
CHAPTER 3: The Structure of Models


One of the two fundamental structures of a system dynamics model consists of socalled laws of balance. A law of balance represents the relation between a system content and the processes responsible for its changes (flows or transports, and production). The second basic structure is formed by special or material laws (also called constitutive laws) which specify the processes in a model. Together, laws of balance and the special laws for flows create complete system dynamics models.
In this chapter we will look behind the scenes of system dynamics models and study their structure in detail. In particular, we will take a close look at the form and function of laws of balance which constitute the backbone of dynamical models, before we turn to their overall mathematical structure.
CONTENTS
The Importance of Storage Elements in Dynamical Systems 86
An Example: The Balance of Amounts of Water 88
Accumulating Quantities and Accounting 96
Describing Amounts Stored in a System 104
Describing Processes 107
The General Law of Balance 109
Constitutive Laws 111
Constitutive Laws and Laws of Balance 115
The Mathematical Structure of Models 117
Boundary Value Problems 122
Model Structure and Modeling Methodology 128
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