Question

Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force.

Answer 1

Frictional force is the one which opposes motion when a body slides or comes in contact with another body. This force will oppose the sliding of a book on the table.

Complete step by step answer
On keeping the book on the table, the book will slide down due to gravity. To counter it, the frictional force will be applied in an upward direction.

As we can see from the diagram, the book slides on the table going downwards and frictional force acts upwards opposite to the direction of sliding. This frictional force acts between table and book and is parallel to the surface of contact – the table. The friction between the book and the desk is due to the interaction of the molecules of the desk and the book.

Additional information
Sliding friction which is also called as kinetic friction acts in a system where two surfaces are in relative motion. Sliding frictional force is less than the static frictional force. In uniform motion, sliding friction can be given by $f = {\mu _k}R$, where f is the sliding frictional force , ${\mu _k}$ is the coefficient of sliding friction and R is the normal reaction on the surface. The coefficient of sliding friction is a bit greater than that of sliding friction coefficient. This is so because for motion, the inertia of rest has to be overcome. For example, we know it requires more force to change the state of a body from rest to motion than to keep the body in motion.

Note Friction is a self-adjusting force. The direction in which the frictional force is applied is always opposite to the direction of motion here, the sliding of a book.