# NCERT Solutions for Class 8 Science Chapter 12 Friction

These NCERT Solutions for Class 8 Science Chapter 12 Friction Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

## Friction NCERT Solutions for Class 8 Science Chapter 12

### Class 8 Science Chapter 12 Friction Textbook Exercise Questions and Answers

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Question 1.
Fill in the blanks:
a. Friction opposes the ……………. between the surfaces in contact with each other.
b. Friction depends on the ……………. of surfaces.
c. Friction produces …………….
d. Sprinkling of powder on the carrom board ……………. friction.
e. Sliding friction is ……………. than the static friction.
a. motion,
b. nature,
c. heat,
d. reduces,
e. less

Question 2.
Four children were asked to arrange forces due to rolling, static and sliding frictions in a ‘decreasing order’.
Their arrangements are given below. Choose the correct arrangement.
a. rolling, static, sliding
b. rolling, sliding, static
c. static, sliding, rolling
d. sliding, static, rolling
c. static, sliding, rolling

Question 3.
Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in ‘increasing order’ will be:
a. wet marble floor, dry marble floor, newspaper and towel.
b. newspaper, towel, dry marble floor, wet marble floor.
c. towel, newspaper, dry marble floor, wet marble floor.
d. wet marble floor, dry marble floor, tow el, newspaper
a. wet marble floor, dry marble floor, newspaper and towel.

Question 4.
Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
When book slides down the desk, a frictional force acts between the book and the surface of the desk. The direction of the frictional force on the book is opposite to the direction of its motion and acts in an upward direction. It is shown in the diagram below:

Question 5.
You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?
We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction between the ground and feet decreases when there is soapy water spilled on the floor. It acts as a lubricant. Hence, it becomes difficult to walk on the soapy floor.

Question 6.
Explain why sportsmen use shoes with spikes.
Shoes with spikes offer more friction because of more irregularities in the surface. This gives better grip to sportsmen while running.

Question 7.
Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the floor, the interlocking of irregularities on the surfaces of box and floor becomes strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply a greater force than Iqbal.

Question 8.
Explain why the sliding friction is less than the static friction.
When something is static, a greater force is required to break the interlocking between two surfaces. When something is in motion, the surfaces do not get enough time to interlock properly and a smaller force is required to keep the object in motion. Hence, sliding friction is less than static friction.

Question 9.
Give examples to show that friction is both a friend and a foe.
Following two examples show that friction is both a friend and a foe:
Friction as a friend:

• We are able to walk because of friction.
• Friction between the tip of the pen and a paper allows us to write.

Friction as a foe:

• Tyres and soles of shoes wear out because of friction.
• Friction between the different parts of machines produces heat. This can damage the machines.

Question 10.
Explain why objects moving in fluids must have special shapes.
When a body moves through a fluid, it experiences a force which tries to oppose its motion through the fluid. This opposing force is known as drag. This frictional force depends on the shape of the body. By giving the objects a special shape, called the streamlined shape, the force of friction acting on it can be minimised. Hence, it becomes easier for a body to move through the fluid.

NCERT Extended Learning Activities and Projects

Question 1.
What role does friction play in the sport of your choice? Collect some pictures of that sport in action where friction is either supporting it or opposing it. Display these pictures with proper captions on the bulletin board of your classroom.
Hint:
Supporting friction:
a. Cycling: The friction between the tyres and the road is necessary for the cycle to function.
b. Running: The friction between the athlete’s feet and the ground surface is necessary otherwise the runner would fall over.
c. Swimming: The friction in the water is required for the swimmer to be propelled forward.

Opposing friction:
a. Swimming: The friction makes it more difficult for the swimmer to swim very fast.
b. Bobsleigh: The aim is to go as fast as possible but the friction between the bottom of the sleigh and the ice slows it down.
c. Diving: The friction between the diver and the water causes wastage of kinetic energy and sound energy whereas the aim is to be as streamlined as possible.

Question 2.
Imagine that friction suddenly vanishes. How would the life be affected. List ten such situations.
Hint:
If frictional force were absent, we may face the following consequences:

• We cannot walk.
• We will not get a grip to hold things, and then we cannot eat, write, hold a pen or pencil, etc.
• Moving things cannot be stopped.
• Buildings cannot be constructed.
• We cannot fix a nail in the wall.
• We cannot stand properly without a grip.
• We would keep slipping.
• Nothing will be steady on ground. Things will not be at proper places because of absence of grip.
• Brakes in the vehicles will be useless.
• Finally, life would become miserable in the absence of friction.

Question 3.
Visit a shop which sells sports shoes. Observe the soles of shoes meant for various sports. Describe your observations.
Hint:
Sports shoes have rough sole and spikes and the reason behind this is to give a better grip and traction on the grassy field. Also, if it is a rainy day, players (in football) can make better cuts with the cleats.

