Explanation:
The fores of magnitude F₁ = 12 N
The frictional force is self adjusting force , and it acts opposite to the applied force .
In this case , the force of friction f = 12 N
Similarly in second case , the force of friction f = 8 N
and in third case the force of friction f = 4 N
( a ) The counter top from beginning
4 N , 8 N and 12 N
( b ) The maximum force applied is 12 N , thus maximum force of static friction is also 12 N