  Using time-resolved multifrequency Very Long Baseline Array data and new KaVA
(KVN and VERA Array) observations, we study the structure and kinematics of the
jet of the flat spectrum radio quasar (FSRQ) 1928+738. We find two distinct jet
geometries as function of distance from the central black hole, with the inner
jet having a parabolic shape, indicating collimation, and the outer jet having
a conical shape, indicating free expansion of the jet plasma. Jet component
speeds display a gradual outward acceleration up to a bulk Lorentz factor
$\Gamma_{\rm max} \approx10$, followed by a deceleration further downstream.
The location of the acceleration zone matches the region where the jet
collimation occurs; this is the first direct observation of an acceleration and
collimation zone (ACZ) in an FSRQ. The ACZ terminates approximately at a
distance of 5.6$\times 10^6$ gravitational radii, which is in good agreement
with the sphere of gravitational influence of the supermassive black hole,
implying that the physical extent of the ACZ is controlled by the black hole
gravity. Our results suggest that confinement by an external medium is
responsible for the jet collimation and that the jet is accelerated by
converting Poynting flux energy to kinetic energy.
