If F is the force acting on a particle having position vector r and τ be the torque of this force about the origin, then (a) r·τ > 0 and F·τ < 0 (b) r·τ = 0 and F·τ = 0 (c) r·τ = 0 and F·τ ≠ 0 (d) r·τ ≠ 0 and F·τ = 0

Answer: (b). τ = r is perpendicular to both r and F. So r·τ = 0 and F·τ = 0. So the answer is (b).

If F is the force acting on a particle having position vector r and τ… | NEET 2009 (AIPMT)

Q44NEET 2009 · AIPMTEasy

Torque, Couple & Equilibrium of Rigid Bodies

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

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Answer: (b)

$\vec{\tau} = \vec{r}\times\vec{F}$ is perpendicular to both $\vec{r}$ and $\vec{F}$ .
So $\vec{r}\cdot\vec{\tau} = 0$ and $\vec{F}\cdot\vec{\tau} = 0$ .
So the answer is (b).