A force F→=αi^+3j^+6k^ is acting at a point r→=2i^-6j^-12k^.The value of α for which angular momentum about origin is conserved is

For the conservation of angular momentum about origin, the torque τ→ acting on the particle will be zero.By definition τ→=r→×F→ Here, r→=2i^-6j^-12k^ and F→=αi^+3j^+6k^∴  τ→=i^j^k^2-6-12α36=i^(-36+36)-j^(12+12α)+k^(6+6α)=-j^(12+12α)+k^(6+6α) But τ→=0∴  12+12α=0   or   α=-1 and 6+6α=0   or   α=-1