Question

As shown in the figure, the $2\text{kg}$ block is subjected to an external force of $42\text{N}$. What is the value of normal reaction ($N$) acting on the $3\text{kg}$ block (exerted on it by the surface), when spring reaches maximum extension? (Take $g=10{\text{m/s}}^2$)

• A. $20\text{N}$
• B. $30\text{N}$
• C. $8\text{N}$
• D. $5\text{N}$

Answer 1

The correct option is C $8\text{N}$

Draw the FBD of both blocks:


When spring reaches maximum extension, acceleration of block of $2kg=0$
Simillarly for block of $3kg$, since it has not broken contact with ground, it's acceleration=$0$

For $2kg$ block: $42–20–kx=0$......(1)
or $kx=22$

Similarly for $3kg$ block:
$N+kx–30=0$......(2)

Putting the value of $kx$ from equation (1) in equation (2), we get
$N=30–22=8N$