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# 3.05 System Selection and Isolation

Every engineering analysis with its event or non-event has a "system" as its focus. System is a theoretical idea patterned after the work of Isaac Newton. The subject of every consideration of thermodynamics is a specific, initially selected amount of matter called the system . System definition amounts to two accomplishments; matter is selected and isolated, meaning ALL matter not-selected is identified.

"Selection and Isolation" of a system for analysis is a process whereby the system is imagined separated from all that is around it. A mental traverse or passage completely around the system matter is conducted noting carefully what is system, noting what the system boundary is and what is surroundings. This process has been described as "isolation of the system."

The system approach logically requires attendant ideas and definitions. The principal terms are listed below:

System:  We perceive matter directly by its existence and since matter exists in space, occasions arise when attention to space (meaning the matter that occupies it at a time t*) is used to define a system and to analyze its events.

Boundary:  Specification of a system establishes a finite, enclosed amount of matter that is separated from all other matter by the system boundary. The boundary of the selected matter comprises the system boundary. All matter outside of the system (all of the universe that is not system) is called surroundings.

Surroundings:  Things happen for the most part to or within localized parts of the universe. Regions of pronounced change are the system. Surroundings are the vastness that remains. Changes of the surroundings are assumed either not to occur or to be specifically prescribed. The mystery of change lies with the system not the surroundings.

Event:  The change of the system matter is called its event. There are a number of event types.

Forces:  For any event of a system, it is anticipated surroundings might, in some way, contribute. Therefore when the system is isolated, potentialities of influence of the surroundings are included by identification of forces (also work and or heat). Forces have been studied widely; more will be presented later.

Universe:  All of space and all matter that exists in that space is called the universe. System plus surroundings equals universe.

BODY:   This simplest model of matter assumes the entire mass of a physical object to reside, to be located at a "point." A body has spatial characteristics - position, velocity, and momentum but it has no "extent." BODY, particle, and point mass are nearly synonymous terms. Newton modeled each planet of the solar system as a body. He created the idea of solar gravity as a central force, then predicted the orbital motions of planets. Clearly Jupiter has size and is more than mass at a point. But the simplification, the model of Jupiter as a "BODY," has sufficed for accurate calculation of its orbital motion. When a model of something physically real is sufficient to answer the questions posed - no finer detail is needed.

In this writing, the noun "body" will usually be written in uppercase letters (BODY)to remind the ourselves that the entity addressed (BODY not body) is a system identified and selected using Newton's Analytic Method. While a "body" might exist in a physical situation, that body selected and isolated (removed from its physical reality to be in an analysis framework) becomes its modeled BODY.

EXTENDED BODY:   Real objects of our everyday lives are not point masses. Common objects have extent; the simplest model of physical reality that has extent is the Extended BODY . The model, extended body, simply adds the reality of size to the model BODY. With an extended body, all of the mass is assumed (or modeled to be) located at the mass center (or geometric center for a uniformly distributed system) of space of the body.

Rigid Body:  Actual matter, as in reality, has distributed mass and it occupies space. In addition real matter deforms; it can be bent, twisted and so by consequence of acting forces. Deformation complicated matters considerably. Consequently the model Rigid Body, meaning no distortion, is usually used for initial analysis.

Uniformly Distributed Fluid:   The model fluid assumes matter (e.g.,liquid water or air) flows (or does not flow) subject to external constraints and gravity. However as it flows, the density of a uniformly distributed fluid remains the same in all of its parts. Uniformly distributed fluid include the idea that density is uniform both spatially and temporally.

Simple Combinations:   A system might be selected to be two bodies or an extended body or a uniformly distributed fluid. In such cases, properties are summed over the components.

Pure Substance:  Nothing is purely one chemical species. Nonetheless, for initial calculations we usually assume 100% purity.

The ideas expressed above are sufficient for us to do elementary calculations similar to what you have done in physics. The general organization below follows the principal properties of thermodynamic analysis: mass, momentum and energy.

The accomplishments of thermodynamics include the accumulated knowledge of its investigations and its proven methods. When those methods are applied, it is to a system. The system is a mental construction, a model of the relevant physical reality. Thermodynamic analysis follows Newton's method in which the system is envisaged as isolated (or extracted from all surrounding matter) as a first step of analysis. Whatever influence surroundings might have on the system event are modeled as constructs. Force, work and heat are "ways of looking at event." They are simple stratagems for understanding. Like "double-entry bookkeeping;" they are constructs.

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