Friction Class 8 Science Chapter 12 as per NCERT Book used in CBSE and other Schools. The lesson covers the complete explanation of class 8 Chapter 12 Friction. Topics covered are friction, causes of friction, factors affecting friction, methods to reduce friction, advantages and disadvantages of friction, fluids and fluid drag. The lesson covers all important questions based on crop production and management. NCERT solutions to book questions have also been provided for convenience of the students.
There might be a number of questions in your mind, as follows –
Question: Have you ever thought that why vehicles slow down when brakes are applied?
Question: Why is it difficult to walk on a smooth and wet floor?
Question: Why do we fall when we step on a banana peel?
You will get answers to all these questions in this chapter on friction.
When we push a box lying on the floor with a small amount of force, it does not move at all. It means that the surface of the floor, on which the box is resting, exerts some forces on the box which act in a direction that is opposite to the force of our push. This natural force between the floor and the bottom of the box which opposes the motion of box on the floor is called friction. The force acting on the two surfaces in contact which opposes the motion of one body over the other is called the force of friction.
Friction Class 8 Video Explanation
Friction
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It is an opposing force that opposes the motion of one body over the surface of another body. Like in case of a ball freely rolling on the ground, the ball eventually comes to stop, because of the force of friction that acts between the ball and the ground. This means that if force acts from right to left, friction acts in left to right direction. Alternatively, if force acts from left to right, friction acts in right to left direction. It comes in notice only when two surfaces are in contact with each other it is called Contact force. It is offered by solids, liquids as well as by gases but it is maximum in the case of solids and least in the case of gases.
Causes of Friction
The causes of friction are molecular adhesion, surface roughness and the ploughing effect. Every object has a rough surface, though the surface may appear to be smooth to the naked eye. But when we observe through a microscope, we find that the surfaces of all the objects have rough edges. Some of the particles on the surface of objects are in the form of tiny hills and valleys. When a body is pulled over another, these tiny hills and valleys present on both the surfaces in contact get entangled with one another.
Like the surface of solids, etc. that appears to be smoother are not actually smooth. They possess some irregularities like ridges & grooves. When these types of surfaces come in contact with other surface, their ridges and grooves get interlocked & opposing force, that is Friction comes into notice which opposes the motion.
Factors affecting friction are as follows-
1. Nature of surface: If there is more roughness, more is the friction. if the surface of an object is smooth, less friction is produced.
For example, a ball rolls slowly and covers a lesser distance on a rough surface such as muddy playground. And obviously it means that to get the ball rolling, we have to apply a force which is greater than the frictional force offered by the ground.
This also explains why we cannot hold a glass with oily hands. Our hands are greasy and smooth and we know that a smooth surface offers lesser interaction with the glass. And hence, it tends to slip.
Another example of this is that while trekking on hilly and watery terrains, we wear groovy sports shoes so that rough shoes establish better locking with the hilly trains, increase friction, provide more grip and decrease chances of a slip.
2. Weight (normal force): If the weight of an object is more, then friction is more and if the weight is less, friction is also less.
3. Surface area in contact: The larger the surface area of contact the more the friction. This means that the more the interlocking the more the friction.
Methods to reduce friction are as follows- 
- In order to reduce friction we need to make the bodies streamlined which means the body that is wider in the middle & narrower at the ends.
Converting into wheels: Wheels have rolling friction. Rolling friction is always less due to less surface area.- By polishing surfaces. As we know that roughness is the cause of friction. To reduce roughness, we need to polish the surface.
Converting into wheels: Wheels have rolling friction. Rolling friction is always less due to less surface area.Q: Why is the sole of your shoe grooved?
Answer: The sole of shoes are grooved because grooved shoes provide better grip on the floor so that we can move safely.
Question: Why are brake pads used in brake system of automobiles and cycles?
Answer: Brake pads are used in order to increase friction so that brake can work in an effective way. When we are driving the vehicle, the pads do not touch the wheels but as soon as we apply brake lever, the pads arrest the motion of rim due to friction and wheels stop moving.
Q: Why do we sprinkle powder on carrom board?
Answer: When powder is sprinkled on carrom board, it smoothens the surface and the stagger moves smoothly. Due to smoothness, the force of friction decreases and stagger moves smoothly.
Q: Why is oil poured on the hinges of door?
Answer: Oil is usually poured on the hinges of door in order to reduce friction so that it can open smoothly. It is due to this that the door works properly without producing any sound or hindrance.
Question: Why are oil and grease applied on moving parts of a machine?
Answer: When oil and grease are applied on the moving parts of a machine, it forms a thin layer and the moving surfaces do not directly rub with each other. So, interlocking can be avoided to great extent. That is the reason oil and grease are recommended for machine.
Q: Why do we suddenly slip if we step on a banana peel accidentally?
Answer: A person slips on the banana peel. This happens because a banana peel has a slippery surface and it offers very less friction to the ground. Thus, whenever a banana peel comes under the feet of a person, the person may slip and trip over because of less friction.
Here are a few more examples:
- It is difficult to walk over a muddy surface. This is because muddy surface offers very less friction to the surface of the feet of a person.
