  Different approaches to the problem of the "quantum friction" force F acting
on an atom moving with velocity v<<c parallel to a dielectric surface have
resulted in different predictions for the way in which F depends on v. For
instance, Scheel and Buhmann [Phys. Rev. A 80, 042902 (2009)] and Barton [New
J. Phys. 12, 113045 (2010)] and others find a force linear in v, while
Intravaia, Behunin, and Dalvit [arXiv:1308.0712, 2013] and others find that the
force varies as v^3 to lowest order in v. We argue that the F proportional to
v^3 prediction results from an oversimplified treatment of the atom as a linear
oscillator, and that F depends on v in just the way predicted by Scheel and
Buhmann and Barton.
