In figure the uniform 480 kg door can swing about a horizontal axis through O. The door is shown in its equilibrium position under the control of a spring AB of stiffness 5 N/mm. How much work must be done to pull the door down to its closed position where it makes an angle of 90º to the vertical? If the door is released from its closed and latched position, what will be its angular velocity as it passes through the equilibrium position?
for diagram please see attachment