  We study the force balance and resulting acceleration of gas in general
relativity basing on simulations of accretion on a stellar-mass, non-rotating
black hole. We compare properties of acceleration in an optically thin,
radiatively inefficient disk, and in an optically thick, super-critical disk
accreting at 10 times the Eddington rate. We study both the average forces
acting at given location and forces acting on a gas parcel along its
trajectory. We show that the acceleration is not a continuous process - in most
cases gas is accelerated only in short-lasting episodes. We find that in the
case of optically thin disks gas is pushed out by magnetic field in the polar
region and by thermal pressure and centrifugal force below the disk surface. In
case of optically thick, radiative accretion, it is the radiation pressure
which accelerates the gas in the polar funnel and which compensates (together
with the centrifugal force) the gravity in the bulk of the disk. We also show
that the Newtonian formulae for the forces are inadequate in the innermost and
in the highly magnetized regions.
