What defines equilibrium in a physical system?

Equilibrium in a physical system is defined as a state in which all forces acting on an object are balanced, resulting in a net force of zero. When an object is in equilibrium, it will either remain at rest or move with a constant velocity, as there is no unopposed force to change its state of motion.

In such a situation, each force acting on the object is countered by an equal and opposite force, leading to a stable condition. This concept can be illustrated with examples such as a book resting on a table, where the gravitational force pulling the book downward is balanced by the normal force exerted by the table upwards.

While there are other aspects, like motion or external conditions, they don't define equilibrium itself. A state of motion can exist at equilibrium if the object is moving at a constant velocity. Imbalance refers to a situation where forces do not cancel out, leading to acceleration, which directly contradicts the definition of equilibrium. A variable state depending on external conditions implies that equilibrium can change, but it does not change the fundamental requirement that forces must be balanced for a state of equilibrium to exist. Thus, the concept of balance among forces is the core element that defines equilibrium in physical systems.