# DAV Class 8 Science Chapter 5 Question Answer – Friction

The DAV Class 8 Science Book Solutions Pdf and DAV Class 8 Science Chapter 5 Question Answer – Friction are essential study tools for DAV public school students in Class 8.

## DAV Class 8 Science Ch 5 Question Answer – Friction

### DAV Class 8 Science Ch 5 Solutions – Friction

Something To Know

A. Fill in the blanks:

Question 1.
Friction always ________ any relative motion between the two surfaces that are in contact with each other.
opposes

Question 2.
Static friction is always ________ than sliding friction.
greater

Question 3.
Sportsmen use shoes, fitted with spikes, to ________ friction between their shoes and the ground.
increase

Question 4.
Sprinkling of talcum powder, on the carom-board , helps to ________ friction.
decrease

Question 5.
Frictional force, on an object moving in a fluid, depends on its ________
viscosity

Question 6.
The shape of an aeroplane is ________ to reduce, the effects of friction, due to air.
streamlined

B. State True or False for the following statements:

1. When a body slides over smooth and wet surfaces, the amount of interlocking of irregularities, of the two surfaces in contact, increases.
Flase

2. If a car moves eastwards, the force of friction acts southwards.
Flase

3. Friction depends on how hard the two surfaces press against each other.
True

4. The sole of shoes and the tyres of the vehicles are ‘treaded’ to decrease the effects of friction.
Flase

5. Lubricants can be liquids, semi-liquids or solids.
True

6. Friction can be increased by using ball bearings between the moving parts of machines.
Flase

7. Fluid friction can be minimized by giving suitable shapes to the objects moving through the fluid.
True

C. Tick (✓) the correct option:

Question 1.
Suppose your writing desk is tilted to a position where a book kept on it just starts sliding down. The figure, showing the correct direction of friction force acting on it is-

Question 2.
Two boys are applying oppositely directed, and equal in magnitude, forces on a box as shown in the figure.

In such a case, the force of friction, that would exist between the lower end of the box and the ground, would be called-
(a) Sliding friction
(b) Fluid friction
(c) Static friction
(d) rolling friction
Static friction

Question 3.
Four students were asked to arrange the (frictional) forces, due to rolling, static and sliding friction, in an increasing order. Their arrangements are listed below. The correct arrangement is-
(a) Rolling, static, sliding
(b) Rolling, sliding, static
(c) Static, sliding, rolling
(d) Static rolling, sliding
(b) Rolling, sliding, static

Question 4.
The energy, ‘used up’ in overcoming friction, gets converted mainly into-
(a) Sound energy
(b) Heat energy
(c) Light energy
(d) Chemical energy
(b) Heat energy

Question 5.
Rahul rolls a ball on a wooden surface. The ball covers a certain distance before coming to rest. To make the same ball cover a (much) longer distance, before coming to rest, Rahul should-
(a) Spread a newspaper on the wooden surface
(b) Spread a towel on the wooden surface
(c) Sprinkle talcum powder on the wooden surface
(d) Spread a jute bag on the wooden surface
(c) Sprinkle talcum powder on the wooden surface

Question 6.
A block is sliding on a horizontal surface. The force of friction between the two can be increased by-
(a) Decreasing the area of contact of the block with the surface.
(b) applying a layer of some lubricant on the surface.
(c) By polishing the surface.
(d) By putting a second identical block on top of the given block.
(d) By putting a second identical block on top of the given block.

D. Answer the following questions in brief:

Question 1.
Define the terms (a) static friction (b) sliding friction.
When a force is applied on an object to move it over a surface, the counter force which opposes the applied force to prevent the motion is called static friction. When one surface is moving over another surface, the frictional force between them is called sliding friction.

Question 2.
State the meanings of the terms (a) rolling friction (b) fluid friction.
When an object is rolling over a surface, the force of friction opposing this motion is called rolling friction.
When an object is moving through a fluid, the frictional force due to the fluid is called fluid friction.

Question 3.
State the likely cause of friction.
The causes of friction are:

• Nature of the two surfaces in contact
• Force pressing the two surfaces towards each other

Question 4.
A box is resting on the floor. To move it, a (variable) force is applied as shown in the diagram below.

