You place a book on your desk, and then you begin to lift one edge of the desk. You make the following observations: At first the book does not move. When you lift the desk edge higher, the book begins to slide slowly. As you lower the desk slightly, the book slows down, then stops. Use your knowledge of forces and friction to explain your observations.

Explanation:

The movement of an object involves two forces: the force applied to the body which causes acceleration of the object called kinetic force and the force that opposes it called the friction. These forces are involved in the movement of the book as:

1. When a book is kept on the desk, since no external force is applied to the book therefore the book remains at rest.

2. When the desk is placed in an inclined position, the inclined position causes the sliding of the book but as the book moves down the force of friction increases which causes the book to come at rest. This force of friction is known as the sliding force.