Problem 6: A force of 5 N is applied perpendicular to the plane of a uniform door 2 m high and 0.6 m wide. Find the torque along the line joining the hinge. (Hinges mean the pivots or turning points of the door. QABZE in Pashto).

Solution

Strategy: This is a simple case of opening (or closing a door). Force is applied perpendicular at one end of the door which rotates about the hinges on the other side to open or close. The width of the door provides the moment arm.

Given                         Force applied = F = 5 N

                                  Moment arm = r = width of the door = 0.6 m

Required                   torque =?         

Angle between moment r and F = 90⁰