If there are variations in pressure or elastic stress within the system,parts of the system may move, or expand or contract. Eventually these motions,expansions or contractions will cease, and when this has happened we say that the system is in mechanical equilibrium. This does not necessarily mean that the pressure is the same at all points. Consider a vertical column of fluid in the earth's gravitational field. The pressure in the fluid is in mechanical equilibrium under the influence of its own weight and an equal upward force arising from the pressure difference between its upper and lower surfaces.
This was written in my book (by Sears & Salinger). But (whatever anyone calls me) I could not understand what they wanted to tell about mechanical equilibrium. Can anyone please help to clarify what they talked about here?
[ Has mechanical equilibrium any relation with the surroundings of the system?]