This document discusses the principles of equilibrium of particles in two dimensions. It defines equilibrium as when a particle is either not moving or moving with uniform motion, meaning the net force on the particle is zero. It presents the equations of equilibrium in two dimensions and describes principles like two-force and three-force equilibrium. Free body diagrams and Lami's theorem, which relates forces to the angles between them, are also covered. An example problem calculates tensions in ropes supporting a crate between two buildings.