Viscosity is, essentially, fluid friction. Like friction between moving solids, viscosity transforms kinetic energy of (macroscopic) motion into heat energy. Heat is energy of random motion at the molecular level, so to have any understanding of how this energy transfer takes place, it is essential to have some picture, however crude, of solids and/or liquids sliding past each other as seen on the molecular scale.
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For Students: Fact Sheet, Homework Problems, Study Guide
Fluids Fact Sheet
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Physics 152: Homework Problems on Fluids
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Fluids: the Essentials
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