Touching a hemispherical dome of radius `R` there is a vertical tower of height `H = 4 r`. A boy pro

Touching a hemispherical dome of radius `R` there is a vertical tower of height `H = 4 r`. A boy projects a ball horizontally at speed `u` from the top of the tower. The ball strikes the dome at a height `(R)/(2)` from ground and rebounds. After rebounding the ball retraces back its path into the hands of the boy. Then the value of `u` is :