Question

A block $A$ of mass $4\, kg$ is placed on another block $B$ of mass $5\, kg$, 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 $12\, N$, 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 $=12\, N$
Therefore maximum acceleration
$a_{\max }=\frac{12 \, N }{4\, kg }=3 \, m / s ^2$
Hence maximum force,
$ F _{\max } =\text { total mass } \times a_{\max }$
$=9 \times 3=27\, N$