Q. Let F be the force acting on a particle having position vector r and $\tau$ be the torque of this force about the origin. Then
A) $\mathbf{r}.\tau = 0\ and\ \mathbf{F}.\tau\ \neq 0$
B) $\mathbf{r}.\tau \neq 0\ and\ \mathbf{F}.\tau = 0$
C) $\mathbf{r}.\tau \neq 0\ and\ \mathbf{F}.\tau \neq 0$
D) $\mathbf{r}.\tau = 0\ and\ \mathbf{F}.\tau = 0$
Explanation
(d) $\tau = \mathbf{r \times F}$
$\tau$ is perpendicular to both $\mathbf{r}\ and\ \mathbf{F}$, so $\mathbf{r}.\tau\ as\ well\ as\mathbf{\ F}.\tau$ has to be zero.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.