Class 8 Science NCERT Solutions for Chapter – 12 Friction

Friction

Question 1.
Fill in the blanks.

(a) Friction opposes the _______ between the surfaces in contact with each other.
Answer:
Relative motion

(b) Friction depends on the _______ of surfaces.
Answer:
Nature

(c) Friction produces _______.
Answer:
Heat

(d) Sprinkling of powder on the carrom board _______ friction.
Answer:
Reduces

(e) Sliding friction is _______ than the static friction.
Answer:
less

Question 2.
Four children were asked to arrange forces due to rolling, static, and sliding frictions in 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

Answer:
(c) Static, sliding, rolling

Question 3.
Alida. runs her toy car on the 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, towel, newspaper.

Answer:
(a) wet marble floor, dry marble floor, newspaper, 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.

Answer:
The frictional force will act parallel to the inclined surface, opposite to the direction of the sliding of the book.
NCERT Solutions for Class 8 Science img 31

Question 5.
You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?

Answer:
Soapy water reduces the frictional force applied by the floor to a great extent. That is why it is difficult to walk on a soapy floor.

Question 6.
Explain why sportsmen use shoes with spikes.

Answer:
Sportsmen use shoes with spikes because spikes produce the desired frictional force and thus help in holding the ground firmly.

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?

Answer:
Seema will have to apply a/larger force because her box is heavier and heavier objects will have to overcome greater frictional force offered by the surface of contact.

Question 8.
Explain why sliding friction is less than static friction.

Answer:
Friction is caused by the interlocking of irregularities in the two surfaces. When the object starts sliding, the contact points on its surface, do not get enough time to lock into the contact points of the other surface. So, the sliding friction is slightly smaller than the static friction.

Question 9.
Give examples to show that friction is both a friend and a foe.

Answer:
Friction is both friend and foe, that is why it is said that friction is a necessary evil.

(1) Friction as a friend :
(a) To hold a glass, we have ridges on our palm, which increase the friction between palm and glass.
(b) We are able to write anything with a pen or pencil because there is friction between the surfaces of paper and the point of a pen (nib) or pencil.
(c) A teacher can write on the blackboard with chalk because of the friction between the black-board surface and the chalk.
(d) If there is no friction, then a moving body would never stop.
(e) A nail could not be fixed in the wall, or a knot could not Be tied, had there been any friction.
(f) No building could be constructed without friction.

(2) Friction as a foe :
(a) Friction wears out materials, whether they are screws, ball-bearings, or soles of shoes.
(b) Friction can also produce heat, which increases the wear and tear of machine parts. It also causes much wastage of energy, because this heat is not utilized.

Question 10.
Explain why objects moving in fluids must have special shapes.

Answer:
Birds and the fishes have to move about in fluids all the time. Thus, their “bodies have evolved to shapes which make them not to lose much energy in overcoming fluid friction. Such shapes are called streamlined. Cars are also designed so that their shapes resemble the streamlined shapes of birds, fishes, etc.

InText Questions and Answers

Question 1.
Have you ever thought why the vehicle slows down when brakes are applied?

Answer:
When brakes are applied, a frictional force acts on the vehicle which slows down the motion of the vehicle and finally brings it to rest.

Question 2.
Have you not seen a ball moving on the ground stop after some time?

Answer:
Yes, I have seen a ball moving on the ground which stopped after some time. The ground applies a force of friction, which slows down the motion of the ball and finally the ball stops.

Question 3.
Why do we slip when we step on a banana peel?

Answer:
Banana peel is very smooth and it does not provide much friction. So, when we step on it, we slip.

Question 4.
Why is it difficult to walk on a smooth and wet floor?

Answer:
A smooth and wet floor does not provide sufficient friction required to walk smoothly. So, it is difficult to walk on Such surfaces.

12.1 Force of Friction Activity 12.1

Question 1.
Does the book stop this time, too?

Answer:
Yes, the book stops this time too.

Question 2.
Can you think of an explanation?

Answer:
The book slides for some time and then stops. The reason is that the surface of the table provides the frictional force to oppose the sliding of the book.

Question 3.
Can we say that a force must be acting on the book opposing its motion?

Answer:
Yes, a force is acting on the book opposing its motion. This force is called the force of friction.

Question 5.
Is the friction the same for all the surfaces? Does it depend on the smoothness of the surface?

