MCQ
What is the torque of the force $\mathop F\limits^ \to   = (2\hat i - 3\hat j + 4\hat k)N$ acting at the pt. $\mathop r\limits^ \to   = (3\hat i + 2\hat j + 3\hat k)\,m$ about the origin
  • A
    $-17\hat i + 6\hat j + 13\hat k$
  • B
     $-6\hat i + 6\hat j - 12\hat k$
  • $17\hat i - 6\hat j - 13\hat k$
  • D
    $6\hat i - 6\hat j + 12\hat k$

Answer

Correct option: C.
$17\hat i - 6\hat j - 13\hat k$
c
Torque of a Force $\vec{F}$ acting on a point with position vector $\vec{r}$ is given by:

$\vec{\tau}=\vec{r} \times \vec{F}$

To, we can find Torque by finding the cross product of $\vec{r}$ and $\vec{F}$

We have:

$\vec{r}=3 \hat{\imath}+2 \hat{\jmath}+3 \hat{k} m$

$\vec{F}=2 \hat{\imath}-3 \hat{\jmath}+4 \hat{k} N$

So, torque will be:

$\vec{\tau}=\left|\begin{array}{ccc}{\hat{\imath}} & {\hat{\jmath}} & {\hat{k}} \\ {3} & {2} & {3} \\ {2} & {-3} & {4}\end{array}\right|$

$\Longrightarrow \vec{\tau}=\hat{\imath}((2)(4)-(-3)(3))-\hat{\jmath}((3)(4)-(2)(3))+\hat{k}((3)(-3)-(2)(2))$

$\Longrightarrow \vec{\tau}=\hat{\imath}(8+9)-\hat{\jmath}(12-6)+\hat{k}(-9-4)$

$\Longrightarrow[\vec{\tau}=17 \hat{\imath}-6 \hat{\jmath}-13 \hat{k}, N m]$

This is the torque of the force acting about Origin.

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