Question

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 1

Answer: (A), (B), (C), (D)

When net external torque on a system of particles about an axis is zero,
i.e., $\tau= r \times F =r F \sin \theta \tau=$ Zero, where $\theta$ is angle between $r$ and $F , \tau$ is unit vector along $\tau$, then all the four statements (a), (b), (c), (d) are compatible.