Sastry CBSE

Statistics for Economics Class 11 NCERT Solutions Chapter 1 Introduction

Question 1.
Mark the following statements as true or false.
(i) Statistics can only deal with quantitative data.
(ii) Statistics solves economic problems.
(iii) Statistics is of no use to Economics without data.
Answer:
(i) False Statistics deals with both quantitative data as well as with qualitative data. Qualitative data describes the attributes.
(ii) True Economists use Statistics as a tool to understand and evaluate an economic problem by analysing past data. Statistical tools help economists to identify causes of an economic problem and devise policies accordingly.
(iii) True Data is the raw material for economic analysis. Statistical analysis of economic variables cannot be undertaken without having any data.

Question 2.
Make a list of activities that constitute the ordinary business of life. Are these economic activities?
Answer:
The following are the activities that constitute the ordinary business of life

• Buying of goods and services.
• Rendering services to a company by employees and workers.
• Selling of goods and services.
• Production process carried out by a firm.

Yes, the above mentioned activities are regarded as economic activities. This is because, these activities are undertaken for monetary gain and are thus economic activities.

Question 3.
The government and policy makers use statistical data to formulate suitable policies of economic development’. Illustrate with two examples.
Answer:
The statistical data provide the base for the government and the policy makers to formulate policies. The statistical data not only help them to analyse and evaluate the outcomes of the past policies but also assist them to take corrective measures and to formulate new policies. Statistical data also help the government to ascertain the relationship between economic variables and form policies accordingly.

For example, if Indian Government aims at increasing the national output, then it formulates its investment expenditure policy based on the capital output ratio in the past few years. Another example could be the preparation of monetary policy. The previous data of inflation and economic growth are taken into consideration for estimating the money supply required in the next period.

Question 4.
“You have unlimited wants and limited resources to satisfy them.” Explain by giving two examples.
Answer:
The problem of scarcity is the most basic economic problem. Human wants are unlimited and resources to satisfy these wants are limited and these limited resources have alternative uses. ‘Scarcity of resources’ implies that there are unlimited wants to be fulfilled by limited resources which leads to lesser supply of resources as compared to demand for them.

The basic concern of an economy is to allocate the scarce resources to the best possible use in order to satisfy maximum wants. The limited resources have alternative uses which along with problem of scarcity makes it necessary for an economy to make a choice among various alternatives.

For example, an economy endowed with a given level of resources has to make a choice between the production of capital goods and consumer goods. The choice of the economy (i.e., what to produce and in what quantities) depends on the need of the economy. While the production of consumer goods will hamper the capital formation in the country for future production, the production of capital goods will not provide sufficient goods for consumption to the present population.

The same problem of scarcity can be felt at an individual level, e.g., with a given amount of money say, ? 10,000, one cannot buy a refrigerator and a washing machine simultaneously. Thus, the individual needs to make a choice between the alternatives according to his/her priority.

Question 5.
How will you choose the wants to be satisfied?
Answer:
An individual may have unlimited wants but these wants are in an order of priority according to their intensity. The wants of highest intensity will be fulfilled first as they provide the highest satisfaction or utility to the individual and hence, the individual attaches the top most priority to these wants.

Further, the choice of want also depends on the need or priority in the given situation, availability of the goods and services which can satisfy the wants and the purchasing power to realise a particular want. Thus, depending on all these conditions, we can say that an individual having a limited budget will fulfil a particular need that would provide him/her the highest possible satisfaction in the given income and given prices of the goods and services required to satisfy the wants. satisfaction in the given income and given prices of the goods and services required to satisfy the wants.

Question 6.
What are your reasons for studying Economics?
Answer:
Human wants are unlimited and resources to satisfy these wants are limited and these limited resources have alternative uses. The basic concern of economics is to allocate the scarce resources to the best possible use in order to derive maximum benefit from the scarce resources. Due to the scarcity of resources having alternative uses, an economy needs to allocate the scarce resources to the areas with maximum possible and optimum returns. The following are the reasons that make the study of economics important
(i) To Study the Consumer Behaviour The theory of consumer behaviour in Economics deals with the study of the behaviour of the consumers in different types of market situations. This theory helps us understand how a rational consumer makes his/her decisions to get the maximum possible satisfaction in the given income and given prices of the goods and services.

(ii) To Study the Production Theory The theory of production studies the production decisions of the producers in different types of market. The theory explains how a producer takes production decisions related to maximisation of output in given cost or the minimisation of cost for a given level of output. The theory highlights how a producer combines different inputs (given their prices) in order to minimise the cost of production and to maximise the profits.

(iii) To Study the Distribution of Income The study of Economics makes us aware about the distribution of national income. In other words, it tells us how the income arising from the total production in an economy is distributed in the form of wage, rent, interest and profit to different factor owners (like labour, land, capital and entrepreneur).

(iv) To Study the Macroeconomic Problems Faced by an Economy Economics proves to be the most powerful tool to understand and analyse the root cause of basic macroeconomic problems faced by an economy like poverty, unemployment, inflation, recession etc. Economics helps us not only in understanding the interrelationship among these problems but also to take various corrective measures.

Question 7.
Statistical methods are no substitute for common sense. Comment.
Answer:
It is absolutely true that statistical methods are no substitute for common sense. Statistical data should not be believed blindly as they can be misinterpreted or misused. The statistical data may involve personal bias or may be subject to manipulations for one’s own selfish motive.

Statistical data and methods are subject to the errors committed by an investigator while surveying and collecting data. Thus, one should use his/her common sense while working with the statistical methods.

This point can be understand with the help of an example A person who wanted to cross a river with his family but did not know how to swim. He knew the average depth of the river to be 125 cm. His height was 175 cm, that of his wife was 152 cm and his two children measured 120 cm and 90 cm respectively in height.

He calculated the average height of his family and found it to be around 134 cm. He analysed that the average depth of the river was less than the average height of his family and concluded that they all could cross the river safely on foot. This resulted in drowning of his children. This example proves that common sense must supersede statistical methods.

Statistics for Economics Class 11 NCERT Solutions

NCERT Solutions for Class 11 Economics

S Chand Science Class 8 Solutions Chapter 16 Light

Lakhmir Singh Science Class 8 Chapter 16 Very Short Answer Type Questions

Question 1.
What is meant by ‘incident ray’?
Answer:
A ray of light that strikes a surface or falls on a surface before being reflected, transmitted or absorbed is known as incident ray.

Question 2.
What is meant by ‘reflected ray’?
Answer:
The light ray reflected by a surface, when an incident ray strikes the same surface is the reflected ray. The reflected ray corresponds to the incident ray always.

Question 3.
How many reflected rays can there be for a given single incident ray falling on a plane mirror?
Answer:
For a single incident ray falling on a plane mirror, there is always a single reflected ray. The angle of incidence is always equal to the angle of reflection, this is by the law of reflection.

Question 4.
What do you understand by the term ‘point of incidence’?
Answer:
The point of incidence is the point where the incident ray strikes the mirror.

Question 5.
What is ‘normal’ in the reflection of light from a plane mirror?
Answer:
Normal is an imaginary line perpendicular to the surface, in the reflection of light from a plane mirror.

Question 6.
Define the angle of incidence.
Answer:
The angle which incident ray makes with the perpendicular to the surface at the point of incidence.

Question 7.
Define the angle of reflection.
Answer:
The angle between the reflected ray and the line perpendicular to the reflecting surface at the point of incidence is known as the angle of reflection.

Question 8.
A ray of light is incident on a plane mirror at an angle of 30°. What is the angle of reflection?
Answer:
The angle of reflection will be 30 degree. This is by the law of reflection that angle of incidence is equal to the angle of reflection.

Question 9.
An incident ray makes an angle of 75° with the surface of a plane mirror. What will be the angle of reflection?
Answer:
The angle made by the normal will be 90 – 75 = 15. Angle made by normal is equal to angle of incidence. So i = 15 degree. And by the law of reflection which states that angle of incidence is equal to the angle of reflection, the angle of reflection will be 15 degree.

Question 10.
A ray of light is incident normally (perpendicularly) on a plane mirror. Where will this ray of light go after reflection from the mirror?
Answer:
When a ray of light is incident normally (perpendicularly) on a plane mirror, the ray reflects and follows the same path.

Question 11.
What is the angle of incidence when a ray of light is incident normally on a plane mirror?
Answer:
The angle of incidence when a ray of light is incident normally on a plane mirror is zero degree because the ray is incident normally.

Question 12.
What is the angle of reflection when a ray of light is incident normally on a plane mirror?
Answer:
The angle of reflection when a ray of light is incident normally on a plane mirror will be zero degree because the angle of incidence will be zero degree as the ray is incident normally. By law of reflection, angle of incidence = angle of reflection.

Question 13.
What is the angle of incidence of a ray of light if the reflected ray is at an angle of 90° to the incident ray?
Answer:
the angle of incidence = angle of reflection.
According to the question, given i + r = 90 degree
i = r
hence i + i = 90 degree
2i = 90 degree.
I = 45 degree.
Therefore, the angle of incidence is 45 degree.

Question 14.
Name the apparatus which is used to obtain a thin beam of light.
Answer:
The apparatus used to obtain a thin beam of light is known as Ray box. The rays are bright enough to be viewed easily. A ray box is a standard source of light used in optics experiments.

Question 15.
What type of reflection of light takes place from:
(a) a rough surface?
(b) a smooth surface?
Answer:
a) Diffused reflection takes place from a rough surface. The reflected rays are not parallel in diffused reflection.
b) Regular reflection takes place from a smooth surface. The reflected rays are parallel in regular reflection.

Question 16.
Which type of reflection of light, regular reflection or diffuse reflection, leads to the formation of images?
Answer:
Regular reflection, wherein the reflected rays are parallel to each other, lead to the formation of images because all the rays get reflected in a particular direction. Regular reflection always occurs from a smooth surface.
Diffused reflection does not lead to the formation of images because the rays do not reflect in one particular direction.

Question 17.
What type of reflection of light takes place from:
(a) a cinema screen?
(b) a plane mirror?
Answer:
a) Regular reflection occurs from a cinema screen because the surface of the screen is smooth.
b) Regular reflection occurs from a plane mirror because the surface of a plane mirror is smoot/ even.

Question 18.
If an object is placed at a distance of 7.5 cm from a plane mirror, how far would it be from its image?
Answer:
The object would be 15cm far away from its image if it is placed at a distance of 7.5cm from the plane mirror.

The object is at 7.5cm from the mirror, then the image of the object is 7.5cm on the other side of the mirror. Hence, the image is a total of 15cm from object (7.5cm to the mirror + 7.5cm to the image).

Question 19.
Is the image of an object in a plane mirror: virtual or real?
Answer:
The image of an object in a plane mirror is virtual. Virtual image cannot be projected onto the screen because it is not real.

A virtual image is a copy of the object, formed on the plane mirror. The light rays never come from the image. They appear to come from the object.

Question 20.
Name the phenomenon responsible for the following effect:
When we sit in front of a plane mirror and write with our right hand, it appears in the mirror that we are writing with the left hand.
Answer:
the phenomenon is known as Lateral inversion. The effect produced by a plane mirror in reversing the images from left to right and right to left while forming images is lateral inversion.

Question 21.
Name a device which works on the reflection of reflected light.
Answer:
Periscope is one such device which works on the reflection of reflected light.

The light from the object falls on one mirror that is placed at 45o to the object and the light gets reflected. This reflected light in turn falls on another mirror and is again reflected until it reaches the eyepiece.

Question 22.
How are the two plane mirrors in a periscope arranged:
(a) with respect to one another?
(b) with respect to sides of the tube?
Answer:
a) The 2 plane mirrors are parallel to one another.

b) The plane mirrors make an angle of 45 degrees with respect to the sides of the tube.

Question 23.
What will be the number of images formed when an object is placed between two parallel plane mirrors facing each other?
Answer:
Infinite number of images are formed when an object is placed between two parallel plane mirrors facing each other. The object undergoes multiple reflection to produce image of the distant object. The distance between the mirror and the object is inconsiderable in formation of infinite images. No matter how much the distance be, number of images formed will be infinite.

