Question

A block A of mass 2kg rests on another block B of mass 8kg which rests on a horizontal floor. The coefficient of friction between A and B is 0.2, while that between B and the floor is 0.5. When a horizontal force of 25N is applied on B, the force of friction between A and B is :-

• A. Zero
• B. 3.9 N
• C. 5.0 N
• D. 49.0N

Answer 1

The correct option is A Zero

First calculate the static friction force.

At bottom surface friction coefficient is $0.5$

$F_1=\mu N_1{N}_1=(2+8)g=10\times 10=100N$

Thus,

$F_1=\mu N_1=0.5\times 100N=50N$

Static friction force for motion of lower block we have to apply minimum 50 N force there is no motion until we apply $50N$ force.

Thus, there is no motion hence there is no friction force between block A and B.