We don't need to divide by 2 if we are using total horizontal displacement . Total horizontal displacement = Horizontal component of velocity * total time of flight . At the top vertical component of velocity is zero so speed at the top = horizontal component only . Hope it helps .
But sir, in Q 9 the question does not specificly say that they are falling in constant speed. What indicates in the question that they are falling in constant speed? Also, initially before the balls reach constant velocity, they must have have an acceleration right? Then the acceleration slowly got reduced to zero. Was that acceleration of heavier mass greater than the smaller mass?
In the question , it's given that Weight=Upthrust and Drag . It means that resultant force is zero so balls are falling at constant speed . As balls have same shape and size it means that upthrust is the same for both of them . As heavier ball has greater mass , it means that Drag on heavier ball is greater and Drag is proportional to speed . Hence , terminal speed( final constant speed ) of heavier ball is higher or we can say heavier ball has greater average acceleration so higher speed .