Question 24.
Name an instrument or toy which works by producing multiple reflections from three plane mirrors to form beautiful patterns.
Answer:
Kaleidoscope is the instrument that works by producing multiple reflections. It consists of 3 plain mirrors inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

Question 25.
State one use of kaleidoscope.
Answer:
A Kaleidoscope is a toy mostly used by children for enjoyment. It is made of a tube with plain mirrors and colored glass pieces inside it at one of the end.

Kaleidoscope having object chambers are used for hiding valuables.

Question 26.
Name the device used to split white light into seven colours.
Answer:
A Glass Prism. It is a device used to split white light into seven colors. It has 2 triangular ends and 3 rectangular sides which help in splitting the white ray of light into a band of 7 colors, also known as spectrum.

Question 27.
What happens when a beam of sunlight is passed through a glass prism?
Answer:
When a beam of sunlight is passed through a glass prism, it splits up into a band of 7 colors, also known as a spectrum.

Question 28.
What type of lens (convex or concave) is present in the human eye?
Answer:
The human eye has convex type of lens. Light that hits the lens of the eye is focused by the lens so that it hits the retina. Retina has photoreceptor cells (rods and cones) which get stimulated and sends signals to the brain for viewing.

Question 29.
What is the range of vision of a normal human eye?
Answer:
The range of vision of a normal human eye is from 25cm to infinity. The minimum distance for an object to be viewed clearly for a normal human eye is 25cm and the view may extend to infinity.

Question 30.
Name the point inside the human eye where the image is not visible.
Answer:
Blind spot is the point in human eye where the image is not visible. This is because there are no photoreceptors, i.e., rods and cones present in the optical disk.

Question 31.
Name the phenomenon which enables us to see movies in a cinema hall.
Answer:
The phenomenon which enables us to see movies in a cinema hall is known as Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.

Question 32.
Name an eye ailment (or eye-disease) caused by the deficiency of vitamin A in the diet.
Answer:
Deficiency of Vitamin A in the diet causes night blindness.

Vitamin A helps in transforming the nerve impulses into images in the retina. Deficiency of vitamin A (also known as retinol) causes night blindness. Night blindness is the inability to see properly at night or in poor light.

Question 33.
What is the name of transparent front part of an eye?
Answer:
Cornea is the transparent front part of an eye. It covers the front portion of the eye.

Question 34.
What is the name of a small opening in the iris of an eye?
Answer:
A small opening in the iris of an eye is known as pupil. Pupil controls the amount of light that enters the eye.

Question 35.
Which part of the eye gives it its distinctive color?
Answer:
Iris, the part of our eye, contains pigment and gives the eye its distinctive color. Iris also helps in controlling the size of the pupil by constricting and dilating it, thus reducing and increasing the amount of light entering the eye.

Question 36.
Write the names of the main parts of the human eye.
Answer:
The main parts of the human eye are-

IRIS- The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light reaching the retina.

RETINA- Retina receives the light focused by the lens, the photoreceptors In the retina (rods and cones) convert this light into signals and the signals are then passed onto the brain for viewing.

LENS- Lens focuses the light rays passing through it onto the retina and thus creates a clear image of the objects in sight.

PUPIL- Pupil controls the amount of light that enters the eye. It is controlled by the iris.

CORNEA- Cornea acts as the outermost layer of the eye. It protects the eye with the eyelids.

CILIARY MUSCLES- The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.

OPTIC NERVE- The optic nerve transmits signals from photoreceptors in the retina to the brain through electric impulses

Question 37.
What happens to the size of the pupil of our eye in dim light?
Answer:
In dim light the size of the pupil of our eye becomes larger or dilates to increase the amount of light going inside the eye. This dilation in size is facilitated by the muscles of iris.

Question 38.
What happens to the size of the pupil of our eye in bright light?
Answer:
In bright light the size of the pupil of our eye constricts and becomes small to reduce the amount of light going inside the eye. This reduction in size is facilitated by the muscles of iris.

Question 39.
State whether the following statements are true or false:
(a) The moon is an illuminated object.
(b) Diffuse reflection means the failure of the laws of reflection of light.
(c) In a kaleidoscope, a pattern seen once can never be seen again.
Answer:
(a) True. Moon is a non-luminous object. It does not have light of its own, it reflects the light of sun and it is an illuminated object.

(b) False. Diffuse reflection does not mean the failure of laws of reflection of light. Diffused reflection is caused by the irregularity of a surface.

(c) True. The pattern once seen through the eye hole can never be seen again in a kaleidoscope. The mirrors in the tube and the colored glass pieces continuously make different patterns.

Question 40.
Fill in the following blanks with suitable words:
(a) The angle of ……………. equals the angle of reflection.
(b) A person 1 m in front of a plane mirror seems to be……… m away from his image.
(c) If you touch your………ear with right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with……….
(d) The size of pupil becomes………..when you see in dim light.
(e) Night birds have………cones than rods in their eyes.
(f) The image of an object persists on the retina of an eye for about………second even after the object has disappeared.
(g) If the still pictures of a moving object are flashed on our eyes at a rate faster than…………..pictures per second, the eye perceives the object as moving.
(h) In a movie, the still pictures in proper sequence are projected on the screen usually at the rate of ………..pictures per second.
Answer:
a) Incidence. This is by law of reflection. The angle of incidence equals the angle of reflection.
b) 2m. Image formed by a plane mirror is far behind the mirror as the object is in front of the mirror.
c) Left ear; left hand. Images formed by a plane mirror are opposite in direction.

d) Large. The size of pupil dilates, facilitated by the iris, so that maximum amount of possible light enters the eye and a person views things clearly.

e) Fewer. Night birds have fewer cones and more rods because rods function in less amount of light. Cones function in bright light.

f) 1/16th. The phenomenon is known as Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second even after the object has disappeared.

g) 16. The is due to the phenomenon Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second, hence if 16 frames are flashed in front of the eyes, they are viewed by the retina as an ongoing action

h) 24. The frame rate for motion pictures is 24 pictures per second. Frame rate is the rate at which consecutive images are displayed in a display.

Lakhmir Singh Science Class 8 Chapter 16 Short Answer Type Questions

Question 41.
Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer:
The objects in a dark room cannot be seen because there is no light. We can only see something when a certain amount of light falls on it. The objects outside the room can only be seen if there is light outside the room.

Question 42.
What makes things visible to us? Why cannot we see a book which is placed (a) behind a wooden screen, and (b) in a dark room?
Answer:
Light makes things visible to us. We see an object when light passing through it is reflected.
We cannot see a book which is placed
a- Behind a wooden screen because the wooden screen is opaque so it does not allow light to pass through it.

b- In a dark room because there is no light to be reflected by the book. Thus, due to no reflection, the book is not visible.

Question 43.
We can see the sun because it is glowing. How are we able to see the moon?
Answer:
We are able to see the moon because the light of sun falls on moon and is reflected onto the earth. Hence, we are able to see the moon.

Question 44.
Name the two types of reflection of light. Which type of reflection makes us see an object from all directions?
Answer:
The two types of reflection of light are Regular reflection of light and Diffused reflection of light.

Regular reflection of light makes us see an object from all directions as the reflected rays are all parallel and in a particular direction so the image of the object can be viewed from all directions. In diffused reflection of light the reflected rays are neither parallel nor in a specific direction so the image of the object isn’t visible from any direction.

Question 45.
A wall reflects light and a mirror also reflects light. What difference is there in the way they reflect light?
Answer:
Reflection of light from a wall is diffused reflection because the surface of the wall is uneven so the light is reflected in different directions and also the reflected rays are not parallel

Reflection of light from a mirror is regular reflection because the surface of mirror is an even surface which is polished. The light gets reflected from the mirror surface in one particular direction and the reflected rays are all parallel.

Question 46.
Explain why, a book lying on a table in a room can be seen from all the parts of the room.
Answer:
A book lying on a table in a room can be seen from all the parts of the room because of reflection by the light falling on the book. The light is reflected by the book and it reaches our eye at any part of the room.

Question 47.
What is the full form of i and r? What is the relation between them?
Answer:
i = angle of incidence and r = angle of reflection
By law of reflection, the angle of incidence is equal to the angle of reflection.

Question 48.
You see your image in a plane mirror? State two characteristics of the image so formed.
Answer:
Characteristics of the image formed by a plane mirror are

The images formed by a plane mirror are virtual and erect
Image formed is of the same size as the object and is laterally inverted.
mage is far behind the mirror as the object is in front of the mirror.

Question 49.
What is a periscope? How many mirrors are there in a periscope?
Answer:
A periscope is a device that reflects the reflected ray again. It consists of a tube attached to a set of mirrors. There are 2 mirrors in a periscope.

The light from the object falls on one mirror that is placed at 45o to the object and the light gets reflected. This reflected light in turn falls on another mirror and is again reflected until it reaches the eyepiece.

Question 50.
State the various uses of a periscope.
Answer:

1. A periscope is used to see over, through or around any object.
2. It can be used to see over a wall, to see around a corner.
3. Periscopes are also used in warfare to keep an eye on the enemy.

Question 51.
Explain how, a hair dresser makes you see hair at the back of your head after the hair cut is complete.
Answer:
The phenomenon that applies here is multiple reflection that provides the image of back of the head to be viewed. A person is made to stand in front of a mirror and another mirror is held at the back of the head. The reflection from the mirror at the back is visible in the mirror in front of the person and hence the hair at the back of the head is visible to the person in the front mirror.

Question 52.
How many images of an object will be formed when the object is placed between two plane mirrors which are inclined at the following angles to one another?
(a) 120°
(b) 45°
(c) 180°
(d) 60°
(e) 90°
Answer:
The formula to calculate the no. of images of an object placed between 2 plane mirrors – (360/θ) − 1; where theta is the angle of inclination.

a- 2 (360/120) – 1 = 3 – 1 = 2

b- 7 (360/45) – 1 = 8 – 1 = 7

c- 1 (360/180) – 1 = 2 – 1 = 1

d- 5 (360/60) – 1 = 6 – 1 = 5

e- 3 (360/90) – 1 = 4 – 1 = 3

Question 53.
Two plane mirrors are set at right angles to each other. A coin is placed in-between these two plane mirrors. How many images of the coin will be seen?
Answer:
When 2 plane mirrors are set at right angles to each other and a coin is placed in-between these two plane mirrors, then three images will be formed, see the diagram below:

The formula for calculating the number of images, when two are kept at an angle θ, is given as: (360/θ) − 1
Now,θ = 90 degree.
Thus, the number of images formed will be
= (360/θ) − 1
= (360/90) – 1
= 4 – 1
= 3

Question 54.
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer:
Infinite number of images of the candle will be formed because the mirrors are placed parallel to each other.

Question 55.
Explain why, when an object is placed between two plane mirrors inclined at an angle, then multiple images are formed.
Answer:
Multiple images are formed due to multiple reflection when an object is placed between two plane mirrors at any angle.
N= (360/ θ) – 1
Say, the angle is 120
N = (360/120) – 1
N = 3 – 1
N = 2

Question 56.
How can you show that white light (say, sunlight) consists of seven colors?
Answer:
White light (say, sunlight) consists of seven colors can be shown by passing this light through a prism. The sunlight passes through the prism and splits into a band of 7 colors, also known as the spectrum.

Question 57.
What information do you get about sunlight from the formation of a rainbow?
Answer:
The information that we get about sunlight from the formation is about dispersion of light. The separation of white light into seven different colors is known as dispersion.

Question 58.
What is meant by dispersion of light’? Name a natural phenomenon which is caused by the dispersion of sunlight in the sky.
Answer:
Dispersion of light is the separation of white light or visible light into different colors is known as dispersion.

Formation of rainbow is a natural phenomenon which is caused by the dispersion of sunlight in the sky. The sunlight splits into 7 colors, namely, violet, indigo, blue, green, yellow, orange and red.

Question 59.
How many plane mirror strips are there in a kaleidoscope? How are they arranged?
Answer:
Kaleidoscope consists of 3 plain mirror strips which are arranged inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

Question 60.
How does eye adjust itself to deal with light of varying intensity?
Answer:
The iris adjusts the size of pupil according to the light of varying intensity.

When the amount of light is very high, the iris contracts the pupil and reduces the amount of light entering the eye. Whereas, when the amount of light is less, the iris expands the pupil so that more light can enter the eye and things can be viewed clearly.

