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. 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 on block B is:

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

Answer 1

The correct option is C $120$N

Force on block A, $F_A=20N$

Force on block B, $F_B=100N$

Coefficient of friction between the blocks, $m_{₁}=0.1$

Coefficient of friction between the block B and the wall, $m_{₂}=0.15$

For block A,

$m₁g=m₁F$

$20=0.1\times F$

$F=200N$

Hence, frictional force on block A in the upward direction$=m₁F=0.1\times 200=20N$

Therefore, force on block B in the downward direction.

$m₂F=m₂g+m₁F=100+20=120N$

Answer C: