Question

A particle of 4 kg is projected from the ground with speed 40 m/s at angle ${30}^{\circ }$. Find average torque about point A during the flight.

• A. 40 N m
• B. zero
• C. $30\sqrt{3}$ N m
• D. $40\sqrt{2}$ N m

Answer 1

The correct option is B zero


Let $L_{iA}$ be the initial angular momentum about A and
$L_{fA}$ be the final angular momentum about $\text{A}$.

$L_{iA}=mu\sin \theta \left(\frac{R}{2}\right)$
$\underset{–}{L_{fA}=mu\sin \theta \left(\frac{R}{2}\right)}$

$L_{iA}=L_{fA}$
$\Delta L=0$

Average torque ${\tau }_{avg}=\frac{\Delta L}{\Delta t}=0$