question_answer 52)

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

Answer:

                  \[\vec{\tau }\,=\vec{r}\times \,\vec{F}\]. The direction of torque due to the weight of the door will be along z direction and not along y-axis (vertical axis given in the question). Hence, weight of the door will not cause any torque about an axis passing through die hinge.