Question 8
The force applied to a table is increased until it is larger than the greatest frictional force the floor can supply. What acts on the table then, starting its motion?
The right answer is the one marked above.
An unbalanced force equal to the difference between the two. Friction cannot grow past its maximum, so once the push exceeds it the two forces no longer cancel and the remainder is left over.
Friction does not disappear — it keeps acting all the while the table slides. It simply stops being big enough to balance the push, and that difference is the unbalanced force of chapter 4.