Question

A door is hinged at one end and is free to rotate about a vertical axis (Fig. 7.10). Does its weight cause any torque about this axis? Give reason for your answer.

Answer 1

No. A force can produce torque only along a direction normal to itself as t = r × f . So, when the door is in the xy-plane, the torque produced by gravity can only be along ±z direction, never about an axis passing through y direction.