
(a) In the pulley arrangement $\left| {\overrightarrow{a}}_{1} \right| = \left| {\overrightarrow{a}}_{2} \right| = a = \left( \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right)g$
But ${\overrightarrow{a}}_{1}$ is in downward direction and in the upward direction $ie$, ${\overrightarrow{a}}_{2} = - {\overrightarrow{a}}_{1}$
$\therefore$ Acceleration of centre of mass
${\overrightarrow{a}}_{CM} = \frac{m_{1}{\overrightarrow{a}}_{1} + m_{2}{\overrightarrow{a}}_{2}}{m_{1} + m_{2}} = \frac{m_{1}\left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack g - m_{2}\left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack g}{(m_{1} + m_{2})}$
$= \left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack^{2}g$