A professor’s office door to 0.91 m wide, 2.0 m high, and 4.0 cm thick; has a mass of 25kg; and pivots on frictionless hinges. A “door closer” is attached to the door and the top of the door frame. When the door is open and at rest, the door closer exerts a torque of 5.2 N * m.

What is the least force that you need to apply to the door to hold it open?

5.2 N-m = F * 0.91

F = 5.71 N