Agriculture Reference
In-Depth Information
those factors that make things happen. They push a system in one direction or
another, such as increased heat, transfer across boundaries, energy conversions,
mass transfer, and flow. Constraints and drivers can be seen as working in opposite
directions.
For example, if we set our boundary at a microscopic, cellular membrane,
the drivers and contraints will include the ability of a nutrient or contaminant
to enter and change within the cell. However, if we look at the same chemical
compound in a building, the boundaries will be the roof, floor and walls, the
drivers and constraints will include air movements, porosity and permeability of
the building materials, and the case of sorption to surfaces.
Motion
The second set of scientific principles that must underpin good green design
concerns mechanics. Sir Isaac Newton described motion in three basic laws. For
design purposes, as in thermodynamics, we are concerned almost exclusively
with macroscopic scales (i.e., a large number of molecules). The first law states:
Every object in a state of uniform motion tends to remain in that state of motion
unless an external force is applied to it.
Galileo also observed this phenomenon, which he called inertia. This is very
important for designers. If we are going to harness energy, we need to understand
that objects will stay in motion unless other forces come to bear. The most
common external force that changes the state of uniform motion is friction.
Thus, any design must see friction as the “enemy” if we want to keep things
going (lubricants and smooth surfaces are needed to fight friction), and as an
essential “friend” if we want to change direction or stop things (e.g., brake shoes
in an automobile).
The second law of motion states: The relationship between an object's mass m ,
its acceleration a , and the applied force F is F
ma . Acceleration and force are
vectors, wherein the direction of the force vector is the same as the direction of
the acceleration vector.
With the second law, we can calculate unknowns from knowns. That is, if we
know the mass of the propellers and the applied force generated by the wind,
we can calculate the acceleration of the propellers in a windmill. This, in turn,
allows us to estimate the amount of energy being generated by the windmill
system.
The third law of motion tells us that for every action there is an equal and
opposite reaction. Like the first law, this tells us that we can expect things to
happen in response to what we do. If we apply a force, there will be an equal
force in the opposite direction.
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