You might have seen a driver of a car or a truck slowing down the vehicle at a traffic signal. You, too, slow down your bicycle whenever needed by applying brakes. Have you ever thought why a vehicle slows down when brakes are applied? Not only vehicles, any object, moving over the surface of another object slows down when no external force is applied on it. Finally it stops. Have you not seen a moving ball on the ground stopping after some time? Why do we slip when we step on a banana peel (Fig. 12.1)? Why is it difficult to walk on a smooth and wet floor?
Gently push a book on a table [Fig. 12.2(a)]. You observe that it stops after moving for some distance. Repeat this activity pushing the book from the opposite direction [Fig. 12.2, (b)]. Does the book stop this time, too? Can you think of an explanation? Can we say that a force must be acting on the book opposing its motion? This force is called the force of friction.
Tie a string around a brick. Pull the brick by a spring balance (Fig. 12.3). You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the of force of friction between the surface of the brick and the floor.
Now wrap a piece of polythene around the brick and repeat the activity. Do you observe any difference in the readings of the spring balance in the above two cases? What might be the reason
Recall now some of your experiences. Is it easier to hold a kulhar (earthen pot) or a glass tumbler? Suppose the outer surface of the tumbler is greasy, or has a film of cooking oil on it; would it become easier or more difficult to hold it? Just think : would it be possible to hold the glass at all if there is no friction?
As you have seen in the previous section,
friction is desirable in some situations.
Have you ever thought why the sole of your shoes is grooved [Fig. 12.11 (a)]? It is done to provide the shoes better grip on the floor, so that you can move safely. Similarly, the treaded tyres of cars,trucks and bulldozers provide better grip with the ground.
Take a few pencils which are cylindrical in shape. Place them parallel to each other on a table. Place a thick book over it (Fig. 12.15). Now push the book. You observe the pencils rolling as the book moves. Do you feel it easier to move the book in this way than to slide it? Do you think that resistance to the motion of the book has been reduced? Have you seen heavy machinary being moved by placing logs under it?
You know that air is very light and thin. Yet it exerts frictional force on objects moving through it. Similarly, water and other liquids exert force of friction when objects move through them. In science, the common name of gases and liquids is fluids. So we can say that fluids exert force of friction on objects in motion through them.