Question 61.
Explain why, we cannot see our surroundings clearly when we enter a darkened cinema hall from bright sunshine but our vision improves after some time.
Answer:
In bright sunshine the size of the pupil of the eye is very small so when we enter a darkened cinema hall from bright sunshine very little amount of light enters the eye. The vision improves at some time later because the pupil dilates and more amount of light enters the eye easily.

Question 62.
How does the eye-lens differ from the ordinary convex lens made of glass?
Answer:
The eye lens is made up of living cells and the focal length of eye lens is adjustable. Whereas, an ordinary convex lens is made of glass and its focal length cannot be changed as it is fixed.

Question 63.
Name the part of the eye:
(a) which controls the amount of light entering the eye.
(b) which converges light rays to form the image.
(c) on which image is formed.
(d) which carries the image to brain.
(e) which changes the curvature (or thickness) of eye-lens to focus objects lying at various distances.
Answer:
(a) Iris controls the amount of light entering the eye by constricting and dilating the pupil in bright and dim light respectively.

(b) Lens. The lens may diverge or converge the light rays to form an image.

(c) Retina. An upside-down image is formed on the retina.

(d) Optic nerve carries the image to brain in form of electric impulses.

(e) Ciliary muscle changes the lens shape to focus objects lying at various distances.

Question 64.
Name the cells on the retina of an eye:
(a) which are sensitive to bright light.
(b) which are sensitive to dim light.
(c) which produce sensation of colour.
Answer:
(a) Cones are sensitive to bright light. Cone shaped cells present in the retina are sensitive to bright light.

(b) Rods are sensitive to dim light. Rod shaped cells present in the retina are sensitive to dim light. They are most important for vision in dim light

(c) Cones are the cells of retina that produce color sensation. These photoreceptor cells are responsible for color vision.

Question 65.
What are rods and cones in the retina of an eye?
Answer:
Rods are rod shaped cells present in the retina are sensitive to dim light. They are most important for vision in dim light.

Cones are cone shaped cells present in the retina are sensitive to bright light. These photoreceptor cells are responsible for color vision.

Question 66.
Name any one defect of the eye. How is it corrected?
Answer:
Myopia (nearsightedness) is the defect of vision wherein a person is unable to see the distant objects clearly as the light is unable to focus on the retina and thus distant objects appear to be blurred. The problem of nearsightedness is that the light is focused in front of the retina

Myopia is corrected by wearing glasses or contact lenses. A concave lens(diverging lens) with a minus power is used in the glass, which moves the image back to retina and thus clears the image. The diverging lens will diverge the light before it reaches the retina. The light will then be converged by cornea and lens thus producing an image on the retina.

Question 67.
What is cataract? How can the vision of a person having cataract be restored?
Answer:
Cataract is the clouding of the lens of human eye, that lies behind the iris and pupil. It leaves to a blurred vision or decrease in vision.

The lens that has becomes cloudy is replaced by an artificial lens to restore the normal and clear vision.

Question 68.
What is meant by ‘persistence of vision’?
Answer:
Persistence of Vision- The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.

Question 69.
Explain how you can take care of your eyes.
Answer:
One can take care of eyes by following or practicing some simple steps-
a- Watch television from a distance.

b- If a dust particle or an insect gets into our eye, we should never rub the eye, instead wash the eye with cold water.

c- Do not look at the sun directly as too much of light i.e. bright light may injure the retina.

d- Read from a normal distance of vision.

e- Never read or study in dim light or low light as it causes headaches.

Question 70.
What should we do if something like a dust particle or an insect gets into our eye?
Answer:
If something like a dust particle or an insect gets into our eye, we should never rub the eye, instead wash the eye with cold water. If washing with cold water does not help, we should consult a doctor immediately.

Question 71.
Name any five food items (including two fruits) which are rich in vitamin A.
Answer:
5 food items rich in vitamin A are- fish, sweet potatoes, carrots, banana, pink grapefruit, apricots.

Question 72.
Explain why, too little or too much light, both are bad for eyes.
Answer:
In the presence of little light the iris has to dilate pupil to facilitate entry of more and more light to view things, which leads to strain on the eyes and causes headaches.
In case of too much light, the retina may get injured and damaged thus impairing vision.

Question 73.
Explain why, an owl can see well in the night (but not during the day) whereas an eagle can see well during day (but not at night).
Answer:
An owl can see well in the night but not during the day because owl is a night bird and night birds have fewer cones and more rods in the retina. Since rods function in less amount of light so an owl can see well in the night. To see well during the day, cones are required which are not present in adequate amount in an owl’s retina.

An eagle can see well during the day but not during the night because eagle has more cones and lesser rods in the retina. Cones function in good amount of light so an eagle can see well during the day. To see well during the night, rods are required which are not present in adequate amount in a n eagle’s retina.

Question 74 A.
What is ‘blind spot’ in the eye?
Answer:
Blind spot is the point in human eye where the image is not visible. This is because there are no photoreceptors, i.e., rods and cones present in the optical disk.

Question 74 B.
What is night blindness? What causes night blindness?
Answer:
Night blindness is the inability to see properly at night or in poor light. In scientific terms, it is called as “nyctalopia”.
The following factors cause night blindness:

1. The deficiency of Vitamin A (retinol) in the diet of a person for a considerable time can lead to the disease called night blindness.
2. The disorder in which the rod cells in the retina gradually lose their ability to respond to the light.
3. It can also happen due to cataract, or clouding of the eye’s lens.
4. Usher syndrome can also cause night blindness, which is a genetic condition that affects both hearing and vision.
5. Nearsightedness or blurred vision can also cause night blindness.

Question 75.
What is lateral inversion? Explain with the help of an example.
Answer:
The effect produced by a plane mirror in reversing the images from left to right and right to left while forming images is lateral inversion.
Example:
The images are reversed by the plane mirror.

Lakhmir Singh Science Class 8 Chapter 16 Long Answer Type Questions

Question 76 A.
What is meant by a luminous object? Name two luminous objects.
Answer:
Objects that have their own light i.e., objects that emit light of their own are known as luminous objects. 2 luminous objects are- stars and sun.

The sun and the stars have their own light. A light bulb is also luminous.

Question 76 B.
What is meant by a non-luminous object? Name two non-luminous objects.
Answer:
Non- luminous objects are those that are themselves not capable of producing their own light but can reflect light from another source. 2 non luminous objects are- wood, plastics etc.

Moon also is incapable of producing its own light and it reflects sunlight and hence is non-luminous.

Question 77 A.
What is the difference between regular reflection and diffuse reflection of light? Name one object which can produce regular reflection of light and another which produces diffuse reflection of light.
Answer:

REGULAR REFLECTION OF LIGHT	DIFFUSED REFLECTION OF LIGHT
1. Regular reflection occurs from a polished or even surface. 2. The reflected rays are parallel always. 3. A plane mirror produces regular reflection of light.	1. Diffused reflection occurs from any uneven surface. 2. The reflected rays are not parallel ever. 3. A wall or piece of paper produces diffused reflection of light.

Question 77 B.
Draw diagrams to show regular reflection of light and diffuse reflection of light.
Answer:

Question 77 C.
Which of the following will cause regular reflection of light and which diffuse reflection of light?
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard
(d) Mirror
(e) Paper
(f) Marble floor with water spread over it.
Answer:
a- polished wooden table- regular reflection since the surface is polished (plane)

b-chalk powder-diffused reflection will occur because the surface of chalk powder is uneven

c- cardboard- diffused reflection of light since cardboard has an uneven surface

d- mirror- regular reflection since the surface of a mirror is plane

e- paper- diffused reflection since the surface of paper is uneven

f- marble floor with water spread over it- regular reflection as the floor with water will behave as a plane surface

Question 78 A.
Draw a diagram to show the reflection of light from a plane mirror. Label the following on the diagram:
(a) Plane mirror
(b) Incident ray
(c) Reflected ray
(d) Point of incidence
(g) Angle of reflection
(e) Normal
(f) Angle of incidence
Answer:

Question 78 B.
State the laws of reflection of light.
Answer:
The laws of reflection of light are

1. The incident ray, the normal ray and the reflected ray all lie in the same plane, and
2. The angle of incidence is always equal to angle of reflection.

Question 79 A.
Draw a labelled diagram showing how a plane mirror forms an image of a point object placed in front of it.
Answer:

Question 79 B.
State the characteristics of the image formed in a plane mirror.
Answer:
Characteristics of the image formed by a plane mirror are-

1. The images formed by a plane mirror are virtual and erect
2. Image formed is of the same size as the object.
3. Image is laterally inverted.
4. Image is far behind the mirror as the object is in front of the mirror.

Question 80 A.
Draw a labelled diagram of the human eye. Label the following parts on this diagram: Cornea, Iris, Pupil, Ciliary muscles, Eye-lens, Retina, Optic nerve, Blind spot.
Answer:

Question 80 B.
What are the functions of the following parts of the eye?
(a) Iris
(b) Eye-lens
(c) Ciliary muscles
(d) Retina
(e) Optic nerve
Answer:
Functions of parts-
IRIS- The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light reaching the retina.

EYE-LENS- Lens focuses the light rays passing through it onto the retina and thus creates a clear image of the objects in sight.

CILIARY MUSCLES- The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.

RETINA- Retina receives the light focused by the lens, the photoreceptors In the retina (rods and cones) convert this light into signals and the signals are then passed onto the brain for viewing.

OPTIC NERVE- The optic nerve transmits signals from photoreceptors in the retina to the brain through electric impulses.

Lakhmir Singh Science Class 8 Chapter 16 Multiple Choice Questions (MCQs)

Question 81.
The angle of reflection is equal to the angle of incidence:
A. always
B. sometimes
C. under special conditions
D. never
Answer:
A. always
According to laws of reflection, angle of reflection is always equal to the angle of incidence. The law of reflection also states that the incident ray, reflected ray and the normal to a surface, all lie in the same plane.

Question 82.
The image formed by a plane mirror is:
A. virtual, behind the mirror and enlarged.
B. virtual, behind the mirror and of the same size as the object.
C. real, at the surface of the mirror and enlarged.
D. real, behind the mirror and of the same size as the object.
Answer:
B. virtual, behind the mirror and of the same size as the object.
The images formed by a plane mirror are virtual, opposite in direction and are of the same size as the object.

Question 83.
The least distance of distinct vision for a young adult with normal vision is about:
A. 25 m
B. 2.5 cm
C. 25 cm
D. 2.5 m
Answer:
C. 25 cm
The least distance of distinct vision means the minimum distance for an object to be viewed clearly. This minimum distance for a young adult with normal vision is 25cm.

Question 84.
The angle between an incident ray and the plane mirror is 30°. The total angle between the incident ray and the reflected ray will be:
A. 30°
B. 60°
C. 90°
D. 120°
Answer:
A. 30°
The angle between the incident ray and the mirror is 30 degrees.
Angle of incidence = angle of reflection.
90 degrees is going directly into the mirror.
So, 90 – 30 = 60 degrees.
The total angle between the incident ray and the reflected ray will be 120 degree.

Question 85.
The image of an object formed by a plane mirror is:
A. virtual
B. real
C. diminished
D. upside-down
Answer:
A. virtual
The image of an object formed by a plane mirror is virtual always suggesting that the light rays do not actually come from the image.

Question 86.
Which of the following is a non-luminous object?
A. sun
B. star
C. moon
D. fire
Answer:
C. moon
Non- luminous objects are those that are themselves not capable of producing their own light but can reflect light from another source. Moon reflects sunlight and hence is non-luminous.

Question 87.
A device which works on the reflection of light from two plane mirrors arranged parallel to one another is:
A. electroscope
B. kaleiodoscope
C. periscope
D. stethoscope
Answer:
C. periscope
Periscopes are used to see objects that are not in direct line of sight.

Question 88.
The number of images formed of an object placed between two plane mirrors inclined at right angles to each other is:
A. two
B. five
C. one
D. three
Answer:
D. three
The relationship between no. of images and angle of mirror is
Images = (360degrees/ angle between the mirrors) – 1
Images = (360 degree/90 degree) – 1
Images = 4 – 1
Images = 3

Question 89.
As the angle between two plane mirrors is decreased gradually, the number of images of an object placed between them:
A. increases gradually
B. decreases gradually
C. first increases then decreases
D. first decreases then increases
Answer:
A. increases gradually
If the angle between 2 plane mirrors is decreased, the number of images of an object placed in between will increase gradually.
The relationship between no. of images and angle of mirror is
Images = (360degrees/ angle between the mirrors) – 1

Question 90.
The deficiency of one of the following in the diet of a person for a considerable time can lead to a disease called night blindness. This one is:
A. vitamin B
B. vitamin D
C. vitamin A
D. vitamin C
Answer:
C. vitamin A
Vitamin A deficiencies occur from fat malabsorption, liver disorders and inadequate food intake.

