The strength, growth, and motion of vortices behind a circular cylinder immersed in a two-dimensional uniform flow with constant acceleration are analyzed. Equations for lift and drag forces are obtained from potential theory in terms of the flow and vortex characteristics. By combining the theoretical equations with the experimental results, drag and inertia coefficients are separated and shown to be a function of the relative displacement of the fluid. The results are striking evidence of the existence of a unique relationship between the drag and inertia coefficients.