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.

A frictional force is created when a book slides across the surface of a writing desk. The direction of the frictional force on the book is opposite to the direction of its motion and acts in an upward direction.

Note:

The force opposing the relative motion between two surfaces in contact is called the frictional force. It is caused by the irregularities on the two surfaces in contact.