  We present a fundamental classification of forces relevant in nonequilibrium
structure formation under collective flow in Brownian many-body systems. The
internal one-body force field is systematically split into contributions
relevant for the spatial structure and for the coupled motion. We demonstrate
that both contributions can be obtained straightforwardly in computer
simulations, and present a power functional theory that describes all types of
forces quantitatively. Our conclusions and methods are relevant for flow in
inertial systems, such as molecular liquids and granular media.
