The aim of this paper is to study the spectral problem of small motions of a pendulum with a cavity filled with a system of ideal capillary fluids when the condition of statically stable in linear approximation is valid. It is proved that this proble
m has a real discrete spectrum with a limit point at and the eigenvalues for this problem are successive minima of variation ratio. It is also proved that if the operator of potential energy of a system ( pendulum + a system of ideal capillary fluids )has a negative eigenvalues, then the solutions of the initial boundary value problem are instable
