Question

A block A of mass 2 kg rests on another block B of mass 8 kg 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. When a horizontal force of 25 N is applied on the block B, the force of friction between A and B is

• A. Zero
• B. 3.9 N
• C. 5.0 N
• D. 49 N

Answer 1

Answer: (A)

Zero

Answer 2

Limiting friction between the block B and the surface

$F _ { B S } = \mu _ { B S } \cdot R$ $= 0.5 ( m + M ) g$ $= 0.5 ( 2 + 8 ) 10$ $= 50 \mathrm {~N}$

but the applied force is 25 N so the lower block will not move i.e. there is no pseudo force on upper block A. Hence there will be no force of friction between A and B.