Physical laws and observations

Because the physical systems of primary interest are static and/or dynamic in nature, the mathematieal models used to represent these systems most often include difference or differential equations. Such equations, based on physical laws and observations, are statements of the fundamental relationships among the important variables that describe the system. Difference and differential equation models are expressions of the way in which the current values assumed by the variables combine to determine the future values of these variables. As reviewed later it is important to relate static and/or dynamic loads on plastic products to operating temperatures. 

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