  Galactic disks are highly responsive systems that often undergo external
perturbations and subsequent collisionless equilibration, predominantly via
phase-mixing. We use linear perturbation theory to study the response of
infinite isothermal slab analogues of disks to perturbations with diverse
spatio-temporal characteristics. Without self-gravity of the response, the
dominant Fourier modes that get excited in a disk are the bending and breathing
modes, which, due to vertical phase-mixing, trigger local phase-space spirals
that are one- and two-armed, respectively. We demonstrate how the lateral
streaming motion of slab stars causes phase spirals to damp out over time. The
ratio of the perturbation timescale ($\tau_{\mathrm{P}}$) to the local,
vertical oscillation time ($\tau_z$) ultimately decides which of the two modes
is excited. Faster, more impulsive ($\tau_{\mathrm{P}} < \tau_z$) and slower,
more adiabatic ($\tau_{\mathrm{P}} > \tau_z$) perturbations excite stronger
breathing and bending modes, respectively, although the response to very slow
perturbations is exponentially suppressed. For encounters with satellite
galaxies, this translates to more distant and more perpendicular encounters
triggering stronger bending modes. We compute the direct response of the Milky
Way disk to several of its satellite galaxies, and find that recent encounters
with all of them excite bending modes in the Solar neighborhood. The encounter
with Sagittarius triggers a response that is at least $1-2$ orders of magnitude
larger than that due to any other satellite, including the Large Magellanic
Cloud. We briefly discuss how ignoring the presence of a dark matter halo and
the self-gravity of the response might impact our conclusions.
