Science

Activity 5.7 Project Tiger was launched by the

[Activity] ## Activity 5. 7 Project Tiger was launched by the government to protect the tigers in the Visit a nearby zoo. Observe the country. The objective of this project conditions provided to the animals. was to ensure the survival and Were they suitable for the animals? maintenance of the tiger population in Can animals live in artificial setting the country. 2024-25 Are tigers still found in this forest? I hope I can see a tiger! Tiger (Fig. 5. 4) is one of the many species which are slowly disappearing from our forests. But, the Satpura Tiger Reserve is unique in the sense that a significant increase in the population of tigers has been seen here. Once upon a Barasingha time, animals like lions, elephants, wild animals has become difficult because of disturbances in their natural habitat. Professor Ahmad tells them that in order to protect plants and animals strict rules are imposed in all National Parks. Human activities such as grazing, poaching, hunting, capturing of animals or collection of firewood, mdicinal plants, etc. are not allowed Tiger Are only big animals buffaloes (Fig. 5. 5) and barasingha facing extinction? (Fig. 5. 6) were also found in the Satpura National Park. Animals whose numbers Madhavji tells Paheli that small are diminishing to a level that they might animals are much more in danger of face extinction are known as the becoming extinct than the bigger endangered animals. Boojho is animals. At times, we kill snakes, frogs, reminded of the dinosaurs which became lizards, bats and owls ruthlessly without extinct a long time ago. Survival of some realising their importance in the ecosystem. By killing them we are harming ourselves. They might be small in size but their role in the ecosystem cannot be ignored. They form part of food chains and food webs. An ecosystem is made of all the plants, animals and microorganisms in an area along with non-living components such as climate, soil, river deltas etc. Wild buffalo 2024-25 Reservoir to relax for some time. Paheli observes some of the birds near the river. Madhavji tells the children that these are migratory birds. These birds I wonder if there is have flown here from other parts of any record of all the world. endangered species! Migratory birds fly to far away areas every year during a particular time because of climatic changes. They fly for laying eggs as the weather in their 5. 9 Red Data Book natural habitat becomes very cold and Professor Ahmad explains about Red inhospitable. Birds who cover long Data Book to the children. He tells distances to reach another land are them that Red Data Book is the known as migratory birds. source book which keeps a record of all the endangered animals and 5. 11 Recycling of Paper plants. Red Data Book is maintained Professor Ahmad draws attention of internationally by an organisation. the children to another cause of India also maintains Red Data Book deforestation. He tells them that it for plants and animals found takes 17 full grown trees to make one in India. tonne of paper. Therefore, we should save paper. Professor Ahmad also tells 5. 10 Migration that paper can be recycled five to seven The excursion party then enters deeper times for use. If each student saves at into the forest under the guidance of least one sheet of paper in a day, we Madhavji. They sit near the Tawa can save many trees in a year. We should save, reuse used paper and recycle it. By this we not only save trees but also save energy and water needed for manufacturing paper. Moreover, the amount of harmful What would happen if we chemicals used in paper making will had no wood? Is there also be reduced. any alternative available to wood? I know that paper is one of the important products we get from forests. I wonder whether Is there any permanent there are any alternatives solution to the problem available for paper! of deforestation? 2024-25


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5.12 Reforestation retain our green wealth for future generations, plantation of more trees is Professor Ahmad suggests

5. 12 Reforestation retain our green wealth for future generations, plantation of more trees is Professor Ahmad suggests that the the only option. answer to deforestation is reforestation. Professor Ahmad told them that in Reforestation is restocking of the India we have the Forest (Conservation) destroyed forests by planting new trees. Act. This act is aimed at preservation The planted trees should generally be and conservation of natural forests and of the same species which were found meeting the basic needs of the people in that forest. We should plant at least living in or near the forests. as many trees as we cut. Reforestation After some rest Madhavji asks the can take place naturally also. If the children to start heading back because deforested area is left undisturbed, it re- it is not advisable to stay in the establishes itself. In natural reforestation jungle after sunset. On getting back, there is no role of human beings. We Professor Ahmad and the children have already caused tremendous thank Madhavji for guiding them damage to our forests. If we have to through this exciting experience. 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT BIODIVERSITY (cid: 220) Wildlife sanctuary, national park and bio- sphere reserve are names given to the areas BIOSPHERE RESERVE meant for conservation and preservation of DEFORESTATION forest and wild animals. DESERTIFICATION (cid: 220) Biodiversity refers to the variety of living organisms in a specific area. ECOSYSTEM (cid: 220) Plants and animals of a particular area are ENDANGERED known as the flora and fauna of that area. SPECIES (cid: 220) Endemic species are found only in a particular area. ENDEMIC SPECIES (cid: 220) Endangered species are those which are facing EXTINCT the danger of extinction. (cid: 220) Red Data Book contains a record of FAUNA endangered species. FLORA (cid: 220) Migration is the phenomenon of movement of a species from its own habitat to some other MIGRATORY BIRDS habitat for a particular time period every year NATIONAL PARK for a specific purpose like breeding. (cid: 220) We should save, reuse and recycle paper to RED DATA BOOK save trees, energy and water. REFORESTATION (cid: 220) Reforestation is the restocking of destroyed forests by planting new trees. SANCTUARY


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[Exercise] ## Exercises 1. Fill in the blanks. (a) A place where animals are protected in their natural habitat is called. (b) Species found only in a particular area is known as. - Migratory birds fly to far away places because of changes. 2. Differentiate between the following. (a) Wildlife sanctuary and biosphere reserve (b) Zoo and wildlife sanctuary - Endangered and extinct species - Flora and fauna 2024-25 3. Discuss the effects of deforestation on the following. (a) Wild animals (b) Environment - Villages (Rural areas) - Cities (Urban areas) (e) Earth (f) The next generation 4. What will happen if. (a) we go on cutting trees. (b) the habitat of an animal is disturbed. - the top layer of soil is exposed. 5. Answer in brief. (a) Why should we conserve biodiversity? (b) Protected forests are also not completely safe for wild animals. Why? - Some tribals depend on the jungle. How? - What are the causes and consequences of deforestation? (e) What is Red Data Book? (f) What do you understand by the term migration? 6. 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. 7. How can you contribute to the maintenance of green wealth of your locality? Make a list of actions to be taken by you. 8. Explain how deforestation leads to reduced rainfall. 9. Find out about national parks in your state. Identify and show their location on the outline map of India. 10. Why should paper be saved? Prepare a list of ways by which you can save paper. S E S I C R E X E 2024-25 S E S I C R E X E 11. Complete the word puzzle. Down 1. Species on the verge of extinction. 2. A book carrying information about endangered species. 5. Consequence of deforestation. Across 1. Species which have vanished. 3. Species found only in a particular habitat. 4. Variety of plants, animals and microorganisms found in an area. Extended Learning - ## Activities and Projects 1. Plant at least five different plants in your locality during this academic year and ensure their maintenance till they grow. 2. Promise yourself that this year you will gift at least 5 plants to your friends and relatives on their achievements, or on occasions like birthdays. Ask your friends to take proper care of these plants and encourage them to gift plants to their friends on such occasions. At the end of the year count the plants that have been gifted through this chain. 2024-25 3. Is it justified to prevent tribals from staying in the core area of the forest? Discuss the matter in your class and note down the points for and against the motion in your notebook. 4. Study the biodiversity of a park nearby. Prepare a detailed report with photographs and sketches of the flora and fauna. 5. Make a list of the new information you have gathered from this chapter. Which information did you find the most interesting and why? 6. Make a list of various uses of papers. Observe currency notes carefully. Do you find any difference between a currency paper and paper of your notebook? Find out where currency paper is made. 7. Karnataka Government had launched 'Project Elephant' to save Asian elephants in the state. Find out about this and other such campaigns launched to protect threatened species. Did You Know? 1. India has more than half of the world's wild tigers, 65% of the Asian elephants, 85% of the great one-horned rhinoceros and 100% of the Asian lions. 2. India is sixth on a list of 12 mega-biodiversity countries in the world. It contains two of the 34 biodiversity hotspots of the world - Eastern Himalayas and the Western Ghats. These areas are very rich in biodiversity. 3. One of the most important factors that threatens wildlife today is habitat destruction due to encroachment. 4. India contains 172 species of animals considered globally threatened or 2. 9% of the world's total number of threatened species. Eastern Himalayas hotspot has merely 163 globally threatened species including several animal and plant species. India contains globally important population of some of Asia's rarest animals such as the Bengal fox, Marbled cat, Asiatic lion, Indian elephant, Asiatic wild ass, Indian rhinoceros, gaur, Wild asiatic water buffalo, etc. For knowing more, you may contact: - Ministry of Environment, Forest and Climate Change, Govt. of India Environment, Forest and Wildlife Department Indira Paryavaran Bhavan, Jor Bagh Road, New Delhi -11003 Website: http: //envfor. nic. in 2024-25


