The position vector of a point which moves in the x-y plane is given by:
r = (0.6 t4 + 0.8 t3 – 1.7 t2) i + (– 0.9 t4 + 1.5 t) j
where r is in meters and t is in seconds. Determine the angle between the velocity v and the acceleration a when t = 1.4 sec.