  Emission in many astrophysical transients originates from a shocked fluid. A
central engine typically produces an outflow with varying speeds, leading to
internal collisions within the outflow at finite distances from the source.
Each such collision produces a pair of forward and reverse shocks with the two
shocked regions separated by a contact discontinuity (CD). As a useful
approximation, we consider the head-on collision between two cold and uniform
shells (a slower leading shell and a faster trailing shell) of finite radial
width, and study the dynamics of shock propagation in planar geometry. We find
significant differences between the forward and reverse shocks, in terms of
their strength, internal energy production efficiency, and the time it takes
for the shocks to sweep through the respective shells. We consider the
subsequent propagation of rarefaction waves in the shocked regions and explore
the cases where these waves can catch up with the shock fronts and thereby
limit the internal energy dissipation. We demonstrate the importance of energy
transfer from the trailing to leading shell through $pdV$ work across the CD.
We outline the parameter space regions relevant for models of different
transients,e.g., Gamma-ray burst (GRB) internal shock model, fast radio burst
(FRB) blastwave model, Giant flare due to magnetars, and superluminous
supernovae (SLSN) ejecta. We find that the reverse shock likely dominates the
internal energy production for many astrophysical transients.
