Let vp be the velocity of ball P just before collision. Therefore, applying conservation of energy for P (between the moment it is released and the moment just before collision),
mgl+0=21mvp2
⇒vp=2gl
=2×10×0.2
=2ms−1
Since both balls have equal masses and the collision is perfectly elastic, the velocities of both the balls will get interchanged.
Therefore, the velocity of ball Q just after the collision is 2ms−1.
