Question

Find out the velocity of the centre of mass of two $1kg$ masses moving toward each other, one mass with velocity $v_1=10m/s$, the other with velocity $v_2=20m/s$.

• A. $0m/s$
• B. $5m/s$ to the left
• C. $10m/s$ to the left
• D. $15m/s$ to the left
• E. $20m/s$ to the left

Answer 1

Answer: (B)

$5m/s$ to the left

Given : $m_1=m_2=1kg$ $v_1=10m/s$ $v_2=-20m/s$

Let the velocity of centre of mass be $V_{cm}$.

Using $V_{cm}=\frac{m_1{v}_1+m_2{v}_2}{m_1+m_2}$

$\therefore$ $V_{cm}=\frac{1\times 10+1\times (-20)}{1+1}=-5m/s$

Thus the centre of mass moves with speed $5m/s$ towards left.