p.3-4 By applying physical laws to a specific system, it may be possible
to develop a mathematical model that describes the dynamics of the system. Such a model may include unknown parameters, which
must then be evaluated through actual tests. Sometimes, however, the physical laws governing the behavior of a system are
not completely defined, and formulating a mathematical model may be impossible. If so, an experimental modeling process can
be used. In this process, the system is subjected to a set of known inputs, and its outputs are measured. Then a mathematical
model is derived from the input-output relationships obtained.
Simplicity of mathematical model versus accuracy of results of analysis. [section title] In attempting
to build a mathematical model, a compromise must be made between the simplicity of the model and the accuracy of the results
of the analysis. It is important to note that the results obtained from the analysis are valid only to the extent
that the model approximates a given physical system.
In determining a reasonably simplified model, we must decide which physical variables
and relationships are negligible and which are crucial to the accuracy of the model... Whether any particular features
are important may be obvious in some cases, but may, in other instances, require physical insight and intuition. Experience
is an important factor in this connection.
Usually, in solving a new problem, it is desirable first to build a simplified
model to obtain a general idea about the solution. Afterward, a more detailed mathematical model can be built and
used for a more complete analysis.
p.55 Force can be defined as the cause which tends to produce a change in motion of a body on which
it acts. To move a body, force must be applied to it. Two types of forces are capable of acting on a body: contact
forces and field forces. Contact forces are those which come into direct contact with a body, whereas field forces,
such as gravitational force and magnetic force, act on a body, but do not come in contact with it.
p.73 In a general way, energy can be defined as the capacity or ability to do work. Energy is found
in many different forms and can be converted from one form into another... A system is said to posses energy when it can do
work... The energy that a body possesses because of its position is called potential energy... Potential energy is
the work done by an external force.
p.109 A block diagram of a dynamic system is a pictorial representation of the functions performed
by each component of the system... Such a diagram depicts the interrelationships that exist among the various components...
In general, the functional operation of a system can be visualized more readily by examining a block diagram than by examining
the physical system itself. A block diagram contains information concerning dynamic behavior
p.492 A system that maintains a prescribed relationship between the output and the reference input by comparing
them and using the difference as a means of control is called a feedback control system or simply a control system.