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RR AA EEPPRROODDUUCCTTIIOONN IINN NNIIMMAALLSS D o you recall the processes of Table 6. 1 digestion, circulation and respiration which you have S. No. Animal Young one studied in your previous classes? These 1. Human Baby processes are essential for the survival of every individual. You have also learnt 2. Cat about the process of reproduction in 3. Dog plants. Reproduction is essential for the continuation of a species. Imagine what 4. Butterfly would have happened if organisms had 5. Hen Chick not reproduced. You will realise that reproduction is very important as it 6. Cow ensures the continuation of similar 7. Frog kinds of individuals, generation after generation. Just as in plants, there are two modes You have already learnt in your by which animals reproduce. These are: previous class about reproduction in (i)Sexual reproduction, and plants. In this chapter, we shall learn (ii)Asexual reproduction. how reproduction takes place in animals. 6. 2 Sexual Reproduction 6. 1 Modes of Reproduction Try to recall reproduction in plants which you studied in Class VII. You will Have you seen the young ones remember that plants that reproduce of different animals? Try to name some sexually have male and female of the young ones by completing reproductive parts. Can you name these Table 6. 1 shown in examples at S. No. parts? In animals also, males and 1 and 5. females have different reproductive parts You must have seen the young or organs. Like plants, the reproductive ones of various animals being born. parts in animals also produce gametes Can you tell how chicks and that fuse to form a zygote. It is the zygote caterpillars are born? How are kittens which develops into a new individual. and puppies born? Do you think that This type of reproduction beginning these young ones looked the same from the fusion of male and female before they were born as they do now? gametes is called sexual reproduction. Let us find out. Let us find out the reproductive parts 2024-25 in humans and study the process of sperm is a single cell with all the usual reproduction in them. cell components. Male Reproductive Organs Head The male reproductive organs include a pair of testes (singular, testis), two sperm ducts and a penis (Fig. 6. 1). The testes Middle piece produce the male gametes called sperms. Millions of sperms are produced by the testes. Look at Fig. 6. 2 which shows the picture of a sperm. Though sperms are very small in size, each has Tail a head, a middle piece and a tail. Does it appear to be a single cell? Indeed, each Human sperm What purpose does the tail in a sperm serve? Sperm duct Female Reproductive Organs The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes) and the uterus (Fig. 6. 3). The ovary produces Penis Oviduct Ovary Uterus Testis Male reproductive organs in humans Female reproductive organs in humans 2024-25 female gametes called ova (eggs) (Fig. 6. 4). In human beings, a single Sperms matured egg is released into the oviduct by one of the ovaries every month. Uterus is the part where development of the baby takes place. Like the sperm, an egg is also a single cell. Ovum Nucleus Fertilisation you know that the zygote is the beginning of a new individual? Fusing Human Ovum nuclei Boojho recalls that the size of eggs in animals varies. The egg may be very small as in humans, much larger as in hens. Ostrich egg is Zygote the largest! The process of fertilisation is the meeting of an egg cell from the mother Fertilisation and a sperm cell from the father. So, The first step in the process of the new individual inherits some reproduction is the fusion of a sperm characteristics from the mother and and an ovum. When sperms come in some from the father. Look at your contact with an egg, one of the sperms brother or sister. See if you can recognise may fuse with the egg. Such fusion of some characters in them similar to those the egg and the sperm is called of your mother or your father. fertilisation (Fig. 6. 5). During Fertilisation which takes place inside fertilisation, the nuclei of the sperm and the female body is called internal the egg fuse to form a single nucleus. fertilisation. Internal fertilisation This results in the formation of a occurs in many animals including fertilised egg or zygote (Fig. 6. 6). Did humans, cows, dogs and hens. 2024-25 female lays hundreds of eggs. Unlike Have you heard of hen's egg, frog's egg is not covered by a test tube babies? shell and it is comparatively very Boojho and Paheli's teacher once told delicate. A layer of jelly holds the eggs them in the class that in some women together and provides protection to the oviducts are blocked. These women eggs (Fig. 6. 7). are unable to bear babies because sperms cannot reach the egg for fertilisation. In such cases, doctors collect freshly released egg and sperms and keep them together for a few hours for IVF or in vitro fertilisation (fertilisation outside the body). In case fertilisation occurs, the zygote is allowed to develop for about a week and then it is placed in the mother's uterus. Complete development takes place in the uterus and the baby is born like any other baby. Babies born through this technique are called test-tube babies. This term is actually Eggs of frog misleading because babies cannot grow in test tubes. As the eggs are laid, the male deposits sperms over them. Each You will be surprised to know that sperm swims randomly in water with in many animals fertilisation takes the help of its long tail. The sperms place outside the body of the female. In these animals, fertilisation takes come in contact with the eggs. This place in water. Let us find out how results in fertilisation. This type of this happens. fertilisation in which the fusion of a male and a female gamete takes place


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[Activity] ## Activity 6. 1 outside the body of the female is called external fertilisation. It is very Visit some ponds or slow-flowing common in aquatic animals such as streams during spring or rainy fish, starfish, etc. season. Look out for clusters of frog's eggs floating in water. Write down the colour and size of the eggs. Why do fish and frogs lay eggs in During spring or rainy season, frogs hundreds whereas a and toads move to ponds and slow- hen lays only one flowing streams. When the male and egg at a time? female come together in water, the 2024-25 How could a single cell Though these animals lay become such a big individual? hundreds of eggs and release millions of sperms, all the eggs do not get fertilised and develop Development of Embryo into new individuals. Fertilisation results in the formation of This is because the eggs and zygote which begins to develop into an sperms get exposed to water embryo. The zygote divides movement, wind and rainfall. repeatedly to give rise to a ball of cells Also, there are other animals in. The cells then begin to form the pond which may feed groups that develop into different tissues on eggs. Thus, production and organs of the body. This developing of large number of eggs and structure is termed an embryo. The sperms is necessary to embryo gets embedded in the wall of the ensure fertilisation uterus for further development of at least. a few of them. The embryo continues to develop in the uterus. It gradually develops body Uterus Ovary Zygote (b) Embedding embryo Uterus Ovulation wall (a) Developing embryo (c) (a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells (enlarged); (c) Embedding of the embryo in the uterus (enlarged) 2024-25 parts such as hands, legs, head, eyes, place inside the egg shell during this ears etc. The stage of the embryo in period? After the chick is completely which all the body parts can be developed it bursts open the egg shell. identified is called a foetus (Fig. 6. 9). In animals which undergo external When the development of the foetus is fertilisation, development of the embryo complete, the mother gives birth to takes place outside the female body. The the baby. embryos continue to grow within their egg coverings. After the embryos develop, the eggs hatch. You must have seen numerous tadpoles swimming in ponds Uterus and streams. Viviparous and Oviparous Animals We have learnt that some animals give birth to young ones while some animals lay eggs which later develop into young ones. The animals which give birth to young ones are called viviparous animals. Those animals which lay eggs are called oviparous animals. The following activity will help you understand better and differentiate Foetus in the uterus between viviparous and oviparous animals. Internal fertilisation takes place in


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[Activity] ## Activity 6. 2 hens also. But, do hens give birth to babies like human beings and cows? You Try to observe eggs of the following know that they do not. Then, how are organisms - frog, lizard, butterfly chicks born? Let us find out. or moth, hen and crow or any other Soon after fertilisation, the zygote bird. Were you able to observe eggs divides repeatedly and travels down the of all of them? Make drawings of the oviduct. As it travels down, many eggs that you have observed. protective layers are formed around it. The hard shell that you see in a hen's The eggs of a few animals are easy egg is one such protective layer. to observe because their mothers lay After the hard shell is formed around them outside their bodies. These are the developing embryo, the hen finally examples of oviparous animals. But you lays the egg. The embryo takes about 3 would not be able to collect the eggs of weeks to develop into a chick. You must a dog, cow or cat. This is because they have seen the hen sitting on the eggs to do not lay eggs. The mother gives birth provide sufficient warmth. Did you know to the young ones. These are examples that development of the chick takes of viviparous animals. 2024-25 Can you now give some more The tadpoles transform into adults examples of viviparous and oviparous capable of jumping and swimming. The animals? transformation of the larva into an adult through drastic changes is called Young Ones to Adults metamorphosis. What about the The new individuals which are born or changes that we observe in our body as hatched from the eggs continue to grow we grow? Do you think we too undergo till they become adults. In some metamorphosis? In human beings, animals, the young ones may look very body parts similar to those present in different from the adults. The life cycle the adults are present from the time of frog is shown in Fig. 6. 10. of birth. Observe the different stages of frog 6. 3 Asexual Reproduction starting from the egg to the adult stage. We find that there are three distinct So far, we have learnt about reproduction stages, that is, egg → tadpole (larva) → in some familiar animals. But what adult. Don't the tadpoles look so about very small animals like hydra and different from the adults? Can you microscopic organisms like amoeba? Do imagine that these tadpoles would some you know how they reproduce? Let us day become frogs? find out. (b) Early tadpole (a) Eggs - Late tadpole - Adult frog Life cycle of frog 2024-25


