Q: Let $\vec{F}$ be the force acting on a particle having position vector $\vec{r}$ and $\vec{T}$ be the torque of this force about the origin. Then
(a) $ \vec{r} . \vec{T} = 0 $ and $ \vec{F} . \vec{T} = 0 $
(b) $ \vec{r} . \vec{T} = 0 $ and $ \vec{F} . \vec{T} \ne 0 $
(c) $ \vec{r} . \vec{T} \ne 0 $ and $ \vec{F} . \vec{T} = 0 $
(d) $ \vec{r} . \vec{T} \ne 0 $ and $ \vec{F} . \vec{T} \ne 0 $
Ans: (a)