Sol.

Consider in isolation a very small volume A of liquid of mass m at the surface of the liquid (Fig.). The motion mentioned will cause this volume to move, as does the whole trolley, with acceleration a. The forces which act on a volume of mass m must produce a resultant equal to ma acting in the same direction as the acceleration a. These forces will be: (1) the weight mg. and (2) the reaction N of the lower levels, normal to the surface of the liquid. The force N must have a value and a direction such that the resultant of N and mg should equal ma. Then in triangle ABC we shall have the relationship ma = mg tan α, hence

tan α = a/g

Consequently, the surface of the liquid, which is perpendicular to the force of reaction N, must make an angle α with the horizontal, in accordance with the above relationship.