Question

Block $A$ of mass $2 \,kg$ is placed over block $B$ of mass $8 \,kg$. The combination is placed over a rough horizontal surface. Coefficient of friction between $B$ and the floor is $0.5$. Coefficient of friction between $A$ and $B$ is $0.4$. A horizontal force of $10\, N$ is applied on block $B$. The force of friction between $A$ and $B$ is $(g = 10\, m\,s^{-2})$

• A. 100 N
• B. 40 N
• C. 50 N
• D. zero

Answer 1

Answer: (D)

Total mass of blocks $A$ and $B=2+8=10\, kg$
Friction between surface and combination of $A$ and $B$
$F =\mu R$
$=0.5 \times 10 \times 10=50\, N$
Here applied force on box $B$ is $10\, N$ that is less than $50\, N$. So the system will be in rest because of this there is no friction between blocks $A$ and $B$.