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 $6\hat{j}\ N\,m$

B $-6\hat{i}\ N\,m$

C $6\hat{k}\ N\,m$

D $6\hat{i}\ N\,m$

The answer is B.

Solution

$\vec{\tau} = \vec{r} \times \vec{F} = 2\hat{k} \times 3\hat{j}$ $= 6(\hat{k} \times \hat{j}) = -6\hat{i}$ N m

Concept torque as r × F

2 questions from this chapter have appeared across 2 exam years.