A toy truck of mass 2 m elastically collides with a toy car of mass...

Mass of toy truck $m_t=2 \mathrm{~m}$ Mass of toy car $\mathrm{m}_c=\mathrm{m}$ Initial speed of truck $v_t=v$ and initial speed of car $u_c=0$ If $v_1 and v_2$ be the final velocity of truck and car after collision, then by law of conservation of momentum, Total initial momentum = total final momentum $$\begin{aligned} \mathrm{m}_{\mathrm{t}} \cdot \mathrm{u}_{\mathrm{t}}+\mathrm{m}_{\mathrm{c}} \mathrm{u}_{\mathrm{c}} & =\mathrm{m}_4 \mathrm{v}_1+\mathrm{m}_{\mathrm{c}} \mathrm{v}_2 \\ 2 \mathrm{mv}+0 & =2 \mathrm{mv}_1+\mathrm{mv}_2 \\ 2 \mathrm{v} & =2 \mathrm{v}_1+\mathrm{v}_2 \end{aligned}$$ For elastic collision, coefficient of restitution, $e=1$ i.e. $$\begin{aligned} & \frac{v_2-v_1}{u_1-u_c}=1 \\ & \frac{v_2-v_1}{v-0}=1 \Rightarrow v=v_2-v_1 \end{aligned}$$ Solving Eqs. (i) and (ii), we have $\mathrm{v}_2=\frac{4 \mathrm{v}}{3}$