NCERT Solutions For Class 8 Science Science Chapter 1: Crop Production and Management Class 8
NCERT Solutions For Class 8 Science Science Chapter 1: Crop Production and Management Class 8
NCERT Solutions For Class 8 Science Science Chapter 1: Crop Production and Management Class 8
in
Select the correct word from the following list and fill in the blanks.
(a) The same kind of plants grown and cultivated on a large scale at a place is called _________.
(b) The first step before growing crops is ________ of the soil.
(d) For growing a crop, sufficient sunlight and _________ and _________ from the soil are essential.
Answer :
(a) The same kind of plants grown and cultivated on a large scale at a place is called _ crop _.
(b) The first step before growing crops is _preparation__ of the soil.
(d) For growing a crop, sufficient sunlight and _ water __ and _ nutrients_ from the soil are essential.
Q2 :
A B
(i) Kharif crops (a) Food for cattle
(ii) Rabi crops (b) Urea and super phosphate
(iii) Chemical fertilisers (c) Animal excreta, cow dung, urine and plant waste
(iv) Organic manure (d) Wheat, gram, pea
(e) Paddy and maize
Answer :
A B
(i) Kharif crops (e) Paddy and maize
(ii) Rabi crops (d) Wheat, gram, pea
(iii) Chemical fertilisers (b) Urea and super phosphate
(iv) Organic manure (c) Animal excreta, cow dung, urine and plant waste
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Q3 :
Answer :
Q4 :
Answer :
It is the first method to be followed before growing a crop. This method is usually employed for loosening the soil to allow the root to
penetrate deep into it. The loosening of the soil helps in the growth of several soil microbes, earthworms etc., which enrich the soil
with humus and other essential nutrients. Plants require nutrients for their proper growth and functioning. The process of loosening
is called tilling or ploughing the soil. Tilling of soil brings the nutrient-rich soil to the top. This helps the plants to utilize the nutrients
for their growth.
(b)Sowing:
Sowing is another important step in crop production. It is the process of placing the seed in or on the soil for future growth. The
seeds that are selected for growing should be of good quality. This will improve the net yield of the crop. Sowing is usually done with
the help of either a traditional tool or a seed drill. The traditional tool is shaped like a funnel. It was used earlier for sowing seeds.
Nowadays, seed drills that make the use of tractors are used for sowing seeds. This tool disperses seeds uniformly and sows seeds
at proper depth. Sowing by this method saves time and also protects the seeds from birds.
(c)Weeding:
Undesirable plants that grow along with the crop are known as weeds. Weeding is the process of removing these
weeds.Xanthium, Parthenium, etc. are some common weeds. Weeds compete with the crop for nutrients, light, and space. As a
result, crop plants get lesser nutrients, light, and space for their development. This in turn, reduces their productivity. Thus, various
weeding methods are employed.
(i) Weeds can be controlled using weedicides. It is a chemical, which is sprayed in the fields to kill all available weeds. Weedicides
are not harmful to crops.
(ii) Tilling before sowing of crops also helps in removing weeds. Tilling uproots the weeds. The best time for removal of weeds is
before they produce flowers and seeds.
(iii) The manual method of removing weeds is with the help of a khurpi. It involves regular uprooting or cutting of weeds close to the
ground.
(d)Threshing:
Threshing is the process of separating grains or seeds from chaff. It is done after harvesting the crop. It is usually carried out with
the help of a machine known as 'Combine'. This machine is a combined harvester and thresher. It harvests plants as well as cleans
grains.
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Q5 :
Answer :
Fertiliser Manure
Manure is a natural substance prepared
Fertilisers are commercially available plant
by the decomposition of animal excreta
nutrients.
and plant wastes.
Manure is known to have a large
They can be organic or inorganic in nature. quantity of organic materials and very
little amount of plant nutrients.
They ensure healthy growth and development
They help in enriching the soil with
of plants by providing them with nitrogen,
organic matter and nutrients.
phosphorus, potassium, etc.
The addition of fertilisers to the soil requires
The addition of manure does not require
special guidelines such as dose time, post
any special guidelines.
addition precautions, etc., to be followed.
A fertiliser does not provide any humus to the Manure provides humus to the soil and
soil. increases soil fertility.
Its excessive use causes water pollution. It It protects the environment and helps in
cannot replenish organic matter of soil. recycling farm waste.
Q6 :
Answer :
Irrigation is the process by which water is supplied to crops at different intervals. The time and frequency of irrigation varies
according to different seasons, crops, and soil types. There are various sources of irrigation such as wells, canals, rivers, dams,
ponds, and lakes.
a) Sprinkler system:
This system is more useful on uneven land, having fewer water supplies. In this method, water is supplied using pipes to one or
more central locations within the field. When water is allowed to flow under high pressure with the help of a pump, it gets sprinkled
on the crops.
b)Drip system:
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In this system, water is delivered at or near the roots of plants, drop by drop. This is the most efficient method of irrigation as there is
no wastage of water at all. This method is important in areas where water availability is poor.
Q7 :
Answer :
If wheat is sown in the kharif season (from June to October), then the whole crop might get destroyed because of many factors such
as lack of optimum temperature, adaptability, availability of pests, etc. Kharif season includes the rainy season, which is not
favourable for the growth of wheat crop. Therefore, wheat crop should not be sown during this season.
Q8 :
Explain how soil gets affected by the continuous plantation of crops in a field.
Answer :
Continuous plantation of crops in a field makes the soil poor in certain nutrients such as nitrogen, phosphorus, potassium, etc.
Plants require nutrients for their proper growth and functioning. When a farmer continues to grow crops one after the other, then all
nutrients available in the soil reduce and the crop yield decreases automatically.
Q9 :
Answer :
Undesirable plants that grow along with crop plants are known as weeds. Xanthium, Parthenium, etc. are some common weeds.
Weeds compete with the crop for nutrients, light, and space. As a result, crop plants get lesser nutrients, light, and space for their
development. This in turn, reduces their productivity. Thus, various weeding methods are employed.
i. Weeds can be controlled using weedicides. It is a chemical, which is sprayed in the fields to kill all available weeds. Weedi cides are not
harmful to crops.
ii. Tilling before sowing of crops also helps in removing weeds. Tilling uproots the weeds. The best time for the removal of weeds is before
they produce flowers and seeds.
iii. The manual method of removing weeds is with the help of a khurpi. It involves regular uprooting or cutting of weeds close to the ground.
Q10 :
Arrange the following boxes in proper order to make a flow chart of sugarcane crop production.
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Answer :
Q11 :
Complete the following word puzzle with the help of clues given below.
Down
Across
Answer :
Down
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1. IRRIGATION
2. STORAGE
5. CROP
Across
3. HARVESTOR
4. GRAM
6. WINNOWING
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NCERT Solutions for Class 8 Science Chapter 2
Micro organisms: Friend and Foe
(b) Blue green algae fix _________ directly from air to enhance fertility of soil.
Answer :
Q2 :
Answer :
Q3 :
Column I Column II
(i) Bacteria (a) Fixing nitrogen
(ii) Rhizobium (b) Setting of curd
(iii) Lactobacillus (c) Baking of bread
(iv) Yeast (d) Causing malaria
(v) A protozoan (e) Causing cholera
(vi) A virus (f) Causing AIDS
(g) Producing antibodies
Answer :
Column I Column II
(i) Bacteria (e) Causing cholera
Q4 :
Can microorganisms be seen with the naked eye? If not, how can they be seen?
Answer :
Micro-organisms are too small to be seen through naked eyes. They can be seen with the help of a magnifying glass or microscope.
For example, fungus that grows on bread is so small that it can be seen only with the help of a magnifying glass or microscope.
Q5 :
Answer :
Q6 :
Name the microorganisms which can fix atmospheric nitrogen in the soil.
Answer :
Bacteria such as Rhizobium and certain blue-green algae present in the soil can fix atmospheric nitrogen and convert it into usable
nitrogenous compounds. These nitrogenous compounds can be easily utilized by plants for the synthesis of plant proteins and other
compounds.
Q7 :
Answer :
Micro-organisms are too small to be seen through naked eyes. However, they are vital to plants and the environment.
Importance of micro-organisms:
They are used in winemaking, baking, pickling, and other food making processes.
Alcoholic fermentation by yeast is widely used in the preparation of wine and bread. A bacterium Lactobacillus, promotes the
formation of curd.
Microbes are used to reduce pollution. For example, decomposers such as bacteria and fungi break down dead bodies and excreta
to form inorganic compounds, which can be absorbed by plants.
They are used to increase the soil fertility by fixing the atmospheric nitrogen with the help of bacterium Rhizobium and some other
blue-green algae.
Microbes also play an important role in the preparation of medicines. Antibiotics are chemicals produced by micro-organisms to kill
bacteria. Streptomycin, for example, is an antibiotic.
Certain microbes are also used in the biological treatment of sewage and industrial effluents.
Q8 :
Answer :
Micro-organisms cause diseases in animals. For example, in humans, bacteria cause diseases such as tuberculosis, cholera,
typhoid, etc. In cattle, the foot and mouth disease is caused by a virus. Also, several microbes cause diseases in plants. For
example, the productivity of wheat, orange, apple, etc. is reduced due to microbial diseases in plants. Certain microbes, on entering
into our body, produce toxic substances. This leads to food poisoning. Some micro-organisms such as fungus spoil our food. For
example, bread when left unused under moist conditions gets spoilt by fungus, producing a white cotton-like growth on the bread.
