We solve the formation-tracking control problem for mobile robots via linear control, under the assumption that each agent communicates only with one "leader" robot and with one follower. We assume that the system is force- controlled (hence we use the dynamic model) as opposed to velocity-controlled (in the kinematic-model case). As in the classical tracking control problem for nonholonomic systems, the swarm is driven by a fictitious robot which moves about freely and which is leader to one robot only. For the case of a fixed spanning-tree topology we show that persistency of excitation on the velocity of the virtual leader is sufficient and necessary to achieve consensus tracking.