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

The friction force acts between two bodies in contact and always resists the relative motion between the bodies. As given in the question the desk is tilted and the book is sliding downwards. The frictional force will work against this motion.

Complete step-by-step answer:
When two bodies are in contact a force acts on those bodies that resist any relative motion between them. This force is called frictional force or friction force. It is an adjustive force. That means it adjusts its magnitude and direction to prevent any relative motion between the bodies in contact. The Friction Force is a result of relative roughness between the bodies. And its magnitude depends upon the normal force acting between the body and the coefficient of friction between the surfaces in contact.
The Friction Force is of different types depending upon the relative motion between the bodies in contact. When there is no relative motion between the bodies and a force tries to initiate motion the Friction Force resists the start of the motion and is called the Limiting Friction.
But if the force applied is greater than the Limiting Friction then the bodies come in motion relative to each other and the resistive friction that comes into effect is called the Kinetic Friction and it opposes the motion, decreasing relative velocity or acceleration. The coefficient of kinetic friction between the surfaces determines the magnitude of the frictional force.
As given in the question the book has started sliding so the friction acting here will be kinetic friction and the direction of the frictional force will oppose the motion and act upwards as shown.

Note: Friction force is essential for many of the day to day activities we perform every day. Some examples might be Walking, driving etc. and thus the frictional force is often called a necessary evil. It, however, leads to loss of quite a lot of energy and can be reduced via the use of lubricants. Lubricants are smoother multilayers coating that is applied on surfaces of objects to reduce friction between them.