friction workbook class 8

1. NCERT Practice Workbook (Science – VIII) 135
Observation : When we bring the rubbed end of straw to near the
suspended straw, an attraction between the straws comes into play.
A repulsion between both straw comes into play when we bring the
rubbed end of straw to near the rubbed end of the suspended straw.
Conclusion and Explanation : This is due to a force called
electrostatic force. Which results due to repulsion between similar
charges or attraction between opposite charges. In this activity, straw
is said to acquire electrostatic charge after it has been rubbed with
sheet of paper. Here, straw is an example of charged body.
Investigation – 2
Aim : To show that liquid exerts equal pressure at the same depth.
Requirements : Discarded soft drink bottle, drill machine, water.
Procedure : Take a discarded soft drink bottle and drill a few equal-
sized holes all around it at the same level. Now, fill the bottle with
water. What do you observe?
Observation : The water rushing out from all the holes falls at the
same distance from the wall of the soft drink bottle.
Conclusion and Explanation : The pressure of a liquid is the same
in all the direction at the same depth.
Friction
12
OBJECTIVE TYPE QUESTIONS
A. Multiple choice questions :
1. The friction that exists between a surface sliding on another
surface, is called the
(a) rolling friction (b) sliding friction (c) static friction
Ans. (b) sliding friction
2. Ball bearings are used to
(a) increase friction (b) decrease friction (c) optimize friction
Ans. (b) decrease friction

2. 136 Solution Book
3. The use of lubricants makes the surface
(a) smooth (b) rough (c) very rough
Ans. (a) smooth
4. The flying plane offering the least frictional force, should be
(a) irregular shape
(b) streamlined shape
(c) symmetrical with many arms
Ans. (b) streamlined shape
5. Friction between the two flat surfaces can be reduced by
(a) painting (b) greasing (c) usingballbearing
Ans. (b) greasing
B. Fill in the blanks with the suitable words given below:
1. Friction is very useful as it helps in the movement of the
bodies.
2. Limiting friction depends upon the nature of surfaces in
contact with each other.
3. Friction always acts in the opposite direction to the direction
of applied force.
4. Static friction is a self adjusting force.
5. The sliding friction is 10 times more than the rolling friction.
6. The boats and aeroplanes are streamlined shape so as to
reduce fluid friction.
C. Match the items in column A with those in column B :
Column A Column B
1. The opposing force called into
play when one body moves on
the surface of another body.
(a) Ball-bearing
2. The friction is called into play
when one body does not move
on the surface of another body after
applying force.
(b) Talcum powder
3. The special shape of the body
due to which it experiences least
fluid friction.
(c) Streamlining

3. NCERT Practice Workbook (Science – VIII) 137
4. A substance which when applied
between two surfaces in contact,
reduces the force of friction
between them.
(d) Static friction
5. A device used for reducing rolling
friction, by making an axle to
move over metal balls placed
around it radially.
(e) Lubricant
6. The substance used as lubricant
on the carrom-board.
(f) Friction
Ans.1. (f) 2. (d) 3. (c) 4. (e) 5. (a) 6. (b)
D. Write ‘True’ or ‘False’ in front of the statements given
below :
1. Lubricants reduce the force of friction between two surfaces
in contact.
2. Sliding friction is another name for dynamic friction.
3. Friction is independent of the nature of surfaces in contact
with one another.
4. The maximum force of friction is called into play when
one body slides over another body that is called sliding
friction.
5. Friction between the two surfaces can be reduced by polishing
the surfaces.
Ans. 1. True 2. True 3. False
4. False 5. True
SUBJECTIVE TYPE QUESTIONS
A. Very short answer type questions :
1. Which opposing force is called into play when one body
moves on the surface of another body?
Ans. Frictional force called into play when one body moves on
the surface of another body.
2. Give the other name for frictional force exerted on fluids.
Ans. The Frictional force exerted by fluids is also called drag.

4. 138 Solution Book
3. What are the substances called that are used to reduce the
friction?
Ans. The substances that are used to reduce the friction are called
lubricants.
4. Name the force required to keep the object moving with the
same speed.
Ans. A body can move with same speed if it is acted upon by a
balanced force.
5. List the types of frictions.
Ans. Static friction, sliding friction, rolling friction
6. What do you sprinkle on the carrom-board to play smoothly?
Ans. We sprinkle fine power on the carrom-board to play smoothly.
7. Name two lubricants which are used in machines.
Ans. Two Lubricants used in machines are oil and grease.
8. What do the kabaddi players rub their hands with, for a better
grip of their opponents?
Ans. Kabaddi players rub their hands with soil for better grip of
their opponents.
9. Which one, is it easier to hold – a kulhar or a glass tumbler?
Ans. A kulhar is easier to hold.
10. In which case the force is greater – the sliding friction or the
static friction?
Ans. In static friction the force is greater.
11. What happens when you rub your palms?
Ans. When you rub your palms together, your palms feel warm.
12. When we use brake pads in the brake system of bicycles,
what happens to the force of friction?
Ans. We use brake system of a bicycle to slow down or stop a
bicycle when brake is applied, work is done by the force of
friction. Which reduces the speed of bicycle.
13. Which type of friction is reduced when we use ball bearings?
Ans. Sliding friction is reduced when we use ball bearings.
B. Short answer type questions :
1. What is friction?
Ans. When a body moves or tries to move over a surface, the
force that resists the motion of the body is called friction or
frictional force.