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[Activity] ## Activity 6. 3 Get permanent slides of hydra. Observe them using hand lens or a microscope. Look out for any bulges from the parent body. Count the number of bulges that you see in different slides. Also, note the size of the bulges. Draw the diagram of hydra, as you see it. Compare it with Budding in Hydra the Fig. 6. 11. In each hydra, there may be one or more bulges. These bulges are the developing new individuals and they are called buds. Recall the presence of buds in yeast. In hydra too the new individuals (a) develop as outgrowths from a single parent. This type of reproduction in which only a single parent is involved is called asexual reproduction. Since new individuals develop from the buds in Dividing nucleus hydra, this type of asexual reproduction (b) is called budding. Another method of asexual reproduction is observed in the microscopic organism, amoeba. Let us see how this happens. You have already learnt about the structure of amoeba. You will recall that (c) amoeba is a single-celled organism. It begins the process of reproduction by the division of its Daughter amoebae nucleus into two nuclei. This is followed by division of its body into two, each part receiving a nucleus. Finally, two amoebae are (d) produced from one parent amoeba. This type of asexual Binary fission in Amoeba 2024-25 reproduction in which an animal there are other methods by which a reproduces by dividing into two single parent reproduces young ones. individuals is called binary fission. You will study about these in your Apart from budding and binary fission, higher classes. Story of Dolly, the Clone Cloning is the production of an exact copy of a cell, any other living part, or a complete organism. Cloning of an animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, Scotland. They successfully cloned a sheep named Dolly. Dolly was born on 5th July 1996 and was the first mammal to be cloned. (a) Finn Dorsett sheep (b) Scottish blackface ewe (c) Dolly Fig. 6. 13 During the process of cloning Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep. Simultaneously, an egg was obtained from a Scottish blackface ewe. The nucleus was removed from the egg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed. The egg thus produced was implanted into the Scottish blackface ewe. Development of this egg followed normally and finally Dolly was born. Though Dolly was given birth by the Scottish blackface ewe, it was found to be absolutely identical to the Finn Dorsett sheep from which the nucleus was taken. Since the nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not show any character of the Scottish blackface ewe. Dolly was a healthy clone of the Finn Dorsett sheep and produced several offspring of her own through normal sexual means. Unfortunately, Dolly died on 14th February 2003 due to a certain lung disease. Since Dolly, several attempts have been made to produce cloned mammals. However, many die before birth or die soon after birth. The cloned animals are many-a-times found to be born with severe abnormalities. 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT ASEXUAL (cid: 220) There are two modes by which animals repro- REPRODUCTION duce. These are: (i) Sexual reproduction, and - Asexual reproduction. BINARY FISSION (cid: 220) Reproduction resulting from the fusion of male and female gametes is called sexual BUDDING reproduction. (cid: 220) The reproductive organs in the female include EGGS ovaries, oviducts and uterus. (cid: 220) The reproductive organs in male include EMBRYO testes, sperm ducts and penis. (cid: 220) The ovary produces female gametes called ova EXTERNAL and the testes produce male gametes called FERTILISATION sperms. (cid: 220) The fusion of ovum and sperm is called fertilisation. FERTILISATION The fertilised egg is called a zygote. (cid: 220) Fertilisation that takes place inside the female FOETUS body is called internal fertilisation. This is observed in human beings and other animals INTERNAL such as hens, cows and dogs. FERTILISATION (cid: 220) Fertilisation that takes place outside the female body is called external fertilisation. This METAMORPHOSIS is observed in frogs, fish, starfish, etc. (cid: 220) The zygote divides repeatedly to give rise to an OVIPAROUS ANIMALS embryo. (cid: 220) The embryo gets embedded in the wall of the SEXUAL uterus for further development. REPRODUCTION (cid: 220) The stage of the embryo in which all the body parts are identifiable is called foetus. SPERMS (cid: 220) Animals such as human beings, cows and dogs which give birth to young ones are called VIVIPAROUS ANIMALS viviparous animals. (cid: 220) Animals such as hen, frog, lizard and butterfly ZYGOTE which lay eggs are called oviparous animals. (cid: 220) The transformation of the larva into adult through drastic changes is called metamorphosis. (cid: 220) The type of reproduction in which only a single parent is involved is called asexual reproduction. (cid: 220) In hydra, new individuals develop from buds. This method of asexual reproduction is called budding. (cid: 220) Amoeba reproduces by dividing itself into two. This type of asexual reproduction is called binary fission. 2024-25


Science

[Exercise] ## Exercises 1. Explain the importance of reproduction in organisms. 2. Describe the process of fertilisation in human beings. 3. Choose the most appropriate answer. (a) Internal fertilisation occurs - in female body. - outside female body. - in male body. - outside male body. (b) A tadpole develops into an adult frog by the process of - fertilisation (ii) metamorphosis (ii) embedding (iv) budding - The number of nuclei present in a zygote is - none (ii) one (ii) two (iv) four 4. Indicate whether the following statements are True (T) or False (F). (a) Oviparous animals give birth to young ones. ( ) (b) Each sperm is a single cell. ( ) - External fertilisation takes place in frog. ( ) - A new human individual develops from a cell called gamete. ( ) (e) Egg laid after fertilisation is made up of a single cell. ( ) (f) Amoeba reproduces by budding. ( ) (g) Fertilisation is necessary even in asexual reproduction. ( ) (h) Binary fission is a method of asexual reproduction. ( ) - A zygote is formed as a result of fertilisation. ( ) (j) An embryo is made up of a single cell. ( ) 5. Give two differences between a zygote and a foetus. 6. Define asexual reproduction. Describe two methods of asexual reproduction in animals. 7. In which female reproductive organ does the embryo get embedded? 8. What is metamorphosis? Give examples. 9. Differentiate between internal fertilisation and external fertilisation. 2024-25 S E S I C R E X E 10. Complete the crossword puzzle using the hints given below. Across 1. The process of the fusion of the gametes. 6. The type of fertilisation in hen. 7. The term used for bulges observed on the sides of the body of hydra. 8. Eggs are produced here. Down 2. Sperms are produced in these male reproductive organs. 3. Another term for in vitro fertilisation. 4. These animals lay eggs. 5. A type of fission in amoeba. Extended Learning - ## Activities and Projects 1. Visit a poultry farm. Talk to the manager of the farm and try to find out the answers to the following. (a) What are layers and broilers in a poultry farm? (b) Do hens lay unfertilised eggs? - How can you obtain fertilised and unfertilised eggs? - Are the eggs that we get in the stores fertilised or unfertilised? (e) Can you consume fertilised eggs? (f) Is there any difference in the nutritional value of fertilised and unfertilised eggs? 2024-25 2. Observe live hydra yourself and learn how they reproduce by doing the following activity: During the summer months collect water weeds from ponds or ditches along with the pond water and put them in a glass jar. After a day or so you may see several hydra clinging to the sides of the jar. Hydra is transparent, jelly-like and with tentacles. It clings to the jar with the base of its body. If the jar is shaken, the hydra will contract instantly into a small blob, at the same time drawing its tentacles in. Now take out few hydras from the jar and put them on a watch glass. Using a hand lens or a binocular or dissection microscope, observe the changes that are taking place in their body. Note down your observations. 3. The eggs we get from the market are generally the unfertilised ones. In case you wish to observe a developing chick embryo, get a fertilised egg from the poultry or hatchery which has been incubated for 36 hours or more. You may then be able to see a white disc-like structure on the yolk. This is the developing embryo. Sometimes if the heart and blood vessels have developed you may even see a red spot. 4. Talk to a doctor. Find out how twinning occurs. Look for any twins in your neighbourhood, or among your friends. Find out if the twins are identical or non-identical. Also find out why identical twins are always of the same sex? If you know of any story about twins, write it in your own words. For more information on animal reproduction, you can visit: - ww. saburchill. com - ww. teenshealth. org/teen/sexual-health Did You Know? An interesting organisation is observed in a honey bee hive, a colony of several thousand bees. Only one bee in the colony lays eggs. This bee is called the queen bee. All other female bees are worker bees. Their main job is to build the hive, look after the young and feed the queen bee adequately to keep her healthy so that she can lay eggs. A queen bee lays thousands of eggs. The fertilised eggs hatch into females, while the unfertilised eggs give rise to males, called drones. It is the job of the worker bees to maintain the temperature of the hive at around 35°C to incubate the eggs. 2024-25


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RR AA AA EEAACCHHIINNGG TTHHEE GGEE OOFF DDOOLLEESSCCEENNCCEE I n the previous chapter, you have no longer a child but are on the way to learnt how animals reproduce. It is becoming an adult. only after 'growing up' to a certain age that human beings and many other animals can reproduce. Why can I wonder how long this humans reproduce only after a certain period marked by changes age? in the body will last! In this chapter, you will learn about changes that take place in the human body after which a person becomes capable of reproduction. It is a strange period of life In Chapter 6, you have learnt about when you are neither a child nor human reproductive organs. Here, we an adult. I wonder whether this shall discuss the role that hormones play period between childhood and in bringing about changes that make a adulthood had a special name! child grow into an adult. Growing up is a natural process. The 7. 1 Adolescence and Puberty period of life, when the body undergoes changes, leading to reproductive Boojho was celebrating his 12th maturity, is called adolescence. birthday. After his friends left, Boojho Adolescence begins around the age of and Paheli began chatting with their 11 and lasts upto 18 or 19 years of age. parents. Paheli studies in an all-girls Since this period covers the 'teens' (13 school. She started laughing. She to 18 or 19 years of age), adolescents remarked that many of Boojho's school are also called 'teenagers'. In girls, friends, whom she met after a year, had adolescence may begin a year or two suddenly shot up in height. Some of earlier than in boys. Also, the period of them were looking very funny with a adolescence varies from person to person. hairy line above their lips. Her mother The human body undergoes several explained that the boys had grown up. changes during adolescence. These Growth begins from the day one is changes mark the onset of puberty. The born. But upon crossing the age of 10 most important change which marks or 11, there is a sudden spurt in growth puberty is that boys and girls become which becomes noticeable. The changes capable of reproduction. Puberty ends taking place in the body are part of when an adolescent reaches reproductive growing up. They indicate that you are maturity. 2024-25 the tallest and who might be the shortest in your class. Paheli and Boojho realised that sudden Age in % of full height increase in height and hairy line above the lips Years Boys Girls in boys were signs of adolescence. They 8 72% 77% wanted to know more 9 75% 81% about other changes at puberty. 10 78% 84% 11 81% 88% 12 84% 91% 7. 2 Changes at Puberty 13 88% 95% Increase in Height 14 92% 98% The most conspicuous change during puberty is the sudden increase in 15 95% 99% height. At this time the long bones, that is, the bones of the arms and the legs 16 98% 99. 5% elongate and make a person tall. 17 99% 100%


Science

[Activity] ## Activity 7. 1 18 100% 100% The following chart gives the average rate of growth in height of boys and girls with age. The figures Calculation for full height (cm) in columns 2 and 3, give the percentage of the height a person Presentheight(cm) · 100 has reached at the age given in % of fullheightatthisage column 1. For example, by the age (as given in the chart) 11, a boy has reached 81% of his probable full height, while a girl Example: has reached 88% of her full A boy is 9 years old and 120 cm height. These figures are only tall. At the end of the growth period representative and there may be he is likely to be individual variations. Use the Table for your friends and work out how tall they are likely 120 · 100 cm = 160 cm tall to be. Find out who is likely to be 75 2024-25


Science

Activity 7.2 There is no need for Paheli to worry.

