- 1 Friction Is A Necessary Evil
- 1.1 Advantages of Friction
- 1.2 1. Friction Enables Us to Walk Without Slipping
- 1.3 2. Friction Enables a Car to Move on Road Without Skidding
- 1.4 3. Friction Enables Us to Apply Brakes and Slow Down or Stop a Moving Car
- 1.5 4. Friction Enables Us to Write and Draw on Paper
- 1.6 5. Friction Enables Us to Pick Up and Hold Things in Our Hands
- 1.7 6. Nails Can be Fixed in a Wall (or Wood) Due to Friction
- 1.8 7. Friction Enables Us to Light a Matchstick
- 1.9 8. Friction Enables Us to Cut Wood with a Saw
- 1.10 Disadvantages of Friction
- 1.11 1. Friction Wears Away the Soles of Our Shoes
- 1.12 2. The Tyres of Vehicles Wear Out Gradually Due to Friction
- 1.13 3. Friction Wears Out the Rubbing Machine Parts
- 1.14 4. Friction Wears Out the Brake Pads of Vehicles Gradually
- 1.15 5. Friction Wears Out Steps of Staircases in Buildings and Foot Over-Bridges
- 1.16 6. Friction Produces Heat Which May Damage Machines
- 1.17 7. Friction Reduces the Efficiency of Machines
- 1.18 8. Friction Slows Down Motion
Physics Topics are often described using mathematical equations, making them precise and quantifiable.
Friction Is A Necessary Evil
Frictional force plays an important role in our daily life. In some cases, friction is useful and we want to keep it but in other cases friction is harmful and we wish to reduce it. We will now discuss the advantages and disadvantages of friction in detail, one by one.
Advantages of Friction
Friction (or frictional force) is necessary because it helps us in performing many of our daily life activities. Friction is useful to us because of the following advantages.
1. Friction Enables Us to Walk Without Slipping
We are able to walk on ground because friction between the sole of our shoes and ground prevents us from slipping over the ground. This happens as follows : In order to take a step forward during walking, we lift one foot off the ground and push the ground backwards with the other foot (see Figure). If there were no friction between the sole of our shoe and ground, then our shoe on the ground would slip backwards. Since we push the ground backwards, the force of friction acts in the opposite direction, forward direction, and prevents our foot from slipping backwards (see Figure). So, it is the force of friction which makes us move forward at each step we take during walking. If there were no force of friction between the soles of our shoes and the ground, it would not be possible to walk (because our shoe would slip everytime we tried to walk).
Walking on slippery ground is difficult because the frictional force on slippery ground is much less which may not be sufficient to prevent us from slipping. It is also difficult to walk on a well polished floor (or oh ice) because the friction on these smooth surfaces is very small (which cannot prevent us from slipping). If a person throws a bucket of water on a smooth marble floor, it would become even more difficult for us to walk on this wet marble floor. This is because the friction on wet marble floor (having a layer of water on it) becomes very small which cannot prevent us from slipping. So, many times people slip on wet marble floor and fall down (see Figure). When we accidently step on a banana peel thrown on the road, we usually slip and fall down. This is because the inner side of banana peel being smooth and slippery reduces the friction between the sole of our shoe and the surface of road. And this small frictional force is not enough to prevent our foot from slipping backwards.
2. Friction Enables a Car to Move on Road Without Skidding
The friction between tyres of a car and the road enables a car to move forward on road without skidding. This happens as follows : When the engine of car makes the wheel of a car turn, the tyre pushes the road backwards at its point of contact with the road (see Figure). The friction between tyre of car and the surface of road acts in the forward direction and prevents it from skidding (see Figure).
In this way, friction provides the forward force which drives the car. If there were no friction between car tyres and road, then the wheels of car would spin at the same place but the car would not move forward at all. The car would stay where it was. It becomes somewhat difficult to drive and control a car on wet road. This is because the presence of water on the surface of a wet road reduces friction and makes it slippery.
And because of reduced friction, there are more chances of skidding of car (especially when brakes are applied suddenly). So, driving a car on wet roads needs extra care. Just like a car, friction also enables a bicycle to move along a road. In fact, all the vehicles are able to move on road because of the presence of friction between their tyres and the surface of road.
