The expression “interpretive framework” describes the norms, beliefs and guidelines that people use to explain what they see. People interpret events by fitting them into pre-existing pictures of similar events, which then act as a frame of reference for understanding the observed behaviour.  In the more restricted sense of scientifically quantifiable measurements, an interpretive framework consists of laws, patterns and archetypes that help understand behaviour. System dynamics is one such framework, based on stocks, flows and feedback.

The Newtonian interpretive framework uses familiar concepts from Newton’s laws of mechanics to help understand the behaviour of dynamical systems, including those of human behaviour. Thus, force, momentum, mass, Newton’s three laws, etc., can be used to explain how people-based systems behave, both in the real world and in the corresponding mathematical models. The framework is seen as complementing other frameworks, such as system dynamics and state-based equilibria and stability.  The name “sociomechanics” encompasses the general principle that the framework of mechanics can be applied to social systems.

### Basis of the Newtonian Framework

Within system dynamics, the Newtonian framework proposes:

• Force represents a causal connection between stocks. Force represents the ability of one stock to cause acceleration in another stock1.
• The momentum of a stock is measured by the stock values of the stocks that influence it.
• The mass of one stock, with respect to another, measures the degree of responsiveness of one stock to changes in another. Thus, stocks have inertial resistance to change.
• There are three laws of stock dynamics, similar to Newton’s laws
• The first law of stock dynamics. Stocks will remain level or change uniformly unless they are acted upon by a force. That is, they are influenced by stocks that are themselves undergoing change.
• The second law of stock dynamics. The acceleration of a stock produced by a net force is proportional to that force and inversely proportional to the stock’s mass.
• The third law of stock dynamics. The force on a stock through a flow has an equal and opposite force on a stock at the other end of the flow.
• The concept of energy can be associated with system forces and stocks2.
• Each stock has kinetic energy, representing how fast that stock changes.
• Feedback loops and exogenous influences inject, remove or exchange energy in stock flow systems.
• Energy is conserved in each stock-flow subsystem.
• The power of a feedback loop represents the rate of change of energy flow.

### References

1. Hayward J. & Roach P.A. (2019). The Concept of Force in Population Dynamics, Physica A: Statistical Mechanics and its Applications, 531, 121736, DOI: 10.1016/j.physa.2019.121736. ↩︎
2. Hayward J. & Roach P.A. (2022). The Concept of Energy in the Analysis of System Dynamics ModelsSystem Dynamics Review, accepted for publication. DOI: 10.1002/sdr.1700 ↩︎