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Name: Natalie D.
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A


Question:
During our inquiry science unit we have the children make "cluck-cluck cups." The purpose is to discover how sound is created and altered. A hole is poked into the bottom of a paper cup, a string is passed through the hole and tied onto a paper clip. Holding the paper cup in one hand with the string hanging down, the student uses his index finger and thumb to slide down on the string and listens for sound. Next he wraps a dry paper towel around the string and pulls down. Next he wets the paper towel and repeats the action. Our explanation includes: When the string is pulled on, it creates friction on the string. The string then vibrates into the cup to create a sound. Changing variables, changes the vibrations and therefore the pitch. Here comes the question. In simple terms, what is happening when water is added? The sound is definitely louder! (Cluck-Cluck!) Is the water creating more friction, therefore more vibration leading to the louder sound? Someone suggested the water was acting as a lubricant. This did not make sense to me. I would think that as a lubricant the vibration would be less and therefore the outcome less noisy. Confused. Can you help?



Replies:
This is a complex experiment and there could be a number of things going on. However, my first reaction is that the "wet" paper towel increases the contact between the string -- towel -- string. Since sound travels faster in liquids than in air, the wet paper towel increases the transmission of the sound. The complicating factor is whether the vibrations in the string change from "up-and-down" oscillations to "back-and-forth" oscillations. What happens if the whole experiment is repeated with a "wet" string compared to a "dry" string? What happens if the string is replaced with a strand of copper wire? Is the sound louder? What about trying different cords, for example, monofilament fishing string vs. multi- braided fishing string? You have a good experiment planned, but the test you are applying -- dry vs. wet paper towel may not be revealing the operating factors.

Vince Calder



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