THERMO Spoken Here! ~ J. Pohl © TOC     NEXT    ~   15

# 1.04 Models of Reality

Every attempt to analyze an event (or non-event) of physical matter involves a number of steps or stages the first of which is system identification, i.e., its selection. Selection is never wrong but some selections prove useless to the answers sought. For example, since all matter belongs to the "universe", the universe as system includes all events. For any event only a tiny part of the univese is involved. Thus the system defined applies generally to matter of the event within the area or locality of its observed or imagined activity. The selection process amounts to construction of what might be called a "physical scenario." With these aspects of reality in mind, we select a system, its boundary and surroundings.

The behaviors of masses of real systems are complex. It is virtually always necessary to simplify the system mass. Also it is expedient to engage the simplest calculations; the numbers gotten, though wrong, aid greatly in understanding and how to upgrade the analysis. Very many models of matter are used in analysis of physical reality. Topics addressed in TSH, HS physics, engineering mechanics, thermodynamics, ideal fluid mechanics use a few models.

Our descriptions are not exhaustive and others do things differently. Understanding of the usefulness and limitations of models happens with use and study.

Models of Mechanics:

• Body:  The simplest, most widely used model of physical matter is the BODY. This is an approximation of physical reality of the system as having its mass but not distributed in space (as in reality). The model, BODY, assumes all of the distributed in space system mass to be located precisely "at a point." Matter modeled to exist "at a point" is said to have no "extent." In this writing, model is emphasized by writing body as BODY. BODY is the model of reality of Newton's Laws of Motion. With the reduction of the real matter to be represented as mass at a point (BODY), so also are the relevant forces of the actual mass reduced to act "at a point."

For cases where the distribution of system mass over its space are significant, the model BODY is inadequate. Two model improvements are the "Extended BODY" and the "Rigid Body."
• Extended BODY:  This model assumes the system mass to be located at a point. The location of the mass is the system "center of mass." Extended BODY is useful with systems that have no rotation nor acceleration of rotation.
• Rigid BODY:

Models of Fluids Mechanics:

• Ideal FLUID:  This model assumes the system mass to be located at a point. The location of the mass is the system "center of mass." Extended BODY is useful with systems that have no.
• Ideal Gas:  This model of a gas prescribes that its properties conform with the Ideal Gas Equation of State: pv = (R/M)T.

Models of Thermodynamics:

• Pure Substance:   The principal assumption of thermodynamic analysis is the system substance (or material) is "pure". That is, the system is composed of only one type of matter, be it elemental, or in cases a pure compound. This model assumes the system mass to be located at a point. The location of the mass is the system "center of mass." Extended BODY is useful with systems that have no rotation nor acceleration of rotation.
• Simple Substance:    The energy of a system can change as a consequence of heat and/or work. As regards work, there are a number of manners in which system work might occur. These "manners" which are called work modes, include compression (for all substances) and might include additionally, magnetic work, electromagnetic work, elastic work, surface work or such. Substances having more than one appreciable work mode are additional mode.

• Complex Substance:    As regards work, there are a number of manners in which system work might occur. These "manners" which are called work modes, include compression (for all substances) and might include additionally, magnetic work, electromagnetic work, elastic work, surface work or such. Substances having more than one appreciable work mode are additional mode.

## 1.04 Models of Reality

The behaviors of masses of real systems are complex. It is virtually always necessary to simplify the system mass. Also it is expedient to engage the simplest calculations; the numbers gotten, though wrong, aid greatly in understanding and how to upgrade the analysis. Very many models of matter are used in analysis of physical reality.

Premise presently unwritted!