Question

A block $A$ of mass $4kg$ is placed on another block $B$ of mass $5kg$, and the block $B$ rests on a smooth horizontal table. If the minimum force that can be applied on A so that both the blocks move together is $12N$, the maximum force that can be applied to $B$ for the blocks to move together will be:

• A. 30 N
• B. 25 N
• C. 27 N
• D. 48 N

Answer 1

Answer: (C)

Minimum force on $A$
$=$ frictional force between the surfaces $=12N$
Therefore maximum acceleration
$a_{\max }=\frac{12N}{4kg}=3m/s^2$
Hence maximum force,
$F_{\max }=\text{ total mass }\times a_{\max }$
$=9\times 3=27N$