The perfect launch

A tank filled with water is placed on guide rails as shown. The top surface is at ground level. A child stands at the edge of the platform and throws a ball horizontally with velocity $u_0$ . The tank also starts to move immediately with velocity $v_0(<u_0$ ) away from the platform. The length of the tank is $l$ . It was observed that the ball falls into the tank and strikes the opposite end at right angle (perpendicular to it). The ball is thrown such that its total energy is minimum at the time of launch. The specific gravity of the ball is $\alpha$ . Neglect viscous drag inside the tank. If a relation between $u_0$ and $v_0$ can be established as follows:

Then compute the value of $1/ab$