[Activity] ## Activity 7. 2 There is no need for Paheli to worry. All parts of the body do not grow at the Use the data given in ## Activity 7. 1 to same rate. Sometimes the arms and legs draw a graph. Take age on the or hands and feet of adolescents look X-axis and per cent growth in height oversized and out of proportion with the on the Y-axis. Highlight the point body. But soon the other parts catch representing your age on the graph. up and result in a proportionate body. Find out the percentage of height You must have noticed that height you have already reached. Calculate of an individual is more or less similar the height you might eventually to that of some family member. This is reach. Tally your graph with the one because height depends on the genes given here (Fig. 7. 1). inherited from parents. It is, however, very important to eat the right kind of food during these growing years. This helps the bones, muscles and other parts of the body get adequate nourishment for growth. You will find nutritional needs of adolescents discussed later in the lesson. Change in Body Shape Have you noticed that boys in your class have broader shoulders and wider chests than boys in junior classes? This is because they have entered the age of Graph showing percentage of height with age puberty when shoulders generally broaden as a result of growth. In girls, Initially, girls grow faster than boys the region below the waist becomes but by about 18 years of age, both reach wider. their maximum height. The rate of In boys, the muscles of the body grow growth in height varies in different more prominently than in the girls. individuals. Some may grow suddenly Thus, changes occurring in adolescent at puberty and then slow down, while boys and girls are different. others may grow gradually. Voice Change Did you notice that sometimes the voice of some of the boys in your class cracks? I am worried. Though I At puberty, the voice box or the larynx have become taller, my begins to grow. Boys develop larger voice face looks much smaller boxes. The growing voice box in boys can compared to my body. be seen as a protruding part of the throat 2024-25 called Adam's apple (Fig. 7. 2). In girls, and pimples on the face at this time the larynx is hardly visible from the because of the increased activity of these outside because of its small size. glands in the skin. Generally, girls have a high pitched voice, whereas boys have a deep voice. In Development of Sex Organs adolescent boys, sometimes, the muscles Look up Fig. 6. 1 and 6. 3 of the of the growing voice box go out of control previous lesson which show sex and the voice becomes hoarse. This state organs of humans. At puberty, male may remain for a few days or weeks after sex organs like the testes and penis which the voice becomes normal. develop completely. The testes also begin to produce sperms. In girls, the ovaries enlarge and eggs begin to Adam's mature. Also ovaries start releasing apple mature eggs. Reaching Mental, Intellectual and Emotional Maturity Adolescence is also a period of change in a person's way of thinking. Adolescents are more independent than before and are also self conscious. Intellectual development takes place and they tend to spend considerable time Adam's apple in a grown up boy thinking. In fact, it is often the time in one's life when the brain has the greatest capacity for learning. Many of my classmates Sometimes, however, an adolescent may have a hoarse voice. Now I feel insecure while trying to adjust to know why? the changes in the body and mind. But as adolescent learners, you should know Increased ## Activity of Sweat and that there is no reason to feel insecure. Sebaceous Glands These changes are a natural part of During puberty the secretion of sweat growing up. glands and sebaceous glands (oil glands) increases. Many young people get acne 7. 3 Secondary Sexual Characters A few glands such as sweat glands, oil glands and salivary glands release You have learnt in Chapter 6, that their secretions through ducts. testes and ovaries are the reproductive Endocrine glands release hormones organs. They produce the gametes, directly into the bloodstream. So, they that is, sperms and ova. In girls, are also termed ductless glands. breasts begin to develop at puberty 2024-25 and boys begin to grow facial hair, that The testes and ovaries secrete sex is, moustaches and beard. As these hormones. You have just learnt that features help to distinguish the male these hormones are responsible for the from the female they are called male and female secondary sexual secondary sexual characters. Boys characters. Further, the sex hormones also develop hair on their chest. In are under the control of hormones from both, boys and girls, hair grows under the pituitary gland (Fig. 7. 3). The the arms and in the region above the pituitary secretes many hormones, one thighs or the pubic region. of which makes ova mature in the ovaries and sperms form in the testes. Both Boojho and Paheli wish to know what initiates changes at puberty. The changes which occur at adolescence are controlled by hormones. Hormones are chemical substances. These are secretions from endocrine glands, or endocrine system. The male hormone or testosterone begins to be released by the testes at the onset of puberty. This causes changes in boys about which you have just learnt, for example, the growth of facial hair. Once puberty is reached in girls, ovaries begin to produce the female hormone or estrogen which makes the breasts develop. Milk secreting glands or mammary glands develop inside the breasts. The production of these hormones is under the control of another hormone secreted from an endocrine gland called pituitary gland. The onset of puberty is controlled by 7. 4 Role of Hormones in hormones Initiating Reproductive Function Paheli and Boojho have now Endocrine glands release hormones understood that puberty marks the into the bloodstream to reach a beginning of the reproductive period particular body part called target site. when one becomes capable of The target site responds to the reproduction. But they want to know if reproductive life, once begun, hormone. There are many endocrine continues, or it ends some time. glands or ductless glands in the body. 2024-25


Science

7.5 Reproductive Phase of Menstrual cycle is controlled by Life in Humans hormones. The cycle includes the maturation

7. 5 Reproductive Phase of Menstrual cycle is controlled by Life in Humans hormones. The cycle includes the maturation of the egg, its release, Adolescents become capable of thickening of uterine wall and its reproduction when their testes and breakdown if pregnancy does not ovaries begin to produce gametes. The occur. In case the egg is fertilised it capacity for maturation and production begins to divide and then gets of gametes lasts for a much longer time embedded in the uterus for further in males than in females. development as you have learnt in In females, the reproductive phase of Chapter 6 (Fig. 6. 8). life begins at puberty (10 to 12 years of age) and generally lasts till the age of 7. 6 How is the Sex of the approximately 45 to 50 years. The ova Baby Determined? begin to mature with the onset of puberty. One ovum matures and is released by one of the ovaries once in about 28 to 30 days. During this period, the wall of the uterus becomes I heard my mother and my aunt talking about my cousin thick so as to receive the egg, in case who is going to have a baby. it is fertilised and begins to develop. They were discussing whether This results in pregnancy. If she would give birth to a boy fertilisation does not occur, the or a girl. I wonder what makes released egg, and the thickened lining the fertilised egg develop of the uterus along with its blood either into a boy or a girl! vessels are shed off. This causes bleeding in women which is called Boy or Girl? menstruation. Menstruation occurs once in about 28 to 30 days. The first Inside the fertilised egg or zygote is the menstrual flow begins at puberty and instruction for determining the sex of is termed menarche. At 45 to 50 years the baby. This instruction is present in of age, the menstrual cycle stops. the thread-like structures, called Stoppage of menstruation is termed chromosomes in the fertilised egg. menopause. Initially, menstrual cycle Chromosomes are present inside the may be irregular. It take some time to nucleus of every cell. All human beings become regular. have 23 pairs of chromosomes in the nuclei of their cells. Two chromosomes out of these are the sex chromosomes, named X and Y. A female has two X Paheli says that the chromosomes, while a male has one X reproductive life of a and one Y chromosome. The gametes woman lasts from (egg and sperm) have only one set of menarche to menopause. chromosomes. The unfertilised egg Is she right? always has one X chromosome. But 2024-25 sperms are of two kinds. One kind has and ovaries to produce their hormones. an X chromosome, and the other kind You have already learnt that the has a Y chromosome. pituitary gland is an endocrine gland. See Fig. 7. 4. When a sperm It is attached to the brain. containing X chromosome fertilises the Apart from the pituitary, the testes egg, the zygote would have two X and the ovaries, there are other chromosomes and develop into a female endocrine glands in the body such as child. If the sperm contributes a Y thyroid, pancreas and adrenals chromosome to the egg (ovum) at (Fig. 7. 5). fertilisation, the zygote would develop into a male child. sperms pituitary gland thyroid gland adrenal gland eggs pancreas position of the ovary in the female boy girl testis Sex determination in humans Now you know that the sex chromosomes of the father determine the Position of endocrine glands in the sex of an unborn baby. The belief that human body the mother is responsible for the sex of Boojho and Paheli had once visited her baby is completely wrong and to their aunt who was a doctor and blame her for this is totally unjustified. remembered that a boy named Kaka had a very big and bulging throat. Their aunt 7. 7 Hormones other than had told them that Kaka was suffering Sex Hormones from 'goitre', a disease of the thyroid Look at Fig. 7. 3 again. The hormones gland. Kaka's thyroid gland was not secreted by the pituitary stimulate testes producing the hormone thyroxine. 2024-25 Their aunt also told them that their controlled by insect hormones. In a uncle was suffering from 'diabetes' frog, it is controlled by thyroxine, the because his pancreas was not producing hormone produced by thyroid. the hormone insulin in sufficient Thyroxine production requires the quantities. Boojho and Paheli then presence of iodine in water. If the water asked their aunt about the adrenal in which the tadpoles are growing does glands, which are also shown in the not contain sufficient iodine, the chart hung on the wall of her clinic. The tadpoles cannot become adults. aunt told them that adrenal glands secrete hormones which maintain the correct salt balance in the blood. If people do not have Adrenals also produce the hormone enough iodine in their diet, adrenalin. It helps the body to adjust will they get goitre caused to stress when one is very angry, by lack of thyroxine? embarrassed or worried. Thyroid and adrenals secrete their hormones when they receive orders from ## Activity 7. 3 the pituitary through its hormones. Pituitary also secretes growth hormone Collect information from magazines which is necessary for the normal or from doctors and prepare a note growth of a person. on the importance of consuming iodised salt. You can also look for this information on the internet. Are there hormones in 7. 9 Reproductive Health other animals also? Have they any role to play in The physical and mental well being of reproduction? an individual is regarded as an individual's health. To keep the body healthy, every human being, at any age, needs to have a balanced diet. The 7. 8 Role of Hormones in person must also observe personal Completing the Life hygiene and undertake adequate History of Insects and physical exercise. Frogs During adolescence, however, these become even more essential as the body You have already learnt about the life is growing. cycle of the frog. The tadpole passes through certain stages to become a frog Nutritional Needs of the Adolescents (Chapter 6). This change from larva to adult is called metamorphosis Adolescence is a stage of rapid growth (Fig. 6. 10). Metamorphosis in insects is and development. Hence the diet for an 2024-25 adolescent has to be carefully planned. ## Activity 7. 5 You have already learnt what a balanced Get ideas from the pictures given in diet is. Recall that a balanced diet means Fig. 7. 6. Prepare charts or posters that the meals include proteins, and paste them in the class so that carbohydrates, fats and vitamins in you are aware of the diet for requisite proportions. Our Indian meal adolescents. You may use your of roti/rice, dal (pulses) and vegetables creative ideas and present it like an is a balanced meal. Milk is a balanced advertisement. You may even food in itself. Fruits also provide organise a competition on this topic. nourishment. For infants, mother's milk provides all the nourishment that they need. Iron builds blood and iron-rich food meat such as leafy vegetables, jaggery, meat,


