\[\[quadratic system incompressibility\]\]). This last equation, though difficult to interpret, is also the location that drag forces appear explicitly, via . Lastly, we emphasise again the importance of geostrophic balance, which offsets pressure gradients by Coriolis circulation, rather than radial flow. Thus radial pressure gradients do not drive stabilising radial motions which might alleviate the self-same gradients (but note that vertical pressure gradients remain in full). Therefore, rotation may not make an explicit appearance in these equations but its influence is crucial, as the next section demonstrates.

Streaming stability in the absence of rotation
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To emphasise the importance of rotation, and geostrophic balance in particular, let us consider the same problem with neither Coriolis force nor background shear, that is, in a nonrotating frame, a uniform flow of both dust and fluid, drifting relative to each other because of a pressure gradient, which is compensated for by an external gravitational field (and/or an inertial, non-Coriolis acceleration). It is actually more straightforward to handle this problem directly in terms of the dust and solid perturbations, with none of the approximations used in §3. We denote by $\mathbf{v}_{p,g}$ the perturbations of $\mathbf{V}_{p,g}$, $\delta_p\equiv\delta/f_p$ the logarithmic density perturbation of the gas, and 