ABSTRACT
Fluids considered as continuum media are in static equilibrium when, independently of any stationary or moving frame of reference, there is no relative motion between any of their parts. Since, by definition, a fluid cannot support shear stresses without deforming continuously, static equilibrium is characterized by the absence of shear stresses or any other mechanism that gives rise to relative motion. Consequently, velocity gradients in static equilibrium do not exist. According to these definitions, a fluid is under static equilibrium even if it is subjected to a rigid-body translation and/or rotation, since these types of bulk motion do not involve relative motion between parts of the fluid. In fact, for these motions there is a reference frame moving and/or rotating with the velocity of the rigid-body motion such that the velocity of any part of the liquid with respect to the frame of reference is zero. Therefore, the only velocity and acceleration that can exist under static equilibrium are uniform and common to all parts of the fluid.
