Glass and plastic cups

At dinner at a nice but not very nice restaurant the cups could have been either thin light glass or a convincing plastic immitation. To determine which kind they were we tapped the cups with a metal knife. The cups chimed, therefore they were made of glass.

The best guess I have for this is that a glass cup is essentialy one cystal of silicon dioxide whereas a plastic cup is several long chain polymers linked together by strong intermolecular forces. The result is that when force is applied to the glass cup the force translates smoothly through the cup and at the opposite end of the cup the force reverses direction and travels back through the cup. Thus the resonance is the velocity of the comperssion wave through the cup divided by the length. A plastic cup doesn't resonate becuase it's non crystaline structure does not have enough uniformity for the pressure waves to bounce back and forth within it.


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