Question

As given in the figure two blocks $A$ & $B$ of weight $20\text{N}$ and $100\text{N}$ respectively are in equilibrium. These are being pressed against a wall by force $F$ as shown. If the coefficient of friction between the two blocks is $0.1$ and between block $B$ and the wall is $0.15$, the frictional force applied by wall on block $B$ is $\left(f_2\right)$ and the frictional force applied by block $B$ on block $A$ is $\left(f_1\right)$.

• A. $f_2=120\text{N}$
• B. $f_2=150\text{N}$
• C. $f_1=100\text{N}$
• D. $f_1=20\text{N}$

Answer 1

Answer: (A), (D)

$f_1=20\text{N}$

FBD of block $A$


For equilibrium of $A$
$f_1=W_A$
$f_1=20\text{N}...\left(1\right)$

To balance $W_A$ force friction will act in upward direction)

FBD of block $B$


For equilibrium of $B$
$f_2=f_1+W_B$
$f_2=20+100$
$f_2=120\text{N}$