Question

2 bodies of different masses of 2 kg and 4 kg are moving with velocities 20 m/s and 10 m/s towards each other due to mutual gravitational attraction. What is the velocity of their centre of mass:

• A. 5 m/s
• B. 6 m/s
• C. 8 m/s
• D. Zero

Answer 1

Answer: (D)

Zero

Answer 2

$m_{1} = 2kg,\mspace{6mu} m_{2} = 4kg,\mspace{6mu}{\overrightarrow{v}}_{1} = 2m/s,\mspace{6mu}{\overrightarrow{v}}_{2} = - 10m/s$

${\overset{\rightarrow}{v}}_{cm} = \frac{m_{1}{\overset{\rightarrow}{v}}_{1} + m_{2}{\overset{\rightarrow}{v}}_{2}}{m_{1} + m_{2}} = \frac{2 \times 20 - 4 \times 10}{2 + 4} = 0\mspace{6mu} m/s$