5. NCERT Practice Workbook (Science – VIII) 139
2. What is static friction? State the laws of static friction.
Ans. The friction acting on an object at rest is called static friction.
Static friction has a limiting value depends on the nature of
surface in contact. It is independent of area of contact as long
as normal reaction remains the same.
3. Why do we use fine talcum powder while playing on carrom-
board?
Ans. While playing on carrom-board, we sprinkle fine power on the
board. The powder makes the surface of the board smooth,
so friction is reduced and the striker moves easily.
4. What is lubricant? How does a lubricant reduce friction?
Ans. The material used to make a surface smooth is called a
lubricant. The most commonly used lubricants are, oil, grease
graphite, boric power etc. When oil or grease is applied
between moving parts, it forms a thin layer, so the surfaces
do not rub directly and friction is reduced.
5. State two advantages and two disadvantages of friction?
(a) Advantages : (i) The friction between paper and pen
enables us to write. Without friction, the pen would keep
slipping. (ii) A matchstick lights up due to friction between
the head of the matchstick and the lighting surface.
(b) Disadvantages : A lot of energy is wasted in overcoming
the force of friction. (ii) The soles of our shoes and tyres
of vehicles wear out due to friction.
6. Why is friction called a necessary evil?
Ans. Friction is wasteful. It causes loss of energy and causes wear
and tear of machines. But on the other hand, our normal daily
life would not be possible if there were no friction. So, we
can say that friction is a necessary evil.
7. Why do new automobile tyres have deep grooves?
Ans. Automobile tyres have deep grooves called ‘treads’. These
treads increases friction between the tyre and the road,
providing a better road grip. This prevents skidding of tyres
on a wet road.
8. Why are the boats and aeroplane given a special shape?
Ans. The bodies of boats and aeroplane are made streamlined to
minimise the force of friction with the medium they travel.

6. 140 Solution Book
9. Why do meteors burn on entering into the atmosphere?
Ans. The friction between earth’s atmosphere and the meteor
generates a lot of heat. This causes the meteor to burn out
in the sky before it can collide with the earth.
10. Why does a ball rolling on the ground slow down?
Ans. A ball rolling on the ground slows down because of the force
of friction acting in the opposite direction.
11. Sliding friction is always less than static friction. Give reason.
Ans. The sliding friction is less than the static friction because
when an object starts sliding the contact points on its surface
do not get enough time to interlock.
C. Long answer type questions :
1. Define the force of friction. Describe all the factors affecting
friction.
Ans. The force acting along the two surfaces in contact which
opposes the motion of one body over the other, is called the
force of friction. Factors affecting friction 1. Nature of surface.
A rough surface has more irregularities than a smooth surface.
This results in greater resistance to the motion, i.e. greater
friction. 2. Mass of a body : A heavier mass will press harder
into the irregularities. So, there will be greater resistance to
motion, that is greater friction.
2. Give the different ways to –
(a) Increase the friction. Friction can be increased by increasing
the roughness of the surface. Tyres of all kinds of vehicles
are provided with deep grooves, so as to increase friction.
The soles of shoes of athletes provided with spikes.
(b) Reduce the friction. Friction can be reduced in various
ways (i) By oiling or lubricating. (ii) Providing wheels or
ball bearings. (iii) Polishing. (iv) Streamlining – Improving
designs.
3. With the help of an activity, explain how wheels reduce friction.
Draw the related figure also.
Ans. A wooden block
with a hook on one
end is pulled by a
spring balance. The

7. NCERT Practice Workbook (Science – VIII) 141
reading of the spring balance noted. Same block is being
pulled with wheels on its lower surface. The reading of the
spring balance is much lower than the previous one.
4. Explain why do the objects moving in air or water have
special shapes. Give suitable examples also.
Ans. When a body moves through air or water (fluid) it experiences
an opposing force which tries to oppose its motion through
the fluid. This opposing frictional force depends on the shape
of the body. By giving object special shape (streamlined
shape) the force of friction acting on it can be minimised. The
streamlined shapes can be seen in boats, ships and aeroplane,
because of which it is easier to move through the fluid.
D. Some other types of questions :
1. If your writing desk is tilted a little, a notebook kept on it
starts sliding down. Show the direction of frictional force
acting on it. Give reason in support of your response. [HOTS]
Ans. When a notebook slides on the writing desk, a frictional force
acts between the book and the surface of
the desk. The direction of friction force
on notebook is opposite to the direction
of its motion and is uphill.
2. Samir has to push a lighter box and Saurabh has to push a
similar heavier box on the same floor. Who will have to apply
a larger force and why? [HOTS]
Ans. 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 surface of
box and floor become strong because two surfaces in contact
are pressed harder. Hence, more force is require to overcome
interlocking. Hence, Saurabh has to apply a greater force than
Samir.
3. One day Aradhya had to take part in a race competition. Rani
gave new spikes-shoes to Aradhya so that Aradhya could
complete her race easily. [Value Based Question]
Answer the following questions based on the above information:
(a) What values were depicted by Rani?
Ans. Concern for others, accept challenges and use knowledge to
overcome weakness.