Science

[Activity] ## Activity 7. 4 Nutritious items of food Make a group with your friends. Write down the items of food in your Personal Hygiene breakfast, lunch and dinner you Everyone should have a bath at least had on the previous day. Identify once everyday. It is more necessary for the items responsible for proper teenagers because the increased activity growth. Also identified the junk of sweat glands sometimes makes the food that you consumed the body smelly. All parts of the body should previous day. be washed and cleaned everyday. If 2024-25 cleanliness is not maintained there are ## Activity 7. 6 chances of catching bacterial infection. Collect data on the number of Girls should take special care of children in your class who exercise cleanliness during the time of menstrual flow. They should keep track of their regularly and who do not exercise menstrual cycle and be prepared for the regularly. Did you notice any onset of menstruation. Use sanitary difference in their fitness and napkin or clean homemade pads. health? Prepare a report on the Change pads after every 4-5 hours as benefits of regular exercise. per the requirement. Say "NO" to Drugs Physical exercise Walking and playing in fresh air keeps Adolescence is a period of much the body fit and healthy. All young boys activity in the body and mind which and girls should take walks, exercise is a normal part of growing up. So do and play outdoor games. not feel confused or insecure. If anybody suggests that you will get Myths, Taboos, Do's and Don'ts relief if you take some drugs, just say You have learnt here and from Chapter 'No' unless prescribed by the doctor. 6 the scientific facts related to human Drugs are addictive. If you take them reproduction. There are many wrong notions which you should now be able once, you feel like taking them again to discard as informed adolescents. For and again. They harm the body in the example, there are myths and taboos long run. They ruin health and regarding bodily changes that happiness. adolescents experience. Some of these You must have heard about AIDS are given below and you can now argue which is caused by a dangerous virus, why these are myths and not facts. 1. A girl becomes pregnant if HIV. This virus can pass on to a she looks at boys during normal person from an infected menstruation. person by sharing the syringes used 2. The mother is responsible for the for injecting drugs. It can also be sex of her child. transmitted to an infant from the 3. A girl should not be allowed to infected mother through her milk. The work in the kitchen during menstruation. virus can also be transmitted through You may come across many other myths sexual contact with a person infected and taboos. Discard them. with HIV. Adolescent Pregnancy You might be knowing that in our country, the legal age for marriage is 18 years for girls and 21 years for boys. This is because teenage mothers are not prepared mentally or physically for motherhood. Early marriage and motherhood cause health problems in the mother and the child. It also curtails employment opportunities for the young woman and may cause mental agony as she is not ready for responsibilities of motherhood. 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT ADAM'SAPPLE (cid: 220) Humans become capable of reproduction after puberty sets in. Children between the ADOLESCENCE ages of 11 and 19 years are called adolescents. ADRENALIN (cid: 220) The onset of puberty brings about growth of the reproductive organs. Hair grow at BALANCED DIET various places on the body. Breasts develop ENDOCRINE GLANDS in girls and facial hair (moustache and beard) appear in boys. Voice of boys becomes hoarse ESTROGEN as voice box enlarges during adolescence. HORMONES (cid: 220) Children gain height during adolescence. (cid: 220) The onset of puberty and maturity of INSULIN reproductive parts are controlled by hormones. LARYNX (cid: 220) Hormones are secretions of endocrine glands which pour them directly into the blood stream. PITUITARY GLAND (cid: 220) Pituitary gland secretes hormones which PUBERTY include growth hormone and hormones that make other glands such as the testes, ovaries, REPRODUCTIVE thyroids and adrenals, secrete hormones. HEALTH Pancreas secretes insulin, thyroid produces thyroxine and adrenals produce adrenalin. SECONDARY SEXUAL (cid: 220) Testosterone is the male hormone and CHARACTERS estrogen, the female hormone. The uterine wall SEX CHROMOSOMES in females prepares itself to receive the developing fertilised egg. In case there is no TARGET SITE fertilisation, the thickened lining of the uterine TESTOSTERONE wall breaks down and goes out of the body along with blood. This is called menstruation. THYROXINE (cid: 220) Sex of the unborn child depends on whether VOICE BOX the zygote has XX or XY chromosomes. (cid: 220) It is important to eat balanced food and maintain personal hygiene during adolescence. 2024-25


Science

[Exercise] ## Exercises 1. What is the term used for chemical secretions of endocrine glands responsible for changes taking place in the body? 2. Define adolescence. 3. What is menstruation? Explain. 4. List changes in the body that take place at puberty. 5. Prepare a Table having two columns depicting names of endocrine glands and hormones secreted by them. 6. What are sex hormones? Why are they named so? State their function. 7. Choose the correct option. (a) Adolescents should be careful about what they eat, because - proper diet develops their brains. - proper diet is needed for the rapid growth taking place in their body. - adolescents feel hungry all the time. - taste buds are well developed in teenagers. (b) Reproductive age in women starts when their - menstruation starts. - breasts start developing. - body weight increases. - height increases. - The right meal for adolescents consists of - chips, noodles, coke. - chapati, dal, vegetables. - rice, noodles and burger. - vegetable cutlets, chips and lemon drink. 8. Write notes on- (a) Adam's apple. (b) Secondary sexual characters. - Sex determination in the unborn baby. 2024-25 9. Word game: Use the clues to work out the words. Across 3. Protruding voice box in boys 4. Glands without ducts 7. Endocrine gland attached to brain 8. Secretion of endocrine glands 9. Pancreatic hormone 10. Female hormone Down 1. Male hormone 2. Secretes thyroxine 3. Another term for teenage 5. Hormone reaches here through blood stream 6. Voice box 7. Term for changes at adolescence S E S I C R E X E 2024-25 10. 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? Age Height (cm) (Years) Boys Girls 0 53 53 4 96 92 8 114 110 12 129 133 16 150 150 20 173 165 Extended Learning - ## Activities and Projects 1. Find out from your elder relatives about their awareness of the legal status of early marriage. You yourself may get information on it from your teacher, parents, a doctor or the internet. Write a two- minute speech explaining why early marriage is not good for the couple. 2. Collect newspaper cuttings and information in magazines about HIV/ AIDS. Write a one page article of 15 to 20 sentences on HIV/AIDS. 3. In our country, according to 2011 census, there are 940 adolescent females for every 100 males. Find out. (a) the concerns of the community regarding this low ratio.


Science

## Remember that the chance of having a boy or a girl is equal. (b) what amniocentesis is and how useful this technique is. Why is its use for identification of sex of the unborn child banned in India? 4. Put your ideas together and write a short note on the importance of knowing facts about reproduction. For more information visit: - ww. teenshealth. org/teen/sexual_health/ - ww. adolescenthealth. com 2024-25


Science

FF PP OORRCCEE AANNDD RREESSUURREE I n Class VII, you have learnt how by a batsman? (Fig. 8. 1). In all these objects move. Do you recall how we situations the ball is either made to can decide whether an object is move faster or slower or its direction moving faster than the other? What does of motion is changed. the distance moved by an object in unit We often say that a force has been time indicate? You also know that a applied on a ball when it is kicked, moving object like a ball rolling on the pushed, thrown or flicked. What is a ground slows down. Sometimes it may force? What can it do to bodies on which change its direction of motion. It is also it is applied? We shall seek answers to possible that the ball may slow down such questions in this chapter. and also change its direction. Have you ever wondered what makes an object 8. 1 Force - APush or a Pull slow down or go faster, or change its Actions like picking, opening, direction of motion? shutting, kicking, hitting, lifting, Let us recall some of our everyday flicking, pushing, pulling are often experiences. What do you do to make a football move? What do you do to used to describe certain tasks. Each make a moving ball move faster? How of these actions usually results in does a goalkeeper stop a ball? A some kind of change in the state of hockey player changes the direction motion of an object. Can these terms of the moving ball with a flick of the be replaced with one or more terms? stick. How do fielders stop a ball hit Let us find out. (a) (b) (c) (a) A goal keeper saving a goal, (b) A hockey player flicking a ball, and - A fielder stopping a ball 2024-25


