Why does the sound change when water is added to a glass, and the glass is tapped with a knife?
Robert P. Ulrich Elementary, California City, CA, USA
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.
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)
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.
- Ian M. A (age 10)
Rockland Country Day School, Hillsdale, NJ United States
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.
(republished on 07/29/06)