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what cannot be compressed
Everything can be compressed, to a greater or lessor extent. Gasses, like air, are pretty squishy. Both solids and liquids are much stiffer but still can be compressed under pressure.
One of the measures of compressibility is the velocity of sound in a substance. For example the velocity of sound in air is 330 meters per second whereas the velocity of sound in steel is about 5,000 meters per second. The relationship is: the square of the velocity is inversely proportional to the product of the density and compressibility.
(published on 10/22/2007)
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