Question 91.
Which of the following is not a part of the human eye?
A. retina
B. auditory nerve
C. optic nerve
D. ciliary muscle
Answer:
B. auditory nerve
Auditory nerve is not a part of the human eye, but a part of human ear. It transmits hearing information between the brain and cochlea.
See the diagram of human ear attached:

Question 92.
How does the eye change in order to focus on near or distant objects?
A. the lens moves in or out
B. the retina moves in or out
C. the lens becomes thicker or thinner
D. the pupil becomes larger or smaller
Answer:
C. the lens becomes thicker or thinner
The shape of the lens is changed by ciliary muscles so that the image forms at a sharp focus at the retina. The lens becomes thicker to focus at near objects and becomes thinner to focus at distant objects.

Question 93.
Which of the following changes occur when you walk out of bright sunshine into a poorly lit room?
A. the pupil becomes larger
B. the lens becomes thicker
C. the ciliary muscle relaxes
D. the pupil becomes smaller
Answer:
A. the pupil becomes larger
When a person walks out of bright sunshine into a poorly lit room, the pupil dilates and becomes larger to allow maximum light to enter the eye and thus helps in viewing things easily in a poorly lit room.

Question 94.
An incident ray makes an angle of 65° with the surface of a plane mirror. The angle of reflection in this case will be:
A. 65°
B. 45°
C. 25°
D. 35°
Answer:
D. 35°
Angle of incidence is always equal to angle of refection. Hence, angle of reflection will be 35°.

Question 95.
Which of the following produces diffuse reflection of light?
A. mirror on a dressing table
B. water surface of a pond
C. screen in a cinema hall
D. polished wooden table
Answer:
B. water surface of a pond
Others, the mirror, the screen and the polished wooden table will give regular reflection as they are even surfaces.

Question 96.
The human eye forms the image of an object at its:
A. cornea
B. iris
C. pupil
D. retina
Answer:
D. retina
The image of an object is formed at the retina of an eye which is the third and the innermost coat of an eye. Light rays are focused on the retina by cornea, lens and the pupil.

Question 97.
The change in converging power of an eye-lens is caused by the action of:
A. iris
B. ciliary muscles
C. optic nerve
D. retina
Answer:
B. ciliary muscles
The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.

Question 98.
The size of the pupil of the eye is adjusted by:
A. cornea
B. ciliary muscles
C. optic nerve
D. iris
Answer:
D. iris
The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light entering the eye.

Question 99.
The defect of vision in which the eye-lens of a person gets progressively cloudy resulting in blurred vision is called :
A. myopia
B. night blindness
C. cataract
D. hypermetropia
Answer:
C. cataract
Cataract is the clouding of the lens of human eye, that lies behind the iris and pupil. It leaves to a blurred vision or decrease in vision.

Question 100.
A person cannot see the distant objects clearly (though he can see the nearby objects clearly). He is suffering from the defect of vision called:
A. hypermetropia
B. myopia
C. night blindness
D. cataract
Answer:
B. myopia
Myopia is the defect of vision wherein a person cannot see the distant objects clearly as the light is unable to focus on the retina and thus distant objects appear to be blurred.

Lakhmir Singh Science Class 8 Chapter 16 Questions Based on High Order Thinking Skills (HOTS)

Question 101.
A man stands 10 m in front of a large plane mirror. How far must he walk before he is 5 m away from his image?
Answer:
The object distance is equal to image distance in a plane mirror.
For the distance of man from his image to be 5m
The distance of man from mirror + distance of image from the mirror = 5m
Since object distance = image distance
Therefore, 2*distance of man from the mirror = 5m
Distance of man from the mirror = 5/2 = 2.5m
The man is 10m from the mirror at the starting, so he will have to walk 10 – 2.5 = 7.5m towards the mirror for being 5m away from his image.

Question 102.
A ray of light strikes a plane mirror XY at an angle of incidence of 65°, is reflected from this plane mirror and then strikes a second plane mirror YZ placed at right angles to the first mirror. What is the angle of reflection for the mirror YZ?
Answer:
The mirrors XY and YZ are at right angles, i.e., perpendicular. When a ray of light strikes the XY mirror with 65 degree angle, the incident angle will be 90 – 65 = 25 degree on the YZ mirror. Since angle of incidence = angle of reflection, the angle of reflection for YZ mirror will be 25 degrees.

Question 103.
The eye of a person exhibits a phenomenon X due to which it can see the image of an object for a short duration of Y even after the object has disappeared from his view. It is due to the phenomenon X that we are able to see moving Z on a television screen. What are X, Y and Z?
Answer:
The phenomenon X is persistence of vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.
Short duration of Y is 1/16 seconds.
Z is pictures. Due to the phenomenon of persistence of vision, we are able to see moving pictures on a television or a cinema screen.

Question 104.
Man A has a defect of vision due to which he cannot see the nearby objects clearly (though he can see the distant objects clearly). On the other hand, man B has a defect of vision due to which he cannot see the distant object clearly (though he can see the nearby objects clearly). The defect in man A can be corrected by using spectacles containing lenses C whereas the defect in man B can be corrected by using spectacles containing lenses D.
(a) Name the defect of vision in man (i) A, and (ii) B.
(b) What type of lenses are (i) C, and (ii) D?
Answer:
a) i-Man A suffers from hypermetropia wherein the image of a nearby object is formed behind the retina so the person has a blurred vision while looking at nearby objects.

ii- Man B suffers from myopia wherein Light from a distant object forms an image before it reaches the retina so the person can see the nearby objects clearly but not the distant objects.

b) C are Convex lens. these lenses are placed in front of a hyper-metropic eye which moves the image forward and focuses directly onto the retina which clears the vision of a nearby object.

D are concave lenses. These lenses are placed in front of a myopic eye which moves the image back to the retina and clarifies the image of a distant object.

Question 105.
A student makes a device P by using three long and narrow strips of plane mirrors inclined at 60° to one another which enables him to see beautiful patterns made by pieces of coloured glass bangles. On the other hand, another student makes a device Q by using two plane mirrors arranged parallel to each other which helps him to see a football match clearly even when some very tall persons are sitting in front of him in the ground. What are P and Q?
Answer:
Device P is a Kaleidoscope which consists of 3 plain mirror strips are arranged inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

The device Q is a Periscope. A periscope is a device that reflects the reflected ray again. It consists of a tube attached to a set of 2 mirrors. A periscope is used to see over, through or around any object. So, the student is able to see a football match clearly even when some very tall persons are sitting in front of him in the ground because the periscope helps him in seeing over, through and around the tall persons sitting in front of him in the ground.

Lakhmir Singh Science Class 8 Solutions

NCERT Solutions for Class 8 Maths Chapter 14 Factorisation Ex 14.4

• Class 8 Maths Factorisation Exercise 14.1
• Class 8 Maths Factorisation Exercise 14.2
• Class 8 Maths Factorisation Exercise 14.3
• Class 8 Maths Factorisation Exercise 14.4
• Factorisation Class 8 Extra Questions

NCERT Solutions for Class 8 Maths Chapter 14 Factorisation Exercise 14.4

Find and correct the errors in the following mathematical statements.

More CBSE Class 8 Study Material

• NCERT Solutions for Class 8 Maths
• NCERT Solutions for Class 8 Science
• NCERT Solutions for Class 8 Social Science
• NCERT Solutions for Class 8 English
• NCERT Solutions for Class 8 English Honeydew
• NCERT Solutions for Class 8 English It So Happened
• NCERT Solutions for Class 8 Hindi
• NCERT Solutions for Class 8 Sanskrit
• NCERT Solutions

S Chand Science Class 8 Solutions Chapter 11 Force and Pressure

Lakhmir Singh Science Class 8 Chapter 11 Very Short Answer Type Questions

Question 1
What is the push or pull on an object known as?
Answer:
Force
Force is a Pull or a push on the object.

Question 2.
Why do the shape and size of a balloon change when filled with air or water?
Answer:
As we know that air and water are the matter and matter has a characteristic property which says that they have mass and occupy certain space. So when air or water is filled in a balloon, they tend to stretch its rubber boundary and occupy the space. Therefore the shape and size of the balloon get change when filled with air or water.

Question 3.
Name the quantity whose unit is ‘Newton’ (N).
Answer:
Force
The unit of force is Newton. Kg ms^-2 = Newton

Question 4.
When a ball is dropped from a height, its speed increases gradually. Name the force which causes this change in speed.
Answer:
Gravitational force
Every object in the universal exerts a force on every other object. this force is known as the gravitational force.

Question 5.
What is the unit of force?
Answer:
Newton
The unit of force is Newton

Question 6.
Give one example where force changes the shape of an object.
Answer:
When you squeeze a toothpaste.
Force can change the shape, size, direction, etc on applying the body.

Question 7.
Identify the actions involved in the following situations as push or pull, or both:
(a) Opening a drawer.
(b) A cricket ball hit by a batsman.
(c) Drawing a bucket of water from a well.
(d) moving a book placed on a table.
(e) A football player taking a penalty corner.
(f) Moving a wheel barrow.
Answer:
(a) PULL
Force is a Pull or a push on the object.

(b) PUSH
Force is a Pull or a push on the object.

(c) PULL
Force is a Pull or a push on the object.

(d) Ans Both push and pull
Pull or push on an object is known as force

(e) push
Pull or push on an object is known as force

(f) push
Pull or push on an object is known as force.

Question 8 A.
Name two contact forces.
Answer:

1. frictional force
2. applied force

The force applied when it is in contact with the object.

Question 8 B.
Name two non-contact forces.
Answer:

1. Gravitational force.
2. Magnetic force.

The force acting on an object without being in contact with it is known as non-contact force.

Question 9.
When a plastic pen is rubbed in dry hair, it attracts tiny pieces of paper. Which force is involved in this process?
Answer:
Electrostatic force due to create unlike charge by rubbing to get attracted

Question 10.
A small device pulls iron nails from a distance. Which type of force is involved in this process?
Answer:
Magnetic force
Iron nails get attracted by magnet without touching them.

Question 11.
Which force can be used to gather iron pins scattered on the floor?
Answer:
Magnetic force
Iron pins get attracted by magnet without touching them.

Question 12.
Name the force which always opposes motion.
Answer:
Friction
Friction force acting between the two surfaces in the contact with each other which oppose the motion

Question 13.
Which force makes a rolling ball stop on its own?
Answer:
Friction force
Friction force acting between the two surfaces in the contact with each other which oppose the motion, due to which stop the moving object.

Question 14.
An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?
Answer:
Electrostatic force
On the rubbing an object to get charged (static charge) and unlike charges get attracted to each other, such type of force is Electrostatic force.

Question 15.
What name is given to the force acting on a unit area of an object?
Answer:
Pressure
Pressure = thrust (force) / area

Question 16.
Name the quantity whose one of the units is Pascal (Pa)?
Answer:
Pressure
The unit of pressure is Pascal (Pa).

Question 17.
What conclusion do you get from the observation that a fountain of water is created at the leaking joint of pipes of the main water supply line?
Answer:
Water exerts force on the bottom of the container.

Question 18.
What type of pressure is involved in the filling of a liquid in a syringe?
Answer:
Air pressure
Pressure = thrust (force)/area

Question 19.
What substance present in our body balances the atmospheric pressure acting on us?
Answer:
Our Blood (Internal body pressure created by blood)

Question 20.
Where will the atmospheric pressure be greater-at ground level or at the top of high mountain ?
Answer:
Ground Level
At mountain air is low.

Question 21.
Name any two devices used in everyday life which work on the existence of atmospheric pressure.
Answer:

1. Drinking Straw works on atmospheric pressure
2. Vacuum Cleaner works on atmospheric pressure.

Question 22.
If a vacuum is created between two Magdeburg hemispheres joined together, they cannot be separated easily, What presses the hemispheres together?
Answer:
The two hemispheres pressed together when vacuum is created between them because air is pumped out of the interior which create the remarkable force between them to stick together.

