The motion of a particle with a spin in spherical harmonic oscillator
potential with spin-orbit interaction is discussed. The attention is f
ocused on the spatial motion of wave packets. The particular case of w
ave packets moving along the circular orbits for which the most transp
arent and pedagogical description is possible is considered. The split
ting of the wave packets into two components moving differently along
classical orbits reflects a strong analogy with the Stern-Gerlach expe
riment. The periodic transfer of average angular momentum between spin
and orbital subspaces accompanying this time evolution is called the
spin-orbit pendulum.