Question

A ball of mass $m$ moving with velocity $v$, collide with the wall elastically as shown in the figure. After impact the change in angular momentum about $P$ is :

• A. $2mvd$
• B. $2mvd\cos \theta$
• C. $2mvd\sin \theta$
• D. $Zero$

Answer 1

The correct option is B $2mvd\cos \theta$

There is a change of velocity only in the $x$ direction, thus angular momentum also changes in $x$ direction only. The initial angular momentum in $x$ direction is $mvdcos\theta$. And since the collision involved is perfectly elastic, thus the change in momentum is simply twice the initial momentum i.e. $2mvdcos\theta$