We study effects of pinning on the dynamics of a vortex lattice in a type IIsuperconductor in the strong-pinning situation and determine theforce--velocity (or current--voltage) characteristic combining analytical andnumerical methods. Our analysis deals with a small density $n_p$ of defectsthat act with a large force $f_p$ on the vortices, thereby inducing bistableconfigurations that are a characteristic feature of strong pinning theory. Wedetermine the velocity-dependent average pinning-force density $\langleF_p(v)\rangle$ and find that it changes on the velocity scale $v_p \simf_p/\eta a_0^3$, where $\eta$ is the viscosity of vortex motion and $a_0$ thedistance between vortices. In the small pin-density limit, this velocity ismuch larger than the typical flow velocity $v_c \sim F_c/\eta$ of the freevortex system at drives near the critical force-density $F_c = \langleF_p(v=0)\rangle \propto n_p f_p$. As a result, we find a generic excess-forcecharacteristic, a nearly linear force--velocity characteristic shifted by thecritical force-density $F_c$; the linear flux-flow regime is approached only atlarge drives. Our analysis provides a derivation of Coulomb's law of dryfriction for the case of strong vortex pinning.
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