Question 23.
What makes a balloon get inflated when air is filled in it?
Answer:
When we filled air in the balloon, the inside pressure in the balloon increases and thereby the surface area of balloon increases. Thus the force acting by air molecule inside outward and the force acting by air from outside inward equals. This balancing force tend to make a balloon get inflated when air is filled in it.

Question 24.
Name the substance whose weight produces atmospheric pressure.
Answer:
Air
Air has mass and occupied space.

Question 25.
Where is the pressure greater, 10m below the surface of the sea or 20m below the surface of sea?
Answer:
20m below the surface of sea.
Pressure increases with height or depth of sea level

Question 26.
What force acting on, an area of 0.5 m² will produce a pressure of 500 Pa?
Answer:
250N
P = F/A
F = P × A
= 500 × 0.5 = 250 N

Question 27.
Can a liquid exert pressure upwards?
Answer:
Yes, but exert more pressure downwards than upwards.

Question 28.
Can a liquid exert pressure sideways?
Answer:
Yes, Liquid exert pressure in all direction.

Question 29.
State whether the following statements are true or false :
(a) The pressure exerted by a liquid depends on the area of base of its container.
(b) A drinking straw works on the pressure exerted by the liquid filled in a soft drink bottle in which it is placed.
Answer:
(a) True
Pressure depends on the area and force.

(b) False
Pressure depends on the area and force.

Question 30.
Fill in the following blanks with suitable words :
(a) To draw water ‘from a well, we have to ………….at the rope.
(b) If the two forces applied to an object are equal and act in opposite directions, the net force acting on the object will be …………….
(c) Force could be a ………….or a ………..
(d) Force has magnitude as well as …………..
(e) A force arises due to…………….between two objects.
(f) A charged body…………an uncharged body towards it.
(g) The north pole of a magnet…………the north pole of another magnet.
(h) Force acting on a unit area is called ………………
(i) The pressure exerted by a liquid ……….with depth.
(J) A drinking straw works on the existence of ……………….pressure.
(k) Atmospheric pressure………….with increasing height.
Answer:
(a) To draw water ‘from a well, we have to apply pull force at the rope.
To draw an object by applying force

(b) If the two forces applied to an object are equal and act in opposite directions, the net force acting on the object will be zero.
The force applied on the body are equal and opposite direction, force cancel out and net force is zero.

(c) Force could be a push or a pull.
A push or pull on an object is known as force.

(d) Force has magnitude as well as direction.
Force is a vector quantity and it depends on direction.

(e) A force arises due to interaction between two objects.
The interaction between two objects due to a force .

(f) A charged body attracts an uncharged body towards it.
Force attracts to each other of two objects when they are unlike charges.

(g) The north pole of a magnet repeals the north pole of another magnet.
Like poles of magnet repeal to each other’s.

(h)Force acting on a unit area is called Pressure.

(i) The pressure exerted by a liquid increases with depth.
Pressure = height × density of object × gravity

(J) A drinking straw works on the existence of atmospheric pressure.
The level of water rises from drinking straw due to atmospheric pressure.

(k) Atmospheric pressure decreases with increasing height.
Pressure is inversely proportional to height

Lakhmir Singh Science Class 8 Chapter 11 Short Answer Type Questions

Question 31.
Define ‘state of motion’ of an object. Name the ‘agent’ which can change the state of motion of an object.
Answer:
The state of motion of an object is defined by the speed acquired by it in a particular direction. Force can change the state of motion of an object.

Question 32.
Give two examples of situations where you push or pull to change the state of motion of objects.
Answer:

1. Pushing a carom board coin
2. Moving a glass placed on table

The force can change the state of motion on applying on it.

Question 33.
What is meant by saying that force is due to an interaction’? Give an example to illustrate your answer.
Answer:
It means for force to come into play, atleast two object must interact with each other. For example: If the man pushes the car with his hands due to which the car starts moving, there is an interaction between man and a car. During this interaction force arises which Acts on the car and makes it move in the direction of applied force.

Question 34.
In a tug of war; when the two teams are pulling the rope, a stage comes when the rope does not move to either side at all. What can you say about the magnitudes and directions of the forces being applied to the rope by the two teams at this stage?
Answer:
Same magnitude of force are acting in opposite direction. Therefore they cancel each other.

Question 35.
What is force? State the various effects of force.
Answer:
A force is a push or pull upon an object resulting from the object’s interaction with another object. Various effects of force are:
a) It can change shape
b) It can change direction
c) It can change position

Question 36 A.
Give one example where force moves a stationary object.
Answer:
Pushing a table
The force can change the state of motion of object.

Question 36 B.
State one example where force stops a moving object.
Answer:
A brake applied on bicycle to stop due to force.

Question 37 A.
Give one example where force changes the speed of a moving object.
Answer:
Pushing a slow moving bicycle increases its speed.

Question 37 B.
Give one example where force changes the direction of a moving object.
Answer:
Hitting a moving ball change its direction.

Question 38.
Why does the shape of an ointment tube change when we squeeze it?
Answer:
The shape changes because our finger applies force on the tube that is why the ointment tube’s shape changed.

Question 39.
What happens to the springs of a sofa when we sit on it?
Answer:
The shape and size of the spring changes, the force due to sitting on the sofa changes the shape and size of the spring.

Question 40.
Name the various types of forces.
Answer:

1. Contact Forces (Frictional Force, Tension Force, Normal Force, Air Resistance Force, Applied Force and Spring Force)
2. Action-at-a-Distance Forces (Magnetic Force, Electrical Force, Gravitational Force)

Question 41.
What is muscular force ? Give one example of muscular force.
Answer:
Muscular force is a force which is generated by our muscles during heavy works, exercises etc which produce lactic acid. The example of muscular force is bouncing a ball.

Question 42.
Which of the following are non-contact forces?
Magnetic force, Frictional force, Gravitational force, Muscular force, Electrostatic force.
Answer:
Non-Contact Forces are the forces exerted by an object that can be experienced by an object even from a distance without any contact with each other magnetic, Electrostatic and Gravitational Forces are Non-Contact Forces

Question 43.
Give two examples from everyday life which show that air exerts pressure.
Answer:

1. Drinking Straw
2. Vacuum Cleaner

Question 44.
What is a rubber sucker? How does it work? State any one use of a rubber sucker.
Answer:
Rubber sucker a device used in toys, sunlight blocker – suction cups, suction clamps and various other places where “sticking” is required for temporary period and when you don’t want to use glue.

The principle on which Rubber suckers work: When the sucker is pressed against the surface, it squeeze out the air that is behind it. The atmospheric pressure on the outside of the sucker holds its rim firmly against the smooth surface.

Question 45.
Why do mountaineers usually suffer from nose-bleeding at high altitudes?
Answer:
At higher altitude the atmospheric pressure decreases therefore the blood vessels exceed the outside pressure which causes blood vessels to get rupture.

Question 46.
Describe one activity to show the existence of atmospheric pressure.
Answer:
Crushing can experiment
When a can filled with hot water is closed and is cooled down rapidly by pouring cold water on it, it will crush instantly. This experiment proves that there is a huge atmospheric pressure exerts on everything on the surface of the earth.

Question 47.
Explain why, water comes out more slowly from an upstairs tap than from a similar tap downstairs.
Answer:
As the depth of water increases the pressure also increases.

Question 48.
What is meant by gravitational force (or force of gravity)? Give its one example.
Answer:
The force of attraction between all masses in the universe, especially the attraction of the earth’s mass for bodies near its surface. For example: When you hold a ball up in the air, the mass of Earth allows the ball to fall to the Earth.

Question 49.
Calculate the pressure when a force of 200 N is exerted on an area of:
(a) 10m² (b) 5m²
Answer:
Given: Force applied is given as: 200N
Now, we know that the Pressure is the force applied perpendicular to the surface of an object per unit area. or Pressure = Force/ Area
(a) Area = 10m²
P = 200N/10m²
or P = 20 Nm² or 20 Pascal

(b) Area = 10m²
P = 200N/5m²
or P = 40 Nm² or 40 Pascal

Question 50.
Which force do the animals apply while moving, chewing and doing other activities?
Answer:
Muscular force

Question 51.
Which force is responsible for raising our body hair when we try to take off a terylene or polyester shirt in the dry weather?
Answer:
Electrostatic force
The force acting on object due to its charged.

Question 52.
Name the type of forces involved in the following :
(a) A horse pulling a cart.
(b) A sticker attached to steel almirah without glue.
(c) A coin falling to the ground on slipping from hand.
(d) A plastic comb rubbed in dry hair picking up tiny pieces of paper.
(e) A moving boat coming to rest when rowing is stopped.
Answer:
(a) Muscular force is a contact force due to action of muscles.
(b) Magnetic force

(c) Gravitational force
Objects fall towards the earth due to its gravity.

(d) Electrostatic force
The force acting on an object due to its charged particles.

(e) Frictional force
Stream of water opposed to moving boat to stop.

Question 53.
Why does a sharp knife cut objects more effectively than a blunt knife?
Answer:
Lesser the area, larger the pressure for the same force applied. Therefore it is easier to cut with sharp knives than blunt ones, with same force.

Question 54.
Explain why, wooden (or concrete) sleepers are kept below the railway line.
Answer:
There are many reasons. Some of them are

1. To keep the lines equidistant from each other.
2. To reduce unwanted vibrations in the track.
3. The sleepers have a large surface area compared to the lines; this larger surface area decreases the pressure due to the weight of the train on the track. If only the rails were used without slippers; they being thinner would extent greater pressure over the bed of stone and move deeper into the ground

Question 55.
Explain why a wide steel belt is provided over the wheels of an army tank.
Answer:
The weight of the tank (its mass being acted upon by the force of gravity) is spread over a larger surface area (the big steel belt), giving it a smaller pressure and therefore it makes it sink in less, allowing it to travel faster over more surfaces where others would with less surface area

Question 56.
Explain why the tip of a sewing needle is sharp.
Answer:
The tip of sewing needle is sharp because with the decrease of area, pressure increases and the needle can easily penetrate the stitching material.

Question 57.
Explain why snow shoes stop you from sinking into snow.
Answer:
The snow shoes have large, flat soles so they exert less pressure (= force /area) on the soft snow and stop the wearer from sinking into it.

Question 58.
Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it.
Answer:
When a man stands on a cushion then only his two feet (having small area) are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion.

Question 59.
Explain why, porters place a thick, round piece of cloth on their heads when they have to carry heavy loads.
Answer:
The round piece of cloth on their heads increases the area on which the force (weight of the load) will act. We know that, the pressure is inversely proportional to area. Therefore, increase in area reduces the pressure and this helps the porter to carry the heavy load easily.

Question 60.
Give one practical application of magnetic force.
Answer:
Magnetic forces are the force due to magnetic field of a magnet. The closing of door of refrigerator works on the application of magnetic forces.

Lakhmir Singh Science Class 8 Chapter 11 Long Answer Type Questions

Question 61 A.
What is meant by a contact force? Explain with the help of an example.
Answer:
A Contact force as the name suggests acts on a point of an object by direct contact. The Contact may be Continuous or sometimes momentary. The former case is known as Continuous force and the latter case is known as impulse force.

Example: Pushing an object along the floor: This is an example of a continuous force. Here, continuously, the direct force is put on the object to move it along the floor.

Question 61 B.
What is meant by a non-contact force? Explain with the help of an example.
Answer:
An imaginary force can pull everything towards the earth surface. All the objects are pulled towards the earth without any physical contact with the objects and bodies. These are non contact forces in the most basic sense.

Example

• Gravitational force
• Magnetic force
• Electrostatics
• Nuclear force

Question 62 A.
Define frictional force (or friction).
Answer:
Frictional Force refers to the force generated by two surfaces that contacts and slide against each other. These forces are mainly affected by the surface texture and amount of force impelling them together. The angle and position of the object affect the amount of frictional force.

If an object is placed flat against an object, then the frictional force will be equal to the weight of the object.

If an object is pushed against the surface, then the frictional force will be increased and becomes more than the weight of the object.

