  We develop simple, physically motivated models for drag-induced dust-gas
streaming instabilities, which are thought to be crucial for clumping grains to
form planetesimals in protoplanetary disks. The models explain, based on the
physics of gaseous epicyclic motion and dust-gas drag forces, the most
important features of the streaming instability and its simple generalisation,
the disk settling instability. Some of the key properties explained by our
models include the sudden change in the growth rate of the streaming
instability when the dust-to-gas-mass ratio surpasses one, the slow growth rate
of the streaming instability compared to the settling instability for smaller
grains, and the main physical processes underlying the growth of the most
unstable modes in different regimes. As well as providing helpful simplified
pictures for understanding the operation of an interesting and fundamental
astrophysical fluid instability, our models may prove useful for analysing
simulations and developing nonlinear theories of planetesimal growth in disks.
