Question

frictinal force changes with change in area of contract explain

Answer 1

Well...that isn't so true. For a given mass frictional force will be same no matter what is the area of contact. This is because no body in this universe is rigid. There is always a change in shape (even microscopic!) in a body when force is applied on it. The frictional force thus balances the area of contact . In other words frictional force doesn't let the area of contact change!!!