Suppose you push your physics book against a wall hard enough to keep it from moving. Does the friction force on the book point (a) into the wall, (b) out of the wall, (c) up, (d) down, or (e) is there no friction force? Explain.
the friction force is always opposed to the movement direction. Here, without friction, your book would move down. As the book is immobile the friction force is pointed upwards and it perfectly counteracts gravity (value m*g).
