Q. The two bodies of mass $m_{1}$ and $m_{2}(m_{1} > m_{2})$ respectively are tied to the ends of a massless string, which passes over a light and frictionless pulley. The masses are initially at rest and the released. Then acceleration of the centre of mass of the system is

A) $\left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack^{2}g$
B) $\left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack^{2}$
C) $g$
D) zero
Explanation

(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
${\overrightarrow{a}}_{CM} = \frac{m_{1}{\overrightarrow{a}}_{1} + m_{2}{\overrightarrow{a}}_{2}}{m_{1} + m_{2}} = \frac{m_{1}\left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack g - m_{2}\left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack g}{(m_{1} + m_{2})}$
$= \left\lbrack \frac{m_{1} - m_{2}}{m_{1} + m_{2}} \right\rbrack^{2}g$

Dr. Abubacker
Reviewed by Dr. Abubacker

Physics Educator & NEET/JEE Specialist

Ph.D. in Physics with 19+ years of teaching experience.