Science

[Activity] ## Activity 8. 1 Table 8. 1 gives some examples of familiar situations involving motion of objects. You can add more such situations or replace those given here. Try to identify action involved in each case as a push and/or a pull and record your observations. One example has been given to help you. Identifying Actions as Push or Pull S. Description of Action: (pushing/ pulling/picking/ Action can be No. the situation hitting/lifting/ lowering/flying/ grouped as a kicking/ throwing/shutting/ flicking) Push Pull 1. Moving a book Pushing Pulling Lifting - Yes Yes placed on a table 2. Opening or shutting a door 3. Drawing a bucket of water from a well 4. A football player taking a penalty kick 5. A cricket ball hit by a batsman 6. Moving a loaded cart 7. Opening a drawer Do you notice that each of the actions can be grouped as a pull or a push or both? Can we infer from this, that to move an object, it has to be pushed or pulled? I learnt in Class VI that a In science, a push or a pull on an magnet attracts a piece of object is called a force. Thus, we can iron towards it. Is attraction also a pull? What about say that the motion imparted to objects repulsion between similar was due to the action of a force. When poles of two magnets? Is it a does a force come into play? Let us pull or a push? find out. 2024-25


Science

8.2 Forces are due to an Fig. 8.3 shows three situations that Interaction may be familiar to you. Can you decide who is

8. 2 Forces are due to an Fig. 8. 3 shows three situations that Interaction may be familiar to you. Can you decide who is pulling and who is pushing in Suppose a man is standing behind a these cases? In Fig. 8. 3 (a), both the girls stationary car. Will the car appear to push each other while move due to his presence? Suppose the man now begins to push the car, that is, he applies a force on it. The car may begin to move in the Fig. 8. 3 (b): Who is pulling whom? the pair of girls in Fig. 8. 3 (b) are trying Fig. 8. 2(a): A man standing behind a stationary car to pull each other. Similarly, the cow and the man in Fig. 8. 3 (c) appear to Fig. 8. 2 (b): A car being pushed by a man direction of the applied force. Note that the Fig. 8. 3 (c): Who is pulling whom? man has to push the car to make it move. pull each other. The girls in the two situations shown here are applying force on each other. Is it also true for the man and the cow? From these examples, we can infer that at least two objects must interact for a force to come into play. Thus, an interaction of one object with another object results in a force between the Fig. 8. 3 (a): Who is pushing whom? two objects. 2024-25


Science

8.3 Exploring Forces Let us try to learn more about forces.

8. 3 Exploring Forces Let us try to learn more about forces.


Science

[Activity] ## Activity 8. 2 Choose a heavy object like a table or a box, which you can move only by pushing hard. Try to push it all The rope may not move if the two teams pull at it with equal force by yourself. Can you move it? Now rope in their direction. Sometimes the rope ask one of your friends to help you simply does not move. Is it not similar to in pushing it in the same direction the situation shown in Fig. 8. 3 (b)? The. Is it easier to move it team that pulls harder, that is, applies a now? Can you explain why? larger force, finally wins the game. Now push the same object, but What do these examples suggest ask your friend to push it from the about the nature of force? opposite side. Does the Forces applied on an object in the object move? If it does, note the same direction add to one another. Now direction in which it moves. Can recall what happened when you and you guess which one of you is your friend pushed the heavy box in the applying a larger force? same direction in ## Activity 8. 2. If the two forces act in the opposite (a) directions on an object, the net force acting on it is the difference between the two forces. What did you observe in ## Activity 8. 2 when both of you were pushing the heavy box from opposite directions? Recall that in the tug-of-war when two teams pull equally hard, the rope does not move in any direction. (b) So, we learn that a force could be larger or smaller than the other or equal to each other. The strength of a force is usually expressed by its magnitude. We have also to specify the direction in which a force acts. Also, if the direction Two friends pushing a heavy load or the magnitude of the applied force (a) in the same direction, (b) in changes, its effect also changes. opposite direction Have you ever seen a game of tug-of Does it mean that the net war? In this game two teams pull at a force on an object is zero if the two forces acting on it in rope in opposite directions (Fig. 8. 5). opposite directions are equal? Members of both the teams try to pull the 2024-25 You might recall similar situations. In general, more than one force may For example, while taking a penalty kick be acting on an object. However, the in football, the player applies a force on effect on the object is due to the net the ball. Before being hit, the ball was at force acting on it. rest and so its speed was zero. The applied force makes the ball move 8. 4 AForce can Change the towards the goal. Suppose, the State of Motion goalkeeper dives or jumps up to save the Let us now find out what happens when goal. By his action the goalkeeper tries a force acts on an object. to apply a force on the moving ball. The force applied by him can stop or deflect


Science

Activity 8.3 the ball, saving a goal being scored. If

[Activity] ## Activity 8. 3 the ball, saving a goal being scored. If the goalkeeper succeeds in stopping the Take a rubber ball and place it on a ball, its speed decreases to zero. level surface such as a table top or These observations suggest that a a concrete floor. Now, gently push force applied on an object may change the ball along the level surface its speed. If the force applied on the object (Fig. 8. 6). Does the ball begin to is in the direction of its motion, the speed move? Push the ball again while it of the object increases. If the force is is still moving. Is there any change applied in the direction opposite to the in its speed? Does it increase or direction of motion, then it results in a decrease? decrease in the speed of the object. Next, place your palm in front of the moving ball. Remove your I have seen palm as soon as the moving ball children competing with one touches it. Does your palm apply a another in moving a rubber tyre force on the ball? What happens to or a ring by pushing it the speed of the ball now? Does it (Fig. 8. 7). I now understand increase or decrease? What would why the speed of the tyre increases whenever it is happen if you let your palm hold pushed. the moving ball? A ball at rest begins to move To move a tyre faster it has to be when a force is applied on it pushed repeatedly 2024-25 Paheli is curious to know whether Let us consider some more application of a force can only change the examples. In a game of volleyball, speed of an object. Let us find out. players often push the moving ball to their team mates to make a winning


Science

Activity 8.4 move. Sometimes the ball is returned

[Activity] ## Activity 8. 4 move. Sometimes the ball is returned to the other side of the court by Take a ball and place it on a level pushing or smashing it. In cricket, a surface as you did in ## Activity 8. 3. batsman plays his or her shot by Make the ball move by giving it a applying a force on the ball with the push. Now place a ruler bat. Is there any change in the in its path as shown in Fig. 8. 8(b). direction of motion of the ball in these In doing so, you would apply a force cases? In all these examples the speed on the moving ball. Does the ball and the direction of the moving ball continue to move in the same change due to the application of a direction after it strikes the ruler? force. Can you give a few more Repeat the activity and try to examples of this kind? obstruct the moving ball by placing A change in either the speed of an the ruler in such a way that it object, or its direction of motion, or both, makes different angles to its path. is described as a change in its state of In each case note your observations motion. Thus, a force may bring a about the direction of motion of the change in the state of motion of an ball after it strikes the ruler. object. State of Motion The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero (a) speed. An object may be at rest or in motion; both are its states of motion. Does it mean that the application of a force would always result in a change in the state of motion of the object? Let us find out. It is common experience that many a time application of force does not result (b) in a change in the state of motion. For example, a heavy box may not move at (a) A ball set in motion by pushing it along a level surface all even if you apply the maximum force and (b) the direction of motion that you can exert. Again, no effect of of the ball after it strikes the force is observed when you try to push ruler placed in its path a wall. 2024-25


Science

8.5 Force can Change the Shape of an Object

8. 5 Force can Change the Shape of an Object


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[Activity] ## Activity 8. 5 Some situations have been given in Column 1 of Table 8. 2 in which objects are not free to move. Column 2 of the Table suggests the manner in which a force can be applied on each object while Column 3 shows a diagram of the action. Try to observe the effect of force in as many situations as possible. You can also add similar situations using available material from your environment. Note your observations in Columns 4 and 5 of the Table. Studying the Effect of Force on Objects Description of How to Apply Diagram Action of Force Situation Force Change in Change in State of Shape Motion Yes No Yes No A lump of Pressing it down dough on a with your hands. plate. Spring fixed to By sitting on the the seat of a seat. bicycle. A rubber band By hanging a suspended from weight or by a hook/nail pulling its free end. fixed on a wall. A plastic or By putting a weight metal scale at the centre of the placed between scale. two bricks. 2024-25 What do you conclude from the help of a stick or a piece of rope. When observations noted in Table 8. 2? What we push an object like a school bag or happens when you apply a force on lift a bucket of water, where does the an inflated balloon by pressing it force come from? This force is caused between your palms? What happens by the action of muscles in our body. to the shape of a ball of dough when it The force resulting due to the action of is rolled to make a chapati? What muscles is known as the muscular happens when you press a rubber ball force. placed on a table? In all these examples you saw that the application It is the muscular force that enables of force on an object may change us to perform all activities involving movement or bending of our body. In its shape. Class VII you have learnt that in the Having performed all the above process of digestion the food gets activities, you would have realised that pushed through the alimentary canal. a force Could it be a muscular force that does it? You also know that lungs expand - may make an object move from rest. and contract while we inhale and - may change the speed of an object exhale air during breathing. Where if it is moving. are these muscles located which - may change the direction of motion make breathing possible? Can you list a few more examples of the force of an object. exerted by the muscles in our body? - may bring about a change in the shape of an object. Animals also make use of muscular - may cause some or all of these force to carry out their physical activities effects. and other tasks. Animals like bullocks, horses, donkeys and camels are used to While a force may cause one or more perform various tasks for us. In of these effects, it is important to performing these tasks they use remember that none of these actions can muscular force (Fig. 8. 9). take place without the action of a force. Thus, an object cannot move by itself, it cannot change speed by itself, it cannot change direction by itself and its shape cannot change by itself. 8. 6 Contact Forces Muscular Force Can you push or lift a book lying on a table without touching it? Can you lift a bucket of water without holding it? Generally, to apply a force on an object, your body has to be in contact with the Muscular force of animals is used to object. The contact may also be with the carry out many difficult tasks 2024-25 Since muscular force can be applied on an object to be always in contact with only when it is in contact with an object, it. Let us find out. it is also called a contact force. Are there other types of contact forces? Let us find 8. 7 Non-contact Forces out. Magnetic Force Friction


