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We study orbit of a pressure-confined cloud in the broad-line region (BLR) ofactive galactic nuclei (AGNs) when the combined effects of the central gravityand anisotropic radiation pressure and the drag force are considered. Physicalproperties of the intercloud gas such as its pressure and dynamic viscosity aredefined as power-law functions of the radial distance. For a drag forceproportional to the relative velocity of a cloud and the background gas, adetailed analysis of the orbits is performed for different values of the inputparameters. We also present analytical solutions for a situation where theintercloud pressure is uniform and the viscosity is proportional to the inversesquare of the radial distance. Our analytical and numerical solutionsdemonstrate decay of the orbits because of considering the drag force so that acloud will eventually fall onto the central region after so calledtime-of-flight. We found that time-of-flight of a BLR cloud is proportional tothe inverse of the dimensionless drag coefficient. We discuss if time-of-flightbecomes shorter than the life time of the whole system, then existence ofmechanisms for continually forming BLR clouds is needed.

On the orbital motion of cold clouds in BLRs