Question

A block A of mass $2kg$ rests on another block B of mass $8kg$ kept on a horizontal floor. The coefficient of friction between A and B is $0.2$ while that between B and floor is $0.5$. When a horizontal force $F$ of $25N$ is applied on the block B, the force of friction between A and B is :

• A. 3 N
• B. 4 N
• C. 2 N
• D. zero

Answer 1

The correct option is D zero

$N^{\prime }=8g+N$

$N=2g$

$\therefore N^{\prime }=10g$

$=100$ Newtons

Now the maximum possible value of friction force on block $B=N^{\prime }{\mu }_2$

$=\left(100\right)\left(0.5\right)$

$=50$ Newtons

But since $f$ (applied) $<$ friction force

there will be no movement so there would not be any acceleration of the blocks $A\&B$ so no frictional force bet $A\&B$.