The nonstationary dynamic of a sphere floating at the Liquid-gas interface in the presence of insoluble surfactants is studied. Taking into account the difference between the diffusion (D-s) and viscous (v) coefficients (D-s/v less than or equal to 10(-3)), a diffusion-limited model for the process is assumed. The additional drag force, Delta F(t), caused by the surfactant gradients on the moving sphere (Marangoni effect) is analyzed. The numerical result shows two successive periods : an initial stage (Delta F approximate to 0), followed by an active reaction of the system, with a well-expressed log-increasing resistance force, Delta F(t) similar to ln t. The problem with the unlimited force is discussed from the viewpoint of the Stokes paradox. The analogy with the dynamic behavior of the same system in steady state, is pointed out, where the resistance force proves to be proportional to Reynolds number Delta F(t) similar to ln Re.