8 NCERT SCIENCE TEXTBOOK SOLUTION

Chapter 13 - Sound Exercise 168 Solution 1 (d) Sound can travel through solids, liquids, and gases. Concept insight : As sound travels by vibrations of particles, so it needs a medium to travel. Solution 2 (c) A man The voice of an adult man is of lower pitch in comparison to the voices of a baby boy, a baby girl and a woman. Since frequency of a sound is directly proportional to its pitch, man's voice is likely to have minimum frequency due to its lowest pitch. Concept insight : Frequency of sound is directly proportional to pitch. Chapter 13 - Sound Exercise 169 Solution 1 (a) True Concept insight : Sound cannot travel in vacuum because vacuum has no molecules which can vibrate and carry sound waves. (b) False Concept insight : The number of oscillations per second of a vibrating object is known as its frequency. (c) False Concept insight : Loudness of a sound is proportional to the amplitude of its ...

Chapter 12 - Friction Exercise 155 Solution 1 (a) Relative motion (b) Nature (smoothness or irregularities) (c) Heat (d) Reduces (e) Smaller Solution 2 (c) static, sliding, rolling Concept insight : When an object has already started moving (sliding), the irregularities on its surface do not get enough time to lock into the irregularities on the surface of the other object completely. Since the interlocking of the two surfaces is less when an object has already started moving, therefore, the sliding friction is smaller than the static friction. It is always easier to roll than to slide an object over another object. So, rolling friction is much less than sliding friction. Solution 3 (a) wet marble floor, dry marble floor, newspaper and towel Concept insight : The rougher the surface, more is the force of friction. Solution 4 When a book slides down on the tilted writing desk, a frictional force acts between the bo...

Chapter 11 - Force and Pressure Exercise 142 Solution 1 Two examples of push force are as follows: (i) A heavy box at rest is pushed to move it from one room to another. This changes the state of motion of the box. (ii) A player pushes a football using his foot. This changes the state of motion of the ball. Two examples of pull force are as follows: (i) Rope is pulled to draw water from a well. This changes the state of motion of the water bucket. (ii) A drawer is pulled to open it. This changes the state of motion of the drawer. Concept insight : Force is a push or pull. Solution 2 Two examples of forces that cause a change in the shape of an object are as follows: (i) Squeezing of a plastic bottle changes the shape of the bottle. (ii) Deformation of clay by pressing it between the hands. (iii) Making a chapati from a ball of dough. Concept insight : Force may cause a change in the state of motion of a body or a change in the shape of the body. ...

Chapter 10 - Reaching the Age of Adolescence Exercise 124 Solution 1 Hormones are chemical substances which are secreted by endocrine glands. They are responsible for changes taking place in the body. Concept insight: Important from Exam Point of View. Solution 2 Adolescence is the time period between the beginning of puberty and adulthood. It begins around the age of 11 and lasts till 18 or 19 years of age. The period of adolescence may vary from person to person.During this time two types of changes takes place in the body: i. Person attains reproductive maturity. ii. Boys and girls develop secondary sexual characters. Concept insight: The period of life, when the body undergoes changes, leading to reproductive maturity, is called adolescence. Solution 3 Menstruation is the process of the shedding of the uterine lining on a regular monthly basis. It begins at puberty. Every month, the uterus prepares itself to receive a...

Chapter 9 - Reproduction in Animals Exercise 110 Solution 1 Reproduction is a biological process through which living organisms produce offspring similar to themselves. Living organisms reproduce to maintain their number and continuation of their species. Concept insight: Reproduction ensures the continuation of similar kinds of individuals. Solution 2 Fertilization involves the fusion of the male and the female gametes. The male and the female gametes are released from the male and the female reproductive organs. Sperms or male gametes are released from the male reproductive organ i.e., the penis. These sperms then enter the female body through the vagina. Then, they travel through the fallopian tubes where they fuse with eggs. Hence, the process of fertilization takes place in the fallopian tubes. Concept insight: Fertilization is the process of fusion of male and female gametes in the fallopian tube of the females. Solution...

Chapter 8 - Cell - Structure And Functions Exercise 98 Solution 1 (a) Unicellular organisms have one-celled body. True Concept insight: Organisms made up of single cell are called unicellular organisms whereas organisms made up of more than one cell are called multicellular organisms. (b) Muscle cells are branched. True Concept insight: Recall different shapes of cells. (c) The basic living unit of an organism is an organ. False Concept insight: Basic unit of life is cell. (d) Amoeba has irregular shape. True Concept insight: Recall different shapes of cells. Solution 2 Concept insight: Recall different types of cells. Solution 3 Concept insight: List the parts of the cell. Solution 4 Cytoplasm is the part of the cell that contains various cell organelles such as mitochondria, ribosomes, Golgi bodies, etc. Cytoplasm is a fluid that fills the cell and occurs between the plasma membrane and the nucleus. Conc...

Chapter 7 - Conservation of Plants and Animals Exercise 86 Solution 1 (a) Sanctuary (b) Endemic (c) Climatic Concept insight: Important from Exam Point of View. Solution 2 (a)  Wildlife sanctuary Biosphere reserve (i) It is an area within which animals are protected from possible dangers such as hunting. Their habitat is also conserved in this area. (i) It is a large protected area constructed for the conservation of biodiversity. (ii) It provides protection and suitable living conditions to wild animals. (ii) It helps in the conservation of various life forms such as plants, animals, and micro- organisms. (iii) Pachmarhi sanctuary is a wildlife sanctuary. (iii) Pachmarhi biosphere reserve is a popular biosphere reserve. Concept insight: List the differences between wildlife sanctuary and biosphere reserve. (b)  Zoo Wild life sanctuary (i) It is a facility in which animals are kept for public exhibiti...