# Application of Friction- Physics Guide for Class 8

## Information about Application of Friction

 Title Application of Friction Class Class 8 Subject Class 8 Physics Topics Covered Advantages of FrictionDisadvantages of FrictionMethods to increase FrictionMethods to decrease FrictionFluid Friction

### Role of Friction

Friction plays a very important role in our daily life. Many of our daily activities depends on the presence of frictional force.
In some cases, friction is useful and necessary. In other cases, friction causes a wastage of energy and damages moving parts of machinery, etc. It is, therefore, harmful and an evil (or nuisance) in such situations.

Let us first look at some situations where friction becomes a necessity.

1. Walking on the ground

It is friction between the ground and our feet (shoes) that enables us to walk. When we walk, we push the ground under our feet in the backward direction (action); friction then provides the forward reaction and makes us move forward. If friction between our feet and ground were absent, it would not be possible to walk. Have you tried walking on a wet smooth floor or an oily floor? Why do we tend to slip on the wet or oily floor? The water/oil on the floor provides a thin layer in between our feet and the floor. This decreases friction between our feet/shoes and the ground. Our feet are then no longer able to grip the floor firmly and push it backward. We, therefore, tend to slip.

2. For rolling
The friction between the tyres/wheels and the road, is necessary for vehicles to move safely. If there were no friction, the tyres of vehicles will go on spinning at the same place and will not move forward at all. If friction becomes less than a specified value, the wheel/tyre can lose their grip of the road. The vehicle may then skid or turn.
3. Performing small day to day activities/tasks
Can you imagine being able to write at all if there were no friction? Whenever we write with pen, or pencil on paper, or with chalk on a blackboard, it is the friction that holds the ink/chalk particles, and makes them stick to the rough surface of paper/blackboard.
We also know from our day to day experience that it is easier to hold an earthen pot, or a paper glass than to hold a (smooth) glass tumbler. Why is it so? It is friction which enables us to hold things safely in our hands. Friction also enables us to keep things along a slope. Imagine everything hurtling down hills and mountains if there were no friction. We would not be able to fix a nail/screw on the wall/wood, or tie a knot, or light a matchstick, had there been no friction between the surfaces.

We now know that friction is needed for so many activities in our daily life. However, it has many disadvantages too and is unwanted in certain situations. Let us now look at some situations where friction is not desirable.
1. Friction consumes a substantial part of the useful energy available to us. As friction opposes any relative motion between two objects in contact; some of the effort (force/energy) applied to the moving object is wasted in overcoming friction.
2. Friction is responsible for a lot of wear and tear of moving parts/objects. We must have seen the worn out steps of 'foot over-bridges' (at railway stations) or the worn out soles of old shoes. Have you ever thought about the cause of these observations? It is friction which 'wears out' surfaces rubbing against each other. For this very reason, the moving parts of old machines need replacements.
3. A significant amount of energy, supplied to a machine, gets wasted in the form of heat energy while overcoming the force of friction.
We all have observed that, when we strike a matchstick against a rough surface, it catches fire. Also, when we vigorously rub our palms together for a few minutes, they become warm. This is because friction leads to production of heat. The energy, required to overcome friction is (mainly) converted into heat. While this Is welcome for warming up our palms on a winter morning, it becomes a problem between the moving parts of a machine. Excessive heat, produced due to friction, can damage the moving parts of a machine.

### Methods of Increasing Friction

We now know that friction is desirable in some situations and undesireable in some other situations. Whenever friction is required, it is increased by making the surfaces rough. Why do you think tyres of vehicles like (cars and bus) have treades on them? Why is the sole of your shoe grooved?
• The treades and grooves improve their grip on the road. This increases the friction to desired value, and helps us to avoid skidding or slipping.
• Sportsmen and players use special types of sport shoes with spikes/cleats (a piece of metal/rubber) on their soles. This helps them to 'get a better grip' on the ground.
• Atheletes and other sportpersons, (when they play), often make use of friction. Gymnasts often apply some coarse substance/chalk powder on their hands. By doing so, they increase the friction between their hands and the uneven bars. This gives them a better grip. For the same reason, Kabaddi (a game) players rub their hands with soil for having a better grip on their opponents.

### Methods of Reducing Friction

We now know that, in very many situations, friction is undesirable and we would want to minimise it.
The following are some common ways used to reduce friction:
1. Polishing
2. Lubrication
3. Ball bearings

#### 1. Polishing

• When we polish a surface, its roughness (unevenness) decreases. The surface becomes smooth and friction gets reduced.
• We also sometimes rub the surfaces with a fine sand paper to reduce their unevenness.

