 Mechanical equilibrium

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 $z$, that balance is $dp/dz=-\rho g$, where $\rho$ is mass density and $g$ is the downward gravitational acceleration. Mechanical equilibrium alone does not establish thermal equilibrium or chemical equilibrium.

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.