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Q & A: Musical glasses

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Most recent answer: 10/22/2007
Q:
Why does the sound change when water is added to a glass, and the glass is tapped with a knife?
- Kimberlea
Robert P. Ulrich Elementary, California City, CA, USA
A:
When the glass is tapped, the sides vibrate in and out. We hear fast vibrations as a high pitch and slow vibrations as a low pitch.

When water is added, the bottom part of the glass can't vibrate as well. The water adds mass without changing the "stiffness" so the glass can't vibrate as quickly, producing a lower pitch.

EJ & Tamara

(published on 10/22/2007)

Follow-Up #1: Correction to ’Musical Glasses’

Q:
I have a very big complaint about your answer to one of your questions. It is the question about filling up containers with water and striking them. Provided that you are striking the glasses, and not blowing over them, your answer is incorrect. I have gone on many Internet sites, which say things that are quite controversial to what you have said, and I have personally tested your theory. In your answer to this question, I see that you have either not tested this yourself before posting your answer on the web, or you have not tested it in a proper manner. In my tests, I have seen that the note, or pitch, or tone becomes lower when you add water to the glass. By the way, the density of the liquid used can, and does, alter the produced pitch. Be my guest, test it yourself, in fact, please do! I am doing this for my school science fair, and, I am very sorry, if a person of your stature does not know some basic facts about the subject. Thank You. Sincerely,
- Ian M. A (age 10)
Rockland Country Day School, Hillsdale, NJ United States
A:
Ian -

Thank you for the correction - I have changed the answer to Musical Glasses so it is now more correct. I believe that the person who answered that question made an honest mistake by thinking of the phenomenon of blowing across the top of the bottle.

Sometimes things that seem very common are not very well studied. For instance, another common science project is to place ice cubes in various liquids and measure how long it takes for them to melt. While it may seem like this is fairly simple, the details of what makes it work are more complicated - there are a lot of factors involved (such as the fluid's convective and heat-conductive properties) that make it very difficult for even professional physicists to predict.

-Tamara

(published on 10/22/2007)

Follow-up on this answer.