  We perform the analytic study of the the buoyancy-drag equation with a
time-dependent acceleration $\gamma(t)$ by two methods. We first determine its
equivalence class under the point transformations of Roger Liouville, and thus
for some values of $\gamma(t)$ define a time-dependent Hamiltonian from which
the buoyancy-drag equation can be derived. We then determine the Lie point
symmetries of the buoyancy-drag equation, which only exist for values of
$\gamma(t)$ including the previous ones, plus additional classes of
accelerations for which the equation is reducible to an Abel equation. This
allows us to exhibit two r\'egimes for the asymptotic (large time $t$) solution
of the buoyancy-drag equation. %\textbf{ It is shown that they describe a
mixing zone driven by the Rayleigh--Taylor instability and the
Richtmyer--Meshkov instability, respectively.
