Question

Figure shows two blocks $4kg$ and $2kg$ placed on a frictionless surface and connected with a spring. An external push gives a velocity of $10m{s}^{-1}$ to the heavier block in the direction of the lighter one. At that instant, the velocity gained by the centre of mass will be

• A. $\frac{2}{3}m{s}^{-1}$
• B. $\frac{5}{3}m{s}^{-1}$
• C. $\frac{14}{3}m{s}^{-1}$
• D. $\frac{20}{3}m{s}^{-1}$

Answer 1

Answer: (D)

$\frac{20}{3}m{s}^{-1}$


At the instant when $4kg$ block receives a push, $2kg$ block does not receive any velocity.
Velocity of centre of mass at that instant,
$V_{cm}=\frac{4\times 10+2\left(0\right)}{4+2}=\frac{40}{6}=\frac{20}{3}m{s}^{-1}$