12N of force required to be applied on A to slip on B. Find the maximum horizontal force F to be applied on B so that A and B moves together.

Explanation

Let `mu` be friction coefficient between A and B
As 12N force on A is required for slipping so `mum_(A)g=12rArrmu=0.3`
maximum force `(F_(B))` applied on B so that `A & B` move together.
`F_(B)=(m_(A)+m_(B))a "where" a=mugrArrF_(B)=(m_(A)+m_(B))mug=(4+5)(0.3)(10)=27N`