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 on $B$ for the blocks to move together will be :

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

Answer 1

Answer: (C)

Here , $m_1=4kg,m_2=5kg$.
Force applied on $A,f=12N$,
This must at least be equal to force of kinetic friction applied on $A$ bt $B$
$f=f_k={\mu }_{k}R={\mu }_k{m}_{1}g$
$12=f_k={\mu }_k\times 4g$
$\therefore {\mu }_k=124g=3g$
As block $B$ is on smooth surface , therefore to move $A$ and $B$ togrther , (maximum) force $F$ required to be applied on $B=$ frictional force applied on a by A
$F=\frac{3}{g}(4+5)g=27N$