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We revisit the issue of directed motion induced by zero average forces inextended systems driven by ac forces. It has been shown recently that adirected energy current appears if the ac external force, $f(t)$, breaks thesymmetry $f(t) = - f(t+T/2)$, $T$ being the period, if topological solitons(kinks) existed in the system. In this work, a collective coordinate approachallows us to identify the mechanism through which the width oscillation drivesthe kink and its relation with the mathematical symmetry conditions.Furthermore, our theory predicts, and numerical simulations confirm, that thedirection of motion depends on the initial phase of the driving, while thesystem behaves in a ratchet-like fashion if averaging over initial conditions.Finally, the presence of noise overimposed to the ac driving does not destroythe directed motion; on the contrary, it gives rise to an activation processthat increases the velocity of the motion. We conjecture that this could be asignature of resonant phenomena at larger noises.Comment: 11 pages, to appear in the proceedings of the SPIE conference  "Fluctuations and Noise 04" (Gran Canaria, Spain, May 2004

Ratchets in homogeneous extended systems: internal modes and the role of noise