- Soles of shoes have grooves on them. Grooved soles offer more friction to the ground which gives better grip while walking. Shoes with worn out soles can be slippery.
- The tyres of vehicles have treads for better grip over the road. When treads are worn out, the tyres need to be replaced with new ones.
- Writing with a pen is possible because paper offers friction to the tip of the pen.
- On oily paper, friction is very less and so, it is difficult to write on such paper.
- While writing with chalk on the blackboard, the rough surface of the blackboard rubs off the chalk because of which some of the particles of chalk get stuck with blackboard and writing become possible.
Advantages and Disadvantages of Friction
Advantages of friction are as follows –
- Friction between pen and paper enables us to write on the paper.
- It enables us to hold pen.
- Friction between our feet and the ground allows our movements like standing, walking and running.
- It holds screw in wood.
- It allows dishes to stand on table & many more.
- Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping
Disadvantages of friction are as follows –
- Friction produces heat, but some of the energy is wasted in overcoming friction.
It causes wear and tear of sole of shoes. For example, the sole of our shoes wears out after prolonged use because of friction with the rough road surface. - The heat product by friction damage rubber parts of machine.
- Friction causes moving objects to stop or slow down.
It causes wear and tear of sole of shoes. For example, Q: It is easy to move an object on a sliding surface than the plane surface. Why?
Answer: The reason is that the resistive force of friction is a function of the normal or perpendicular force of the object on the surface. When the surface is at an angle, that force is reduced according to the angle of inclination.
Q: How does a fish swim easily in water?
Answer: The body of a fish has a shape similar to a boat i.e., streamlined body. Thus, streamlined shapes of fishes offer less resistance against the current of water. It facilitates them to swim easily in water.
Q: Why is friction called ‘a necessary evil’?
Answer: We cannot walk, write and drive without friction. In certain cases like the motion of machines, wear and tear of machines, loss of energy, friction is an ‘evil’.
Q: What is meant by lubrication? Why is it important?
Answer: The use of lubricants like oil and grease helps to reduce friction by forming a thin film between the different parts of a machine. This film covers up the scratches and bumps present on the surfaces of different parts and thus, makes the surface more even than before. This reduces interlocking between the two surfaces, and hence, the parts of the machine run smoothly.
Q: Which is easier to hold in hand: an earthen pot or a glass tumbler. Why?
Answer: An earthen pot is easier to hold in hand because it has a rougher surface.
Q: When we try to push a very heavy box kept on the ground, it does not move at all. Which force is preventing this box to move forward? Where does this force act?
Answer: When we try to push a very heavy box kept on the ground, it does not move at all. This is because the weight of the box and frictional force acting due to roughness between the contact surfaces is preventing it from moving forward.
Fluids
Liquids and gas are called fluids. The substances which have the ability to flow are called fluids. These fluids also exert force of friction on objects in motion through them.
Fluid friction is friction that acts on objects that are moving through a fluid.
Fluid drag
It is defined as the force of friction exerted by fluids.
The frictional force of an object depends upon various factors which are written as follows-
- The speed of the object with respect to the fluid
- The shape of the object
- The nature of fluid
Let us discuss them one by one-
1. The speed of the object with respect to fluid
The more the speed of object, the more the friction. Alternatively, the less the speed of the object, the less the friction.
2. 
The shape of the object
The more is the surface area of object, more is the friction.
3. The nature of the fluid
The more the particles are closer, more is the density. Hence, more is the friction offered. As we know that oil is lighter than water and when we mix both of them in a container, oil floats on the top due to low density and water is at the bottom of the container as it has high density. If an object passes through oil, its density will be different and if an object pass through water, its density will also be different.
The following steps can be taken to reduce fluid drag-
Object are given special shapes and that is they are made streamlined. Streamlined shape is the shape which is narrow from ends and wider from middle.
Here is the most important question based on the above factors:
Q. Explain why objects moving in fluids should have streamlined shape.
Answer: When objects move through fluids, they have to overcome the friction acting on them, due to which they loose energy. So, efforts are made to minimize the friction. For this, the objects are given special shapes. As we know, birds and fishes have to move about in fluids all the time. Their bodies must have evolved to shapes which would make them lose less energy in overcoming friction. Such shapes are called streamlined shapes. This is the reason why ships, boats and aeroplanes have a streamlined shape so that minimum energy is lost while moving through water and air respectively.
Q: Why do birds and fishes have streamlined bodies?
Answer: They have streamlined bodies in order to overcome the friction offered to them by fluids that is air and water. As streamlining reduces the surface area and we know that lesser the surface area, lesser is the friction offered.
Types of friction
There are three types of friction which are written as follows:
- Static friction
- Sliding friction
- Rolling friction
Static friction
It is the friction which comes into notice when one body is just in contact with other body but there is no relative motion.
We can say that it is the opposing force that comes into play when one body tends to move over the surface of another, but the actual motion has not started is called static friction. The magnitude of static friction remains equal to the applied external force and the direction is always opposite to the direction of motion.
Sliding friction
It is the friction offered when one body slides over another body. For example, a wooden block is moved over the flat surface of the table. Static friction is always high in its magnitude.
