A small toy airplane is flying in the xy-plane parallel to the ground. In the time interval to t = 0 to t = 1.00 s, its velocity as a function of time is given by v = (1.20 m/s²)tî + [12.0 m/s – (2.00 m/s²)t]j. At what value of t is the velocity of the plane perpendicular to its acceleration?