Question

suppose your writing desk is tilted a little a book kept on it is sliding down the direction of frictional force acting on it​

Answer 1

Answer:

Here the two surfaces in contact are the book and the desk. As mentioned before, the book slides and moves in a downward direction. Hence the relative motion is in downward direction. Since friction opposes the relative motion, the direction of frictional force acting here will be opposite to the motion of the book.