(a) Give the (likely) value of the force of limiting friction.
2F

(b) Why does the box stay at rest in the cases corresponding to Fig (a) and Fig (b)?
In these situations, static friction is equal to or greater than the applied force. Hence, the box stays at rest in these cases.

(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.
In all the cases, direction of applied force is towards right. Hence, the direction of frictional force is opposite to this, i.e. towards left.

Question 5.
Name the kind of frictional force, that comes into play, when a book, kept on a collection of cylindrical pencils, is moved by pushing it.
Rolling friction

Question 6.
Why are objects given special shapes when they are moving through fluids? Write the name given to these special shapes.
A streamlined shape helps in overcoming the fluid friction. Hence, object that are moving through fluids are given special shapes.

Question 1.
Define force of friction. List the factors affecting the force of friction. Explain with examples.
When two surfaces are in contact and one of them is moving in relation to the other, a counter force opposes this motion. This force is called force of friction. Following factors affect the force of friction:

• Nature of the two surfaces in contact
• Force pressing the two surfaces towards each other

It is difficult to walk over a path which is covered with pebbles. But it is easier to walk over a smooth road. This happens because rough surface gives more friction than a smooth surface. It is easier to slide a light box compared to a heavier box. This shows that the force pressing the two surfaces towards each other affects the friction.

Question 2.
A ball is moved to roll down an inclined wooden plane from a given height. At the foot of the inclined plane, it moves on a horizontal surface, differently ‘covered’, one by one, as shown in the following figures.

In which case, is the ball likely to move the longest distance? Give reason for your answer.
The ball is likely to move the longest distance in case ‘C” because of the plane glass sheet. A glass sheet is highly smooth compared to a woollen cloth or a newspaper. So, the glass sheet would offer the least amount of friction.

Question 3.
Explain why sliding friction is slightly less than static friction.
Once an object begins to move over a surface, the interlocking between the two surfaces gets removed. Due to this, sliding friction is slightly less than static friction.

Question 4.
Give reasons for the following:
(a) We tend to slip when we step on a banana peel.
A banana peel provides very low amount of friction. Due to this, it can easily move over a surface. Hence, we tend to slip when we step on a banana peel.

(b) It is easier to push a lighter box than a similar heavy box on the same floor.
A lighter box results in less amount of force that is pressing the box against a surface. On the other hand, a heavier box results in more amount of force pressing the box against a surface. So, it is easier to push a lighter box than a similar heavy box on the same floor.

(c) The force, needed to start a car, is (somewhat) greater than the force needed to keep it moving with a uniform speed.
When a car is static and is about to move, static friction comes into play. When a car is moving, sliding friction comes into play. We know that static friction is more than sliding friction. Due to this, the force needed to start a car is somewhat greater than the force needed to keep it moving with a uniform speed.

(d) Sportsmen use shoes fitted with spikes.
Spikes help in increasing the friction by creating more interlocking with the surface. This helps a sportsman to get better grip while running. Hence, sportsmen use shoes fitted with spikes.

(e) Ball bearings are used in machines.
Ball bearings change sliding friction into rolling friction. Rolling friction is less than sliding friction. Hence, ball bearings are used in machines to reduce the amount of friction.

(f) Tyres of the vehicles need to be changed regularly.
Tyres of vehicles wear off in the long run due to friction. Thus, tyres’ surface becomes smooth after prolonged use; which is not good for driving. Hence, tyres of the vehicles need to be changed regularly.

(g) Machine parts are frequently oiled or greased.
Oil or grease helps in reducing friction. This helps in smooth running of the machine parts. This also prevents wear and tear of machine parts. Hence, machine parts are frequently oiled or greased.

(h) Sports cars, aeroplanes and boats are designed to have a streamlined shape.
Streamlined shape helps in reducing fluid friction. This helps in increasing the performance of sports cars, aeroplanes and boats. Hence, sports cars, aeroplanes and boats are designed to have a streamlined shape.

(i) Metal chains are wrapped on tyres when they run on icy roads.
Icy surface offers least amount of friction. It is very difficult to drive over an icy surface. Metal chains are wrapped on tyres to increase friction. This helps in better control on vehicle while driving on icy roads.