3. Friction Enables Us to Apply Brakes and Slow Down or Stop a Moving Car
The brakes of a car work by friction. In a disc brake, a steel disc attached behind each car wheel spins between two small brake pads. When the brakes are applied to the running car by pushing the brake pedal, the brake pads press against the discs of the rotating car wheels. This produces friction between brake pads and the discs, making the wheels to slow down and ultimately stop. The brakes of a bicycle also work by friction. When the bicycle is running, the brake pads of bicycle do not touch the wheels (there is a gap between them).
But when we press the brake lever to apply brakes to the running bicycle, then the brake pads rub on the rims of the bicycle wheels (see Figure). The friction between brake pad and rim prevents the wheel from moving ahead. Due to this, the running bicycle slows down and finally stops. If there were no friction, then once a vehicle started moping, it would never stop (because brakes would not work without friction). In fact, had there been no friction between the tyres of vehicles and the road, the vehicles could not be started or stopped or turned to change the direction of motion.
4. Friction Enables Us to Write and Draw on Paper
We are able to write and make drawings on paper because there is friction between the tip of pencil (or pen) and paper. The pencil has a thin, black core made of carbon (or graphite) which is called ‘pencil lead’. When we write with a pencil, friction with paper rubs off carbon particles from the ‘pencil lead’ which stick to the paper and leave black marks on paper (which we see as our writing on paper) (see Figure).
Similarly, when we write with a pen, the particles of ink rub off from the pen’s refill due to friction with paper, stick on the paper and leave marks of writing on paper. If there were no friction between the pencil (or pen) and paper, writing and drawing on paper would not have been possible. We cannot write with a pencil on a glass sheet because the glass surface is very smooth due to which the friction between the tip of pencil and glass surface is much less. This friction is not sufficient to rub off black graphite particles from the tip of pencil. A teacher is able to write on blackboard with a chalk due to friction between the blackboard and the chalk. When the teacher starts to write on the blackboard with a chalk, the rough surface of blackboard rubs off some chalk particles which stick to the blackboard and appear as writing on the blackboard.
5. Friction Enables Us to Pick Up and Hold Things in Our Hands
We are able to pick up this book and hold it in our hands due to friction between the book and the hands. Similarly, we can hold a glass tumbler in our hands because of friction (between the glass tumbler and our hands). If the outer surface of a glass tumbler is oily or greasy (having a film of cooking oil on it), then it becomes more difficult to hold it. This is because the presence of a film of oil on the outer surface of glass tumbler reduces the friction between glass tumbler and our hands. Due to less friction, the oily glass tumbler tends to slip from our hand and it becomes more difficult to hold. It is easier to hold a kulhar (an earthen pot) in our hand than a glass tumbler.
This is because due to the rough surface of kulhar, the friction between kulhar and our hand is much more which makes it easier to hold it. On the other hand, due to the smooth surface of glass tumbler, the friction between glass tumbler and hand is much less which makes it comparatively difficult to hold it. If there were no friction, it would not be possible to hold a book, a glass tumbler, a kulhar or any other object in our hands.
6. Nails Can be Fixed in a Wall (or Wood) Due to Friction
We are able to fix nails in a wall due to friction. When we hammer a nail into the wall, it is the friction between the surface of nail and wall which holds the nail tightly in the wall. Without friction, nails could not be fixed in a wall to hold things. Nails and screws are also held in wood by friction. If there were no friction, then nails and screws could not be used to hold pieces of wood together and hence we could not make any furniture (like table, chair, etc.). Thus, friction enables nails and screws to hold things together. Friction enables knots to be tied in strings (ropes, etc.). In other words, knots in ropes are held together by friction. Friction enables a person to climb a tree or pole without sliding down all the time. An oily or greasy pole has much less friction due to which it becomes difficult to climb up a greasy pole. Friction enables a ladder to be leaned against a wall and not slip down to the floor. Friction helps in the construction of buildings. Without friction, no building could be constructed. Friction enables the belts to drive machines in factories. Without friction, belts would not drive machines.
