This paper proposes an interpretative framework for system dynamics models using concepts from Newtonian mechanics. By considering the second derivative form of a model, it is shown that Newton's three laws of motion have their equivalent in system dynamics, with forces between stocks being determined using the loop impact method. The concepts of mass, inertia, momentum and friction are explored as to their usefulness in understanding model behaviour. The Newtonian framework is applied to two standard system dynamics models-inventoryworkforce and economic long-wave-where their behaviour is analyzed using force dominance on the stocks. Results show improved intuitive understanding of system behaviour compared with existing dominance methods, particularly for models with exogenous effects, oscillations and many loops. The framework is commended for further exploration.address the limitations raised above. Firstly, the concept of force has a clear intuitive connection with variable behaviour, enabling informal explanations in terms of model structure. Secondly, the force concept is easily extended to exogenous inputs, enabling comparison with forces from stocks within feedback loops. Thirdly, using stock-to-stock pathways as the main structural element, implicit in the PPM and loop impact methods, enables the unique identification of system forces. Finally, concepts of mass and friction can help explain oscillations arising from higher-order feedback. The proposed Newtonian interpretative framework builds on Hayward and Boswell's (2014) loop impact method by recasting the concept of impact in terms of the classic definition of force. Sato (2016) notes that force has been used informally in SD in the past (e.g. Forrester, 1961;Goodman, 1974) and more recently (e.g. Richardson, 2011). The proposed interpretative framework will provide a formal definition of force and other Newtonian concepts for the SD methodology.The structure of the paper is as follows. Firstly, the concept of loop impact, as introduced by Hayward and Boswell (2014), is investigated further by discussing the notion of the "impact" of a cause on motion generally. The concept is reinterpreted as the impact of a force on a stock. Secondly, laws of SD are proposed by analogy with Newton's laws of motion, with the concepts of force, momentum, mass, inertia and friction explored as to their usefulness in understanding the behaviour of any SD model. Thirdly, a notation is introduced to enable the analytical computation of force impact. Finally, the ideas presented are applied to two existing SD models to evaluate their use. (Models are available in an online supplement as supporting information.)The concept of "impact"The interpretive framework described in this paper employs the effect that one stock has on another as its principal focus of system behaviour analysis. The proposal is to use Hayward and Boswell's (2014) definition of loop impact as a measure of the "force" of one stock on the "motion" of another stock. In order to aid understanding of the c...

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This paper details the development of a first-principles hybrid model capable of simulating transient thermal performances of a large scale reheating furnace. In particular, the new modelling approach combines the advantages of the classical zone method of radiation analysis and Computational Fluid Dynamics (CFD) in a robust manner, and overcomes the difficulties of incorporating three-dimensional flow field within a zone method based model. The developed model has been validated with comprehensive experimental data collected during an instrumented bloom trial period that includes a long production delay. The results suggest that the model predictions were in good agreement with the actual measurements, and that the model was able to respond correctly with respect to the encountered production delay during the trial.

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