A professor's office door is 0.90m wide, 2.2m high, 4.2cm thick; has a mass of 28kg , and pivots on frictionless hinges. A "door closer" is attached to door and the top of the door frame. When the door is open and at rest, the door closer exerts a torque of 5.5N*m .
What is the moment of inertia of the door?
If you let go of the open door, what is its angular acceleration immediately afterward?