An additional effect which usually has to be taken into account is the strong dependence of the friction force on the relative tangential velocity (the Stribeck curve) [2,7]. Generally speaking, when the friction coefficient is independent of the magnitude of the sliding velocity, the friction force has the usual retarding effect upon the motion maintained by an external force. In this case, the friction force switches between negative and positive values depending on the sense of the sliding velocity. Under these conditions, the friction force has the effect of positive damping which is associated with energy