The top view of a solid door is shown in Figure. The door has a mass of 40 kg and is 2.1 m high, 1.2 m wide and 0.05 m thick. Its door closer has a tensional spring constant of 13.6 N-m/rad. The door will close as fast as possible without oscillating if the tensional damping coefficient c in the closer is set to the critical damping value corresponding to a damping ratio of ζ = 1. Determine this critical value of c.