A body can be in translational equilibrium yet still start rotating if
- A. its mass is very small
- B. the net force is zero but the net torque is not
- C. the forces are all vertical
- D. it is moving at constant speed
Explanation
Two equal and opposite forces acting along different lines form a couple: they cancel as vectors, so there is no acceleration, but they produce a turning effect about the centre. This is exactly how a steering wheel is turned. Complete equilibrium therefore requires the second condition as well as the first.
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