Using ab initio approach to the theory of electromagnetic interaction of a small particle and spherical whispering gallery mode resonator, we derive the optical forces experienced by the particle and it's resulting motion. The form of the forces differs from that expected by the traditional gradient/scattering approach due to the modification of the field by the particle. The main effect of the confinement of the field in a cavity consists in making the component of the optical force usually interpreted as gradient, manifestly non-conservative, and hence not presentable in the gradient form. It is shown how the standard gradient/scattering formalism can be modified for the cavity confined optical field.
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