Question 4.
A toy to play with: Take an empty match box. Take out its tray. Cut a used refill of a ball pen of the same width as the tray as shown in the figure below. Fix the refill with two pins on the top of the tray as shown in figure. Make two holes on the opposite sides of the tray. Make sure that the holes are large enough to allow a thread to pass through them easily. Take a thread about a metre long and pass it through the holes as shown. Fix beads at the two ends of the thread so that it does not come out. Insert the tray in the outer cover of the matchbox.
Suspend the match box by the thread. Leave the thread loose. The match box will start falling down due to gravity. Tighten the thread now and observe what happens.
Explain your observation. Can you relate it to friction?

Hint:
Do it yourself.

Activity 1

Objective: To show that a rough surface exerts greater frictional force than a smooth surface.
Materials Required: A brick, a string, spring balance, polythene, jute bag.
Procedure:

• Tie a string around a brick.
• Pull the brick by a spring balance. You need to apply some force.
• Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor.
• Now wrap a piece of polythene around the brick and repeat the activity and observe.
• Repeat the activity by wrapping a piece of jute bag around the brick and observe again.

Observation: In the first case, as the brick surface in contact with the ground is very rough, the brick experienced greater frictional force. But in the second case, when polythene is wrapped around the brick, the surface becomes smoother. So, in this case, the frictional force is less. The reading of spring balance is more than that of when brick was wrapped with polythene. Jute is also rough and exerts a larger frictional force.

Conclusion: A rough surface exerts a greater frictional force than a smooth surface.

Activity 2

Objective: To study that force of friction depends upon the nature of the two surfaces in contact.
Materials Required: A wooden board, a pencil cell, bricks or books, a piece of cloth and sand.
Procedure:

• Make an inclined plane on a smooth floor, or on a table. (You may use a wooden board supported by bricks or books.)
• Put a mark with a pen at any point A on the inclined plane.
• Now let a pencil cell roll down from this point.
• Observe how far does it move on the plane surface before coming to rest. Note down the distance.
• Now spread a piece of cloth over the table. Make sure that there are no wrinkle in the cloth. Try this activity again.
• Repeat this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity.

Observation: The distance covered is minimum in the case when thin layer of sand is spread over the table. The distance covered is maximum on the plane surface. The distance covered by the pencil cell is different every time because of the difference in the amount of friction provided by the different surfaces.

Conclusion: Friction depends upon the nature of surfaces in contact.

Spring balance: It is a device used for measuring the force acting on an object. It consists of a coiled spring and a pointer moving on a graduated scale. When a force is applied, stretching of spring takes place. The reading on the scale indicated by the pointer gives the magnitude of the force.

Types of Friction:
i. Static Friction: The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object moving is called static friction.
ii. Sliding Friction: Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box than to make a static box move.

iii. Rolling Friction: When an object rolls over a surface, the resistance produced to its motion is called rolling friction. Rolling friction is less than both the static and the sliding friction. It is always easy to roll than to slide a body over a surface.

Friction-A Necessary Evil: Friction is a necessary evil because it has both harmful and beneficial effects.
Benefits of friction:

• We can walk easily because ground offers friction. In less friction, walking becomes almost impossible. Therefore, friction is necessary even for a simple task like walking. It is difficult to move on a wet muddy track or wet marble floor. This is because these surfaces offer very little friction to the surface of the feet of a person.
• Writing with pen is possible because paper provides friction to the tip of pen.
• Writing with chalk on the blackboard is also possible because of friction. The blackboard surface rubs off some chalk particles which stick to it.
• Moving objects can be stopped because of friction.
• Friction between tyres of automobiles and road enables them to be started, stopped or change their direction of motion.
• Construction of building, fixing a nail on the wall, all require friction.

Evils of friction:

• Friction produces heat. Hence, when a machine is operated, heat generated due to friction causes much wastage of energy.
• Friction opposes the relative motion between objects.
• It causes wear and tear of materials whether they are shoes, ball bearings, screws, etc.
• Excessive friction may even result in unwanted fires, such as forest fires.

Ways to Increase Friction:
i. Rubber grips are put on bicycle handles, electrical tools and on many other tools. Increased friction helps in giving better grip on these objects.
ii. A surface can be made rough with the help of sand paper in order to increase friction.
iii. Soles of shoes have grooves on them. Grooved soles give more friction to the ground which gives better grip when we walk. Shoes with worn out soles may be slippery.

iv. The tyres of vehicles have treads for better grip over the road. It provides friction to the surface of tyres. When treads are worn out, the tyres need to be replaced with new ones.
v. Holding a bottle cap with a thick towel helps in increasing friction. Increased friction makes it easier to open the bottle cap.

