• question_answer Block A of mass 2 kg is placed over block B of mass, 8 kg. The combination is placed over a rough horizontal surface. Coefficient of friction between B and the floor is $0.5$. Coefficient of friction between A and B is $0.4$. A horizontal force of 10 N is applied on block B. The force of friction between A and B is: A)  zero                  B)  50 N C)  40 N                            D)  100 N

Net friction force between block and surface is: $F=\mu \,R=0.5\,\times 10\times 10=50\,N$ Applied force is 10 N and it is less than 50 N System is at rest no friction between A and B