Three objects A, B and C are kept in a straight line on a frictionless horizontal surface.

Sol. Collision between A and B

$\mathrm{m} \times 9=\mathrm{mv}_{1}+2 \mathrm{mv} \mathrm{v}_{2}$ (from momentum conservation)

$\mathrm{e}=1=\frac{\mathrm{v}_{2}-\mathrm{v}_{1}}{9}$

$\Rightarrow \mathrm{v}_{2}=6 \mathrm{~m} / \mathrm{sec}, \mathrm{v}_{1}=-3 \mathrm{~m} / \mathrm{sec}$

collision between $\mathrm{B}$ and $\mathrm{C}$

$2 \mathrm{~m} \times 6=4 \mathrm{mv}$ (from momentum conservation)

v = 3 m/s