Glass does the same thing. As your finger slips, it tries to make it back to its resting position, but it has some velocity when it gets there, so it doesn’t stop, it overshoots and then starts slowing down and accelerating in the opposite direction. We can’t see this with the naked eye because it’s so fast and so small of a change. But we can hear it. The frequency that you hear is called the natural frequency of that object… more accurately, it’s the natural frequency of that object in this particular environment. We can change the environment (like adding water) to change the natural frequency, and make different notes.
The glass resonates at a certain frequency. Think of the finger as a bow being drawn across a violin string. So as you rub your finger across the rim it cause the glass to vibrate making a sound. The pitch of the sound can be regulated by the amount of water in the glass. More water means a higher pitch. So by choosing the correct glasses and measuring the correct amount of water one can essentially create a scale of all the notes and therefore play a melody.
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