Q92 / 180 · NEET 2020 (Phase 1) · Physics

Find the torque about the origin when a force of $3 \hat{j} N$ acts on a particle whose position vector is $2 \hat{k} m$ .

A
$6 \hat{j} N m$
Correct answer
B
$- 6 \hat{i} N m$
C
$6 \hat{k} N m$
D
$6 \hat{i} N m$

Solution

$\vec{F} = 3 \hat{j} N, \vec{r} = 2 \hat{k}$

$\vec{τ} = \vec{r} \times \vec{F} = 2 \hat{k} \times 3 \hat{j} = 6 (\hat{k} \times \hat{j})$

$= 6 (- \hat{i})$

$\vec{τ} = - 6 \hat{i} N m$

Hence, the correct answer is option: 'B'.

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