# Friction – Types & Role

Objects do not come to rest on their own. What brings an object to rest is an invisible force called friction. Friction is a force that opposes the motion of an object. When objects are in contact with each other, friction acts in a direction opposite to the motion of the moving object. The moving object slows down and finally stops.

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## Types of Friction

There are four kinds of friction. When you begin to push a box along the floor, there is static or starting friction. As you keep it moving there is kinetic or sliding friction. If you mount the box on wheels and push it, you get rolling friction. If you put oil or other liquid between the box and floor, you get fluid friction.

Static or Starting Friction

The force of friction that comes into play between the surfaces of two bodies before the body starts moving is called static friction.

Sliding Friction (Kinetic Friction)

When solid objects slide over each other, slide friction acts between their surfaces. Sliding a book across a table produces this type of friction.

Once the motion has begun, the force of friction decreases. A smaller force is now necessary to maintain uniform motion.

The amount of sliding friction depends upon two factors:

The weight of the object that is moving. And the types of surfaces that are in contact.

The sliding friction is less than static friction.

Rolling Friction

An object such as a ball or a wheel rolls over a surface rather than slides. The friction produced here is called rolling friction. Rolling friction tends to be less than sliding friction. So wheels are often placed under objects that are to be moved. Just imagine how much force would have to be used if automobiles had to overcome sliding friction instead of rolling friction.

Fluid Friction

Water, oil and air are examples of fluids. All liquids and gases are fluids. When an object moves across or through a fluid, the fluid exerts a force called fluid friction. Air resistance is an example of fluid friction. Air resistance makes a falling object fall more slowly. For example, a piece of paper dropped from a height comes to the ground slowly due to air friction.

Fluid friction is usually less than sliding friction. Substances called lubricants, which are slippery substances such as grease, change sliding friction to fluid friction. This reduces friction.

## Methods of Reducing Friction

Friction can be reduced in the following ways:-

1. By making the surfaces smooth and by polishing them.
2. By applying lubricants, such as oil, grease or graphite.
3. By using wheels and ball bearings. Use of ball bearings and roller bearings converts the sliding friction into rolling friction.
4. Friction due to air and water is reduced by adopting streamlined shapes, such as shapes of aeroplanes and boats which are narrow in front.

The main advantages of friction are as follows:

1. Writing and drawing is made possible because of the friction between the pen or pencil and the paper.
2. The burning of matchstick is possible only because of the friction between the rough surface of a matchbox and the matchstick.
3. We are able to walk because friction prevents us from slipping.
4. Without friction the wheels of a car would not grip the road.
5. The brakes of the vehicles would not work without friction between the rim and the brake-shoe.

The main disadvantages of friction are as follows:

1. Machine parts which rub with each other wear out.
2. Energy is wasted in overcoming friction in the machines.
3. Due to friction between the sole of the shoe and the floor, the sole wears out.
4. The tyres of vehicles wear out because of the friction between the tyres and the road.

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