#### 2. Lubrication

• We all know that when a few drops of oil are poured on the hinges of a door, the door moves much more smoothly.
• Bicycle and motor mechanics use grease between the moving parts of these machines. In all these cases, we want to reduce friction in order to increase efficiency.
• Oil-like substances, which help to reduce friction when put on a surface, are called lubricants.
Lubricants can be:
• liquids (like oils)
• semi-liquids (like grease)
• solids (like talcum powder).
When we apply a libricant between the moving parts of a machine, a thin layer of this lubricant is formed between the two surfaces. As the surfaces now do not rub against each other directly, friction is reduced.
• Interlocking of irregularities between the (now) changed surfaces reduces considerably and movement becomes smooth.
• Oiling/greasing of machines results in less wear and tear, and hence, less energy wastage. This helps to increase the efficiency of machines.
• Sometimes we use solids (in the form of powders) as lubricants.
For example, when we play a game of carrom-board, we often sprinkle talcum powder on the carrom-board.
• By sprinkling talcum powder (on carrom-board), the friction between the 'striker' and the 'board' is very much reduced and the 'striker' moves smoothly on the board.
• In some machines, it may not be advisable to use oil as a lubricant. An air cushion between the moving parts is also often used to reduce friction.
• Compressed and purified air can also act as a lubricant. It provides an elastic cushion between the moving parts, thus reducing friction. It has the added advantage of preventing dust and dirt from collecting on the moving parts. It is these properties of air that play a very important role in the smooth ride of a hovercraft.

#### 3. Ball-bearings

We know that rolling friction is smaller than sliding friction. Sliding can be replaced by rolling, (in most machines) by use of ball-bearings. For example, we use ball-bearings in shafts of motors, dynamos, axles of vehicles and so on.

### Fluid Friction

• We now know that whenever a solid object moves over some solid surface (of another object), frictional force (solid friction) comes into play. This opposes the relative motion between the two surfaces in contact.
But what happens when an object moves through air? What do you think?
The air also exerts a force of friction even though air itself is very light and thin. We call this friction as air resistance. It also opposes the motion of objects through it. Like air, water and other liquids also exert a force of friction when objects move through them.

Thus we can say that fluids (collective term for liquids and gases) too exert force of friction (fluid friction/drag) on objects moving through them.
The force of (fluid) friction on an object, in a fluid, depends on the:
1. nature of the fluid.
2. shape of the moving object (the area of contact).
3. speed of the moving object (with respect to the fluid).

Air resistance, for example, increases with an increase in the speed of the object moving through it.

Activity 6
• Take two large sheets of aluminium foil/thick paper that are identical in size and shape.
• As both the sheets are made of the same material, it can be assumed that they have the same mass.
• Now, crumble one sheet into a tight ball.
• Hold both—the ball and the other sheet of foil- head high and drop them both at the same time.
What do we observe? Which piece of foil falls with a greater speed?
You will find that the piece, that has been crumbled into a ball, falls with a greater speed. This shows that it experiences much less air resistance. This, in turn, implies that greater the surface area of a body, the more is the resistance that the air offers to its motion. In other words, fluid friction depends on the shape of the object moving through the fluid.

We, thus, realise that fluid friction can be reduced to a great extent by (suitably) adjusting the shapes of bodies.

• We also observe that when objects move through fluids with a higher speed, a larger force (due to fluid friction) opposes their motion. They can lose a considerable part of their useful energy to overcome this fluid friction.
• To minimise this energy loss, the bodies/objects are given special shapes.
• The birds and fishes have to move about in fluids almost all the time. Their bodies must, therefore, have evolved to shapes, which would make them lose less energy in overcoming fluid friction.
• If we look carefully at the shapes of an aeroplane and a boat we find that the shape of an aeroplane has some similarities to the shape of birds and the shape of a boat is somewhat similar to that of a fish.
• Nature, thus, gives many useful ideas to mankind. Scientists often put them to use in designing the shapes of objects.
• Sports vehicles are often so designed (made more pointed towards front) that air flows smoothly over their surfaces in such a way that every air particle passes a particular point with the same speed and in the same direction. Such a flow is called streamline flow.
• All such special shaped bodies are called streamlined bodies. A streamlined flow of air, over the surface of a vehicle, reduces friction (air resistance) and helps it to acquire a faster speed.