Question

The elevator shown in figure is descending with an acceleration of $2{\text{m/s}}^2$. The mass of the block $A=0.5\text{kg}$. The magnitude of force exerted by the block $A$ on block $B$ is

• A. $2\text{N}$
• B. $4\text{N}$
• C. $6\text{N}$
• D. $8\text{N}$

Answer 1

The correct option is B $4\text{N}$


From the FBD of block A, we can see that $N$ is the normal reaction exerted on block A by block B.
$\therefore$ $N$ will be also the magnitude of force exerted by block A on block B (By newton's $3^{rd}$ law)
Again by applying Newton's $2^{nd}$ law on block A in its direction of acceleration:
$F_{net}=m\times a$
$5-N=0.5a$
$\Rightarrow N=5-1=4\text{N}$
Hence option (B) is correct.