Ways to Reduce Friction:
i. Lubricating oil and grease are used in machines to reduce friction between the moving parts.

ii. Some machines use air cushion for reducing friction because oil is not ideal to be used in such machines.
iii. Graphite powder is used in some machines to reduce friction.
iv. Ball bearing converts sliding friction into rolling friction. You have read that rolling friction is less than sliding friction. Thus, ball bearings help in reducing friction.
v. Using wheels to drag load also covert sliding friction into rolling friction, there reducing its effect to a great extent.

Fluid Friction: When an object moves through a fluid (liquid or gas), the fluid creates friction. Friction created by a fluid is called fluid friction or drag. Force of friction is directly proportional to the density of a fluid. Hence, friction in oil will be greater than in water. Similarly, friction in water will be greater than that in air.

Streamlined Shape: A shape which is wide in middle and tapered at the ends is called a streamlined shape. Such a shape reduces drag or fluid friction. Hence, a streamlined body can easily move through a fluid. Birds and fishes are naturally endowed with streamlined body. Boats and airplanes are also made streamlined so that they can easily move through air or water.

### Class 8 Science Chapter 12 Friction Additional Important Questions and Answers

Question 1.
What is friction?
The force which opposes the relative movement between two surfaces in contact is called friction.

Question 2.
Give an example to show that friction produces heat.
Rubbing of palms together generates heat and makes us feel warm. This shows that friction produces heat.

Question 3.
Truck tyres are treaded to increase friction so as to provide a better grip on the ground.

Question 4.
Why is sliding replaced by rolling in most of the machines?
Because rolling friction is smaller than sliding friction and facilitates smooth running of the machine.

Question 5.
Why do you think rolling friction is less than sliding friction?
Rolling friction is less than sliding friction because in rolling, the surface area in contact is less than that in the sliding friction. Moreover, while rolling, the surfaces do not rub against each other properly.

Question 6.
Can we reduce friction to zero by using lubricants?
No, it is not possible to eliminate friction entirely. No surface can have zero friction.

Question 7.
How can fluid friction be minimised?
Fluid friction can be minimised by making the shape of the body streamlined.

Question 8.
Why is it difficult to walk on ice?
It is difficult to walk on ice because ice offers very little friction between the feet and the ice surface.

Question 9.
Why does the force of friction exist?
Friction is produced by the interlocking of the irregularities on the two surfaces in contact.

Question 10.
Why do the soles of shoes wear out?
The soles of shoes wear out due to the friction between the soles and the ground.

Question 11.
Why are the soles of shoes grooved?
Soles of shoes are grooved so as to provide better grip between the shoes and the ground.

Question 12.
Give two examples where rolling friction is utilised.
Rolling friction is utilised by using ball bearings between the hub and the axle of a ceiling fan and in the bicycle.

Question 13.
On which factors does the friction depend?
Friction depends on the nature and pressure of surfaces in contact.

Question 14.
What is meant by air resistance?
The friction between a moving object and the air through which it moves is known as air resistance.

Question 15.
What is a streamlined shape?
A shape which is wide in middle and tapered at the ends is called a streamlined shape.

Question 16.
Name the device used for measuring the force acting on an object.
Spring balance.

Question 17.
Define viscous drag.
The opposition offered to the motion of an object by a fluid is called viscous drag.

Question 1.
It is easier to open a bottle cap by gripping it with a towel. Why?
Rough surface of towel helps in increasing friction and thus provides a better grip for application of a longer force. The towel, thus, makes it easier to open the bottle cap.

Question 2.
Why is it difficult to drive on a wet road?
It is very difficult to drive on a wet road because of less friction. A thin film of water is made over the surface of road. This reduces friction, because of which the driver finds it difficult to control the vehicle while driving on a wet road.

Question 3.
Why is friction said to be a necessary evil?
Friction is all pervasive. Even a simple task, like writing on a page, cannot be done without friction. Walking on road will not be possible without friction. But friction causes wear and tear of materials, and wastage of energy. Thus, friction is beneficial as well as harmful. Flence, friction is called a necessary evil.

Question 4.
Boats and aeroplanes are given streamlined shape. Why?
A shape which is wide in middle and tapered at the ends is called a streamlined shape. Such a shape reduces drag or fluid friction. Hence, a streamlined body can easily move through a fluid. Birds and fishes are naturally endowed with streamlined body. Boats and airplanes are also made streamlined so that they can easily move through air or water.

Question 5.
Give two situations where it is desirable to increase friction.
Two situations where it is desirable to increase friction are:
a. Tyres have designs and patterns with grooves on the surface to increase friction with the road. This prevents slipping of the tyres on the wet road.
b. Spikes are provided in the soles of shoes used by players and athletes to increase friction so that they get a firm grip on the ground and do not slip while running.

