Pushing a table
A table is standing on a level floor and you set out to push it. Watch what the frictional force does while you push — it turns out to be cleverer than you might expect.
Figure to be added
Push gently: nothing happens
Push with only a very small force and the table does not move at all. The floor is exerting a force on the table opposing yours, and the two are equal and opposite, so they balance. The table stays where it is.
Push a little harder: still nothing
Now push slightly harder than before. If the table still does not move, the reason can only be that the frictional force has grown by itself, by just enough to balance your larger push.
Friction adjusts itself
Friction is a force that arranges itself, on its own, to be exactly large enough to balance the force applied. Push with 5 N and it answers with 5 N; push with 12 N and it answers with 12 N.
Keep going: the table moves
Increase the push further and at one particular moment the table begins to move. That happens because friction cannot go on adjusting itself for ever — beyond a certain limit it cannot grow any more.
Once your push is bigger than that limit, the two forces no longer cancel. What is left over is the difference between them: an unbalanced force. And an unbalanced force, as chapter 4 taught, is exactly what starts the motion.
The chapter in one line
Friction rises to match whatever you apply, up to a maximum. Below that maximum nothing moves; above it, the excess is an unbalanced force and the object accelerates.