question_answer 42)

                  The net external torque on a system of particles about an axis is zero. Which of the following are compatible with it?                 (a) The forces may be acting radially from a point on the axis.                 (b) The forces may be acting on the axis of rotation.                 (c) The forces may be acting parallel to the axis of rotation.                 (d) The torque caused by some forces may be equal and opposite to that caused by other forces.                

Answer:

                  (a, b, d) \[\vec{\tau }\,=\,\vec{r}\times \,\vec{F}=\,r\,F\,\,\sin \,\theta ,\] where \[\theta \] is the angle between \[\vec{r}\] and \[\vec{F}.\]