Which of the following best describes the concept of equilibrium in mechanics?

The concept of equilibrium in mechanics refers to a state where all the forces acting on a system are balanced, which means there is no net force causing the system to accelerate. In this condition, an object remains at rest or continues to move at a constant velocity. This fundamental principle ensures that there is a stable state when external forces are countered accurately by internal forces.

In equilibrium, the critical aspect is the balance of forces. For example, consider a book resting on a table; the gravitational force pulling the book downward is perfectly counteracted by the normal force exerted by the table surface beneath it. Because these forces balance each other out, the book remains at rest.

This understanding is pivotal when analyzing mechanical systems, as it forms the basis for static equilibrium and dynamic equilibrium scenarios. Other options, while discussing various mechanical states, do not accurately align with the precise definition of equilibrium in the context of mechanics.