Problems in aircraft dynamics such as stability and response of the rigid airplane may be affected by fuel motion in the tanks. Such problems might also arise in connection with missiles. In this report the response of the fuel to simple harmonic motions of a rectangular tank in translation, pitching and yawing is studied. The shape of the free surface and the values of forces and moments are obtained. 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. With the aid of this mechanical analogy the complete dynamic system can be handled by any desired process.