  In order to study constraints imposed on kinematics of the Crab pulsar's jet
we consider motion of particles along co-rotating field lines in the
magnetosphere of the Crab pulsar. It is shown that particles following the
co-rotating magnetic field lines may attain velocities close to observable
values. In particular, we demonstrate that if the magnetic field lines are
within the light cylinder, the maximum value of the velocity component parallel
to the rotation axis is limited by $0.5c$. This result in the context of the
$X$-ray observations performed by {\it Chandra X-ray Observatory} seems to be
quite indicative and useful to estimate the density of field lines inside the
jet. Considering the three-dimensional (3D) field lines crossing the light
cylinder, we found that for explaining the force-free regime of outflows the
magnetic field lines must asymptotically tend to the Archimedes' spiral
configuration. It is also shown that the 3D case may explain the observed jet
velocity for appropriately chosen parameters of magnetic field lines.