Question 62 B.
Explain why frictional force is said to be a contact force.
Answer:
If life an object which is moving over a horizontal surface does not continue with constant velocity when the accelerating force is removed. Instead, the object slows down and eventually stops. The deceleration is caused by a frictional force between the object and the surface on which it travels. A frictional force always acts to oppose the motion of an object over a surface and is an example of a contact force. When we try to slide one object over another, we find that there is a force which opposes the motion. This force is called friction. Whenever there is relative motion of two surfaces in contact, there is frictional resistance. When a body moves over another, it has to overcome friction. This needs energy. Hence, some energy is always wasted in overcoming friction. When two parts of a machinery rub against each other heat are produced, and the efficiency of the machine decreases. There will be a greater wear and tear in the different parts of a machine due to friction.

Question 62 C.
Explain, why magnetic force is said to be a non-contact force.
Answer:
Magnetic force is acted upon a body by the magnetic lines of force. These are imaginary closed line of lines of force which emanate from the N pole f the magnet and goes into the S pole of the magnet. So, a magnet can influence a magnetic material like a piece of iron even when they are not in contact thus magnetic force is a non contact force.

Question 63 A.
Define pressure. What is the relation between pressure, force and area? State the units in which pressure is measured.
Answer:
Pressure is defined as a measure of the force applied over a unit area. Pressure often is expressed in units of Pascals (Pa), newtons per square meter (N/m² or kg/m.s²), or pounds per square inch. Other units include the atmosphere (atm), torr, bar, and meters sea water (msw).
In equations, pressure is denoted by the capital letter P or the lowercase letter p.
Pressure is a derived unit, generally expressed according to the units of the equation:
P = F / A
where P is pressure, F is force, and A is area
P is pressure
F is the component of force perpendicular to the surface
A is the area of the surface
When a force is constant over an area, the pressure acting on that area is simply
Pressure is a scalar quantity, thus it acts in all directions at any given point. In order for pressure to create a force, the pressure must be integrated over some area.

Question 63 B.
Explain why school bags are provided with wide straps to carry them.
Answer:
A wider strap distributes the force throughout a greater surface area. You can do a comparison using a simple physics equation.
Pressure = Force/Area
So take two different straps and measure the area of them.
Then find out how much weight the pack will be holding.
Then divide the Weight by the Area and you end up with pressure.

Question 64 A.
What is meant by atmospheric pressure? What is the cause of atmospheric pressure?
Answer:
ATMOSPHERIC PRESSURE: The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force forced on a surface by the air above it as gravity pulls it to Earth.

CAUSES:

1. Height of altitude
2. Density of air

Question 64 B.
Why are our bodies not crushed by the large pressure exerted by the atmosphere?
Answer:
Because the atmosphere also pressurize the body. Diving down 100 meters will pressurize the gasses in the body and they will compress, like the air in our lungs and if the lung volume gets below half a litre the lungs can get partly filled with blood plasma. But sorry no bone crushing. But if you take a big breath and hold it while you ascend from 100 meters to the surface, your ribs may start to break

Question 64 C.
Explain why atmospheric pressure decreases as we go higher up above the earth’s surface.
Answer:
The air molecules at lower altitudes are more compressed by gravity than the ones at the higher altitudes. The pressure level is highest right at the surface of the earth because the air at this level is supporting the weight of all the air above it. More weight above means a greater downward gravitational force. As we move up through levels of the atmosphere, the air has less air mass above it and gravity isn’t strong enough to pull down a greater number of particles. So the balancing pressure decreases. This is why atmospheric pressure drops as we rise in altitude.

Question 65 A.
How does the pressure of a liquid depend on its depth? Draw a labelled diagram to show that the pressure of a liquid (say, water) depends on its depth.
Answer:
All the liquids exert pressure on the base or bottom and walls of their container. All the liquid have weight. When we pour liquid into a vessel, then the weight of liquid pushes down on the base of the vessel producing a pressure.

The liquid exerts pressure on the base of the vessel.

The pressure exerted by a liquid changes with depth in the liquid. The pressure exerted by a liquid increases with increasing depth inside the liquid. The pressure exerted by a liquid is small just under the surface of the liquid. But as we go we deeper in a liquid the pressure of liquid increases. As the depth of liquid increases the weight of liquid column pushing down from above increases and hence the pressure also increases.

Question 65 B.
Explain why the walls of a dam are thicker near the bottom than at the top.
Answer:
A dam is thicker at the bottom than at the top because the forces exerted are strongest close to the ground. Water pressure increases proportionally with depth, so the lower portion of the dam has greater water pressure exerted upon it. The increasing thickness of the lower dam also helps it support its own weight.

Lakhmir Singh Science Class 8 Chapter 11 Multiple Choice Questions (MCQs)

Question 66.
Which of the following is not an example of muscular force?
A. a porter carrying a load on a wheel-barrow.
B. an apple falling from a tree.
C. a child riding a bicycle.
D. a person drawing water from a well.
Answer:
B. an apple falling from a tree.
An apple falling from a tree is an example of gravitational force.

Question 67.
Which of the following is not an example of the force of gravity?
A. a leaf falling from a tree.
B. a boy pushing a cart on a level plane.
C. a diver jumping into a swimming pool.
D. a stone falling from the top of a cliff.
Answer:
B. a boy pushing a cart on a level plane.
A boy pushes a cart on a level of plane due to force.

Question 68.
When we press the bulb of a dropper with its nozzle kept in water air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to :
A. pressure of water
B. gravity of the earth
C. shape of rubber bulb
D. atmospheric pressure
Answer:
D. atmospheric pressure
Atmospheric pressure exerted by air .

Question 69.
A rectangular wooden block has length, breadth and height of 50 cm, 25 cm and10 cm, respectively. This wooden block is kept on ground in three different ways, turn by turn. Which of the following is the correct statement about the pressure exerted by this block on the ground?
A. the maximum pressure is exerted when the length and breadth form the base
B. the maximum pressure is exerted when length and height form the base
C. the maximum pressure is exerted when breadth and height form the base
D. the minimum pressure is exerted when length and height form the base
Answer:
C. the maximum pressure is exerted when breadth and height form the base
Pressure = force / area

Question 70.
Which of the following are contact forces ?
A. Friction
B. Gravitational force
C. Magnetic force
D. Muscular force
A. A and B
B. B and C
C. A and D
D. B and D
Answer:
C. A and D
Force applied when it is in contact with the object.

Question 71.
If we release a magnet held in our hand, it falls to the ground. The force responsible for this is :
A. muscular force
B. magnetic force
C. electrostatic force
D. gravitational force
Answer:
D. gravitational force
Force applied when it is in non contact with the object.

Question 72.
Which of the following force is utilized in reducing air pollution by removing dust, soot and fly-ash particles from the smoke coming out of chimneys of factories ?
A. magnetic force
B. gravitational force
C. electrostatic force
D. frictional force
Answer:
C. electrostatic force
Charged particles gets removed by electrostatic force .

Question 73.
The same force Facts on four different objects having the areas given below, one by one. In which case the pressure exerted will be the maximum?
A. 20 m²
B. 50 m²
C. 10 m²
D. 100 m²
Answer:
C. 10 m²

Question 74.
Which of the following represent correct values for the normal atmospheric pressure?
A) 101.3 kilopascals
B) 76 mm of mercury
C) 101.3 pascals
D) 76 cm of mercury
A. A and B
B. B and C
C. A and D
D. B and D
Answer:
D. B and D
P = F/A

Question 75.
Which of the following does not work on the existence of atmospheric pressure ?
A. rise of iodine solution in the glass tube of dropper
B. rise of cold drink in a long plastic straw
C. sticking of suction hook on the wall of a room
D. rise of mercury in glass tube of thermometer
Answer:
A. rise of iodine solution in the glass tube of dropper
P = height × density × gravity

Question 76.
The magnitude of force is expressed in the unit of force called:
A. pascal
B. kelvin
C. newton
D. Magdeburg
Answer:
C. newton
The unit of force is newton.

Question 77.
Which of the following change appreciably when a batsman hits a moving cricket ball?
A. Shape
B. Direction
C. Size
D. Speed
A. A and B
B. B and C
C. A and C
D. B and D
Answer:
D. B and D
The force can change the direction and Speed of motion.

Question 78.
Which of the following is not an effect of force?
A. a force can change the speed of a moving object
B. a force can change the direction of a moving object
C. a force can change the composition of a moving object
D. a force can change the shape and size of an object
Answer:
B. a force can change the direction of a moving object
The force can change the direction of motion.

Question 79.
Which of the following is not a non-contact force?
A. electrostatic force
B. gravitational force
C. frictional force
D. magnetic force
Answer:
B. gravitational force
Friction force is an example of contact force.

Question 80.
Which of the following scientists gave the idea of the existence of gravitational force?
A. Einstein
B. James Watt
C. Faraday
D. Newton
Answer:
D. Newton
Newton propound the universal laws of gravitational

Question 81.
Some mustard oil is kept in a beaker. It will exert pressure:
A. downwards only
B. sideways only
C. upwards only
D. in all directions
Answer:
D. in all directions
Fluid exerted pressure in all direction

Question 82.
A pressure of 10 kPa acts on an area of 0.3 m². The force acting on the area will be :
A. 3000 N
B. 30 N
C. 3 N
D. 300 N
Answer:
A. 3000 N
Force = pressure × area
= 10 × 1000 × 0.3 = 3000N

Question 83
The magnitude of atmospheric pressure is equal to the pressure exerted by a :
A. 76 mm tall column of mercury
B. 760 mm tall column of alcohol
C. 76 cm tall column of mercury
D. 760 cm tall column of mercury
Answer:
C. 76 cm tall column of mercury
The correct solution is option (c)
The standard temperature and pressure (STP), refers to nominal conditions in the atmosphere at sea level, which supports 760 millimeters in a mercurial barometer (760 mm Hg)

Question 84.
The atmospheric pressure is usually measured in the unit of :
A. newtons per square meter
B. pascal
C. cm of mercury
D. mm of mercury
Answer:
D. mm of mercury
The level of mercury is measured in mm.

Question 85.
When a force of 5 N acts on a surface, it produces a pressure of 500 Pa. The area of surface then must be
A. 10 cm²
B. 50 cm²
C. 100 cm²
D. 0.01 cm ²
Answer:
C. 100 cm²
Pressure = thrust / area
Area = Pressure / Thrust = 500/ 5 = 100 cm²

Lakhmir Singh Science Class 8 Chapter 11 Questions Based on High Order Thinking Skills (HOTS)

Question 86.
Two tiny holes are made in a plastic bucket, one near the middle part and the other just above bottom when this bucket is filled with water, the water rushes out from the bottom hole much faster than from the upper hole. What conclusion do you get from this observation?
Answer:
Pressure exerted by water increases with creasing depth

Question 87.
What is common in the working of the devices such as a drinking straw, a syringe, a dropper and a rubber sucker?
Answer:
All these devices work on the existence of atmospheric pressure

Question 88.
A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad (Ignore the frictional force due to air resistance).
Answer:
Upward force applied by the rocket engine and downward gravitational force applied by the earth

Question 89.
One student says that water exerts pressure on the bottom of the bucket but another student says that water exerts pressure on the sides of the bucket. What would you like to say?
Answer:
Water exerts pressure on the bottom of the bucket as well as on the sides of the bucket

Question 90.
Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.
Answer:
Upward muscular force applied by hand and downward gravitational force applied by earth ; The two forces being equal and opposite balance each other and hence do not bring a change in the state of motion.

Lakhmir Singh Science Class 8 Solutions

S Chand Science Class 8 Solutions Chapter 12 Friction

Lakhmir Singh Science Class 8 Chapter 12 Very Short Answer Type Questions

Question 1.
Name the force which always opposes motion.
Answer:
The friction always opposes the motion.

Question 2.
Why does a fast car slow down if its engine is switched off?
Answer:
When the engine of a car is switched off, then there is no energy available for the engine to oppose the frictional forces. Thus, the engine slows down and eventually comes to rest.

Question 3.
Which type of surfaces produce
(a) least friction, and
(b) too much friction?
Answer:
(a) Smooth surfaces offer least friction.
(b) Rough surfaces offer high friction.

Question 4.
What is the direction of force of friction acting on a moving object ?
Answer:
The friction forces always act in the opposite direction of motion.

