Origin of Force Field Analysis
I seem to remember from my early days in OD that Lewin based his model on engineering principles of force analysis taught in Statics and Dynamics of Forces in Mechanical Design. For example, in designing a bridge and determining how to support it you would try to find the load on the bridge, ie forces opposing the structure remaining stable, and thus the supporting forces needed to resit or balance the structure and keep it from collapsing, thus maintaining an equilibrium situation. Lewin applied this concept which primarily dealt with physical forces to more emotional and organizational forces for his model.