When you rub your finger along the rim of a glass, it resonates, why is this? Also, why does the pitch of this sound get lower when you add water to the glass?

Asked by: Sly Fox

Answer

As your finger travels around the glass it sticks and slides at different points. Each stop and start causes the glass to vibrate. These vibrations are at the natural frequency of the glass which is a complex function of its mass and shape. This is why adding the water lowers the pitch. You can demonstrate this effect by thrumming a ruler on your desk. Now clip a pen to the end of the ruler and thrum it again. Notice the drop in pitch. This is because the additional mass drops the natural frequency of the instrument.

The stopping and starting effect is initially harder to accept. Your finger is travelling smoothly isn't it? Well, no. To see this for yourself try doing the trick with a drop of oil on the edge of the glass. Your finger will travel much more smoothly and the glass won't ring. If your finger is dry it will be stopping and starting too much, oiled it stops and starts too little. Fortunately for children everywhere water makes it work just fine.
Answered by: Mark Bruckard, B.S., Physicist, Canberra, Australia