First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 60

Recall that Newton's third law of motion states that for every acting force
between two bodies, there is an equal but opposite reacting force on the same
line of action. The equation for this law is

F reacting =− F acting

(2.10)

As mentioned, a force that is particularly important to green systems is friction ,
which is a force that always resists motion or impending motion. Friction acts
parallel to the contacting surfaces. When bodies come into contact with one
another, friction acts in the direction opposite that which is bringing the objects
into contact.

Fluids

Green engineers are keenly interested in fluids. The obvious fluids that are im-
portant at all scales, from molecular to global, are water and air. To identify a
hazard associated with the chemical, or to take advantage of a fluid in a design
the fluid properties must be understood. For example, if a contaminant's fluid
properties make it insoluble in water and blood, the target tissues are more likely
to be lipids. If a chemical is easily absorbed, the hazard may be higher. However,
if it does not change phases under certain cellular conditions, it could be more
or less toxic, depending on the organ.
The fluid properties of an agent, whether chemical or biological (e.g., mold
and pollen), help us to determine if the contaminant is likely to be found in the
environment (e.g., in the air as a vapor, sorbed to a particle, dissolved in water,
or taken up by biota). Likewise, if a fluid is easily compressible if may be useful
in cooling systems.
Physical transport is a function of the mechanics of fluids, but it is also a chem-
ical process, such as when and under what conditions transport and chemical
transformation processes become steady state or nearly steady state (e.g., seques-
tration and storage in the environment). Thus, transport and transformation of
contaminants and nutrients depend on the characteristics of fluids.
A fluid is a collective term that includes all liquids and gases. 2 A liquid is
matter that is composed of molecules that move freely among themselves without
separating from each other. A gas is matter composed of molecules that move
freely and are infinitely able to occupy the space with which they are contained
at a constant temperature. Engineers define a fluid as a substance that will deform
continuously upon the application of a shear stress (i.e., a stress in which the
material on one side of a surface pushes on the material on the other side of the
surface with a force parallel to the surface).