  Based on a fully relativistic framework and the assumption of local
equilibrium, we describe a simple mechanism of quantum friction for a particle
moving parallel to a dielectric. The Cherenkov effect explains how the bare
ground state becomes globally unstable and how fluctuations of the
electromagnetic field and the particle's dipole are converted into pairs of
excitations. Modelling the particle as a silver nano-sphere, we investigate the
spectrum of the force and its velocity dependence. We find that the damping of
the plasmon resonance in the silver particle has a relatively strong impact
near the Cherenkov threshold velocity. We also present an expansion of the
friction force near the threshold velocity for both damped and undamped
particles.
