On the basis of the Bogolyubov-Mitropol'skii method of averaging, we study the problem of stability of the vertical rotation of a body suspended from a string.
Statement of the Problem. Basic Matrix EquationIn the collection of efficient procedures aimed at the approximate investigation of the problems of nonlinear mechanics (and, in particular, of the mathematical models of various oscillatory processes), an important place is occupied by the method of averaging based on the differential equations presented in a special (standard) Bogolyubov-Mitropol'skii form [1]:where x = colon(x 1 , . . . , x n ) and X = colon(X 1 , . . . , X n ) are n -dimensional vector columns and ε is a small real nonnegative parameter. The procedure of averaging is carried out on the right-hand side of Eq. (1). In some cases, the stability (instability) of the averaged system is accompanied by a similar state in the original system [not necessarily represented in the standard form (1)]. However, there are no general theorems substantiating the applicability of the method of averaging in these cases and the procedure of formal replacement of a given set of equations by a different set does not always give correct results, as indicated, e.g., by Chetaev [2]. In [3], Bogolyubov, for the first time, established a correspondence between the solutions of the exact and averaged equations and formulated theorems based on the standard form (1). Therefore, the reduction of the original equations of the analyzed problem to the standard form should be regarded as a necessary stage in the substantiation of the method of averaging [4].In what follows, we use the following basic matrix equation [5]:where x = colon(x 1 , . . . , x 2m ) is the required vector and J = J T , D = D T , H = −H T , Π = Π T , and P = −P T (the symbol (·) T denotes the operation of transposition) are given constant 2m × 2m matrices. Equation (2) describes the behavior of numerous mechanical systems under the action of dissipative, gyroscopic, potential, and, in particular, nonconservative positional forces. In systems containing gyroscopes, J is regarded as a positive-definite matrix of the moments of inertia of the system about the corresponding axes [6-10]. The equations of motion of a body suspended from a string studied in what follows represent a special case of the general equation (2).