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[Activity] ## Activity 8. 6 Recall some of your experiences. A ball Take a pair of bar magnets. Place rolling along the ground gradually slows the longer side of one of the magnets down and finally comes to rest. When over three round shaped pencils or we stop pedalling a bicycle, it gradually wooden rollers as shown in slows down and finally comes to a stop. Fig. 8. 10. Now bring one end of the A car or a scooter also comes to rest other magnet near the end of the once its engine is switched off. Similarly, magnet placed on the rollers. Make a boat comes to rest if we stop rowing sure that the two magnets do not it. Can you add some more such touch each other. Observe what experiences? happens. Next, bring the other end In all these situations no force appears to be acting on the objects, yet of the magnet near the same end their speed gradually decreases and of the magnet placed on the rollers they come to rest after some time. What (Fig. 8. 10). Note what happens to causes a change in their state of motion? the magnet placed on the rollers Could some force be acting on them! every time another magnet is Can you guess the direction in which brought near it. the force must be acting in each case? The force responsible for changing the state of motion of objects in all these examples is the force of friction. It is the force of friction between the surface of the ball and the ground that brings the moving ball to rest. Similarly, friction between water and the boat brings it to a stop once you stop rowing. The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion. Since the force of friction arises due to contact between surfaces, it is also an example of a contact force. You will learn more about this force in Chapter 9. You may be wondering whether it is Observing attraction and repulsion between two magnets essential for the agent applying a force 2024-25 Does the magnet on the rollers begin to move when the other magnet is brought near it? Does it always move in the direction of the approaching magnet? What do these observations suggest? Does it mean that some force must be acting between the two magnets? You have learnt in Class VI that like poles of two magnets repel each other and unlike poles attract each other. Attraction or repulsion between objects can also be seen as another form of pull or push. Do A straw rubbed with paper attracts you have to bring the magnets in contact another straw but repels it if it has for observing the force between them? A also been rubbed with a sheet magnet can exert a force on another of paper magnet without being in contact with it. The force exerted by a magnet is an A straw is said to have acquired example of a non-contact force. electrostatic charge after it has been Similarly, the force exerted by a rubbed with a sheet of paper. Such a magnet on a piece of iron is also a straw is an example of a charged body. non-contact force. The force exerted by a charged body on another charged or uncharged body Electrostatic Force is known as electrostatic force. This


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Activity 8.7 force comes into play even when the

[Activity] ## Activity 8. 7 force comes into play even when the bodies are not in contact. The Take a plastic straw and cut it into electrostatic force, therefore, is another nearly two equal pieces. Suspend one example of a non-contact force. You will of the pieces from the edge of a table learn more about electric charges in with the help of a piece of thread Chapter 12. (Fig. 8. 11). Now hold the other piece of straw in your hand and rub its Gravitational Force free end with a sheet of paper. Bring You know that a coin or a pen falls to the the rubbed end of the straw near the ground when it slips off your hand. suspended straw. Make sure that the two pieces do not touch each other. Leaves and fruits also fall to the ground What do you observe? when they get detached from the plant. Next, rub the free end of the Have you ever wondered why it is so? suspended piece of straw with a When the coin is held in your hand it sheet of paper. Again, bring the is at rest. As soon as it is released, it piece of straw that was rubbed begins to move downwards. It is clear that earlier with paper near the free end the state of motion of the coin undergoes of the suspended straw. What do you a change. Can this happen without a observe now? force acting on it? Which is this force? 2024-25 Objects or things fall towards the Try cutting vegetables with a blunt earth because it pulls them. This force knife and then with a sharp knife. Which is called the force of gravity, or just is easier? gravity. This is an attractive force. The Do you get the feeling that the area force of gravity acts on all objects. The over which the force is applied (for force of gravity acts on all of us all the example, the pointed end of the nail) time without our being aware of it. Water plays a role in making these tasks easier? begins to flow towards the ground as The force acting on a unit area of a soon as we open a tap. Water in rivers surface is called pressure. flows downward due to the force pressure = force / area on which it acts of gravity. At this stage we consider only those forces which act perpendicular to the Gravity is not a property of the earth surface on which the pressure is to alone. In fact, every object in the be computed. universe, whether small or large, exerts a force on every other object. This force is known as the gravitational force. I now understand why porters place a round piece 8. 8 Pressure of cloth on their heads, Is there any relation between pressure when they have to carry and force? Let us find out. heavy loads (Fig. 8. 13). By doing this they increase the Try to push a nail into a wooden area of contact of the load plank by its head. Did you succeed? Try with their head. So, the now to push the nail by the pointed end pressure on their head is (Fig. 8. 12). Could you do it this time? reduced and they find it easier to carry the load. Pushing a nail into a wooden plank A porter carrying a heavy load 2024-25 Note that the area is in the this process a few more times. Can denominator in the above expression. you see any relation between the So, the smaller the area, larger the amount of the bulge in the rubber pressure on a surface for the same force. sheet and the height of the water The area of the pointed end of the nail column in the pipe? is much smaller than that of its head. The same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank. Can you explain now why shoulder bags are provided with broad straps and not thin strap? And, why the tools meant for cutting and piercing always have sharp edges? Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts? Let us find out. 8. 9 Pressure Exerted by Liquids and Gases Pressure exerted by water at the


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[Activity] ## Activity 8. 8 bottom of the container depends on the height of its column Take a transparent glass tube or a plastic pipe. The length of the pipe/


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[Activity] ## Activity 8. 9 tube should be about 25 cm and its diameter should be 5-7. 5 cm. Also Take a plastic bottle. You can take take a piece of thin sheet of a good a discarded water or soft drink quality rubber, say, a rubber bottle. Fix a cylindrical glass tube, balloon. Stretch the rubber sheet a few cm long near its bottom as tightly over one end of the pipe. shown in Fig. 8. 15. You can do so Hold the pipe at the middle, keeping by slightly heating one end of the it in a vertical position (Fig. 8. 14). glass tube and then quickly inserting it near the bottom of the Ask one of your friends to pour some bottle. Make sure that the water water in the pipe. Does the rubber does not leak from the joint. If there sheet bulge out? Note also the is any leakage, seal it with molten height of the water column in the wax. Cover the mouth of the glass pipe. Pour some more water. tube with a thin rubber sheet as you Observe again the bulge in the did in ## Activity 8. 8. Now fill the bottle rubber sheet and the height of the upto half with water. What do you water column in the pipe. Repeat observe? Why does the rubber sheet 2024-25 fixed to the glass tube bulge this time? Pour some more water in the bottle. Is there any change in the bulge of the rubber sheet? Liquids exert equal pressure at the A liquid exerts pressure on the walls same depth of the container Note that the rubber sheet has been Can you now say that liquids exert fixed on the side of the container and not pressure on the walls of the at the bottom. Does the bulging of the container? rubber sheet in this case indicate that water Do gases also exert pressure? Do exerts pressure on the sides of the container they also exert pressure on the walls of as well? Let us investigate further. their containers? Let us find out.


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[Activity] ## Activity 8. 10 Take an empty plastic bottle or a cylindrical container. You can take I have seen fountains of water a used tin can or a used plastic coming out of the leaking joints bottle. Drill four holes all around or holes in pipes supplying near the bottom of the bottle. Make water. Is it not due to the pressure exerted by water on the sure that the holes are at the same walls of the pipes? height from the bottom (Fig. 8. 16). Now fill the bottle with water. What do you observe? When you inflate a balloon, why do Do the different streams of water you have to close its mouth? What coming out of the holes fall at the happens when you open the mouth of same distance from the bottle? What an inflated balloon? Suppose you have does this indicate? a balloon which has holes. Would you 2024-25 be able to inflate it? If not, why? Can we But, how large or small is the say that air exerts pressure in all atmospheric pressure? Let us get an directions? idea about its magnitude. Do you recall what happens to the air in the bicycle tube when it has a ## Activity 8. 11 puncture? Do these observations suggest Take a good quality rubber sucker. that air exerts pressure on the inner walls It looks like a small rubber cup (Fig. of an inflated balloon or a tube? So, we 8. 18). Press it hard on a smooth find that gases, too, exert pressure on plane surface. Does it stick to the the walls of their container. surface? Now try to pull it off the surface. Can you do it? 8. 10 Atmospheric Pressure You know that there is air all around us. This envelop of air is known as the atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure. We know that pressure is force per unit area. If we imagine a unit area and a very long cylinder standing on it filled with air, then the force of gravity on the air in this cylinder is the atmospheric pressure (Fig. 8. 17). A rubber sucker pressed on a surface When you press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because the pressure of atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. This activity Unit area might give you an idea about the magnitude of atmospheric pressure. In fact, it would not be possible for any Atmospheric pressure is the force of gravity on air in a column of unit area human being to pull the sucker off the 2024-25 surface if there were no air at all between the sucker and the surface. Does it give you an idea how large the atmospheric pressure is? If the area of my head were 15 cm × 15 cm, how much force air will exert on my head? The force due to air in a column of the height of the atmosphere and area 15 cm × 15 cm (Fig. 8. 19) is nearly equal to the force of gravity on an object of mass 225 kg (2250N). The reason we are not crushed under this force of gravity is that the pressure inside our bodies is also equal to the atmospheric Pressure of atmosphere on your head pressure and balances the pressure from outside. Did you know? Otto von Guericke, a German scientist of the 17th century, invented a pump to extract air out of a vessel. With the help of this pump, he demonstrated dramatically the force of the air pressure. He joined two hollow metallic hemispheres of 51 cm diameter each and pumped air out of them. Then he employed eight horses on each hemisphere to pull them apart (Fig. 8. 20). So great is the force of air pressure that the hemispheres could not be pulled apart. Horses pulling the hemispheres 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT ATMOSPHERIC (cid: 220) Force could be a push or a pull. PRESSURE (cid: 220) A force arises due to the interaction between two objects. CONTACT FORCE (cid: 220) Force has magnitude as well as direction. ELECTROSTATIC (cid: 220) A change in the speed of an object or the di- FORCE rection of its motion or both implies a change in its state of motion. FORCE (cid: 220) Force acting on an object may cause a change FRICTION in its state of motion or a change in its shape. (cid: 220) A force can act on an object with or without GRAVITATIONAL being in contact with it. FORCE (cid: 220) Force per unit area is called pressure. GRAVITY (cid: 220) Liquids and gases exert pressure on the walls of their containers. MAGNETIC FORCE (cid: 220) The pressure exerted by air around us is MUSCULAR FORCE known as atmospheric pressure. NON-CONTACT FORCE PRESSURE PULL PUSH


