Question 7
A table is pushed with a small force and does not move. It is then pushed with a slightly larger force, and it still does not move. What must have happened?
The right answer is the one marked above.
The frictional force also increased by itself. For the table to stay at rest the forces must still balance, so friction must have grown to match the larger push.
This self-adjusting behaviour is what section 5.2 calls static friction. Note it cannot go on for ever — push hard enough and the table moves.