Abstract
The method of normal forms is applied to the nonlinear equations of the liquid motion in a rectangular tank. The basic idea underlying the method of normal forms is the use of local coordinate transformations so that the dynamical system can take the "simplest form". In this study, the 1st and 2nd modes are considered, and the transformed equations of O(1) are obtained. It can be explained from these equations that the mean value of the 2nd mode takes the negative value. Comparison with the direct numerical integration of the original nonlinear equations of motion shows the validity of the present analysis.
