  An investigation of the kinematics of a rotating relativistic plasma stream
in the perpendicular rotator model of the pulsar magnetosphere is presented. It
is assumed that the plasma (ejected from the pulsar) moves along the pulsar
magnetic field lines and also corotates with them. The field lines are
considered to be radial straight lines, located in the plane which is
perpendicular to the pulsar rotation axis. The necessity of taking particle
inertia into account is discussed. It is argued that the "massless"
("force-free") approximation cannot be used for the description of this
problem. The frame selection is discussed and it is shown that it is convenient
to discuss the problem in the noninertial frame of ZAMOs (Zero Angular Momentum
Observers). The equation of motion and the exact set of equations describing
the behaviour of a relativistic plasma stream in the pulsar magnetosphere is
obtained. The possible relevance of this investigation for the understanding of
the formation process of a pulsar magnetosphere is discussed.
