Answer: The net external torque about the reference point is zero.
- A The net external torque about the reference point is zero
- B The net external force on the system is zero, regardless of torque
- C The total kinetic energy of the system stays constant over time
- D The entire system remains permanently at rest with no motion
Correct answer: A. The net external torque about the reference point is zero
Explanation: Conservation of L requires Στ_ext = dL/dt = 0. Forces can still act as long as their torques cancel.
Torque about axis O equals the force F multiplied by the perpendicular distance r⊥ from the axis to the line of action of the force, producing rotation.
Concept context
Moment of inertia, torque, angular momentum, rolling motion. Very high JEE weightage.
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