Question

Given in the figure are two blocks A and B of weight 20N and 100N 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 is0.15, the frictional force applied by the wall on block B is

Answer 1

In the vertical direction, weight is balanced by frictional forces.

As the block is in equilibrium balance forces are in horizontal and vertical direction.

For the system of blocks (A+B)

F = N

for block A, f_A​ = 20N

for block B, f_B ​= f_A​ + 100 = 120N

Answer 2

Applying the equilibrium condition for block A in horizontal direction.

N_AB = F — (1)

Similarly, equilibrium condition for block A in vertical given,

f₁ = 20 N — (2)

Now for block B,

N = N_AB = F

f₂ = f₁ + 100

f₂ = 20 + 100

f₂ = 120 N