Compute the force on the step (in terms of N, 
, and
k) using the pressure term from the continuity equation for momentum.
Explain physically the choice in signs which enables you to
get the correct direction for the force on the step.
Hint: The continuity equation which you derived on Problem Set 7 for momentum may be rewritten in the form
where the momentum density of the particles is
the pressure term is
and 
is the local force, and 
is the probability density.