Rolling friction
It is the friction when one body rolls over the surface of another body. This force slows down the motion of a rolling object. This opposing force comes into play when one body actually rolls over the surface of another body. For example, roller skates have wheels that help in moving very fast.
Both sliding and rolling friction are called dynamic friction. Static friction is more than that of rolling friction and sliding friction. Rolling friction is always less than that of sliding friction. It is very easy and convenient to pull heavy luggage (like heavy suitcase etc.) fitted with rollers.
NCERT Book Solutions
MULTIPLE CHOICE QUESTIONS (MCQS)
1.The frictional force exerted by a fluid is called as?
(a) brag
(b) drag
(c) drab
(d) none
2.Which of the following does not have a streamlined shape?
(a) aeroplane
(b) bird
(c) boat
(d) bus
3. Let us suppose Neha has applied some oil on her hands. Which of the following objects will become the most difficult to hold in her hands?
(a) Earthen cup (Kulhar)
(b) Glass Tumbler
(c) Thermocol tumbler
(d) Wooden cup
4.The use of lubricants makes the surface.
(a) smooth
(b) rough
(c) very rough
(d) none
5. What happens when oiling and greasing between moving parts of a machine?
(a) increases the friction
(b) reduces the friction
(c) changes the sliding friction into rolling friction
(d) changes the static friction into sliding friction
6. Friction can:-
(a) reduce the efficiency of the machine
(b) produce heat
(c) wears off of the machine parts
(d) all of these
7. Friction can be increased by which of the following?
(a) making the surface smooth
(b) using dry surfaces
(c) decrease the weight
(d) All of these
8. The ball travels a longer distance on a
(a) plastic sheet
(b) glass sheet
(c) wood plank
(d) rough surface
9. Friction due to air and water can be reduced by suitable designing of the shape of the object. This is called as?
(a) ball bearing
(b) streamlining
(c) lubricants
(d) friction
10. How friction can be reduced?
(a) polishing
(b) lubricating
(c) streamlining the object
(d) all of the above
11. Why it is very difficult to walk on ice?
(a) friction is low
(b) friction is high
(c) pressure is low
(d) pressure is high
12. Sliding friction is less than:-
(a) static friction
(b) rolling friction
(c) both (a) & (b)
(d) none of these
Answer:
1 (d) none
2 (d) bus
3 (b) glass tumbler
4 (a) smooth
5 (b) reduces the friction
6 (d) all of these
7 (d) all of these
8 (b) glass sheet
9 (b) streamlining
10 (d) all of the above
11 (a) friction is low
12 (a) static friction
Fill in the blanks with suitable words.
1. Friction is caused by the ________ of the two surfaces in contact.
2. The body of a fish or a bird is ________.
3. Use of ________ reduces the friction in machines.
4._________ friction is less than sliding friction.
5._________ includes both liquids and gases.
6.__________ offers the least force of friction.
7. Force of friction depends on the smoothness or_______ of a surface.
8.________ helps to write and light a matchstick.
9. A ________ is a machinery part containing small metal balls.
Answers:
- movement
- streamlined
- lubricants
- rolling
- friction
- smooth surface
- roughness
- friction
- ball bearing
State whether the following statements are True (T) or False (F)-
- The use of ball bearing makes the surface rough.
- Friction is a contact force.
- It is difficult to walk on the road covered with ice.
- Friction produces heat which damages the machines.
- Friction can be reduced by making the surface smooth.
- Friction causes wearing off of the machine parts.
Answer:
- False
- True
- True
- True
- True
- True
Check points:
1. What is friction?
Answer: The force acting on the two surfaces in contact which opposes the motion of one body over the other is called the force of friction.
2. Name the force that causes wear tear of tyres of moving vehicles.
Answer: It is the frictional force that causes wear tear of tyres of moving vehicles
3. What do we use on carrom board to reduce friction?
Answer: We use talcum powder on carrom board to reduce friction.
4. Can we eliminate friction completely?
Answer: No, we cannot eliminate friction completely.
5. What is used to move heavy luggage easily?
Answer: Wheels are used to move heavy luggage easily.
6. Give three examples of streamlined objects.
Answer: The three examples of streamlined objects are Aeroplane, Birds and Racing cars.
7. Which type of friction comes into play when a book kept on cylindrical pencils is moved by pushing?
Answer: Static friction comes into play when a book kept on cylindrical pencils is moved by pushing.
8. Why is it more difficult to walk properly on a well-polished floor?
Answer: It is more difficult to walk on a well – polished floor because a well-polished floor offers less friction. It is difficult to walk without the friction between our shoes and the ground. As we try to step forward, we push our foot backwards. Friction holds our shoe to the ground, allowing us to walk.
9. What does friction do to the soles of our shoes?
Answer: The soles of shoes are worn out due to the effect of friction. The soles of shoes increase the friction between surfaces because it makes the surface of the shoes rough. Thus, this is due to the fact that friction causes wear and tear of objects and reduces the life of the object.
10. What are things like oils, creams and grease called?
Answer: Things like oils, creams and grease are called lubricants.