Q9 :
What are antibiotics? What precautions must be taken while taking antibiotics?
Answer :
Antibioticsare medicines produced by certain micro-organisms to kill other disease-causing micro-organisms. These medicines are
commonly obtained from bacteria and fungi. Streptomycin, tetracycline, penicillin, etc. are common antibiotics.
(i) Antibiotics should be taken under the supervision of a well qualified doctor.
(ii) Course (intake) of antibiotics should be completed as per the prescription given by the doctor.
(iii) Antibiotics should be taken in the right amount and at the right time. A wrong dose of antibiotics makes the drug ineffective. Also,
excessive consumption of drugs may kill the useful bacteria present in our body.
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Answer :
There are some fibres that are prepared by man by using chemicals. These are called synthetic fibres. These are made of small
units that join together to form long chains. Examples of synthetic fibres are rayon, nylon, polyester, acrylic, etc.
Q2 :
(c) its fibres can also be woven like those of natural fibres.
Answer :
(c) its fibres can also be woven like those of natural fibres.
Q3 :
(a) Synthetic fibres are also called _______ __or _______ __fibres.
(b) Synthetic fibres are synthesised from raw materials called _______ __.
Answer :
(b) Synthetic fibres are synthesised from raw materials called __petrochemicals__.
Q4 :
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Give examples which indicate that nylon fibres are very strong.
Answer :
Nylon fibres are very strong. It is used for making ropes used for climbing rocks and for making parachutes. Their usage shows that
nylon fibres have high tensile strength.
Q5 :
Answer :
The characteristics that make plastics favourable for storing food items are:
Q6 :
Answer :
Q7 :
Answer :
(a) Saucepan handles are made of thermosetting plastics because these plastics do not get softened on heating. Also,
thermosetting plastics such as bakelite are poor conductors of heat.
(b) Thermosetting plastics such as bakelite are poor conductors of heat and electricity. Therefore, they are used for making electric
plugs, switches, plug boards, etc.
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Q8 :
Categorize the materials of the following products into 'can be recycled' and 'cannot be recycled'.
Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, plastic covering on
electrical wires, plastic chairs, electrical switches.
Answer :
Q9 :
Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana,
giving your reason.
Answer :
Rana should buy shirts made from cotton. This is because cotton is a good absorber of water. It can soak the sweat coming out of
our body and expose it to the environment. Thus, it helps in evaporating the liquid (sweat), thereby cooling our body.
Q10 :
Answer :
Plastics are not corroded even if they come in contact with strong chemicals. This is because of their non-reactive nature with most
materials. For example, the cleansing chemicals that we use at home are stored in plastic bottles, instead of metal containers.
Q11 :
Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.
Answer :
No. The handle and bristles of a tooth brush should be made of different materials. The handle of a toothbrush should be hard and
strong, while the bristles should be soft and flexible.
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Q12 :
Answer :
Plastics are non-biodegradable. Once introduced into the environment, they take several years to decompose. Plastics add to the
environmental pollution. They cannot be burnt as when burnt, they release poisonous gases. Plastic bags thrown in the garbage
dump are swallowed by animals like cows. These plastic bags choke their respiratory system and can even prove fatal. Therefore,
we should avoid plastics as far as possible.
Q13 :
Match the terms of column A correctly with the phrases given in column B.
A B
(i) Polyester (a) Prepared by using wood pulp
(ii) Teflon (b) Used for making parachutes and stockings
(iii) Rayon (c) Used to make non-stick cookware
(iv) Nylon (d) Fabrics do not wrinkle easily
Answer :
A B
(i) Polyester (d) Fabrics do not wrinkle easily
(ii) Teflon (c) Used to make non-stick cookware
(iii) Rayon (a) Prepared by using wood pulp
(iv) Nylon (b) Used for making parachutes and stockings
Q14 :
Answer :
Raw materials for natural fibres are mainly derived from plants and this means cutting a lot of trees. This leads to deforestation. But
raw materials of synthetic materials are mainly petrochemicals. Hence, manufacturing synthetic fibres helps in the conservation of
forests.
Q15 :
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Answer :
We will design a circuit to see that thermoplastics are poor conductors of electricity. We need a bulb, some wires, a battery, a piece
of metal, and a plastic pipe. Set up the circuit first with the metal and then with the plastic pipe (as shown in the figure). After you
switch on the current, you will observe that the bulb glows in the former case. In the latter case, the bulb does not glow. Hence, a
plastic pipe (which is a thermoplastic) is shown to be a poor conductor of electricity.
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Answer :
(a) Zinc
Q2 :
Answer :
However, mercury metal- a liquid at room temperature - cannot be drawn into wires and is not ductile.
Q3 :
Answer :
Q4 :
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Answer :
Q5 :
Some properties are listed in the following Table. Distinguish between metals and non-metals on the basis of these
properties.
Answer :
Q6 :
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(b) Immersion rods for heating liquids are made up of metallic substances.
Answer :
(a) Aluminium foils are used to wrap food items because aluminium metal is malleable. Therefore, it can be beaten into thin foils.
(b) Metals are good conductors of heat and electricity. Therefore, immersion rods for heating liquids are made of metallic
substances.
(c) A metal can displace a less reactive metal from its salt in an aqueous solution. But zinc is more reactive than copper. Therefore,
copper cannot displace zinc from its salt solution.
(d) Sodium and potassium are stored in kerosene because they are highly reactive elements. They can easily catch fire even when
in contact with air.
Q7 :
Answer :
Lemon pickle cannot be stored in aluminium utensils because lemon pickle contains acids, which can react with aluminium (metal)
liberating hydrogen gas. This can lead to the spoiling of the pickle.
Q8 :
Match the substances given in Column A with their uses given in Column B.
A B
(i) Gold (a) Thermometers
(ii) Iron (b) Electric wire
(iii) Aluminium (c) Wrapping food
(iv) Carbon (d) Jewellery
(v) Copper (e) Machinery
(vi) Mercury (f) Fuel
Answer :
A B
(i) Gold (d) Jewellery
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Q9 :
Answer :
(a) When dilute sulphuric acid is poured on a copper plate, there will be no reaction between copper and dilute sulphuric acid as
copper is less reactive and hence no products will be formed.
(b) Iron being more reactive displaces copper from copper sulphate solution. In this reaction, the blue colour of copper sulphate
fades and there is deposition of copper on the iron nail.
Q10 :
Saloni took a piece of burning charcoal and collected the gas evolved in a test tube.
(b) Write down word equations of all the reactions taking place in this process.
Answer :
(a) Add a few drops of water in the test tube containing gas. Now, cover the test tube and shake it well. After shaking, test the
solution with blue litmus and red litmus. It will turn blue litmus red. Thus, the gas is acidic in nature.
Carbon dioxide reacts with water to form carbonic acid, which turns blue litmus paper red.
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Q11 :
One day Reeta went to a jeweller's shop with her mother. Her mother gave an old gold jewellery to the goldsmith to polish.
Next day when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a
reason for the loss in weight?
Answer :
To polish a gold ornament, it is dipped in a liquid called aqua regia (a mixture of hydrochloric acid and nitric acid). On getting the
environment of aqua regia, the outer layer of gold dissolves and the inner shiny layer appears. The dissolving of the layer causes a
reduction in the weight of the jewellery.
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NCERT Solutions for Class 8 Science Chapter 5
Coal and Petroleum
Answer :
The advantages of using compressed natural gas (CNG) and liquified petroleum gas (LPG) as fuels are:
(i) They can be burnt directly.
(ii) They can be transported easily through pipe lines.
(iii) They are clean fuels and do not give smoke when burnt.
(iv) They give a lot of heat energy when burnt.
Q2 :
Answer :
Q3 :
Describe how coal is formed from dead vegetation. What is this process called?
Answer :
Millions of years ago, dense forests got buried under the soil due to natural processes like storms, floods, and earthquakes. These
got compressed as more and more soil got deposited over them. When they got buried deep in the soil, they were exposed to very
high pressure and temperature. Under these conditions, these slowly got converted into coal. This process of formation of coal from
dead vegetation is called carbonization.
Q4 :
Answer :
Q5 :
Tick True/False against the following statements.
Answer :
Q6 :
Answer :
Fossil fuels require millions of years to form from the dead vegetation and animals that get buried deep inside the Earth. They
require high temperature and pressure for their formation, which cannot be provided in the laboratory. Thus, fossils are limited.
Therefore, the use of fossil fuels at this rate will lead to their exhaustion.
Q7 :
Answer :
Uses of coke:
Q8 :
Answer :
Petroleum was formed from dead organisms that got buried in the sea millions of years ago. These dead bodies got covered with
layers of sand and clay. Lack of air, high temperature, and high pressure transformed these dead organisms into petroleum and
natural gas.
Q9 :
The following Table shows the total power shortage in India from 1991 - 1997. Show the data in the form of a graph. Plot
shortage percentage for the years on the Y-axis and the year on the X-axis.