Answer:
No, friction is not the same for all the surfaces. Smoother surfaces provide less friction, whereas rough surfaces provide more friction.

12.2 Factors Affecting Friction Activity 12.2

Question 1.
Do you observe any difference in the readings of the spring balance in the above two cases?

Answer:
The reading of spring balance in the second case is less than that in the first case.

Question 2.
What might be the reason for this difference?

Answer:
In the first case, as the brick surface in contact with the ground is very rough, the surface experienced greater frictional force. But, in the second case, when polythene is wrapped around the brick, the surface becomes smoother. So, in this case, frictional force, experienced by the surface of the brick is less.

Question 3.
Repeat this activity by wrapping a piece of jute bag around the brick. What do you observe?

Answer:
The reading of spring balance is more than that of When the brick was wrapped with polythene and also when a brick was dragged uncovered.

Activity 12.3

Question 1.
How far does it move on the table before coming to rest?

Answer:
It moves up to the end of the inclined plane and comes to rest on the table.

Question 4.
In which case the distance covered is the minimum?

Answer:
The distance covered is minimum in the case when a thin layer of sand is spread over the table.

Question 5.
Why is the distance covered by the pencil cell different every time? Try to reason why?

Answer:
It is due to the amount of friction provided by the different surfaces. The distance covered with a thin layer of sand is least because this surface provides the maximum friction.

Question 6.
Does the distance cover depend on the nature of the surface on which the cell moves?

Answer:
Yes, the distance covered by the pencil cell depends on the nature of the surface on which it moves.

Question 7.
Could the smoothness of the surface of the cell also affect the distance travelled by it?

Answer:
Yes, the smoothness of the sliding surface influence the distance travelled.

Question 8.
I shall try the activity by wrapping a piece of sandpaper around the cell.

Answer:
Then the force, of friction will increase.

12.3 Friction: A Necessary Evil

Question 9.
Is it easier to hold a Kuchar (earthen pot) or a glass tumbler?

Answer:
It is easier to hold a. Kuchar than a glass tumbler as glass tumbler surface is smooth and provides lesser friction.

Question 10.
Suppose the outer surface of the tumbler is greasy or has a thin layer of cooking oil on it; would it become easier or more difficult to hold it?

Answer:
It would become more difficult to hold a glass tumbler with a film of cooking oil on its surface because this surface will provide no friction or least friction.

Question 11.
Would it be possible to hold the glass at all if there is no friction?

Answer:
No, not at all.

Question 12.
Can you imagine being able to walk if there were no friction at all?

Answer:
No, anybody cannot think of walking if there were no friction at all.

Question 13.
Could it be possible to write with chalk if they were no friction between the chalk and the board?

Answer:
No, if there were no friction between the chalk and the board, it was not possible to write on the board. ,

Question 14.
Vigorously rub your palms together for a few minutes (Fig. 12.9, Textbook P. 150). How do y6u feel?

Answer:
The palms warm up.

12.4 Increasing and Reducing Friction

Question 15.
Have you ever thought why the sole of your shoes is grooved [Fig? 12.11 (o), Textbook Page 151] ?

Answer:
Grooves are provided in the sole of the shoe to have a better grip on the floor so that one can move safely.

Question 16.
Why do you sprinkle the fine powder on the carrom board?

Answer:
The fine powder is sprinkled to minimise the friction on the surface of the carrom board so that the striker moves faster on it.

12.5 Wheels Reduce Friction Activity 12.4

Question 1.
Do you feel it easier to move the book in this way than to slide it?

Answer:
Yes, the book can move faster in this way than sliding it on the surface.

Question 2.
Do you think that resistance to the motion of the book has been reduced? Have you seen heavy machinery being moved by placing logs under it?

Answer:
Yes, resistance to the motion of the book has been reduced because of rolling. Yes, I have seen heavy machinery being moved by placing logs under which makes it roll and not slide.

Question 17.
Can you now understand why the wheel is said to be one of the greatest inventions of mankind?

Answer:
The wheel provides rolling friction which is smaller than the sliding friction. So, the wheel reduces the costs of conveyance to the least. That is why the wheel is said to be one of the greatest inventions of mankind.

12.6 Fluid Friction

Question 18.
Do you find any similarity in the shape of an aeroplane and of a bird?

Answer:
Yes, both the aeroplane and bird have streamlined bodies to minimise the force of friction provided by air while flying in the sky.

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