Learning Forum

[Karthik Remella]

Hi! I noticed that when I try to push open a door by applying a force closer to the hinge, I need to apply more force than when I am pushing on the door knob. Why is that? Since the weight of the door remains unchanged in both the cases, why do I need more force in the first case?

[prajput]

Hi @Kremella! If you were lifting the door up (against gravity) then the weight of the door would matter. But in this case, since the door rotates about an axis (its hinge), it is the torque applied that matters. Torque is calculated as force applied normal to a lever (door in this case) multiplied by the lever arm (distance between the hinge and point for force application). Opening the door requires are certain amount of torque. So as the lever arm increases the force required decreases. This why applying the force at the door knob (away from the hinge) needs smaller forces to get that amount of torque, making it easier to open the door.