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[Exercise] ## Exercises 1. Give two examples each of situations in which you push or pull to change the state of motion of objects. 2. Give two examples of situations in which applied force causes a change in the shape of an object. 3. Fill in the blanks in the following statements. (a) To draw water from a well we have to __ at the rope. (b) A charged body __ an uncharged body towards it. - To move a loaded trolley we have to __ it. - The north pole of a magnet __the north pole of another magnet. 2024-25 4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms. muscular, contact, non-contact, gravity, friction, shape, attraction (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. - The type of force responsible for a change in the state of motion of the arrow is an example of a __ force. - While the arrow moves towards its target, the forces acting on it are due to __ and that due to __ of air. 5. 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. (b) Taking out paste from a toothpaste tube. - A load suspended from a spring while its other end is on a hook fixed to a wall. - An athlete making a high jump to clear the bar at a certain height. 6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron? 7. 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? 8. 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. 9. 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. 10. When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to (a) pressure of water. (b) gravity of the earth. - shape of rubber bulb. - atmospheric pressure. S E S I C R E X E 2024-25 Extended Learning - ## Activities and Projects 1. Make a 50 cm × 50 cm bed of dry sand about 10 cm in thickness. Make sure that its top surface is levelled. Take a wooden or a plastic stool. Cut two strips of graph paper each with a width of 1 cm. Paste them vertically on any leg of the stool - one at the bottom and the other from the top. Now gently put the stool on the sand bed with its legs resting on the sand. Increase the size of sand bed if required. Now put a load, say a school bag full of books, on the seat of the stool. Mark the level of sand on the graph strip. This would give you the depth, if any, to which the legs of stool sink in sand. Next, turn the stool upside down so that now it rests on its seat on the sand bed. Note the depth to which the stool sinks now. Next, put the same load on the stool and note the depth to which it sinks in the sand. Compare the pressure exerted by the stool in the two situations. 2. Take a tumbler and fill it with water. Cover the mouth of the tumbler with a thick card similar to that of a postcard. Hold the tumbler with one hand while keeping the card pressed to its mouth with your other hand. Turn the tumbler upside down while keeping the card pressed to its mouth. Make sure that the tumbler is held vertical. Gently remove the hand pressing the card. What do you observe? Does the card get detached allowing the water to spill? With a little practice you will find that the card continues to hold water in the tumbler even after it is not supported by your hand. Also try this activity by using a piece of cloth to hold the tumbler in an upside down position (Fig. 8. 21). Fig. 8. 21 2024-25 3. Take 4-5 plastic bottles of different shapes and sizes. Join them together with small pieces of glass or rubber tube as shown in Fig. 8. 22. Keep this arrangement on a level surface. Now pour water in any one of the bottles. Note whether the bottle in which water is poured gets filled first or all the bottles get filled up simultaneously. Note the level of water in all the bottles from time to time. Try to explain your observations. Fig. 8. 22 2024-25


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FF RRIICCTTIIOONN Y ou might have seen a driver of a 9. 1 Force of Friction car or a truck slowing down the


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[Activity] ## Activity 9. 1 vehicle at a traffic signal. You, too, slow down your bicycle whenever Gently push a book on a table needed by applying brakes. Have you. You observe that it stops ever thought why a vehicle slows after moving for some distance. down when brakes are applied? Not Repeat this activity pushing the only vehicles, any object, moving over book from the opposite direction the surface of another object slows. Does the book stop down when no external force is this time, too? Can you think of an applied on it. Finally it stops. Have explanation? Can we say that a force you not seen a moving ball on the must be acting on the book ground stopping after some time? opposing its motion? This force is Why do we slip when we step on a called the force of friction. banana peel (Fig. 9. 1)? Why is it difficult to walk on a smooth and wet floor? (a) (b) A boy falls down when he steps on a Fig. 9. 2 (a), (b): Friction opposes relative banana peel motion between the surfaces of the book and You will find the answers to such the table questions in this chapter. 2024-25 You saw that if you apply the force for this difference? Repeat this along the left, friction acts along the activity by wrapping a piece of jute right. If you apply the force along the bag around the brick. What do you right, the friction acts along the left observe? direction. In both cases the force opposes the motion of the book. The force of Spring Balance friction always opposes the applied force. In the above activity, the force of Spring balance is a device used for friction acts between the surface of the measuring the force acting on an object. It consists of a coiled spring book and the surface of the table. which gets stretched when a force is Is the friction the same for all the applied to it. Stretching of the spring surfaces? Does it depend on the is measured by a pointer moving on smoothness of the surfaces? Let us find a graduated scale. The reading on the out. scale gives the magnitude of the force. 9. 2 Factors affecting Friction


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[Activity] ## Activity 9. 2 Tie a string around a brick. Pull the brick by a spring balance (Fig. 9. 3). You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor.


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[Activity] ## Activity 9. 3 Make an inclined plane on a smooth A brick is being pulled by spring floor, or on a table. You may use a balance wooden board supported by bricks, or books. . Put a mark Now wrap a piece of polythene with a pen at any point A on the around the brick and repeat the inclined plane. Now let a pencil cell activity. Do you observe any move down from this point. How far difference in the readings of the does it move on the table before spring balance in the above two coming to rest? Note down the cases? What might be the reason distance. Now spread a piece of cloth 2024-25 Does the distance covered depend on over the table. Make sure that there the nature of the surface on which the are no wrinkles in the cloth. Try the cell moves? activity again. Could the smoothness of the surface of the cell also affect the distance travelled by it? Mark A I shall try the activity by wrapping a piece of sandpaper around the cell. Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which (a) appear very smooth have a large number of minute irregularities on them (Fig. 9. 5). Irregularities on the two surfaces lock into one another. When Mark A we attempt to move any surface, we have to apply a force to overcome interlocking. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved. (b) The pencil cell covers different distances on different surfaces Repeat this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity. Surface of book In which case is the distance covered the minimum? Why is the distance Surface of table covered by the pencil cell different every time. Try to reason why? Discuss the Surface irregularities result. 2024-25 We see that the friction is caused by friction and you find it somewhat easier the interlocking of irregularities in the to move the box already in motion than two surfaces. It is obvious that the force to get it started. of friction will increase if the two 9. 3 Friction: ANecessary surfaces are pressed harder. You can experience it by dragging a mat when Evil nobody is sitting on it, and when a Recall now some of your experiences. Is preson is sitting on it. it easier to hold a kulhar (earthen pot) or a glass tumbler? Suppose the outer surface of the tumbler is greasy, or has a thin layer of cooking oil on it; would it become easier or more difficult to hold it? Just think: would it be possible to hold the glass at all if there is no friction? Recall also how difficult it is to move on a wet muddy track, or wet marble floor. Can you imagine being able to walk at all if there were no friction? You could not write with pen or pencil if there were no friction. When You have to push on the box to keep it your teacher is writing with chalk on moving the blackboard, its rough surface rubs Recall your experience when last time off some chalk particles which stick to you moved a heavy box from one place to another (Fig. 9. 6). If you have no such experience, get that experience now. What is easier - to move the box from rest, or to move it when it is already in motion? The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction. When the box starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So, the sliding A nail is fixed in the wall due to friction is slightly smaller than the static friction 2024-25 the black board. Could it happen if there were no friction between the chalk and the board? If an object started moving, it would never stop if there were no friction. Had there been no friction between the tyres of the automobiles and the road, they could not be started or stopped or turned to change the direction of motion. You could not fix a nail on the wall (Fig. 9. 7) or tie a knot. Without friction no building could be constructed. Rubbing of your palms makes you feel warm for a few minutes. You can cite various other examples in which friction Soles of shoes wear out due to friction On the other hand, friction is an evil, too. It wears out the materials whether they are screws, ball bearings or soles of shoes (Fig. 9. 8). You must have seen worn-out steps of foot over-bridges at railway stations. Friction can also produce heat. Vigorously rub your palms together for Stricking a matchstick produces fire a few minutes (Fig. 9. 9). How do you by friction feel? When you strike a matchstick produces heat. In fact, when a machine against the rough surface, it catches fire is operated, heat generated causes much (Fig. 9. 10). wastage of energy. We shall discuss the You might have observed that the jar ways of minimising friction in the of a mixer becomes hot when it is run following section. 2024-25