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 floor is 0.5. If a horizontal force F of 25 N is applied on the block B, find the force of friction between A and B.

• A. 0
• B. $4N$
• C. $25N$
• D. $50N$

Answer 1

The correct option is A 0

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 $50N$ 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

option A is correct