Answer :
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Answer :
Combustion is a process of reaction of a substance with oxygen. There are certain conditions required for combustion to take place.
They are:
Q2 :
Answer :
(c) Fuel must be heated to its __ignition temperature__ before it starts burning.
Q3 :
Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer :
Combustion of fuels like petroleum causes formation of un-burnt carbon particles along with carbon monoxide gas. These harmful
pollutants enter the air and cause respiratory diseases. Compressed Natural Gas (CNG) produces these harmful products in very
less quantity. It is a comparatively cleaner fuel. Therefore, the use of CNG has reduced pollution in our cities.
Q4 :
Answer :
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Wood has been a traditional fuel for both domestic and industrial use. However, it produces a lot of smoke that can cause
respiratory problems. Also, wood is obtained from trees. Thus, using wood as a fuel causes deforestation. Therefore, slowly wood is
replaced by LPG, which is a liquefied form of petroleum gas. It does not give out smoke and other pollutants and is a cleaner fuel.
Again, fuel efficiency of LPG is more than that of wood. The calorific value of LPG is 55000 kJ / kg, while that of wood is between
17000 to 22000 kJ / kg. Hence, LPG is favoured over wood.
Q5 :
Give reasons.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Answer :
(a) Water is a good conductor of electricity. If it is used for controlling a fire involving electrical equipments, then the person dousing
the fire might get an electric shock. Also, water can damage electrical equipments.
(b) LPG is a better domestic fuel as it does not produce smoke and un-burnt carbon particles, which cause respiratory problems.
(c) A piece of paper wrapped around aluminium pipe does not catch fire easily. This is because aluminium, being a metal, is a good
conductor of heat. Therefore, heat is transferred from the paper to the metal and the paper does not attain its ignition temperature.
Q6 :
Answer :
Q7 :
Answer :
Q8 :
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It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer :
Green leaves have a lot of moisture in them. This moisture does not allow them to catch fire easily. However, dry leaves have no
moisturein them. Therefore, they catch fire easily.
Q9 :
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer :
Goldsmiths use the outermost part/zone of the flame to melt gold and silver. This is because the outermost zone of the flame
undergoes complete combustion and is the hottest part of the flame.
Q10 :
In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the
calorific value of the fuel.
Answer :
The calorific value of fuel is the amount of heat produced by the complete combustion of 1 kg of fuel.
Now,
= 40,000 kJ/kg
Q11 :
Answer :
Combustion is a chemical process in which a substance reacts with oxygenand gives out energy during the process in the form of
either heat or light or both. Rusting of iron is an exothermic process as heat is released during rusting. Hence, it is a kind of slow
combustion.
Q12 :
Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the
wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will
get heated in a shorter time?
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Answer :
The water in the Ramesh's beaker will heat up in a shorter time. This is because the outermost zone of a flame is the hottest zone,
while the yellow zone (in which Abida had kept the beaker) is less hot.
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(a) A place where animals are protected in their natural habitat is called a __ .
(c) Migratory birds fly to far away places because of ____ _ changes.
Answer :
(a) A place where animals are protected in their natural habitat is called a sanctuary__.
(c) Migratory birds fly to far away places because of __climatic__ changes.
Q2 :
Answer :
(a)
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Blue whale, tiger, leopard, etc. are examples Dodo, passenger pigeon, etc. are examples
of endangered species. of extinct species.
(d)
Flora Fauna
It refers to all living plants in a particular It refers to all animals living in a particular
area. area.
Sal, teak, mango, etc. form the flora of Leopard, wolf, wild dog, etc. form the fauna
Pachmarhi biosphere reserve. of Pachmarhi biosphere reserve.
Q3 :
(b) Environment
(e) Earth
Answer :
Deforestation is the removal of trees or other vegetation from an area for industrial, agricultural, or other purposes. Trees and other
vegetation form the habitat of many animals. Hence, if the habitat of wild animals is destroyed, then their numbers would
automatically decline.
Plants absorb CO2 from the atmosphere to perform photosynthesis. If plants are destroyed, then the level of CO2 in the atmosphere
will rise. As a result, CO2 will trap more heat radiations, thereby adding to global warming. An increase in the temperature of the
Earth will disturb the natural water cycle. As a result, there will be a change in the rainfall pattern. This could lead to floods or
droughts.
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Roots of plants hold soil particles together. In the absence of plants, the top layer of the soil will be easily removed by the action of
high speed winds or water flow. Thus, deforestation increases the chances of soil erosion. As a result, soil loses humus and become
less fertile. Hence, a fertile land, which is a source of living for farmers in villages, gets converted into a desert.
Deforestation in cities can increase the risk of many natural calamities such as floods and droughts in that area. Also, it can lead to
global warming due to an increase in the level of CO2 in the atmosphere as a result of vehicular and industrial pollution. This
increase in temperature can disturb the natural water cycle of an area
As a result of deforestation, chances of desertification, droughts, floods, etc. increase. Deforestation can also increase the level of
CO2 in the Earth. It will lead to an increase in temperature i.e., global warming. As a result, the entire natural water cycle will get
disrupted. This again increases the risk of natural calamities.
Deforestation is slowly changing our environmental conditions. It is responsible for global warming, soil erosion, greenhouse effect,
drought, floods, and many other global problems. As a result, the next generation will have to face severe consequences of
deforestation.
Q4 :
Answer :
(a) If we go on cutting trees, the natural habitat of many animals will get completely destroyed. As a result, the biodiversity of many
areas will be severely affected. Also, there will be an increase in the temperature of the Earth as a result of global warming, which
can disturb the natural water cycle. As a result, there will be a change in the rainfall pattern. This could lead to floods or droughts.
This will also increase the risk of soil erosion, desertification, and natural calamities.
(b) The habitat of an animal provides it with necessities such as shelter, food, and protection. If the habitat of an animal is disturbed,
then it will be forced to go to other places in search of food and shelter. The animal could get killed by other animals in this process.
(c) If the top layer of soil is exposed, then it will gradually expose the lower layer of soil, which is hard and rocky in nature. This type
of soil is less fertile as it contains less humus. Continued soil erosion will make the land barren or infertile.
Q5 :
Answer in brief:
(b) Protected forests are also not completely safe for wild animals. Why?
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Answer :
(a) Biodiversity refers to the number and variety of various life forms such as plants, animals, and micro-organisms in an area.
Plants and animals depend on each other for survival. This means that the destruction of either of the two will affect the life of the
other. Hence, we need to conserve biodiversity to maintain the balance of nature.
(b) Protected forests are not completely safe for wild animals because people who live near or adjacent to forests use resources
from forests to fulfil their own requirements. In this process, wild animals are killed and sold for lucrative amounts of money.
(c) Tribals gather food, fodder, and fallen branches of trees from forests. Hence, they depend on forests for their daily requirements.
(i) Forests are cleared for accommodating expanding urban areas and for fulfilling their ever-increasing requirements.
(ii) Forests are destroyed to clear land for crops and cattle grazing.
Consequences of deforestation:
(e) Red Data Book is a source book that maintains an international list of all endangered animal and plant species. This book is
maintained by IUCN (International Union for Conservation of Nature and Natural resources).
(f) Migration refers to the movement of an organism or a group of organisms from its natural habitat to another place at a particular
time every year. Organisms migrate from one place to another to avoid inhabitable climatic conditions or for breeding.
Q6 :
In order to meet the ever-increasing demand in factories and for shelter, trees are being continually cut. Is it justified to cut
trees for such projects? Discuss and prepare a brief report.
Answer :
No. It is not at all justified to cut trees to meet the ever increasing demands of human population. Forests are the habitat of several
organisms including wild animals. They provide us with good quality air as they give out O2 and absorb the harmful CO2 gas from the
atmosphere. In the process, they prevent the excessive heating of the atmosphere. They prevent soil erosion and natural calamities
such as floods and droughts. They increase the fertility of the soil and help conserve biodiversity. The cutting of forests to meet the
demands of growing human population will lead to global warming, soil erosion, greenhouse effect, droughts, floods, and many
more problems. The destruction of forests will disturb the balance of nature. Hence, forests must be conserved.
Q7 :
How can you contribute to the maintenance of green wealth of your locality? Make a list of actions to be taken by you.
Answer :
I can help in maintaining the green wealth of my locality by taking care of the plants and trees growing in or around my locality. I can
plant more and more trees. I can also encourage the people in my locality to plant more trees by informing them about the
importance of growing trees. I can make young children aware of the effects that deforestation has on our environment and on our
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planet. I can also ask them to water the plants daily, which will take very little of their time. I believe planting new trees is as
important as taking care of the existing trees.
Q8 :
Answer :
Deforestation is the removal of trees or other vegetation from an area for industrial, agricultural, or other purposes. Plants or trees
absorb CO2 from the atmosphere. If plants are destroyed, then the level of CO2 in the atmosphere will rise. The high levels of CO2 in
the atmosphere will trap more heat radiations, leading to global warming. This increase in temperature of the Earth will disturb the
natural water cycle. As a result of disruption in the water cycle, there will be a change in the rainfall pattern. The reduced amount of
rainwater can cause droughts.
Q9 :
Find out about national parks in your state. Identify and show their location on the outline map of India.