Question 5.
Name a device which is used to measure force acting on an object.
Answer:
Dynamometer is used to measure the forces acting on any object.

Question 6.
What is a spring balance?
Answer:
A spring balance is used to measure the weights of object with the help of tension force.

Question 7.
Out of sliding friction, static friction and rolling friction:
(a) which one is the smallest?
(b) which one is the largest?
Answer:
(a) Rolling friction is the smallest in magnitude.
(b) Static friction is the largest in magnitude.

Question 8.
Which type of friction comes into play when a book kept on cylindrical pencils is moved by pushing?
Answer:
Rolling friction comes into play when a book kept on cylindrical pencils is moved by pushing.

Question 9.
Why is it more difficult to walk properly on a well-polished floor?
Answer:
The well-polished floor offers very less resistance. Therefore, the shoes slip on it and its very difficult to walk properly.

Question 10.
Why is it difficult to walk on a wet marble floor?
Answer:
It is very difficult to walk on a wet floor because the friction is less and the required grip cannot be maintained.

Question 11.
Which force is responsible for the wearing out of car tyres?
Answer:
Frictional force is responsible the wearing of car tyres.

Question 12.
What prevents you from slipping every time you take a step forward?
Answer:
Frictional forces prevent us from slipping every time you take a step forward.

Question 13.
Name the force which helps things to move and stop.
Answer:
Frictional forces helps things to move and stop.

Question 14.
What enables us to fix nails in a wall and knots to be tied?
Answer:
Friction enables us to fix nails in a wall and knots to be tied.

Question 15.
What makes the steps of foot over-bridges at Railway Stations to wear out slowly ?
Answer:
Frictional force is responsible the wearing of footsteps of over-bridges.

Question 16.
What is done to increase friction between the tyres and road?
Answer:
The tyre’s surface has treads on it. Treads improve the grip of the tyre on the road.

Question 17.
Why do gymnasts apply a coarse substance to their hands?
Answer:
Gymnasts apply a coarse substance to their hands to increase the frictional forces on their hands so that they can hold firmly.

Question 18.
Why do kabaddi players rub their hands with dry soil?
Answer:
Kabaddi players rub their hands with dry soil before playing so as to increase friction. This enables them to grip the other player’s hand properly.

Question 19.
Name the device which is used between the hubs and axles of bicycle wheels to reduce friction.
Answer:
Ball bearing is used between the hubs and axles of bicycle wheels to reduce friction.

Question 20.
What is the purpose of using ball bearings in machines?
Answer:
Ball bearing reduce friction between the support loads and axles of a vehicle.

Question 21.
Name any two machines in which ball bearings are used.
Answer:
Car and bicycle.

Question 22.
Name the device which is attached to heavy luggage (such as a heavy suitcase) to move it easily by pulling.
Answer:
Pulley.

Question 23.
Name one example from everyday life where wheels (or rollers) are used to reduce friction.
Answer:
Car

Question 24.
Why does oiling the axles of a bicycle make the bicycle move more easily?
Answer:
Oiling the-axles of a bicycle reduces the friction and thus the bicycle moves easily.

Question 25.
State one way in which the friction between the wheel and its axle can be reduced.
Answer:
The ball bearing help in reducing the friction between the wheels and its axle.

Question 26.
Name two common lubricants.
Answer:
Castor oil and Grease are the two common lubricants.
Note: A lubricant is a substance that smoothens a surface. It decreases the friction of a surface.

Question 27.
Why do we sprinkle fine powder on carrom board?
Answer:
Sprinkling fine powder on carrom board reduces friction between carom and striker.

Question 28.
Which force gets reduced when the two surfaces in contact are polished to make them smooth?
Answer:
Frictional force gets reduced when the two surfaces in contact are polished to make them smooth.

Question 29.
Why is the surface of a slide polished to make it smooth?
Answer:
The surface of a slide polished to make it smooth in order to reduce friction.

Question 30.
Name the force which increases when the two surfaces in contact are made more rough.
Answer:
Frictional force increases when the two surfaces in contact are made more rough.

Question 31.
What is the special name of frictional force exerted by fluids (like air or water)?
Answer:
Drag is the special name of frictional force exerted by fluids.

Question 32.
What is the name of ‘special shape’ which is given to objects moving through air (or water) to reduce drag?
Answer:
The special shape is more like a point shaped so that they cut the fluid friction.

Question 33.
Why are grooves provided in the soles of shoes?
Answer:
Grooves in the soles of shoes help in increasing the friction when we walk on the roads. The grooves in the shoe sole help in maintaining a grip with the ground.

Question 34.
Why are treads made in the surface of tyres?
Answer:
Treads are made on the surface of tyres help in increasing the friction. These treads help in maintaining a firm grip on the road which reduces the risk of slipping on the road thus avoiding accidents.

Question 35.
Fill in the following blanks with suitable words :
(a) Friction always opposes…………..between the surfaces in contact with each other.
(b) Sliding friction is …………..than the static friction.
(c) Friction produces…………….
(d) Friction prevents our foot from…………..over the ground.
(e) Sprinkling of powder on the carom board………….friction.
(f) Ball bearings reduce friction because they……………rather than ‘slide.
(g) The friction when something moves through a liquid or gas is called……
(h) Cars and speedboats are………..to reduce drag.
(i) Shapes that are designed to reduce air resistance are called…………shapes.
(j) Objects which can move quickly through the water have a ………shape.
(k) The shape of an aeroplane is similar to that of a…………in flight.
Answer:
(a) Friction always opposes motion between the surfaces in contact with each other
(b) Sliding friction is less than the static friction.
(c) Friction produces heat
(d) Friction prevents our foot from slipping over the ground
(e) Sprinkling of powder on the carom board reduces friction.
(f) Ball bearings reduce friction because they roll rather than ‘slide.
(g) The friction when something moves through a liquid or gas is called drag.
(h) Cars and speedboats are streamlined to reduce drag.
(i) Shapes that are designed to reduce air resistance are called streamlined shapes.
(j) Objects which can move quickly through the water have a streamlined shape.
(k) The shape of an aeroplane is similar to that of a bird in flight.

Lakhmir Singh Science Class 8 Chapter 12 Short Answer Type Questions

Question 36.
When we try to push a very heavy box kept on ground, it does not move at all. Which force is preventing this box to move forward? Where does this force act?
Answer:
Whenever we push a very heavy box kept on the ground, it does not move at all because frictional force balances the force that we apply. So, there is no net movement of this box. It acts in a direction opposite to the force we apply.

Question 37.
Suppose your writing table (or desk) is tilted a little. A book kept on the table starts sliding down. Draw a diagram to show the direction of force of friction acting on the book.
Answer:

Question 38.
Which will cause more friction: a rough surface or a smooth surface? Why?
Answer:
A rough surface will cause more friction than a smooth surface. Because, rough offers more resistance to the other surface in contact as compared to the smooth surface.

Question 39.
Explain why sliding friction is less than static friction.
Answer:
The sliding friction is less than static friction because, in sliding friction, the interlinking of the two surfaces is for very short time.
Friction mainly arises due to the interlocking of irregularities between two surfaces, as shown below in the diagram:

In smooth surfaces, there are very fewer irregularities, thus friction is very very less. But, on rough surfaces, the irregularities are very large.

In sliding friction, the time of interlocking is very less, as one object slides over another object. Whereas, in static friction, one object is moved over another object thus the irregularities time is more. Thus, the two surfaces could not properly come in contact with each other in sliding due to which the magnitude of static friction is more than sliding friction.

Question 40.
What is meant by ‘rolling friction’?
Answer:
Rolling friction is the friction or drag which opposes the motion when a body rolls.

Question 41.
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 larger force than Iqbal.

Because Seema’s box is heavier and Seema will have to apply greater force to overcome the larger friction in case of the heavier box.

Question 42.
Why does a man slip when he steps on a banana peel thrown on the road?
Answer:
While walking on the road, a man slips if he steps on a banana peel because the banana peel acts a lubricant between the road and the man’s shoe which reduces the friction. Thus, man slips on it.

Question 43.
Car wheels often spin on icy roads. Explain why.
Answer:
A car spins on icy road because the treads of the car can no longer maintain the proper friction to keep it moving. Due to ice, the friction reduces and the car spins.

Question 44.
Explain why:
(a) A pencil will write on paper but not on glass.
(b) climbing a greasy pole is very difficult.
Answer:
(a) A pencil is made up of graphite. The paper surface offers more friction to the pencil when we write on the paper, in this way when the pencil is rubbed on the paper, some bits of graphite come on the paper and we are able to write. But glass offers no friction to the pencil, therefore we can’t write on a glass with pencil.

(b) Climbing a greasy pole is very difficult because the greasy pole offers very less friction to the feet of the person climbing on it. Du to which, the person slips down again and again.

Question 45.
Why does a matchstick light when we strike it on a rough surface ?
Answer:
When a matchstick is rubbed on a rough surface, the surface produces friction. And we know friction produces heat, therefore the matchstick lightens up.

Question 46.
Why is it difficult to light a matchstick by striking it on a smooth surface ?
Answer:
The smooth surface offers very less friction, due to which very less heat is produced by it and it is not sufficient to lighten up the matchstick.

Question 47.
Which parts wear away first in shoes? Give a reason for your answer.
Answer:
The heels of the shoes get wear and tear first than the rest of the shoe sole. This is because, the heel of the shoe is interacted with the ground more than the other part and due to friction, the treads of the shoe sole wear out.

Question 48.
Why do brake pads of bicycles have to be replaced quite often?
Answer:
The brake pads of bicycles have to be replaced quite often because they gather more dirt, dust and the filthy matter on itself. So to allow proper functioning, they have to be replaced.

Question 49.
A pencil eraser loses tiny pieces of rubber each time you use it. Why does this happen ?
Answer:
This is because the paper surface offers more friction to the pencil rubber when we rub on the paper, some bits of rubber come on the paper.

Question 50.
What happens when you rub your hands vigorously for a few seconds ? Why does this happen ?
Answer:
By rubbing our hands against each other we create friction between our hands and this friction produces heat. Therefore, out hands become warm.

Question 51.
Explain how, friction enables us to walk without slipping.
Answer:
When step forward and walk, we are applying force (denoted by “F”) on the ground. Now, in return ground also applies the reaction force to us. But we can fall if friction (denoted by “f”) is not present as the large ground can apply a large force . This is explained in the diagram:

Question 52.
Which is easier to hold in hand: a kulhar (earthen pot) or a glass tumbler? Why?
Answer:
It is easier to hold a kulhar (an earthen pot) because it more rough surface as it is made from mud. Mud offers more friction than glass and we are able to from a grip on the earthen plot.

Question 53.
How does a bicycle stop when its brakes are applied ?
Answer:
When we apply brake sot the bicycle, a force of friction generates between the rim of the bicycle and the brakes. The friction is produced in opposite direction of motion. Thus, the bicycle is stopped.

Question 54.
Explain why, the soles of our shoes wear out gradually.
Answer:
The shoes rub on the ground while walking. The ground offers friction due to which the sole of shoes wear and tear out.

Question 55.
Why do tyres of cars wear out gradually ?
Answer:
The tyres of the car wear out due to friction produced between he tyres and the car when a car is moving.

Question 56.
State two advantages and two disadvantages of friction.
Answer:
Disadvantages of Friction:

1. Friction always opposes he motion.
2. Friction causes wearing and tearing.

Advantages of Friction:

1. Friction help us to walk.
2. A horse can’t pull a cart without friction.

Question 57.
Explain why, sportsmen use shoes with spikes.
Answer:
The sports people use shoes with spikes, spikes increase the friction with the ground and does not let the person slip or fall when running fast.

Question 58.
How will you reduce friction between those machine parts which rub against each other? Give the simplest method.
Answer:
By applying any lubricant , say grease, one can reduce friction between those machine parts which rub against each other to avoid wear and tear.

Question 59.
What is meant by lubrication ? Why is it important?
Answer:
Lubrication means applying some additive to the parts of a machine to reduce the friction. Its important because it reduces the friction between the parts of the machine. But excess lubrication can generate more heat.
Example- Castor Oil, Grease.

Question 60.
Explain why, wheels are so useful.
Answer:
Wheels are important because they are used for transportation. Heavy machines can be moved from one place to another with the help of wheels because wheels can be rolled and the amount of force is also comparatively less.

