The rotation of rod OA about O is defined by the relation ?= 0.5e^ -0.8t*sin3pt, where ? and t are expressed in radians and seconds, respectively. Collar B slides along the rod so that its distance from O is r= 1 + 2t- 6t^2 + 8t^3, where r and t are expressed in feet and seconds, respectively. When t= 0.5 s, determine the acceleration of the collar relative to the rod.