Answer :
One of the national parks located near Delhi is the Corbett National Park.
Q10 :
Why should paper be saved? Prepare a list of ways by which you can save paper.
Answer :
Paper should be saved because it takes around seventeen full grown trees to make one tonne of paper. Trees, as we know, are
important to maintain a balance of nature. Therefore, in order to save trees and prevent the impact of their loss on living organisms,
we need to save paper.
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Q11 :
Down
5. Consequence of deforestation.
Across
Answer :
Down
1. ENDANGERED
5. DESERTS
Across
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1. EXTINCT
3. ENDEMIC
4. BIODIVERSITY
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Indicate whether the following statements are True (T) or False (F).
Answer :
Q2 :
Make a sketch of the human nerve cell. What function do nerve cells perform?
Answer :
The function of a nerve cell is to transmit messages to the brain and also to take away messages from the brain to the receptor
organs. Thus, it controls the working of different parts of the body.
Q3 :
(a) Cytoplasm
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Answer :
(a) Cytoplasm:
It is a fluid that fills the cell and occurs between the plasma membrane and the nucleus. Cell organelles such as mitochondria,
ribosomes, Golgi bodies, etc. are suspended in the cytoplasm. The cytoplasm helps in the exchange of materials between cell
organelles.
The nucleus is a spherical structure generally present at the centre of a cell. The nucleus is composed of the following components:
It is a double-layered membrane which separates the contents of the nucleus from the cytoplasm. The nuclear membrane has
nuclear pores that allow the transfer of specific substances in and out of the nucleus.
(ii) Nucleolus:
(iii) Chromosomes:
These are thread-like structures that carry genes. Genes contain information necessary for the transfer of characteristics from the
parents to the offspring. Thus, chromosomes play an important role in the inheritance of characteristics.
Q4 :
Answer :
Cytoplasm is the part of the cell that contains various organelles such as mitochondria, ribosomes, Golgi bodies, etc. Cytoplasm is a
fluid that fills the cell and occurs between the plasma membrane and the nucleus.
Q5 :
Answer :
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Prokaryotes Eukaryotes
Most prokaryotes are unicellular. Most eukaryotes are multicellular.
The nucleus is poorly defined due to the The nucleus is well defined and is
absence of a nuclear membrane. surrounded by a nuclear membrane. Q6 :
Where
Nucleolus is absent Nucleolus is present.
are
Cell organelles such as plastids, Cell organelles such as plastids, chromos
omes
mitochondria, golgi bodies, etc. are absent. mitochondria, golgi bodies, etc. are present. found in
a cell?
Bacteria and blue-green algae are Fungi, plant, and animal cells are eukaryotic State
prokaryotic cells. cells. their
function.
Answer :
The nucleus contains thread-like structures called chromosomes. Chromosomes play an important role in the inheritance of
characters. They carry genes that help in the transfer of characters from the parents to the offspring.
Q7 :
Answer :
Cells constitute various components of plants and animals. A cell is the smallest unit of life and is capable of all living functions.
Cells are the building blocks of life. This is the reason why cells are referred to as 'the basic structural and functional units of life'. All
cells vary in their shapes, sizes, and activities they perform. In fact, the shape and size of the cell is related to the specific function it
performs.
Q8 :
Answer :
Chloroplasts are found only in plant cells. They contain a green pigment called chlorophyll. This green pigment is important for
photosynthesis in green plants. This chlorophyll pigment traps solar energy and utilizes it to manufacture food for the plant.
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Q9 :
Across
Down
1. Green plastids.
7. A group of cells.
Answer :
Across
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1. CHLOROPHYLL
3. ORGANELLE
6. PROTOPLASM
8. GENES
Down
1. CHLOROPLASTS
2. ORGAN
4. MEMBRANE
5. VACUOLE
7. TISSUE
Q10 :
Make sketches of animal and plant cells. State three differences between them.
Answer :
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They are generally small in size. They are usually larger than animal cells.
Cell wall is absent. Cell wall is present.
Vacuoles are small in size. Vacuoles are larger in size.
No other animal cell possesses plastids Plastids are present.
except for the protozoan Euglena.
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NCERT Solutions for Class 8 Science Chapter 9
Reproduction in Animals
Exercise : Solutions of Questions on Page Number : 110
Q1 :
Answer :
Reproduction is a biological process through which living organisms produce offspring similar to themselves. Living organisms
reproduce to maintain their number and for the continuation of their species. Thus, reproduction ensures the continuation of similar
kinds of individuals.
Q2 :
Answer :
Fertilization involves the fusion of the male and the female gamete. The male and the female gametes are released from the male
and the female reproductive organs. Sperms or male gametes are released from the male reproductive organ i.e., the penis. These
sperms then enter the female body through the vagina. Then, they travel through the fallopian tubes where they meet the eggs.
Hence, the process of fertilization takes place in the fallopian tubes.
During fertilization, the haploid nucleus of the sperm and that of the ovum fuse with each other to form the zygote. This zygote
divides to form an embryo which in turn develops into a foetus.
Q3 :
(i) fertilization
(ii) metamorphosis
(iii) embedding
(iv) budding
(i) none
(ii) one
(iii) two
(iv) four
Answer :
(b) (ii) A tadpole develops into an adult frog by the process of metamorphosis.
Q4 :
Indicate whether the following statements are True (T) or False (F).
Answer :
(d) A new human individual develops from a cell called gamete. (F)
Q5 :
Zygote Foetus
It is a fertilized egg formed after the It is a stage of the embryo that shows all the main fusion
of the sperm with the egg. recognizable body parts of a mature organism.
The zygote divides several times to An embryo gradually develops into a foetus. form
an embryo.
Q6 :
Answer :
Asexual reproduction is a mode of reproduction that does not involve the fusion of the male and the female gamete. It requires only
one parent, and the offsprings produced are exact copies of their parents.
It is a type of asexual reproduction in which a single cell divides into two halves. Organisms that reproduce through binary fission
are bacteria and Amoeba. In Amoeba, the division of cells can take place in any plane. It involves the division of its nucleus into two
nuclei, which is followed by the division of its body into two halves. Each half of the body receives a nucleus.
Budding involves the formation of a new individual from the bulges, known as buds formed on the parent body. This method of
reproduction is common in Hydra.In Hydra, thecells divide rapidly at a specific site and develop as an outgrowth, called the bud.
These buds, while being attached to the parent plant, develop into smaller individuals. When these individuals become mature
enough, they detach from the parent's body and become independent individuals.
Budding in Yeast
Q7 :
Answer :
The embryo gets embedded in the wall of the uterus. The embryo while it is still attached to the uterus gradually develops various
body parts such as hands, legs, head, eyes, etc. The embryo is then called a foetus.
Q8 :
Answer :
Metamorphosis is a biological process of transforming a larva into an adult. This involves relatively sudden and abrupt changes in
the animal's structure. Frogs and insects are examples of organisms showing metamorphosis. The life cycle of a frog has three
distinct stages:
The tadpole that emerges from the egg contains gills, a tail, and a small circular mouth. They can swim freely in water. The tadpole
grows and involves abrupt changes in its structure and develops into a mature frog. A tadpole's metamorphosis begins with the
development of limbs, lung development, and finally the absorption of the tail by the body.
Q9 :
Answer :
Q10 :
7. The term used for bulges observed on the sides of the body of Hydra.
Down
Answer :
Across
1. FERTILIZATION
6. INTERNAL
7. BUDS
8. OVARY
Down
2. TESTIS
3. ZYGOTE
4. OVIPAROUS
5. BINARY
.
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What is the term used for secretions of endocrine glands responsible for changes taking place in the body?
Answer :
Hormones are chemical substances which are secreted by endocrine glands. They are responsible for changes taking place in the
body.
Q2 :
Define adolescence.
Answer :
Adolescence is the time period between the beginning of puberty and adulthood. During this period, the body undergoes several
changes alongside reproductive maturity. It begins around the age of 11 and lasts till 18 or 19 years of age. The period of
adolescence may vary from person to person.
Q3 :
Answer :
Menstruation is the process of the shedding of the uterine lining on a regular monthly basis. It begins at puberty and is the
reproductive cycle of the female body. Every month, the uterus prepares itself to receive a fertilized egg. Therefore, the inner lining
of the uterus becomes thick and is supplied with blood to nourish the embryo. If the egg is not fertilized, then the lining of the uterus
breaks down and gets released in the form of blood through the vagina. This lasts for about two to eight days. This cycle occurs
every month and is known as the menstrual cycle.
Q4 :
Answer :
Changes at puberty:
(ii) Broadening of shoulders and widening of chest in boys. In girls, the region below waist becomes wider.
(iii) In boys, under the influence of hormones, the larynx becomes prominent, the vocal cords become longer and thicker. These
changes cause the voice to become hoarse.
(iv) Appearance of hair in areas such as underarms, face, hands, and legs.
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(iv) Testis grows and starts producing sperms in males, whereas in females, the ovary enlarges and starts producing matured eggs.
Q5 :
Prepare a table having two columns depicting names of endocrine glands and hormones secreted by them.
Answer :
Q6 :
What are sex hormones? Why are they named so? State their function.
