Where friction acts
Two more things the definition settles: where the force acts, and which way it points.
Where: along the touching surfaces
Friction does not act somewhere inside the object or at its centre. It acts along the surfaces that are in contact — between the legs of the table and the floor, between the pencil and the table top, between your shoe and the ground.
Which way: opposite the motion
Push the table to the right and friction acts on it to the left. Push it to the left and friction turns around and acts to the right. It has no direction of its own — it takes the opposite of whichever way the object moves or tries to move.
Solids, liquids and gases
Frictional forces act in the motion of liquids and gases too. A boat is resisted by the water it moves through, and a moving vehicle by the air. Those are real frictional forces.
What this chapter covers
Everything from here on is about friction between solid objects only. The liquid and gas cases behave differently enough to be worth their own treatment later.
Where we go next
You have seen friction do three different things already: hold a still object still, reach a maximum and give way, and then keep acting on the object as it slides. Those are its three states, and section 5.2 takes them one at a time.