  We study the dynamics of a mobile impurity in a quantum fluid at zero
temperature. Two related settings are considered. In the first setting the
impurity is injected in the fluid with some initial velocity ${\mathbf v}_0$,
and we are interested in its velocity at infinite time, ${\mathbf v}_\infty$.
We derive a rigorous upper bound on $|{\mathbf v}_0-{\mathbf v}_\infty|$ for
initial velocities smaller than the generalized critical velocity. In the limit
of vanishing impurity-fluid coupling this bound amounts to ${\mathbf
v}_\infty={\mathbf v}_0$ which can be regarded as a rigorous proof of the
Landau criterion of superfluidity. In the case of a finite coupling the
velocity of the impurity can drop, but not to zero; the bound quantifies the
maximal possible drop. In the second setting a small constant force is exerted
upon the impurity. We argue that two distinct dynamical regimes exist --
backscattering oscillations of the impurity velocity and saturation of the
velocity without oscillations. For fluids with $v_{c {\rm L}}=v_s$ (where $v_{c
{\rm L}}$ and $v_s$ are the Landau critical velocity and sound velocity,
respectively) the latter regime is realized. For fluids with $v_{c {\rm L}} <
v_s$ both regimes are possible. Which regime is realized in this case depends
on the mass of the impurity, a nonequilibrium quantum phase transition
occurring at some critical mass. Our results are equally valid in one, two and
three dimensions.
