Question

Two blocks of mass $2kg$ are connected by a massless ideal spring of spring constant $k=10N/m$. The upper block is suspended from roof by a light string $A$. The system shown is in equilibrium. The string $A$ is now cut, the acceleration of upper block just after the string $A$ is cut will be $\left(g=10m/s^2\right)$

• A. $0m/s^2$
• B. $10m/s^2$
• C. $15m/s^2$
• D. $20m/s^2$

Answer 1

Answer: (D)

Before the string $A$ is cut, let $x$ be elongation in the spring.

As system is in equilibrium. (see Figure $(a)$) Then for lower block,
$kx=mg=20N$
Just after the string $A$ is cut (see Figure $(b)$)
For upper block,
$ma=kx+mg$
$2a=20+20$
$a=20m/s^2$