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. (like maybe in a diagram )

Image.

Explanation:

If the table is tilted to the left, for example, the book will logically start sliding leftwards. But, the friction between the bottom of the book (part placed on table) and the table, will fight back, trying to make the book stay on place. In other words, it will be fighting in the oposite direction, which in our case would be to the right.

Hope it helped,

BiologiaMagister

see earlier answer