that describes the relative difficulty with which a fluid may flow. [P7.6]
is: the quantity that describes the relative difficulty with which a fluid may flow. [P7.6]
is defined: by the relation (a special case of Newton's law of viscosity)
where 
is the shear stress applied in a given direction, 
is the velocity gradient in a perpendicular direction, and the minus sign indicates that the velocity decreases with distance from the plane over which the shear stress is applied. [P7.6]
is sometimes called: the viscosity of the fluid. [P7.6]
has as its SI unit: 
or equivalently 
or 
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