Question

Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Answer 1

When a book on a tilted writing desk starts sliding down, the direction of the frictional force will act opposite to the direction of motion. In this case, the frictional force will act upward along the incline of the desk. The purpose of this frictional force is to oppose the sliding motion and prevent the book from accelerating uncontrollably down the tilted surface. It is essentially a kinetic friction force that opposes the relative motion between the book and the desk.
Therefore, friction force acts opposite to the motion of the book.