Question

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.

Answer 1

If you make the freebody diagram, you will see that the only forces in the vertical direction are the weight of the book, which is downward, and an unknowm force upward that is responsible to keep the book at rest.

That unknown force upward, that opposes the movement of the book, is the friction force of the wall on the book, which by the way is proportional to the push force that you exert on the book.

Then the answer is that the friction force on the book point is up.