Question

A ball of mass $m=2\text{kg}$ is travelling under the influence of gravity. At height $H=5\text{m}$ speed of ball is $u=10\text{m/s}$ as shown in figure. Find the torque due to gravity about $x$ axis.

• A. $20\text{N-m}$
• B. $10\text{N-m}$
• C. $0\text{N-m}$
• D. $30\text{N-m}$

Answer 1

The correct option is C $0\text{N-m}$



We know,
$\overrightarrow{\tau }$ about an axis$=\overrightarrow{r}\times \overrightarrow{F}$
where $r$ is perpendicular distance of $F$ from the axis.
At height $H$ perpendicular distance between ball and $x$ axis is $H$
$\therefore \overrightarrow{r}=H\hat{j}$
$\overrightarrow{F}=-mg\hat{j}$
$\overrightarrow{\tau }=\overrightarrow{r}\times \overrightarrow{F}=0$