Question

Given in the figure are two blocks $A$ and $B$ of weight $20\,N$ and $100\,N$, respectively. These are being pressed against a wall by a force $F$ as shown in figure. If the coefficient of friction between the blocks is $0.1$ and between block $B$ and the wall is $0.15$, the frictional force applied by the wall in block $B$ is

• A. 100 N
• B. 120 N
• C. 80 N
• D. 150 N

Answer 1

Answer: (B)

Note It is not given in the question, best assuming that both blocks are in equilibrium. The free body diagram of two blocks is as shown below,

Reaction force $R=$ applied force $F$
For vertical equilibrium of $A$;
$f_{1}=$ friction between two blocks
$=W_{A}=20\, N$
For vertical equilibrium of $B$;
$f_{2}=$ friction between block $B$ and wall
$=W_{B}+f_{1} $
$=100+20$
$=120\, N$