Problems in aircraft dynamics such as stability and response of the rigid airplane may be affected by fuel motion in the tanks. Such problems also might arise in connection with missiles. In this paper the response of the fuel to simple harmonic motions of a rectangular tank in translation, pitching, and yawing is studied. Using the force and moment expressions, simple mechanical systems equivalent to the fuel are constructed. These systems respond to motions of the tank walls in the same fashion as the fuel, producing identical forces and moments. The use of such mechanical analogies should simplify in many cases the analysis of the complete dynamic system.