Question

A ball of mass $m$ moving with constant velocity $u$ collides with a smooth horizontal surface at $O$ as shown in the figure. The $y-$axis is drawn normal to the horizontal surface at the point of impact $O$ and $x$-axis is along the surface. About which point angular impulse will be zero during collision?

Neglect gravity and friction.

• A. $A$
• B. $B$
• C. $C$
• D. None of these

Answer 1

The correct option is B $B$


We know that, angular impulse, $dL=\tau dt$

So, angular impulse will be zero when external torque $\tau$ is zero.

The torque may be developed about any point due to the normal force acting on the ball along $OB$.

Normal force will develop torque about points $A$ and $C$, but as it passes through point $B$, therefore, torque about it will be zero which will result into zero angular impulse about $B$.

Hence, option $\left(B\right)$ is the correct answer.