(j) Rollers are used for transporting luggage.
We know that rolling friction is less than sliding friction. Rollers help in converting sliding friction into rolling friction. This makes it easier to move heavy luggage. Hence, rollers are used for transporting luggage.

Question 5.
State and explain, with examples, how friction can help the cause of motion in certain situations.
Walking on a frictionless surface is almost impossible. Road’s surface is always made rough so that walking or driving becomes easier. Writing on a smooth and shiny surface is very difficult. But writing on a rough surface is quite easy. These examples show that friction is the cause of motion in certain situations.

Value Based Question

Ramit and Kush, both students of Class VIII, would often strongly argue with each other about the different aspects of their day to day school life. However, they made a very good ‘double team’, of their school, for the interschool badminton tournaments. When playing together, they would forget all their differences and play as a ‘team’ to ensure their school’s victory.
Their science teacher, who was also their badminton coach, would quote their example. He would explain to his students how the force of friction can be a ‘good friend’ and ‘a source of help’ even though it was usually viewed only as a ‘trouble maker’.

Question 1.
State the values displayed by Ramit and Kush.
Team work, and overcoming difference of opinions.

Question 2.
Give two examples of situations in which the force of friction is a ‘source of help’.
Opening a bottle of pickle can be difficult because of the layer of oil on bottle cap. But holding the cap with a rough towel helps in opening the bottle. The towel helps in increasing the friction, and thus helps in opening the bottle cap.

Treads of tyres of a tractor are very large. A tractor needs to traverse through muddy fields where lot of friction is required for smooth movement of tractor. This example shows that a friction can be a source of help.

Question 3.
Stage a play in which different members of two teams give examples to show that the force of friction is (i) a friend (ii) a foe.
Do it yourself.

Something To Do

Question 1.
Imagine that friction force vanish suddenly. Write a short story/play on how would our lives be affected.
Do it yourself

Question 2.
List some sports/games where friction is a ‘help’ or ‘a source of trouble’. Discuss your list with your friends with some supporting picture.
Do it yourself

Question 3.
In the Activity 1 of this chapter, what is likely to happen if you change the angle which the tray makes with the table top? Does the angle, made by the inclined plane with respect to horizontal, affect the sliding? Discuss your findings with your teacher. Does the angle (at which sliding just starts) depend on the nature of the two surfaces in contact? Find the answer using different materials/toy cars and so on.
Do it yourself

### DAV Class 8 Science Chapter 5 Solutions – Friction Important Questions and Answers

I. Fill in the blanks:

1. Friction works in ________ direction of motion.
opposite

2. In case of ________ friction, the two surfaces are not in relative motion.
static

3. In case of ________ friction, the two surfaces are in relative motion.
sliding

4. The motion of a wheel has to encounter ________ friction.
rolling

5. A fish has ________ body shape.
streamlined

II. Tick (✓) the correct answer:

Question 1.
Which of these surfaces would offer the least friction?
(a) Sand paper
(b) Velvet paper
(c) Wax paper
(d) Normal paper
(c) Wax paper

Question 2.
Which of these vehicles runs on an air cushion?
(a) Metro rail
(b) Hovercraft
(c) Helicopter
(d) Aeroplane
(b) Hovercraft

Question 3.
Which of these can be a good lubricant?
(a) Oil
(b) Grease
(c) Air cushion
(d) All of these
(d) All of these

Question 4.
Which of the following surfaces is safe to walk on?
(a) Marble floor
(b) Paved footpath
(c) Ice skating ring
(d) Wet floor
(b) Paved footpath

Question 1.
Name the factors which affect friction.
Factors affecting friction are:

• Nature of two surfaces in contact.
• Forces pressing the two surfaces together.

Question 2.
What is rolling friction?
When a body rolls over a surface, the force of friction which comes into play is called rolling friction.

Question 3.
How can we say that friction is a necessaiy evil?
Friction leads to wear and tear of many things and decreases efficiency. So, friction is an evil. Due to this, friction is called a necessary evil.

Question 4.
What is fluid friction?
When an object moves through a fluid (liquid or gas), the friction offered by the fluid is called fluid friction. Fluid friction is also known as drag.

Question 5.
What do you understand by streamlined bodies?