Mechanical equilibrium requires force balance for the mechanical displacements that are allowed. Two uniform fluids separated by a freely moving, unloaded, frictionless piston with equal face areas require equal pressures. A supported piston can sustain a pressure difference, and a fluid in gravity instead obeys hydrostatic equilibrium. For an upward height coordinate , that balance is , where is mass density and is the downward gravitational acceleration. Mechanical equilibrium alone does not establish thermal equilibrium or chemical equilibrium.
Two fluids at a stationary, freely movable interface require equal pressures when surface tension and other normal stresses are negligible. This local pressure equilibrium does not require equal temperatures or mass densities. Curvature with nonzero surface tension or an applied load changes the normal-stress balance. Comparing points at different heights also requires the hydrostatic equilibrium condition.
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Mechanical equilibrium refers to a state in which the net force and net torque acting on a system are both zero. When a system is in mechanical equilibrium, it is either at rest (static equilibrium) or moving with a constant velocity (dynamic equilibrium). There are two key conditions for mechanical equilibrium: 1. **First Condition - Translational Equilibrium**: The sum of all forces acting on an object must be zero.