8. 142 Solution Book
(b) How do spikes-shoes help in running?
Ans. Spikes-shoes provide better grip on the ground. This is
because force of friction between the shoes and the ground
increases with the help of spikes. On the other hand in normal
shoes there are more chances that you will slip because of
less friction between the shoes and the ground. This is how
spikes-shoes help in running.
4. Suppose you spill a bucket of soapy water on a smooth
floor accidently. Would it make easier or more difficult for
you to walk on the floor? Give reason in support of your
response. [HOTS]
Ans. It is difficult to walk on the soapy floor. We are able to walk
because of the friction between our feet and the ground. During
walking, we push the ground in the backward direction, the
force of friction pushes in the forward direction and allows
us to walk. The force of friction between the ground and feet
decreases when these is soapy water spilled on the floor.
5. Put a few pencils parallel to each other on a table. Put a book
on it. Now push the book. You notice that pencils start rolling
as the book moves. You feel it is easier to move the book in
this way than to slide it. Why? Give reason in support of your
answer. [Activity Based Question]
Ans. The book moves easily when placed on a cylindrical pencil
(or rollers). This is because rolling friction is smaller than
the sliding friction.
OLYMPIAD BASED QUESTIONS
1. Which of the following arrangement of frictions is given in
a decreasing order?
(a) Sliding, static, rolling (b) Static, sliding, rolling
(c) Rolling, sliding, static (d) Rolling, static, sliding
Ans. (b) Static, sliding, rolling
2. When the applied force is thrice on an object and the object
is still at rest, then friction becomes
(a) Doubled (b) Halved
(c) remains same (d) Thrice
Ans. (c) remains same

9. NCERT Practice Workbook (Science – VIII) 143
3. Why does a horse need to pull harder during the first few
steps in pulling the cart?
(a) Static friction is greater than sliding friction.
(b) Sliding friction is greater than rolling friction.
(c) No frictional force acts after the cart comes in motion.
(d) Sliding friction is greater than static friction.
Ans. (a) Static friction is greater than sliding friction.
4. If a vehicle going on a horizontal road towards east, the
frictional force on the vehicle by the road
(a) is towards east as the vehicle is accelerating.
(b) is zero as the vehicle is moving with a uniform velocity.
(c) must be towards east.
(d) must be towards west.
Ans. (d) must be towards west.
5. In which of the following cases the friction is helpful?
(a) Use a piece of cloth to open tightly closed lid of a bottle.
(b) Lighting a matchstick
(c) Applying the brakes on a bicycle
(d) All of these.
Ans. (d) All of these.
V. PRACTICAL WORK
Investigation – 1
Aim : Friction opposes relative motion between the two bodies in
contact irrespective of the direction of the motion.
Requirements : A book, a table.
Procedure : Take a smooth table and a book. Place the book upon
the table lengthwise. Now, gently push the book. What do you
observe? You observe that it stops after moving for some distance.
(a) (b)
Repeat this activity by pushing the book from the opposite direction.
What do you observe?

10. 144 Solution Book
Observation : Book stops after moving for some distance.
Conclusion and Explanation : There is an invisible force between
the surface of the book and table which opposes the motion of the
book. This force is called frictional force. If we apply force along
the right direction, the friction force acts along the left direction.
Investigation – 2
Aim : To show that weight of an object increases friction.
Requirements : A spring balance, weights of 50 gf and 100 gf,
wooden block provided with a hook.
Procedure : Place wooden block on a smooth surface.
Fix spring balance to hook and try to slide wooden block by pulling
spring balance. Read and record the force shown by spring balance.
Now place the 50 gf weight on the wooden block and repeat the
same activity. Read and record the force of friction. Repeat the
activity with 100 gf weight (100 gf + 50 gf). Read and record the
force of friction in every case.
Observation : __________________________________________
_______________________________________________________
_______________________________________________________
Conclusion and Explanation : As the weight of wooden block
increases, the magnitude of frictional force increases. Frictional force
is directly proportional to the weight of the body which moves.
Sound
13
OBJECTIVE TYPE QUESTIONS
A. Multiple choice questions :
1. Sound can travel through
(a) gases only
(b) solids only
(c) solids, liquids and gases
Ans. (c) solids, liquids and gases