7. Friction Enables Us to Light a Matchstick
When we rub a matchstick against the rough side of a matchbox, then friction between the head of matchstick and rough side of matchbox produces heat. This heat burns the chemicals present on the head of matchstick due to which the matchstick lights up (see Figure). So, the matchstick catches fire and starts burning. The burning of a matchstick would not be possible without friction.
It is difficult to light a matchstick by striking it on a smooth surface because enough friction is not provided by a smooth surface to produce sufficient heat (for the match stick to catch fire and light up).
8. Friction Enables Us to Cut Wood with a Saw
We are able to cut wood because there is friction between the saw blade and log of wood. If there were no friction between the saw blade and log of wood, then cutting of wood would not be possible.
Disadvantages of Friction
Friction causes objects to wear away. Tiny pieces break off a surface when it rubs against another surface. The wear and tear due to friction can happen quickly or slowly depending on the nature of materials of the two surfaces. Friction (or frictional force) is harmful to us and considered an evil because of the following disadvantages.
1. Friction Wears Away the Soles of Our Shoes
When we walk on the road, there is friction between the soles of our shoes and the surface of road. When the soles of our shoes rub against the rough surface of road, then tiny pieces of the soles keep on breaking off slowly due to which the soles of our shoes wear out gradually (see Figure). The wear and tear of the soles of our shoes ultimately causes holes in the soles. The shoes get damaged and become unfit to wear.
2. The Tyres of Vehicles Wear Out Gradually Due to Friction
When the vehicles (like cars, buses, trucks, etc.) run on the road, there is friction between the surface of tyres and the surface of road. The rubbing of tyres with road keeps on breaking tiny pieces of rubber from the tyre’s surface gradually. Ultimately, all the treads present on the surface of a tyre are worn out and the tyre becomes baldy (see Figure). Such baldy tyres have to be replaced by new tyres. Thus, the tyres of vehicles wear out due to friction with the road. The tyres of bicycles also wear out gradually due to friction with the road.
3. Friction Wears Out the Rubbing Machine Parts
There are many moving parts in machines (like gears, etc.) which rub against each other constantly. Due to friction, the rubbing parts of a machine wear out gradually (see Figure). Friction also wears out moving parts like ball bearings of bicycles (and other machines). The worn out or damaged parts of machines have then to be replaced by new ones.
4. Friction Wears Out the Brake Pads of Vehicles Gradually
When the brakes of a vehicle (like car) are applied, a lot of friction is produced between the brake pads and moving part of the wheel (like disc). This friction wears out the brake pads gradually. Due to this, the brake pads of vehicles have to be replaced quite often (otherwise the brakes will not function properly). The brake pad of a bicycle is made of rubber. The new, soft rubber pad of a bicycle is shown in Figure (a). In a few months, the bicycle brake pad gets worn out due to friction between brake pad and moving rim
of bicycle wheel (on applying brakes). The worn out brake pad of the bicycle is shown in Figure (b). The brake pads of a bicycle have to be replaced quite often because they wear away due to friction.
5. Friction Wears Out Steps of Staircases in Buildings and Foot Over-Bridges
When a lot of people use the staircase in a building every day, the friction between soles of their shoes and the stone steps wears away hard stone steps very, very slowly (see Figure). Many times we see worn out steps of foot over-bridges at Railway Stations. The steps of foot over-bridges at Railway Stations (and other crowded places) wear out due to the friction caused by the shoes of extremely large number of people who use these over-bridges all the time.
6. Friction Produces Heat Which May Damage Machines
If we rub our hands together quickly for a few seconds, they feel warm. This is because friction between the hands produces heat (which makes them feel warm). Similarly, when we operate a mixer and grinder for a few minutes, its jar becomes hot. Here again heat is produced by friction. When the moving parts of a machine rub together, a lot of heat is produced due to friction between them. This heat may damage the machine gradually.
7. Friction Reduces the Efficiency of Machines
Some of the energy supplied to run a machine is wasted in overcoming friction between its moving parts and some of the energy is wasted in heat generated by the machine. This wastage of energy reduces the efficiency of a machine.
8. Friction Slows Down Motion
Friction reduces the motion of moving parts of a machine. In fact, all the moving things (such as cars, buses, aeroplanes, boats and ships, etc.) are slowed down by friction.