Answer :
Sex hormones are chemical substances produced by sex organs. For example, testosterone is the male sex hormone produced by
the testis, and oestrogen is the female sex hormone produced by the ovary. These hormones affect the sexual features of an
organism. Hence, they are known as sex hormones.
Testosterone: This hormone brings about secondary sex characters in boys such as the growth of a beard, the voice becoming
hoarse, development of reproductive organs, etc.
Oestrogen: This hormone is responsible for the development of secondary sexual characters in females such as the enlargement
of breasts, development of female reproductive organs, etc.
Q7 :
(ii) proper diet is needed for the rapid growth taking place in their body.
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Answer :
(a) (ii) Adolescents should be careful about what they eat, because proper diet is needed for the rapid growth taking place in their
body.
(b) (i) Reproductive age in women starts when their menstruation starts.
(c) (ii) The right meal for adolescents consists of chapatti, dal, vegetables.
Q8 :
Write notes on -
Answer :
In human males, the larynx grows larger during puberty and can be seen as a protruding part of the throat. This protrusion is known
as the Adam's apple. In boys, under the influence of sex hormones, the larynx becomes prominent. As a result, the vocal cords
become longer and thicker, causing the voice to become hoarse. However, in females, the larynx is of a small size and is hardly
visible. Therefore, girls have a high pitched voice, while the voice of boys is low pitched.
(b) Secondary sexual characters are those features that help in distinguishing the male and the female body from each other.
They are physical or behavioural characteristics that appear in humans at the time of puberty.
(iii) Growth of hair in genital area and other parts of the skin.
(i) Increase in breast size and darkening of the skin of nipples present at the tip of the breasts.
The sex of a baby is determined by the type of male gamete that fuses with the female gamete.
All human beings have 23 pairs of chromosomes in their nuclei. Out of these 23 pairs, the last pair is known as the sex
chromosome.
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The human males have 23 pairs of chromosomes including XY sex chromosomes. Therefore, the male gamete has 22
chromosomes and either an X or Y sex chromosome.
Females have 23 pairs of chromosomes including XX sex chromosomes. Therefore, their gametes can only have 22 chromosomes
and one X sex chromosome.
Thus, as the mother provides only X chromosome, the sex of the baby is determined by the type of male gamete (X or Y) that fuses
with the X chromosome of the female.
Q9 :
Across
9. Pancreatic hormone
Down
1. Male hormone
2. Secretes thyroxin
6. Voice box
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Answer :
Across
3. ADAM'S APPLE
4. ENDOCRINE
7. PITUITARY
8. HORMONE
9. INSULIN
10. ESTROGEN
Down
1. TESTOSTERONE
2. THYROID
3. ADOLESCENCE
5. TARGET SITE
6. LARYNX
7. PUBERTY
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Q10 :
The table below shows the data on likely heights of boys and girls as they grow in age. Draw graphs showing height and
age for both boys and girls on the same graph paper. What conclusions can be drawn from these graphs?
Height (cm)
Age (Years)
Boys Girls
0 53 53
4 96 92
8 114 110
12 129 133
16 150 150
20 173 165
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Answer :
The graph depicts the relation between the age and height of both boys and girls. During puberty, there is a sudden increase in
height of both boys and girls. On the basis of the above graph, it can be observed that during the age of 4-8 years, girls have less
height as compared to boys. However, as soon as girls reach 12- 13 years, their height shows a sudden increase and becomes
more than boys. In later years, growth in both sexes becomes stable. Growth during puberty is under the control of hormones.
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NCERT Solutions for Class 8 Science Chapter 11
Force and Pressure
Exercise : Solutions of Questions on Page Number : 142
Q1 :
Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer :
Q2 :
Give two examples of situations in which applied force causes a change in the shape of an object.
Answer :
Two examples of forces that cause a change in the shape of an object are as follows:
(i) Squeezing of a plastic bottle changes the shape of the bottle.
(ii) Deformation of clay by pressing it between the hands.
Q3 :
(d) The north pole of a magnet ________the north pole of another magnet.
Answer :
(a) To stretch the bow, the archer applies a force that causes a change in its _______.
(b) The force applied by the archer to stretch the bow is an example of ________force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a _______force.
(d) While the arrow moves towards its target, the forces acting on it are due to ________and that due to________of air.
Answer :
(a) To stretch the bow, the archer applies a force that causes a change in its __shape__.
(b) The force applied by the archer to stretch the bow is an example of __muscular__force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a __contact__ force.
(d) While the arrow moves towards its target, the forces acting on it are due to __gravity__and that due to __friction__of air.
Q5 :
In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force
in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
Answer :
(a) In squeezing a piece of lemon, we make use of muscular force to extract its juice. This muscular force acts on the lemon.
As a result, the shape of the lemon gets changed.
(b) We use our muscular force to take out paste from a toothpaste tube. The muscular force acts on the toothpaste tube. As a
result, the shape of the tube gets changed.
(c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets
stretched. Hence, its shape gets changed.
(d) An athlete pushes the ground with his feet. His feet exert a muscular force on the ground. This allows him to jump over
the bar. As a result, his state of motion gets changed.
Q6 :
A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of
iron?
Answer :
When a blacksmith hammers a hot piece of iron, he uses his muscular force. This muscular force changes the shape of the iron so
that it can be given a desired shape.
Q7 :
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 :
On rubbing an inflated balloon with a piece of synthetic cloth, it becomes charged. A charged body attracts an uncharged body.
When this charged balloon is pressed against a wall, it sticks to the wall. The force acting between the charged balloon and the wall
is the electrostatic force.
Q8 :
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 :
We make use of muscular force to hold a bucket of water above the ground. This muscular force acts against the force of gravity
that pulls the bucket towards the ground. The two forces are equal in magnitude but opposite in direction. Therefore, the net force
on the bucket is zero. Hence, there is no change in its state of motion.
Q9 :
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.
Answer :
The two forces acting on the rocket are the force of gravity, which pulls the rocket towards the ground, and the force of friction due to
earth's atmosphere, which opposes its motion.
Q10 :
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
Answer :
(a) Friction opposes the ________between the surfaces in contact with each other.
Answer :
(a) Friction opposes the __motion__between the surfaces in contact with each other.
Q2 :
Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their
arrangements are given below. Choose the correct arrangement.
Answer :
Friction comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. Static
friction comes into play when we try to move an object which is at rest. Sliding friction comes into play when an object slides over
the surface of another object. In sliding friction, the time given for interlocking is very small. Hence, interlocking is not strong.
Therefore, less force is required to overcome this interlocking. Because of this reason, sliding friction is less than static friction.
Similarly, the area of contact in case of rolling friction is smallest as compared to static or sliding friction. This area of contact
changes gradually because of rolling. Hence, rolling friction is lesser than both static and sliding friction. Thus, the correct sequence
is - static, sliding, rolling.
Q3 :
Alida runs her 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 orderwill be
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(a) wet marble floor, dry marble floor, newspaper and towel.
Answer :
(a) wet marble floor, dry marble floor, newspaper and towel
Force of friction depends on the nature of surfaces in contact. The rougher the surface, the more is the friction between the surfaces
in contact and vice-versa. Roughness present in the given surfaces can be arranged in an ascending order as wet marble floor, dry
marble floor, newspaper and towel. Hence, the correct sequence of these surfaces when arranged according to the increase in the
force of friction acting on the car is - wet marble floor, dry marble floor, newspaper and towel.
Q4 :
Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force
acting on it.
Answer :
When a book slides on the writing desk, a frictional force acts between the book and the surface of the desk. The direction of
frictional force on the book is opposite to the direction of its motion and acts in upward direction, as shown in the following figure.
Q5 :
You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on
the floor? Why?
Answer :
We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the
backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction
between the ground and feet decreases when there is soapy water spilled on the floor. Hence, it becomes difficult to walk on the
soapy floor.
Q6 :
Answer :
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Sportsmen use shoes with spikes because these shoes give them a better grip while running. This is because the force of friction
between the shoes and the ground increases with the help of spikes.
Q7 :
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 :
Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the
floor, the interlocking of irregularities on the surfaces of box and floor become strong. This is because the two surfaces in contact
are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply
a greater force than Iqbal.
Q8 :
Answer :
Friction comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. In
sliding, the time given for interlocking is very small. Hence, interlocking is not strong. Therefore, less force is required to overcome
this interlocking. Because of this reason, sliding friction is less than static friction.
Q9 :
Answer :
Advantages of friction:
(ii) Friction between the tip of the pen and a paper allows us to write.
Disadvantages of friction:
(ii) Friction between the different parts of machines produces heat. This can damage the machines.
Q10 :
Answer :
When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. This
opposing force is known as the drag force. This frictional force depends on the shape of the body. By giving objects a special shape,
the force of friction acting on it can be minimised. Hence, it becomes easier for the body to move through the fluid.
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Answer :
Sound requires a medium to travel through. Solid, liquid and gas provide the medium for sound. Hence, sound can travel through
solids, liquids and gases.
Q2 :
Answer :
(c) A man
The voice of an adult man is of lower pitch in comparison to the voices of a baby boy, a baby girl and a woman. Since frequency of a
sound is directly proportional to its pitch, man's voice is of minimum frequency in comparison to a boy, a girl, or a woman's voice.
