Question

Given $\overrightarrow{F}=(4\hat{i}-10\hat{j})$ and $\overrightarrow{r}=(5\hat{i}-3\hat{j})$. Then torque $\overrightarrow{\tau }$ is

• A. $-62\hat{j}$
• B. $62\hat{k}$
• C. $38\hat{i}$
• D. $-38\hat{k}$

Answer 1

The correct option is D $-38\hat{k}$

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

=$(5\hat{i}-3\hat{j})\times (4\hat{i}-10\hat{j})$

=$20(\hat{i}\times \hat{i})+12(\hat{i}\times \hat{j})+50(\hat{j}\times \hat{i})+30(\hat{j}\times \hat{j})$

=$0+12\hat{k}-50\hat{k}+0$

=$-38\hat{k}$