Place a book on the table. Give a push to this book towards the right side as shown in Figure 3. We will find that the book moves through some distance to the right side and then stops. Since the book moving towards right side stops on its own, this means that the force of friction is acting on it the opposite direction (towards left) which is opposing its motion and making it stop. Thus, when the force is applied by our push to move the book).
Let us now give a push to the book towards the left side as shown in Figure 4. We will find that the book moves some distance towards the left side and then stops. Since the book moving towards left side stops on its own, this means that the force of friction is acting on it in the opposite direction (towards right) which is opposing its motion and making it stop. Thus, when the force is applied by our push to move the book towards left side, then the force of friction acts towards the right side (in opposite direction to the motion of book).
In the above activity we observe that when the motion of the book is towards right side, then the force of friction acts on it towards the left side. And when the motion of book is towards left side, the force of friction acts on it towards the right side. So, from this activity we conclude that the force of friction acts in a direction opposite to the direction of motion of an object. This is why the force of friction always opposes the motion of an object. In both the cases described above, the force of friction opposes the motion of book on the surface of the table. The force of friction occurs between the surface of table and the surface of book in touch with each other.
In the above examples, we have applied the force of push of our hand to move a book lying on a horizontal table top. The book can also be moved by the force of gravity of earth provided the table is tilted a little. So, if we tilt the table somewhat, then the book kept on it will start sliding down slowly. In this case, the force of gravity is acting in the downwards direction, the force of friction must act on it in the opposite direction upward direction (as shown in Figure 3).
Cause of Friction
Every object has a rough surface, though the surface may appear to be smooth to the naked eye. When we see through microscope, it is found that the surfaces of all the objects have rough edges. Some of the particles on the surface of objects are in the form of tiny hills while others form grooves (see Figure 5). The tiny hills and grooves on the surfaces of objects are called irregularities of the two surface’. When we try to move one object another object, the ‘irregularities of the two surfaces opposes the motion of one object over the other and gives rise to force of friction. Thus, friction is caused by the interlocking of irregularities in the surfaces of the two objects which are in contact with other.