Question

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{i}\, Nm $
• B. $6 \hat{j} \,Nm $
• C. $-6 \hat{i} \,Nm $
• D. $6 \hat{k} Nm $

Answer 1

Answer: (C)

Given: $\vec{F}=3 \hat{j}\,N$,
$\vec{r}=2\hat{k}$
We know, $\vec{\tau}=\vec{r}\times\vec{F}=2\hat{k}\times3\hat{j}$
$=6\left(\hat{k} \times\hat{j}\right)=6\left(-\hat{i}\right)$
$\vec{\tau}=-6 \hat{i}\,Nm$