Task: Instance Description Understanding (ontology: Earth_Environment/SWEET)

Which of the following definitions best describes the instance 'equilibrium'?

A. Motions describing an equlibrium between pressure gradient force, Coriolis force and the turbulent drag force.
B. In mechanics, a state in which the vector sum of all forces, that is, the acceleration vector, is zero. In hydrodynamics, it is usually further required that a steady state exist throughout the atmospheric or fluid model. the instance may be stable or unstable with respect to displacements therefrom. See also hydrostatic the instance, geostrophic the instance, instability. 2. In thermodynamics, any state of a system that would not undergo change if the system were to be isolated.
C. the instance is most commonly thought of as an aqueous substance that can accept hydrogen ions. [Wikipedia]
D. A system or flow that evolves slowly in time compared to the rotation period of the earth, has a length scale of the deformation radius or larger, and undergoes only limited vertical excursions.

Answer: In mechanics, a state in which the vector sum of all forces, that is, the acceleration vector, is zero. In hydrodynamics, it is usually further required that a steady state exist throughout the atmospheric or fluid model. the instance may be stable or unstable with respect to displacements therefrom. See also hydrostatic the instance, geostrophic the instance, instability. 2. In thermodynamics, any state of a system that would not undergo change if the system were to be isolated.