structural:scholar_edition
Static equilibrium

For an object to be in static equilibrium, not only must the sum of the forces be zero, but also the sum of the torques (moments) about any point. For a two-dimensional situation with horizontal and vertical forces, the sum of the forces requirement is two equations: Σ H = 0 and Σ V = 0, and the torque a third equation: Σ τ = 0. That is, to solve statically determinate equilibrium problems in two-dimensions, three equations are used.