  The Newtonian concept of force may be useful in some aspects of the dynamics
of many-particle quantum systems such as fissioning nuclei. Following
Ehrenfest's method, we show that the quantum kinetic force between parts of an
extended quantum system can be described by an operator acting on the boundary
between the two subsystems. The contribution to the force due to a short-ranged
particle interaction can also be treated in the same way. This includes
interaction effects treated in density functional theory by local functionals.
The force operators are applied to several simple models to demonstrate the
method.
