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. 120 N
• B. 80 N
• C. 100 N

Answer 1

The correct option is A

120 N

Given:

Force on block A, F_A = 20 N

Force on block B, F_B = 100 N

Coefficient of friction between the blocks, m₁ = 0.1

Coefficient of friction between the block B and the wall, m₂ = 0.15

For block A,

m₁ g = m₁F

20 = 0.1 × F

∴ F = 200 N

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

Therefore, force on block B in the downward direction.

m₂F = m₂ g + m₁F = 100 + 20 = 120 N