Some large-scale properties of liquids

A study of the mechanics of liquids will involve consideration of the movement of small masses or elements of a liquid (liquid particles) as a result of forces acting on them. 

The mass of a given element will depend on the density of the liquid in the region of the element. For any substance, if the mass of a volume V ism, the mean density p is defined as  

The motion of an element of liquid may be specified in terms of the flow velocity, as described in section 4.3, remembering that this is the drift velocity of the element or liquid particle, and not a molecular velocity. 

In section 2.1 a discussion was given of the internal forces that may be transmitted through a solid. The ability to transmit forces is a property of a condensed phase so that a liquid may transmit internal forces and these can be classified as for solids. 

Any element of a liquid experiences forces across its surface that arise from the presence of the surrounding liquid. These forces are the contact or surface forces and arise from the molecular action between adjacent elements or particles of liquid. There may also be forces produced by the effects of long-range fields such as the gravitational field. These forces act throughout the liquid on all its elements and are called body forces. 

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