Mechanical equilibrium

Mechanical equilibrium is a state in which the net force and net torque on a body are zero, leaving it with no linear or angular acceleration.

Newton's laws of motion: Three classical laws relating a body's motion to forces acting on it. The second law connects zero net force with zero linear acceleration.
Force: An interaction that can change an object's motion or deform it. Mechanical equilibrium requires the vector sum of all external forces to vanish.
Statics: The branch of mechanics that studies bodies at rest or in mechanical equilibrium. It applies force and torque balance to structures and other stationary systems.
Mechanical stability: The tendency of a mechanical system to remain near an equilibrium state after a small disturbance. Equilibrium describes balance; stability describes what happens after displacement.
Torque: The rotational effect of a force about an axis or point. Zero net torque is the rotational balance condition for equilibrium.
Vector addition: The operation of combining vectors according to their magnitudes and directions. Forces cancel only when their vector sum, not merely their magnitudes, is zero.
Support reaction: A force exerted by a support that constrains a body's motion. Unknown support reactions are solved by imposing equilibrium on a structure.
Metastability: A long-lived state that is locally stable but not the system's lowest-energy state. A body can balance in a state that is stable only against small disturbances.
Center of mass: The weighted average position of a system's mass, which tracks its translational motion. A body's net force determines the acceleration of its center of mass.
Rigid body: An idealized body whose internal distances remain fixed under applied forces. Its equilibrium includes both translation and rotation of the whole body.

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