  We analyze the ponderomotive action experienced by a small spherical particle
immersed in an optical field, in relation to the internal energy flows (optical
currents) and their spin and orbital constituents. The problem is studied
analytically, based on the dipole model, and numerically. Three sources of the
field mechanical action - energy density gradient and the orbital and spin
parts of the energy flow - differ by the ponderomotive mechanism, and their
physical nature manifests itself in the optical force dependence on the
particle radius a. If a is much less than the radiation wavelength, the optical
force behaves as a^n and integer n can be used to classify the sources of the
mechanical action. This classification correlates with the multipole
representation of the field-particle interaction: The gradient force and the
orbital-momentum force appear due to the electric or magnetic dipole moments
per se, the spin-momentum force emerges due to interaction between the electric
and magnetic dipoles or between the dipole and quadruple moments (if the
particle is polarisable electrically but not magnetically or vice versa). In
principle, the spin and orbital currents can be measured separately by the
probe particle motion, employing the special choice of particles with necessary
magnetic and/or electric properties.
