Current friction stir welding (FSW) process modeling research is mainly concerned with the detailed analysis of local effects such as material flow, heat generation, etc. These detailed thermomechanical models are typically solved using finite element or finite difference schemes and require substantial computational effort to determine temperature, forces, etc., at a single point in time, or for a very short time range. Dynamic models describing the total forces acting on the tool throughout the entire welding process are required for the design of feedback control strategies and improved process planning and analysis. In this paper, empirical models relating the process parameters (i.e., plunge depth, travel speed, and rotation speed) to the process variables (i.e., axial, path, and normal forces) are developed to understand their dynamic relationships. First, the steady-state relationships between the process parameters and the process variables are constructed, and the relative importance of each process parameter on each process variable is determined. Next, the dynamic characteristics of the process variables are determined using recursive least-squares. The results indicate the steady-state relationship between the process parameters and the process variables is well characterized by a nonlinear power relationship, and the dynamic responses are well characterized by low-order linear equations. Experiments are conducted that validate the developed FSW dynamic models.