A ball is projected with a velocity 3i^+4j^ m/s .  The time after which its velocity vector is normal to initial velocity vector is (g = 10 m/s2)

when velocity becomes perpendicular to initial velocity horizontal velocity will remain same but vertical velocity changes.Let the velocity is 3i^+vj^  as they are perpendicular their dot product will be zero.3i^+4j^.3i^+vj^=0 9+4v=0⇒⇒v=-94=4-gt gt=4+94=254 t=2540=0.625alternate method v is perpendicular to u after t=ugsinθ=u2gusinθ=2510×4=0.625