Sol. When there are no springs, the acceleration of both vessel and water is governed only by the earth's attraction, and therefore it is the same for both in every position and during oscillation the water moves along with the vessel, i.e. the surface of the water remains flat and motionless relative to the vessel (Fig. a).

Also, when the vessel passes through the central position, the surface of the liquid is horizontal, since the vessel has no acceleration in this position, and the surface of the liquid must be perpendicular to the line of action of the force of gravity action on the liquid.

Soft springs attached to the vessel somewhat alter the vessel's acceleration, but do not affect the motion of the water, whose acceleration will depend, as before, only on the earth's attraction. In this case, the surface of the water will not remain motionless, but it will oscillate in relation to the vessel (Fig. b). As it were a wave passes over the water.