Question

Two blocks of mass $2\, kg$ are connected by a massless ideal spring of spring constant $k=10\,N / m$. 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 $\left(g=10\, m / s ^{2}\right)$

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

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,
$k x=m g=20\, N$
Just after the string $A$ is cut (see Figure $(b)$)
For upper block,
$m a=k x+ m g$
$2 a=20+20$
$a=20\, m / s ^{2}$