A particle is projected from ground with a velocity of 40 m/s making an angle of 30o with horizontal. Then

Velocity vector will be perpendicular to a the initial velocity vector if angle of projection is greater than 45o.Since u→ is perpendicular to V→,u→.v→=0i.e. ucosαi^+usinαj^.ucosαi^+usinα−gtj^=0⇒ t=ugsinαNow t>T⇒ugsinα>2usinαg⇒α<45o