Q3 :
In the following statements, tick 'T' against those which are true, and 'F' against those which are false.
(b) The number of oscillations per second of a vibrating object is called its time period. (T / F)
(e) The lower the frequency of vibration, the higher is the pitch. (T / F)
Answer :
(a) True
Sound requires a medium to travel through. Since vacuum is devoid of any medium, sound cannot travel through it.
(b) False
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The number of oscillations per second of a vibrating object is known as its frequency.Time period is the time required to complete
one oscillation.
(c) False
Loudness of a sound is proportional to the square of the amplitude of its vibration. When the amplitude of vibration of a sound is
large, the sound is very loud. The sound is feeble for small amplitude.
(d) True
Humans cannot hear sounds of all frequencies. Humans can hear a sound whose frequency falls in the range of 20 Hz-20,000 Hz.
The sound having frequency out of this range is inaudible to humans.
(e) False
The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases
and vice-versa. A sound is said to be high pitched if its frequency of vibration is high, and is low pitched for a small frequency of
vibration.
(f) False
Unwanted or unpleasant sounds are known as noise. Sounds that are melodious and pleasing to ear are known as music.
(g) True
Unwanted or unpleasant sounds are known as noise. If one is subjected to loud unpleasant sound continuously for a long time, then
it may cause temporary hearing impairment.
Q4 :
Answer :
(a) Time taken by an object to complete one oscillation is called __time period__.
(Shrillness is also called pitch of the sound. Pitch is directly proportional to the frequency of vibration. Hence, shrillness is
determined by the frequency of vibration.)
Q5 :
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer :
Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by
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The time required to complete one oscillation is known as time period.It is given by the inverse of the frequency.
Time period
Q6 :
The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is
the time period of the vibration?
Answer :
The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.
Time period
Time period
Q7 :
Identify the part which vibrates to produce sound in the following instruments.
Answer :
(a) Dholakis a musical instrument. It consists of a stretched membrane called its head. When the head is beaten gently, the
stretched membrane sets into vibration. Since sound is produced when an object vibrates, the dholakproduces a sound.
(b) Sitaris a musical instrument. It consists of stretched strings. When a string is plucked, it sets into vibration. Since sound is
produced when an object vibrates, the sitarproduces a sound.
(c) Fluteis a hollow pipe. When air is blown over its mouth, the air inside the pipe is set into vibration. As a result, a pleasant sound
is produced.
Q8 :
What is the difference between noise and music? Can music become noise sometimes?
Answer :
The sound that is pleasing to the ear is called music. For example, the sound produced by violins, pianos, flutes, pungs, etc.
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Q9 :
Answer :
Q10 :
Answer :
Noise pollution can lead to a number of health-related problems. Some of them are as follows:
(iii) Hypertension
(v) Stress
Q11 :
Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the
roadside. Which house would you suggest your parents should buy? Explain your answer.
Answer :
There will be more noise in the house which is along the roadside. This is because noise produced by transportation vehicles may
cause trouble to the residents. The intensity of noise decreases with the distance between the source and the listener. Hence, it is
better to take the house that is three lanes away from the roadside.
Q12 :
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Answer :
Larynx is a part of the throat. It is responsible for production of sound. A sketch of a human larynx is shown in the following figure.
Larynx moves when we swallow something. Inside the larynx, there are two vocal cords. There is a small gap between them. This
small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to
vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.
Q13 :
Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier
and thunder is heard later. Can you explain?
Answer :
The speed of sound is less than the speed of light. Hence, light reaches us before the sound during a lightning, which is
accompanied by thundering.
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(a) Most liquids that conduct electricity are solutions of ________, _______ and ________.
(b) The passage of an electric current through a solution causes _________ effects.
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to
the__________ terminal of the battery.
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called_________.
Answer :
(a) Most liquids that conduct electricity are solutions of __acids__, __bases__ and __salts__.
(The solutions of acids, bases or salts are conducting in nature. They allow the current to pass through.)
(b) The passage of an electric current through a solution causes __chemical__ effects.
(When an electric current passes through a solution, the solution decomposes into its positive and negative ions. This process of
decomposition of the solution is a chemical effect.)
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the__negative__ terminal
of the battery.
(When an electric current passes through a copper sulphate solution, the solution decomposes into positively charged copper ions
and negatively charged sulphate ions. These positively charged copper ions get attracted towards the plate which is connected to
the negative terminal of a battery.)
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called__electroplating__.
Q2 :
When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the
reason?
Answer :
The deflection in the compass needle shows that current is flowing through the wounded wire and hence, through the circuit. The
circuit is complete since free ends of the tester are dipped in a solution. The solution is certainly a conducting solution. This is the
reason why the compass needle shows a deflection.
Q3 :
Name three liquids, which when tested in the manner shown in Fig. 14.9, may cause the magnetic needle to deflect.
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Answer :
Liquids like lemon juice, salt water and vegetable oil allow electricity to pass through them. Hence, these liquids can be used as in
the beaker to show the given effect.
Q4 :
The bulb does not glow in the setup shown in Fig. 14.10. List the possible reasons. Explain your answer.
Answer :
(i) Liquid in the beaker is non-conducting. In such case, the electric current would not be able to pass through the liquid. Hence, the
circuit is not complete.
(ii) Electric current in the circuit is very weak. This can happen if the material used for making the circuit is not a good conductor of
electricity or the battery does not have sufficient energy to generate electricity.
Q5 :
A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the
tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that
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Answer :
The amount of current flowing through a conducting solution depends on the conductivity of the solution. With more conductivity,
more current passes through the solution and vice-versa. Hence, the conductivity of liquid A is more than the conductivity of liquid B.
Q6 :
Does pure water conduct electricity? If not, what can we do to make it conducting?
Answer :
No. Pure water does not conduct electricity. This is because pure water is devoid of any salts. Pure water can conduct electricity
when a pinch of common salt is added to it, as salt solution is conducting in nature.
Q7 :
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why
they do this.
Answer :
Water may conduct electricity. If the electrical supply for the area is not shut off and water is poured over electrical appliances, then
electricity may pass through water and harm the firemen. That is why, in case of a fire, the firemen shut off the main electrical supply
for the area before they use the water hoses.
Q8 :
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why
they do this.
Answer :
Water may conduct electricity. If the electrical supply for the area is not shut off and water is poured over electrical appliances, then
electricity may pass through water and harm the firemen. That is why, in case of a fire, the firemen shut off the main electrical supply
for the area before they use the water hoses.
Q9 :
A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass
needle deflects more in the case of seawater. Can you explain the reason?
Answer :
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Sea water contains more dissolved salts than the drinking water. Hence, it is more conducting than the drinking water. Because of
this reason, the compass needle deflects more in seawater than in the drinking water.
Q10 :
Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Answer :
No. It is not safe to repair electrical appliances outdoors during heavy downpour. This is because rain water contains dissolved
salts. Therefore, rain water can conduct electricity. The electrician may get electrical shocks while working outdoors during rain.
Q11 :
Paheli had heard that rain water is as good as distilled water. So, she collected some rain water in a clean glass tumbler
and tested it using a tester. To her surprise, she found that the compass needle showed deflection. What could be the
reasons?
Answer :
Rain water contains dissolved salts. This makes it a conducting solution. There are no dissolved salts present in the distilled water.
Hence, rain water can allow electricity to pass through it while distilled water cannot.
Q12 :
Answer :
(i) Chromium plating is done on different parts of cars, buses and motor cycles to give them shiny appearance.
ii. A fine layer of gold is deposited on the silver ornaments and they are called gold-plated ornaments.
ii. Iron used in constructing a building is coated with a layer of zinc. This protects iron from corrosion and rusting.
Q13 :
The process that you saw in Activity 14.7 is used for purification of copper. A thin plate of pure copper and a thick rod of
impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which
electrode should be attached to the positive terminal of the battery and why?
Answer :
Copper ion is positively charged. It is attracted towards the plate which is connected to the negative terminal of the battery. As
copper ions are transferred to the thin copper plate, this thin pure copper plate must be connected to the negative terminal of the
battery. Consequently, impure copper rod is connected to the positive terminal of the battery.
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Answer :
Only non-conducting materials can be easily charged by friction. Copper is a highly conducting materials. Therefore, a copper rod
cannot be charged easily by friction.
Q2 :
When a glass rod is rubbed with a piece of silk cloth the rod
(b) becomes positively charged while the cloth has a negative charge.
(d) becomes negatively charged while the cloth has a positive charge.
Answer :
(b) The rod becomes positively charged, while the cloth has a negative charge.
When an object is charged by rubbing it against another object, the two objects get oppositely charged. By convention, it is
considered that the charged acquired by the glass rod is positive and charged acquired by the cloth is negative. Therefore, the rod
becomes positively charged and the cloth becomes negatively charged.
Q3 :
Answer :
(a) False
Like charges repel each other. It is the unlike charges that attract each other.
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(b) True
A charged glass rod has positive charges on its surface while a charged plastic straw has negative charges on its surface. Since
unlike charges attract each other, a charged glass rod attracts a charged plastic straw.
(c) False
During a lightning, the lightning conductor conducts all the atmospheric charges to the Earth directly, leaving the building safe.
