On a smooth inclined plane, making an angle a with horizontal, a trolley containing a liquid of density p slides down. What is the angle of inclination 0 of free surface with horizontal ?

the trolley is a non-inertial reference frame. the vertical component of acceleration of liquid in it is gcos²(alpha), and the horizontal component is gsin(alpha)cos(alpha). the resultant makes an angle (pi/2)-(alpha) with the horizontal. the free surface is perpendicular to the resultant. so the answer is: alpha is the angle