Question 61.
Why are lubricants (oil or grease) applied to rubbing surfaces of machines?
Answer:
Lubricants (oil or grease) are applied to rubbing surfaces of machines because when the parts rub against each other, heat is produced. To avoid the wearing and tearing of the machine parts, lubrication is done. Because it reduces the friction and thus less heat is produced.

Question 62.
Explain with the help of diagrams, how the use of oil reduces friction between two surfaces in contact with each other.
Answer:
If we oil between the two surfaces, say part1 and part 2 of a machine, then the oil layer will allow less contact of the two surfaces with each other as you can see in the figure below:

In this way, the friction between the two surfaces is decreased.

Question 63.
Why are cars, aeroplanes and rockets streamlined ?
Answer:
The cars, aeroplanes and rockets are streamlined so as to reduce the fluid friction. The air drag obstructs the vehicle to move forward and the fuel is consumed mainly for opposing this opposite frictional force. Therefore, the objects are given special shape called streamline shape, which itself cuts the air friction.s

Question 64.
Explain why, a speedboat has a streamlined shape.
Answer:
A speedboat has a streamlined shape so as to reduce the fluid friction as it moves in the waters.

Question 65.
What are fluids? Name two common fluids.
Answer:
Fluids are those whose which do not have any shape and flow like liquids. Example- Air and Blood.

Lakhmir Singh Science Class 8 Chapter 12 Long Answer Type Questions

Question 66 A.
Define friction. What are the factors affecting friction? Explain with examples.
Answer:
The force that opposes the motion of an object is called friction. Example, when we walk, friction acts in opposite direction we apply force to walk.

The factor affecting friction are:

1. The weight of the object.
2. The type of contact between the two surfaces.

Question 66 B.
What is the cause of friction? Explain with the help of a labelled diagram.
Answer:
The cause of friction is surface roughness and adhesion.

When we apply force, F on a book, the book slide (Fig A). Then acts a force of friction, f (Fig B)

The friction is produced due to the contact of the two surfaces of the table and the book. It acts in opposite direction of the force to stop it. And, we see after some distance, the book again comes to the rest (Fig C).

Question 67 A.
Give examples to show that friction depends on the nature of two surfaces in contact.
Answer:
The friction depends on the roughness as well as smoothness of the surface. If the surface is rough, then the surfaces in contact have more interlockings and the thus the friction is more. Thus, the object is not able to move easily due to more friction.

If the surface is smooth, then the surfaces in contact have very less interlockings and the thus the friction is less. Thus, the object is able to move easily due to less friction.

Question 67 B.
Give an example to show that friction depends on the force with which the two surfaces are pressed together.
Answer:
Suppose, a person is pushing a heavy box, then he is applying say force F1.

Now if the same person pushes two boxes of same mass as the previous case. Then, in this case he will have to apply more force to overcome the larger friction as in the earlier case. Thus is because friction depends on the mass of the object. Heavier the mass, more the force is exerted by the object on the surface and thus more friction is produced.

Question 68 A.
What is the difference between static friction and sliding friction? For’ a given pair of objects, which of the two is greater?
Answer:
When the body is in stationary state or is at rest, the frictional forces acting on the body is known as static frictional forces.

But when the object is pushed and it moves, the friction that comes into action is sliding friction.

The static friciton is greater than sliding friction because more force is required to remove the objectf rom rest state into dynamic state. Once the obejct comes into motion, it is easier to keep the object moving by applying comparetively lesser force.

Question 68 B.
How can a very heavy machine be moved conveniently from one place to another in a factory? (No crane is available for this purpose).
Answer:
A very heavy machine can be moved from one place to another with the help of wheels. Because then rolling friction comes into action and its value is the least among static and sliding friction.

The following diagram shows, how a heavy cylinder can be rolled easily.

Question 69 A.
What is drag? Give two examples of a drag force.
Answer:
The drag is the other name for fluid friction. It resists the motion of any object that travels in fluid.

Examples: 1. A boat travelling in water.
2. An aeroplane moving in the sky.

Question 69 B.
How can you reduce the drag on something moving through the air?
Answer:
The objects are built in special streamline shape to reduce the drag while moving in the air. Example, birds and Aeroplane have streamlined shapes.

Question 70 A.
What is meant by ‘streamlined shape’? Name an object which usually has a streamlined shape.
Answer:
A streamline shape is the one in which the body is pointed at the ends and broad-shaped in the middle.

Example : Speedboat has streamlined shape.

Question 70 B.
Explain why, objects moving in fluids should have streamlined shape.
Answer:
The objects moving in fluids have streamlined shape so as to reduce the air drag or fluid friction that opposes their motion in any fluid.

Lakhmir Singh Science Class 8 Chapter 12 Multiple Choice Questions (MCQs)

Question 71.
A boy runs his 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, 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
As the roughness of the surface increases, the friction force increases.

Question 72.
Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. The correct arrangement is :
A. Rolling, Static, Sliding
B. Rolling, Sliding, Static
C. Static, Sliding, Rolling
D. Sliding, Static, Rolling
Answer:
C. Static, Sliding, Rolling
The static friction > Sliding Friction > Rolling friction.

Question 73.
A big wooden box is being pushed on the ground from east to west direction. The force of friction due to ground will act on this box towards:
A. north direction
B. south direction
C. east direction
D. west direction
Answer:
D. west direction
The force of friction acts opposite to the direction of motion.

Question 74.
A spring balance can be used to measure
A) Mass of an object
B) Force acting on an object
C) Density of an object
D) Weight of an object
A. A and B
B. Band C
C. B and D
D. only D
Answer:
C. B and D
A spring balance can measure both the force acting on it as well as the weight of the object.

Question 75.
The friction between two surfaces does not depend on one of the following. This one is:
A. amount of surface area of the two objects which is in contact with each other
B. weight of the object which tends to move on the surface of other object
C. degree of smoothness of surfaces of two objects in contact with each other
D. degree of roughness of surfaces of two objects in contact with each other
Answer:
A. amount of surface area of the two objects which is in contact with each other
Friction does not depend on the area of contact.

Question 76.
If the sliding friction between two surfaces is found to be 8 N, then the static friction between these two surfaces is most likely to be:
A. 5 N
B. 10 N
C. 4 N
D. 2 N
Answer:
B. 10 N
The static friction is always greater than sliding friction.

Question 77.
Which of the following is not an advantage of friction?
A. it enables drawing to be made on paper
B. it enables fallen things to be picked up
C. it enables rubber pads to be rubbed off
D. it enables vehicles to move on ground
Answer:
C. it enables rubber pads to be rubbed off
Friction does wear and tear of the objects.

Question 78.
Which of the following statements is incorrect ?
A. static friction is greater than rolling friction
B. sliding friction is less than rolling friction
C. rolling friction is less than static friction
D. static friction is greater than sliding friction
Answer:
D. static friction is greater than sliding friction
The static friction > Sliding Friction > Rolling friction.

Question 79.
If the static friction between two surfaces X and Y is found to be 20 N, then the rolling friction between these two surfaces should most likely be :
A. 25 N
B. 20 N
C. 5 N
D. 50 N
Answer:
C. 5 N
The relationship between the three types of friction is:
The static friction > Sliding Friction > Rolling friction Thus the rolling friction will be less than 20N

Question 80.
If the static friction between two surfaces P and Q is measured to be 50 N, then the sliding friction between these two surfaces is most likely to be :
A. 75N
B. 45N
C. 55N
D. 65N
Answer:
B. 45N
The static friction > Sliding Friction > Rolling friction.

Question 81.
Which of the following will produce the maximum friction?
A. rubbing of sand paper on glazed paper
B. rubbing of sand paper on glass table top
C. rubbing of sand paper on aluminium frame
D. rubbing of sand paper on sand paper
Answer:
A. rubbing of sand paper on glazed paper
As the roughness of the surface increases, the friction increases.

Question 82.
Four similar cars having exactly the same mass are running at the same speed on the same road when brakes are applied at the same time. The cars come to a stop after covering distances of 5 m, 5.5 m, 4.8 m and 5.2 m respectively. The friction between the brake pads and discs will be the maximum in the car which travels the distance of:
A. 5 m
B. 5.5 m
C. 4.8 m
D. 5.2 m
Answer:
C. 4.8 m
As the distance travelled is the least in 4.8 m , thus friction would have been the largest.

Question 83.
The weight of an object can be measured by a:
A. beam balance
B. analytical balance
C. spring balance
D. physical balance
Answer:
C. spring balance
The spring balance measures weight of an object.

Question 84.
A book is lying on the horizontal table top. If we tilt the table a little, then the book starts sliding down slowly. This happens because:
A. sliding friction is greater than static friction
B. sliding friction is less than force of gravity
C. static friction is greater than sliding friction
D. force of gravity is less than sliding friction
Answer:
B. sliding friction is less than force of gravity
The book starts sliding down slowly sliding slowly, because the sliding friction is less than force of gravity.

Question 85.
A body shape which offers very little resistance to the flow of air (or water) around it is called:
A. trimlined shape
B. steamlined shape
C. streaklined shape
D. streamlined shape
Answer:
D. streamlined shape
A body shape which offers very little resistance to the flow of a fluid around it, is called streamlined shape.

Question 86.
Which of the following should be used to reduce friction on a carrom board?
A. a lubricating oil
B. a dry lubricant
C. a layer of grease
D. a ball bearing
Answer:
B. a dry lubricant
Using talcum powder to reduce the friction on carom board.

Question 87.
Which of the following does not have a streamlined shape?
A. Aeroplane
B. boat
C. bird
D. bus
Answer:
D. bus
Bus is not streamlined shape. It is a cuboid shape.

Question 88.
The frictional force exerted by a fluid is called:
A. brag
B. drab
C. drag
D. tread
Answer:
C. drag
The frictional force exerted by fluids is called drag force.

Question 89.
A person has applied some mustard oil on his hands. Which of the following objects will become most difficult for him to hold in his hand?
A. Earthen cup (kulhar)
B. thermocol tumbler
C. glass tumbler
D. wooden cup
Answer:
C. glass tumbler
Mustard oil reduces the friction on his hands. So, Glass tumbler will be most difficult to handle.

Question 90.
Ball bearing is a device which usually converts:
A. rolling friction into sliding friction
B. static friction into sliding friction
C. sliding friction into rolling friction
D. rolling friction into static friction
Answer:
C. sliding friction into rolling friction
Because rolling friction allows to move heavier objects from one place to other easily.

Lakhmir Singh Science Class 8 Chapter 12 Questions Based on High Order Thinking Skills (HOTS)

Question 91.
When a pencil cell is released from a certain point on an inclined wooden board, it travels a distance of 35 cm on floor A before it comes to rest. When the same pencil cell is released from the same point on the same inclined board, it travels a distance of 20 cm on floor B before coming to rest. Which floor, A or B, offers greater friction? Give reason for your answer.
Answer:
Floor B offers greater friction because it stops the moving pencil cell stop at a lesser distance of 20 cm on the floor. More the friction, more the surface is rough.

Question 92.
A car is moving towards North. What will be the direction of force of friction acting on this car due to surface of road ?
Answer:
The direction of force of friction acting on this car due to surface of road is along the south direction i.e. opposite to the direction of motion.

Question 93.
You spill a bucket of soapy water on a marble floor accidently. Would it make easier or more difficult for you to walk on the floor? Why?
Answer:
It will make more difficult to walk on the floor. As soapy solution decreases the friction on the floor.

Question 94.
What kind of friction comes into play:
(a) when a block of wood kept on table moves slowly ?
(b) when a block of wood kept on table just tends to move (or slip)?
(c) when a block of wood kept on cylindrical iron rods moves?
Answer:
(a) Sliding friction

(b) Static friction

(c) Rolling friction

Question 95.
Explain why, it is easier to drag a mat on floor when nobody is sitting on it but much more difficult to drag the same mat when a person is sitting on it.
Answer:
When no person is sitting on the mat, then the mat and floor are not pressed together.

Thus, there is no friction.
But, when a person is sitting on the mat.

Then due to the weight of the person, the mat and floor are pressed together harder, increasing the friction too much. Thus, then it is very difficult to drag the person.

Lakhmir Singh Science Class 8 Solutions