|MadSci Network: Physics|
I noticed, as a teenager, that the pitch created by a spoon contacting a coffee cup while making instant coffee rose as I stirred it. Figured that it probably had to do with heat from the boiling water I'd poured into the cup acting on either the cup or the spoon... or the level of the liquid rising in the cup as a result of the stirring; reducing the "free vibrating" surface area above the liquid level. Tried the same procedure w/out stirring, w/ a cold spoon tapping the outside of the cup, w/ hot water only (in case the density of the coffee might be a factor)... isolated it to the cup. Tried just pouring boiling water in a glass, tapping the outside w/ a cold spoon, similar effect. My best guess is that the ceramic glaze on the cup (being, for all intents and purposes, glass... I reckon) attempts to expand as it is heated by the boiling water, but is constrained by low elasticity? Resulting in increased internal tension? Raising the frequency of vibration-- much as tightening a guitar string does? My life took me in directions far from furthering this investigation, to a career in the Air Force, and now to England, so I have never gone any further in my quest to answer this nagging question. I have posed it to everyone I've met w/ a background in physics or frequency propogation, but have never had it satisfactorily answered. Stumbled across your site tonight, and thought I'd give it one last go. Here's hoping...
Re: why does the sound made by a spoon 'clinking' a cup of coffee rise in pitch
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