  In numerical computations of astrophysical flows one frequently has to deal
with situations which require the use of a corotating coordinate system. Then,
in addition to the centrifugal force, the Coriolis terms have to be included in
the Navier-Stokes equations. In usual numerical treatments of solving the
equations of motion these non-inertial forces are included explicitly as source
terms. In this letter we show that this procedure, if applied to the angular
momentum equation, may lead to erroneous results. Using the specific sample
calculation of a protoplanet embedded in a protostellar disk, we then
demonstrate that only a conservative treatment of the Coriolis term in the
angular momentum equation yields results which are identical to those obtained
by computations performed in the inertial frame.
