Question

If $\vec{F}$ is the force acting on a particle having position vector $\vec{r}$ and $\vec{\tau}$ be the torque of this force about the origin, then

• A. $\vec{r}\cdot \vec{\tau} > 0$ and $\vec{F}\cdot \vec{\tau} < 0$
• B. $\vec{r}\cdot \vec{\tau} = 0$ and $\vec{F}\cdot \vec{\tau} = 0$
• C. $\vec{r}\cdot \vec{\tau} = 0$ and $\vec{F}\cdot \vec{\tau} \ne 0$
• D. $\vec{r}\cdot \vec{\tau} \ne 0$ and $\vec{F}\cdot \vec{\tau} = 0$

Answer 1

Answer: (B)

$\vec{r}\cdot \vec{\tau} = 0$ and $\vec{F}\cdot \vec{\tau} = 0$

Answer 2

$\vec{\tau}=\vec{ r } \times \vec{ F }$
$\vec{\tau}$ is perpendicular to $\vec{ r }$ and $\vec{ F }$.