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: We have two surfaces sliding on each other. We know that frictional force always opposes the motion of the surfaces that are in contact with each other. To find the direction of frictional force, we find the direction of relative motion between the two surfaces. Direction of frictional force will be against this direction.

Complete answer:
In the question it is said that we have a book kept on a slanted writing desk, and the book starts to slide down.

When a book slips on the writing desk, there is a frictional force acting between the book and the surface of the desk.
We know that friction opposes the relative motion between the surfaces in contact with each other. 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.
Therefore the direction of frictional force will be in the upward direction.

The direction of frictional force, when the book kept on a tilted desk slides down is as shown in the figure above.

Note:
The resistance to motion of one object relative to another object is known as frictional force.
There are two kinds of friction; kinetic friction and static friction.
Friction depends on the nature of the surface; on smooth surfaces friction will be less and on rough surfaces friction will be high.