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

Hint
In this question, the concept of the friction force will be used, that is it is the force which opposes the motion of the object just focus on the definition of the frictional Force and draw the diagram of Frictional Force accordingly to get the answer.

Complete step by step answer
Firstly, to understand what friction is, we focus on the definition of friction.
So, Friction is resistance to motion of one object moving relative to another. This is not a fundamental force like electromagnetic or gravitational force. But scientists believe that it is the result of the EM (Electromagnetic) attraction between charged particles in two touching surfaces. For every general rule of friction, there are many exceptions. Example, while two rough surfaces (such as sandpaper) rubbing against each other may get more frictional force. Smoothly polished materials such as plates of glass, which have been carefully cleaned from all surface particles, may stick to each other very strongly.
Now we focus on what is frictional force.
The force generated by two surfaces that slides and contacts against each other. The factors affecting the frictional force are various. Main two factors are:
1 In a flat surface, if an object is placed against another object, then the frictional force will be equal to the weight of the object.
2 If an object is pushed against the surface, then the frictional force is increased, and it becomes more than the weight of the object.
Now, in the given question, it is said that the writing desk is tilted a little and a book kept on it starts sliding down. So, the figure for the same can be given as:

So, from the above figure, it is clearly observed that the frictional force acts between the surface of the desk and the book. The direction of the frictional force on the book is opposite to the direction of its motion and acts in the upward direction.

Note
As we know that frictional forces are mainly affected by the surface texture and amount of force impelling them together. The angle and position of the object affect the amount of frictional force.