Question

In the figure given below, there are two blocks A and B of weight $20N$ and $100N$ respectively. These are being pressed against a wall by an external force $F$ as shown in figure. If the coefficient of static friction between the blocks is $0.1$ and between block B and the wall is $0.15$, then the frictional force applied by the wall on block B will be

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

Answer 1

The correct option is A $120N$


$N_{BA}$= Normal reaction on block B due to block A= $N_{AB}$= Normal reaction on block A due to block B

The direction of friction on block A will be vertically upward The friction on block B due to block A will be vertically downward. On block B, friction due to contact surface of wall will be upward.

Assume friction is limiting.

Applying the equilibrium condition for block A in horizontal direction:
$N_{AB}=F$..........(1)
Similarly, equilibrium condition for block A in vertical gives;
$f_1=20N$..........(2)


Now for block B,
$N=N_{AB}=F$ &
$f_2=f_1+100=20+100=120N$