HOW FRICTION IS INCREASED OR DECREASED?

HOW FRICTION IS INCREASED OR DECREASED?

As you have seen in the previous section, friction is desirable in some situations.

Have you ever thought why the sole of your shoe 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.

Figure 12.11: (a) Soles of shoes and (b) tyres are treaded to increase friction

We deliberately increase friction by using brake pads in the brake system of bicycles and automobiles. When you are riding a bicycle, the brake pads do not touch the wheels. But when you press the brake lever, these pads arrest the motion of the rim due to friction. The wheel stops moving. You might have seen that kabaddi players rub their hands with soil for a better grip on their opponents. Gymnasts apply some coarse substances on their hands to increase friction for better grip.

In some situations, however, friction is undesirable and we would want to minimize it.

Why do you sprinkle the fine powder on the carrom board (Fig. 12.12)? You might have noticed that when a few drops of oil are poured on the hinges of a door, the door moves smoothly. A bicycle and a motor mechanic uses grease between the moving parts of these machines. In all the above cases, we want to reduce friction in order to increase efficiency. When oil, grease, or graphite is applied between the moving part of a machine, a thin layer is formed there and moving surfaces do not directly rub against each other (Fig. 12.13). The interlocking of irregularities is avoided to a great extent. Movement becomes smooth. The substances which reduce friction are called lubricants. In some machines, it may not be advisable to use oil as a lubricant. An air cushion between the moving parts is used to reduce friction.

Figure 12.12: Powder is sprinkled on the carrom board to reduce friction

Figure 12.13: Action of lubricant

Can we reduce friction to zero by polishing surfaces or using a large amount of lubricants?

Friction can never be entirely eliminated. No surface is perfectly smooth. Some irregularities are always there.

Source: This topic is taken from NCERT TEXTBOOK

WHEELS REDUCE FRICTION

WHEELS REDUCE FRICTION

You must have seen attaches and other pieces of luggage fitted with rollers. Even a child can pull such pieces of luggage (Fig. 12.14). Why is it so? Let us find out.

Figure 12.14: Rolling reduces friction

Activity 12.4:

*   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?

Figure 12.15: Motion of the book on rollers

When one body rolls over the surface of another body, the resistance to its motion is called rolling friction. Rolling reduces friction. It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull luggage fitted with rollers. Can you now understand why the wheel is said to be one of the greatest inventions of mankind?

Since the rolling friction is smaller than the sliding friction, sliding is replaced in most machines by rolling by the use of ball bearings. Common examples are the use of ball bearings between hubs and the axles of ceiling fans and bicycles (Fig. 12.16).

Figure 12.16: Ball bearings reduce friction

Source: This topic is taken from NCERT TEXTBOOK