Hence, lightning conductors protect a building from lightning.
(d) False
Although the causes of earthquakes is known, but no instrument could be invented to detect it till now. Hence, earthquakes cannot
be predicted in advance.
Q4 :
Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.
Answer :
Whena sweater is taken off, the woollen sweater gets charged because of the friction between the sweater and the body. Hence,
one can hear a crackling sound during the given process.
Q5 :
Explain why a charged body loses its charge if we touch it with our hand.
Answer :
When we touch a charged object, our body conducts its charges to the earth. That is why a charged body loses its charge, if we
touch it with our hand. This phenomenon is known as electric discharge.
Q6 :
Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale.
Would it be recorded by a seismograph? Is it likely to cause much damage?
Answer :
The destructive energy of an earthquake is measured by the Richter scale. This scale has the readings from 1 to 10.
If the Richter scale gives a reading of magnitude 3, then the earthquake is not likely to cause much damage. Generally, earthquake
of magnitudes higher than 5 is considered destructive in nature.
Q7 :
Answer :
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(i) Stay in a completely closed place. If you are moving in a car, then remain there until the lightning is over. Close the windows of
the car immediately.
(ii) Do not touch any electrical wires, telephone cables, metal pipes, etc.
(iii) Do not bath in running water. This may cause an electric shock.
Q8 :
Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by
another charged balloon?
Answer :
The nature of charges present on the surface of charged balloons are similar. Since like charges repel each other, two charged
balloons repel each other. When a charged body is brought near an uncharged body, the uncharged body acquires charges on its
surface caused by the induction of charges. The charges are of opposite nature in relation to the charged body. Since unlike
charges attract each other, a charged body always attracts an uncharged body. Hence, an uncharged balloon is attracted by
another charged balloon.
Q9 :
Describe with the help of a diagram an instrument which can be used to detect a charged body.
Answer :
An electroscope can be used to detect whether a body is charged or not. The following figure shows a simple electroscope.
It consists of a metal rod. At one end of the rod, two leaves of aluminium foil are fixed and at the other end, there is a metal disc.
The leaves of aluminium foil are placed inside a conical flask and the flask is corked to isolate the leaves from air.
When the metal disc is touched with a charged body, the aluminium strips move away from each other. This happens because
some of the charges of the body are transferred to the strips through the metal rod. This method of charging a body is called
charging by conduction. The nature of charges on both the leaves and the charged body are the similar. Hence, both the leaves of
the aluminium foil will move away from each other. If the body was not charged, then the leaves of the foil would remain as they
were before. They would not repel each other.
Q10 :
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List three states in India where earthquakes are more likely to strike.
Answer :
The three states in India where earthquakes are more likely to strike are Jammu and Kashmir, Gujrat, and Assam.
Q11 :
Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?
Answer :
(i) Try to find an open field away from tall buildings, installations, tall trees, and electric wires and poles.
(ii) If travelling in a bus or a car, then do not come out when an earthquake strikes. Ask the driver to drive in an open field.
Q12 :
The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out
on that day. Would you carry an umbrella? Explain.
Answer :
No. We should not carry an umbrella in a thunderstorm. During thunderstorms, which are accompanied with lightning, electric
discharge from the clouds can travel through the metallic rod of the umbrella. This may give an electric shock to the person who is
carrying it. Hence, it is not safe to carry an umbrella during lightning.
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Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer :
If we are in a dark room, then it is not possible for us to see objects in the room. However, objects outside the room are visible to us.
An object becomes visible when light reaches our eye after being reflected from the object. If there is no light in the room, then the
objects inside the room cannot reflect any light. Hence, we cannot see in a dark room. If there is light present outside the room, then
we can see the objects outside the room.
Q2 :
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer :
Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other
for parallel incident rays. Irregular or diffused reflection takes place from an irregular surface. In diffused reflection, the reflected rays
are not parallel to each other for parallel incident rays. This happens because of the presence of irregular microscopic surfaces.
Hence, parallel incident rays reflect in different directions. However, each ray obeys the laws of reflection. Therefore, laws of
reflections are not violated in diffused or irregular reflections.
Q3 :
Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes.
Justify your answer in each case.
(c) Cardboard surface (d) Marble floor with water spread over it
Answer :
A polished surface is an example of a smooth surface. A polished wooden table has a smooth surface. Hence, reflections from the
polished table will be regular.
Chalk power spread on a surface is an example of an irregular surface. It is not smooth. Therefore, diffused reflection will take place
from chalk powder.
Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface.
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Marble floor with water spread over it is an example of a regular surface. This is because water makes the marble surface smooth.
Hence, regular reflection will take place from this surface.
Although a piece of paper may look smooth, but it has many irregularities on its surface. Due to this reason, it will give a diffused
reflection.
Q4 :
Answer :
Laws of reflection:
(ii) The incident ray, the reflected ray and the normal to the reflective surface at the point of incidence all lie in the same plane.
Q5 :
Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same
plane.
Answer :
Place a plane mirror on the table. Take a paper sheet and make a small hole in its centre. Make sure that the light in the room is not
bright. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Draw a
normal line on the second sheet from the mirror. Now, light a torch on the mirror through the small hole such that the ray of light falls
on the normal at the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction.
You can easily observe the incident ray, reflected ray and the normal to the mirror at the point of incidence on the sheet placed on
the table. This shows that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the
same plane.
Q6 :
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(a) A person 1 m in front of a plane mirror seems to be ____ __ m away from his image.
(b) If you touch your ________ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right
ear is touched with your _________.
(c) The size of the pupil becomes ________ when you see in dim light.
(d) Night birds have _________ cones than rods in their eyes.
Answer :
(a) A person 1 m in front of a plane mirror seems to be __2 __ m away from his image.
(Object distance and image distance are the same from a plane mirror. The image of a person 1 m in front of a mirror is 1 m back to
the mirror. Hence, the image is 1 + 1 = 2 m away from the person.)
(b) If you touch your __left__ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is
touched with your _left hand_.
(c) The size of the pupil becomes __large__ when you see in dim light.
(In dim light, the amount of light entering the eye is very little. To increase the amount of light, the pupil expands.)
(d) Night birds have __less__ cones than rods in their eyes.
(Night birds can see in the night, but not in the day. They have on their retina a large number of rod cells and only a few cones.)
Q7 :
Answer :
(a) The angle of incidence is always equal to the angle of reflection. This is the first law of reflection.
Q8 :
(b) virtual, behind the mirror and of the same size as the object.
(d) real, behind the mirror and of the same size as the object.
Answer :
(b) Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object.
The image formed by a plane mirror is of the same size as the object. The image is formed behind the mirror. The image cannot be
obtained on a screen. Hence, it is a virtual image.
Q9 :
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Answer :
Construction of a kaleidoscope:
Three rectangular mirror strips of dimensions 15cm x 4cm (l x b) are joined together to form a prism (fig a). This prism is fixed into a
circular cardboard tube. The circular cardboard tube should be slightly longer that the prism (fig b). This circular tube is now closed
at one end with a cardboard disc. This disc has a hole in it through which we can see (fig c) .At the other end of the circular tube, a
plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken
pieces of coloured glass are placed. This end is now closed by a round glass plate allowing enough space for the coloured glass
pieces to move (fig d).
Q10 :
Answer :
Q11 :
Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis
of the teacher's advice?
Answer :
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Laser light is harmful for the human eyes, because its intensity is very high. It can cause damage to the retina and lead to blindness.
Hence, it is advisable not to look at a laser beam directly.
Q12 :
Answer :
To protect our eyes, the given points should be taken into account:
(iv) Clean your eyes with cold water quickly if dust particles or small insects enter your eye. Do not rub your eyes.
(v) Maintain a distance of at least 25 cm between the book and your eyes while reading.
Q13 :
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Answer :
If the reflected ray is at an angle of 90° to the incident ray, then the angle of incidence is 45 °. According to the law of reflection, the
angle of incidence is equal to the angle of reflection. Therefore, the angle of incidence and the angle of reflection both are =
45°.
Q14 :
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer :
Infinite or multiple images of the candle will be formed because of multiple reflections between the mirrors. When two mirrors are
placed parallel to each other, then infinite numbers of images are formed.
Q15 :
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the
reflected ray from the second mirror.
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Answer :
The first law of reflection is used to obtain the path of reflected light.
It can be observed that the given ray of light will reflect from the second mirror at an angle 60°.
Q16 :
Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also can he
see the image of objects situated at P, Q and R?
Answer :
A plane mirror forms a virtual image behind the mirror. The image is as far behind the mirror as the object is in front of it. A cannot
see his image because the length of the mirror is too short on his side. However, he can see the objects placed at points P and Q,
but cannot see the object placed at point R (as shown in the given figure).
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Q17 :
(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(d) When Paheli moves from B to C, where does the image of A move?
Answer :
(a) Image of the object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of
A from the mirror. Image of A is shown in the given figure.
(d) Image of the object at A will not move. It will remain at the same position when Paheli moves from B to C.
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(a) An asteroid
(b) A satellite
(c) A constellation
(d) A comet
Answer :
(c) Constellation
A constellation is not a member of the solar system.Constellations are groups of stars that form recognisable shapes.
