Question

The torque of the force F about O is defined as T=r×F, then select the correct option. (where r is displacement vector about O)

(1) r.F must be zero
(2) T.F must be non zero
(3) T.(r-F) must be zero
(4) T.r must be non zero

Answer 1

i think 3 is the answer

Answer 2

If it completes the full circle round then it is zero as displacement is zero . So 1. Is correct