Question 6.
Advantages of friction are as follows:

• Friction between pen and paper enables us to write on the paper.
• It enables us to hold various things.
• Friction between our feet and the ground allows our movements like walking, jogging, etc.
• It holds the screw in a wood.
• It allows dishes to stand on table.
• Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping.

Question 7.
Disadvantages of friction are as follows:

• Friction opposes motion, so 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 damages rubber parts of machines, and also results in energy loss.
• Friction causes moving objects to stop or slow down.

Question 8.
Why is it easier to tie a jute knot than a silk knot?
The coefficient of friction of jute on jute is much more than that of silk on silk. Hence, two strands of a jute rope can ‘grip’ each other much more strongly than the two strands of a silk rope. This makes it easier to tie a knot in a jute rope as compared to that in a silk rope.

Question 9.
What is spring balance?
It is a device used for measuring the force acting on an object. It contains a coiled spring. The spring gets stretched when a force is applied on it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of force.

Question 10.
How does lubrication reduce friction?
Friction is due to roughness of surfaces. Any process that makes the contact surfaces smooth will reduce friction. Friction can be decreased by adding lubricants to the surfaces like oil, grease or graphite. The sliding surfaces then have a thin layer of the lubricants between them. The friction is then between the surfaces and the lubricant layer which is much less. For example, graphite is used as a lubricant in machines where the moving parts reach very high temperatures.
While playing carrom, fine talcum powder on the carrom board reduces friction.

Question 11.
Why is it not easy to move an object from its static position?
When an object is at rest, it has better hold of the surface on which it is placed. In the static position, the irregularities of the surfaces are interlocked properly, due to which more force is required to overcome friction. Hence, it is not easy to move an object from its static position.

Question 12.
Why is it easier to pull luggage fitted with rollers?
When a body rolls over the surface of another body, the friction is reduced. It is always easier to roll than to slide over another body. That’s why it is easier to pull luggage fitted with rollers.

Question 13.
Aman and Rohan are riding their bicycles on the same concrete road. Aman has new tyres on his bicycle while Rohan has tyres that are old and used. Aman suggested Rohan to change the tyres of his bicycle as soon as possible.
a. Which of the two boys is more likely to slip while moving through a patch of the rod which has lubricating oil spilled over it.
b. Why has Aman advised Rohan to change the tyres of his bicycle?
a. Rohan.
b. This is because the worn out tyres do not offer friction and chances of skidding of vehicle increase.

Question 1.
Discuss the different types of friction.
Following are the three types of friction:
a. Static Friction: The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object move is called static friction.

b. Sliding Friction: Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box compared to pushing a static box.

c. Rolling Friction: When an object rolls over the surface of another object, the friction created is called rolling friction. Rolling friction is less than sliding friction.

Question 2.
Suggest some methods to increase friction.
Some methods to increase friction are as follows:

• Rubber grips are put on bicycle handles, electrical tools and on many other tools. Increased friction helps in giving a better grip on these objects.
• A surface can be made rough with the help of sandpaper, in order to increase friction.
• Soles of shoes are made rough to increase friction. This helps in providing a better grip on the surface while walking.
• Treads of tyres are deliberately made so that a good road grip can be achieved with the help of friction.
• Holding a bottle cap with a thick towel helps in increasing friction. Increased friction makes it easier to open the bottle cap.

Question 3.
Suggest some methods to reduce friction.
Some methods to reduce friction are as follows:

• Lubricating oil and grease are used in machines to reduce friction between the moving parts.
• Some machines use air cushions for reducing friction because use of oil is not ideal in such machines.
• Graphite powder is used in some machines to reduce friction.
• Ball bearing converts sliding friction into rolling friction. Rolling friction is less than sliding friction. Thus, ball bearing helps in reducing friction.

Question 4.
Define fluid. Write the factors on which fluid friction depends.
The gases and liquids are together called fluids. The friction exerted by fluids on an object is called fluid friction. The fluid friction is also called drag. The factors on which fluid friction depends are as follows:

• The fluid friction of an object depends on its speed with respect to the fluid.
• The fluid friction of an object also depends on the shape of the object.
• The fluid friction also depends on the nature and density of the fluid.

Picture-Based Questions

Question 1.
Observe the following figure and answer the questions.

a. What kind of friction do these things encounter.
b. What type of shape do they have?
c. How does this shape help than.
a. Fluid friction.
b. They have a streamlined shape.
c. Streamlined shape reduces fluid friction and makes their movement easy.

Question 2.
Draw a diagram to show the surface irregularities.