Q2 :
(a) Sirius
(b) Mercury
(c) Saturn
(d) Earth
Answer :
(a) Sirius
Q3 :
(a) we can see only that part of the moon which reflects light towards us.
(c) the shadow of the Earth covers only a part of the moon's surface.
Answer :
(a) Phases of the moon occur because we can see only that part of the moon which reflects light towards us.
Moon does not produce its own light. We are able to see the moon because the sunlight falling on it gets reflected towards us. Thus,
we see only that part of the moon from which the light of the sun gets reflected towards us.
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Q4 :
(c) A group of stars that appear to form a pattern in the sky is known as a __ _________.
(f) Asteroids are found between the orbits of ________ __ and _________.
Answer :
(c) A group of stars that appear to form a pattern in the sky is known as a __constellation__.
(f) Asteroids are found between the orbits of __Mars __ and __Jupiter.
(Asteroids occupy a large gap between the orbits of Mars and Jupiter)
Q5 :
Answer :
(a) False
Stars are not a member of the solar system. The sun and the celestial bodies revolving around it form the solar system.
(b) True
(c) False
(d) True
(e) False
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They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
(f) False
Constellation Orion can be seen during winters around late evenings. It is one of the most magnificent constellations in the sky,
visible to the naked eyes.
Q6 :
A B
(i) Inner planets (a) Saturn
(ii) Outer planets (b) Pole star
(iii) Constellation (c) Great Bear
(iv) Satellite of the Earth (d) Moon
(e) Earth
(f) Orion
(g) Mars
Answer :
A B
(i) Inner planets (g), (e) Mars, Earth
(ii) Outer planets (a) Saturn
(iii) Constellation (c), (f) Great Bear, Orion
(iv) Satellite of the Earth (d) Moon
Q7 :
In which part of the sky can you find Venus if it is visible as an evening star?
Answer :
Venus appears in the western sky after sunset and is called the evening star.
Q8 :
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Answer :
Q9 :
Answer :
The two well known constellations are Ursa Major and Orion.
Q10 :
Draw sketches to show the relative position of prominent stars in (a) Ursa Major and (b) Orion
Answer :
(a) Ursa Major appears like a big dipper. There are three bright stars in the handle and four stars in the bowl of the dipper (as shown
in the given figure).
(b) Orion appears like a hunter. Three bright stars appear in the belt, while five bright stars are arranged in the form of a
quadrilateral (as shown in the given figure).
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Q11 :
Name two objects other than planets which are members of the solar system.
Answer :
(i) Asteroids
A collection of a large number of small objects, gases and dust are revolving around the sun. They occupy a large gap between the
orbits of Mars and Jupiter. However, these are not planets. These celestial objects are known as asteroids.
(ii) Meteors
Meteors are small celestial objects that are seen as bright streaks of light in the sky. They brunt out on entering the Earth's
atmosphere because of the heat produced by friction. This results in bright streaks in the sky. They are not planets.
Q12 :
Explain how you can locate the Pole Star with the help of Ursa Major.
Answer :
In order to locate the Pole star in the sky, first of all Ursa Major or Big Dipper constellation must be located. The bowl of the Big
Dipper consists of four bright stars (as shown in the given figure).
Consider two stars at the end of this bowl. Now, draw an imaginary straight line towards the Northern direction connecting these two
stars (as shown in the given figure).
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This imaginary line meets a star called the Pole Star. The length of the imaginary line from the bowl is about five times the distance
between the two stars of the bowl.
Q13 :
Answer :
No. The Earth rotates from West to East on its axis. Hence, all stars in the sky (except the Pole star) appear to move from East to
West. With reference to the Earth, the Pole star does not appear to move in the sky because it is located above the axis of rotation
of the Earth in the north direction. It appears to remain stationary at a point in the sky.
Q14 :
Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight
light years away from the Earth?
Answer :
The distance of the stars from the Earth and the distance between the stars are very large. It is inconvenient to express these
distances in kilometer (km). Thus, these large distances are expressed in light years. One light year is the distance travelled by light
in one year. One light year is equal to 9.46 x 1012km.
A star is located eight light years away from the Earth. This means that the distance between the star and the Earth is equivalent to
the distance travelled by light in eight years, i.e., a star is located 8 x (9.46 x 1012) = 7.6 x 1013km away from the Earth.
Q15 :
The radius of Jupiter is 11 times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How
many Earths can Jupiter accommodate?
Answer :
Earth and Jupiter can be considered as two spheres with radii Rand R'respectively. Given that the radius of Jupiter is 11 times the
radius of the Earth.
Thus, R' = 11 R
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Hence, this ratio suggests that Jupiter can accommodate 1331 number of Earths within it.
Q16 :
Boojho made the following sketch (Fig. 17.29) of the solar system. Is the sketch correct? If not, correct it.
Fig. 17.29
Answer :
Planets of the solar system in sequence of their distances from the sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
and Neptune. Therefore, Boojho's sketch of the solar system is not correct because he has interchanged the positions of Mars and
Venus and also the positions of Uranus and Neptune. Also, he has shown the Asteroid belt in the gap between the orbits of Jupiter
and Saturn. This is not correct. The asteroid belt is located between the orbits of Mars and Jupiter. The correct sketch of the solar
system is shown in the given figure.
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Answer :
(i) Agricultural chemicals: Farmers use excessive amounts of pesticides and fertilizers to increase crop production. These
chemicals get carried away to the water bodies due to rains and floods which lead to water pollution.
(ii) Industrial wastes: Industries release harmful chemical wastes into water sources, thereby polluting them.
(iii) Sewage wastes: Waste materials from kitchens, toilets, and laundry sources are also responsible for contaminating water.
Q2 :
Answer :
(i) Avoiding the use of cars as much as possible and by using public transport whenever possible.
(iii) By using clean fuels such as LPG and CNG instead of diesel and petrol.
(iv) Always disposing the garbage properly and not burning it.
Q3 :
Answer :
No. Clear and transparent water is not always fit for drinking. Water might appear clean, but it may contain some disease causing
micro-organisms and other dissolved impurities. Hence, it is advised to purify water before drinking. Purification can be done by
water purifying systems or by boiling the water.
Q4 :
Make a list of measures that would help your town to ensure the supply of clean water to all its residents.
Answer :
To ensure the supply of clean water to all residents the following steps must be taken:
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(i) The main water source must be built in clean surroundings and should be maintained properly.
(ii) Chemical methods such as chlorination must be used for purifying water.
Q5 :
Answer :
Pure air contains around 78% nitrogen, 21% oxygen, and 0.03% carbon dioxide. Other gases such as argon, methane, ozone, and
water vapours are also present in small quantities. When this composition of air is altered by the addition of harmful substances or
gases such as nitrogen dioxide, sulphur dioxide, carbon monoxide, and particulate matter, then the air is said to be polluted.
Q6 :
Explain circumstances leading to acid rain. How does acid rain affect us?
Answer :
Burning of fossil fuels such as coal and diesel releases a variety of pollutants such as sulphur dioxide and nitrogen dioxide into the
atmosphere. These pollutants react with water vapours present in the atmosphere to form sulphuric acid and nitric acid respectively.
These acids come down with the rain, thereby resulting in acid rain.
(ii) Acid rains corrode buildings and structures especially those made of marble such as Taj Mahal.
Q7 :
(c) Methane
(d) Nitrogen
Answer :
(d) Nitrogen
Q8 :
Answer :
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Greenhouse effect may lead to global warming, i.e., an overall increase in the average temperature of the Earth. Greenhouse effect
is caused by greenhouse gases. Examples of greenhouse gases include carbon dioxide, methane, and water vapour. When solar
radiations reach the Earth, some of these radiations are absorbed by earth and then released back to the atmosphere. Greenhouse
gases present in the atmosphere trap these radiations and do not allow heat to leave. This helps in keeping our planet warm and
thus, helps in human survival. However, an indiscriminate increase in the amount of greenhouse gases can lead to excessive
increase in the Earth's temperature leading to global warming.
Q9 :
Prepare a brief speech on global warming. You have to deliver the speech in your class.
Answer :
Global warming is an increase in the average temperature of the Earth's surface. It occurs as a result of an increased concentration
of greenhouse gases in the atmosphere. The greenhouse gases include carbon dioxide, methane, and water vapour. These gases
trap solar radiations released back by the Earth. This helps in keeping our planet warm and thus, helps in human survival. However,
an increase in the amount of greenhouse gases can lead to an increase in the Earth's temperature leading to global warming.
Q10 :
Answer :
Acid rain is a major threat to the beauty of the Taj Mahal. When acid rains fall on the monument (that is completely made of marble),
they react with marble to form a powder-like substance that is then washed away by the rain. This phenomenon is known as marble
cancer. Also, the soot particles emitted from the Mathura oil refinery located near Agra is leading to the yellowing of the marble.
Q11 :
Why does the increased level of nutrients in the water affect the survival of aquatic organisms?
Answer :
An increase in the level of nutrients in a water body leads to an excessive increase in the population of algae in the water body.
When these algae die, they serve as food for decomposers. A lot of oxygen is utilised in this process, consequently leading to a
decrease in the level of oxygen dissolved in the water body. This in turn causes fishes and other aquatic organisms to die.
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