The second order sliding mode control of uncertain dynamical systems can be expressed in terms of the stabilization of uncertain second-order non-linear systems with incomplete state availability. The existence of the solution strictly depends on the assumptions made on the uncertainties, and global results are not always attainable. In this paper the class of systems for which a suitable variable structure control algorithm ensures finite time global convergence to the sliding manifold is identified. Simulation results highlight the effectiveness of the proposed method.