KSEEB Solutions for Class 8 Science Chapter 8 Friction

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KSEEB Solutions for Class 8 Science Chapter 8 Friction

Friction Textbook Questions and Answers

Question 1.
Fill in the blanks.

1. Friction opposes the _______ between the surfaces in contact with each other.
2. Friction depends on the _______ of surfaces.
3. Frcition produces _______
4. The sprinkling of powder on the carrom board _______ friction.
5. Sliding friction is _______ than the static friction.

Answer:

1. Friction opposes the relative motion between the surfaces in contact with each other.
2. Friction depends on the nature of surfaces.
3. Friction produces heat.
4. The sprinkling of powder on the carrom board reduces friction.
5. Sliding friction is less than static friction.

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
(а) 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, 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.
Answer:

Fig. Frictional force is acting opposite to the movement of book i.e. upwards.

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 floor reduces the frictional force applied by the floor to the maximum. 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 a spike to increase the friction so, that their shoes do not slip while they run or play.

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 experience more frictional force because the heavy object will be pressed hard against the opposite surface and produces more friction.

Question 8.
Explain why sliding friction is less than static friction.
Answer:
Suppose a box kept on the surface has to be pushed. If the box is slid then, when the box starts sliding, the contact points on its surface do not get enough time to lock into the contact points on the floor. So the sliding friction is slightly less than the static friction and we find it easier to move the box already in motion than to get it started (i.e., static friction.)

Question 9.
Give examples to show that friction is both a friend and a foe.
Answer:
Friction is a friend because:

• It allows us to walk comfortably on the ground.
• It allows us to grip and catch different objects.
• Things don’t move from their places because of friction.
• Speeds of moving objects can be minimized or stopped by friction only.

Friction is a foe because:

• It wears out materials, whether they are screws, ball- bearings, or soles of shoes.
• It also produces 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:
When objects move through fluids, they have to overcome friction acting on them. In this process. They lose energy. To minimize friction, objects are given special shapes.

Friction Additional Questions and Answers

Question 1.
Why the vehicle slows down when brakes are applied?
Answer:
When brakes are applied frictional force acts on the vehicle which slows down the motion of the vehicle and finally brings it to rest.

Question 2.
Why does a ball moving on the ground stopping after some time?
Answer:
A ball moving on the ground stops after some time because the ground applies a force of friction, which slows down the motion of the ball, and finally the ball stops.

Question 3.
What is a force of friction?
Answer:
Force of friction opposes the relative motion between two surfaces in contact and it acts on both surfaces.

Question 4.
What is spring balance?
Answer:
Spring balance is a device used for measuring the force acting on an object.

Question 5.
How is the force measured by spring balance?
Answer:
Spring balance consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.

Question 6.
Is it easier to hold a kulhar (earthern pot) or a glass tumbler?
Answer:
It is easier to hold a kulhar than a glass tumbler as the glass tumbler surface is smooth and provides lesser friction.

Question 7.
Suppose the outer surface of the tumbler is greasy or has a film 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 8.
How does friction cause problems for us?
or
Why is friction called evil?
Answer:
Friction is evil. It causes a lot of problems for us. It wears out the objects or materials on which it acts. The soles of our shoes get worn due to friction. The floors, machines, their parts, clothes, metals, etc. get worn due to the friction they bear. Friction spoils the parts of the machines. They are, therefore, regularly lubricated to minimize friction. Friction produces heat, which further damages the objects on which it is exerted, The grooves of the tires are finished due to regular friction they bear, which is offered by their regular motion on roads. Except for this wear and tear, friction utilize a large amount of energy spent to overcome it.

Question 9.
Why the sole of the shoe is grooved?
Answer:
The sole of the shoe is grooved to provide the shoes better grip on the floor so that we can move safely.

Question 10.
What is rolling friction?
Answer:
When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.

Question 11.
Rolling increases or reduces friction?
Answer:
Rolling reduces friction.

Question 12.
Why the wheel is said to be one of the greatest inventions of mankind?
Answer:
Wheels have reduced friction and it becomes easier to take things from one place to another.

Question 13.
What is drag?
Answer:
The frictional force exerted by fluids is called drag.

Question 14.
Does air have friction?
Answer:
Yes, the air has friction.

Question 15.
Do you find any similarity in the shape of a bird and airplane?
Answer:
Yes, both the airplane and bird have streamlined bodies to minimize the force of friction provided by air while flying in the sky.

Activities

Activity 1.
Gently push a book on a table [Fig. (a)]. You observe that it stops after moving for some distance. Repeat this activity pushing the book from the opposite direction [Fig., (b)].

Question (i).
Does the book stop this time, too?
Answer:
Yes, the book stops this time too.

Question (ii).
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 (iii).
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 is a force of friction.

Activity 2.
Tie a string around a brick. Pull the brick by a spring balance (Fig.). 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 beteen the surface of the brick and the floor.

Question (i).
Now wrap a piece of polythene around the brick and repeat the activity. 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 first case is more than that in the second case.

Question (ii).
What might be the reason for this difference?
Answer:
In the first case, the brick surface in contact with the ground is very rough and therefore surface has more frictional force. In the second case, after wrapping polythene around the brick, the surface becomes smoother, and hence frictional force becomes less.

Question (iii).
Repeat this activity by wrapping a piece of jute bag around the brick. What do you observe?
Answer:
We observe that the reading of spring balance is more than that of when a brick was wrapped with polythene.

Activity 3.
Make an inclined plane on a smooth floor, or on a table. You may use a wooden board supported by bricks, or books. [Fig. (a)]. Put a mark with a pen at any point A on the inclined plane. Now let a pencil cell move down from this point. How far does it move on the table before coming to rest? Note down the distance. No spread a piece of cloth over the table. Make sure that there are no wrinkles in the cloth. Try the activity again [Fig.(b)].

Repeat this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity.

Question (i).
In which case is the distance covered by the pencil cell minimum?
Answer:
The distance covered by the pencil is minimum in the case when a thin layer of sand is spread over the table.

Question (ii).
Why is the distance covered by the pencil cell different every time? Try to reason why?
Answer:
The distance covered by the pencil cell is different every time because the amount of friction provided by different surfaces is different.

Question (iii).
Does the distance cover depend on the nature of the surface on which the cell moves?
Answer:
Yes.

Question (iv).
Could the smoothness of the surface of the cell also affect the distance travelled by it?
Answer:
Yes.

Question (v).
What would happen if this activity is done by wrapping a piece of sandpaper around the cell?
Answer:
The force of friction will increase and hence distance covered by the pencil cell will be lesser.

Activity 4
Take a few pencils which are cylindrical in shape. Place them parallel to each other on a table. Place a thick book over it (Fig.). Now push the book. You observe the pencils rolling as the book moves. Have you seen heavy machinery being moved by placing logs under it?

Question (i).
Do you feel it easier to move the book in this way than to slide it?
Answer:
Yes, it is easier to move the book in this way than to slide it because in this way frictional force is less.

Question (ii).
Do you think that resistance to the motion of the book has been reduced?
Answer:
Yes

Question (iii).
Have you seen machinery being moved by placing logs under it?
Answer:
Yes.