A combined spherical pendulum and linear pendulum system is developed to produce the same dynamic in-line and cross-axis reaction weight as liquid exhibiting rotary liquid slosh. An approximate solution of the dynamic equations for the spherical pendulum is used along with cross-axis weight measurements from slosh experiments to develop mass and damping parameters for the spherical pendulum. Similar measurements of in-line weight responses are used to develop the corresponding parameters for the linear pendulum solution. A verification of the spherical pendulum solution is accomplished by comparing the approximate solutions with those predicted by a time-step integration of the nonlinear governing equations. It is found that a constantparameter combined system model cannot be used to represent typical rotary slosh over the entire frequency range within which this type of liquid motion response occurs. Therefore, a model is developed with some parameters constant, and others are allowed to vary as necessary to match the force data.