NCERT Class 8 Science Exemplar Chapter 13 - Sound

Chapter 13 - Sound

Multiple Choice Questions
Question 1: A list of mediums is given below.
i) wood
ii) water
iii) air
iv) vacuum
In which of these mediums can sound travel?
a) i & ii only
b) i, ii & iii only
c) iii & iv only
d) ii, iii & iv only
Answer:
b) i, ii & iii only

Question 2: The loudness of sound depends on:
a) its amplitude.
b) its frequency.
c) its time period.
d) its speed.
Answer:
a) its amplitude.

Question 3: Which of the following statements are correct?
i) Sound is produced by vibrations.
ii) Sound requires a medium for propagation.
iii) Light and sound both require a medium for propagation.
iv) Sound travels slower than light.
a) i & ii only
b) i, ii & iii only
c) ii, iii & iv only
d) i, ii & iv only
Answer:
d) i, ii & iv only

Question 4: An object is vibrating at 50 hertz. What is its time period?
a) 0.02 s
b) 2 s
c) 0.2 s
d) 20.0 s
Answer:
a) 0.02 s

Question 5: In order to reduce the loudness of a sound we have to
a) decrease its frequency of vibration of the sound.
b) increase its frequency of vibration of the sound.
c) decrease its amplitude of vibration of the sound.
d) increase its amplitude of vibration of the sound.
Answer:
c) decrease its amplitude of vibration of the sound.

Question 6: Loudness of sound is measured in units of
a) decibel (dB)
b) hertz (Hz)
c) metre (m)
d) metre/second (m/s)
Answer:
a) decibel (dB)

Question 7: The loudness of sound is determined by the
a) amplitude of vibration
b) ratio of amplitude and frequency of vibration
c) frequency of vibration
d) product of amplitude and frequency of vibration
Answer:
a) amplitude of vibration

Question 8: 1 hertz is equal to
a) 1 vibration per minute
b) 10 vibrations per minute
c) 60 vibrations per minute
d) 600 vibrations per minute
Answer:
c) 60 vibrations per minutes

Question 9: Pitch of sound is determined by its
a) frequency
b) amplitude
c) speed
d) loudness
Answer:
a) frequency

Question 10: Ultrasound has frequency of vibration
a) between 20 and 20,000 Hz
b) below 20 Hz
c) above 20,000 Hz
d) between 500 and 10,000 Hz
Answer:
c) above 20,000 Hz

Very Short Answer Questions
Question 11: Lightning can be seen the moment it occurs. Paheli observes lightning in her area. She hears the sound 5 s after she observed lightning. How far is she from the place where lightning occurs? (speed of sound = 330 m/s).
Answer:

Light always travels faster than sound, so we see the lighting first and then the sound. We can detect the how far the lightning had happened from us.
Distance = Time × Speed
Let distance be x.
Time = 5 seconds
Speed = 330 m/s
= 330 × 5 = 1650 m

Therefore, the distance between Paheli and the lightning is 1650 m.

Question 12: Does any part of our body vibrate when we speak? Name the part.
Answer:
Yes, the body part that vibrates when we speak is larynx or the vocal cords.

Question 13: Boojho saw a cracker burst at night at a distance from his house. He heard the sound of the cracker a little later after seeing the cracker burst. Give reason for the delay in hearing the sound.
Answer:
The delay in hearing of sound is because the speed of light is much faster than the speed of sound. Therefore, Boojho heard the sound of the cracker a little later after seeing the cracker burst.

Question 14: When we hear a sound, does any part of our body vibrate? Name the part.
Answer:
Yes, the body part that vibrates when we hear a sound is ear drum.

Question 15: Name two musical instruments which produce sound by vibrating strings?
Answer:
The two musical instruments which produce sound by vibrating strings are sitar and ektara.

Short Answer Questions
Question 16: A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?
Answer:

Number of oscillations = 10
Time taken = 20 seconds
Frequency = Number of oscillations/Time taken
                    = 10/20 = 0.5 Hz
Time period = 1/Frequency
                      = 1/0.5
                      = 2 seconds

Question 17: We have learnt that vibration is necessary for producing sound. Explain why the sound produced by every vibrating body cannot be heard by us?
Answer:
Sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible. On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear. Thus, for human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz. Hence, if the sound produced by a vibrating body is in the audible range, the sound produced will be heard by us otherwise we will not be able to hear the sound even though the body is vibrating.

Question 18: Suppose a stick is struck against a frying pan in vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.
Answer:
Yes, the frying pan will vibrate but we will not be able to hear the sound of vibration because sound cannot travel in vacuum. Sound can’t travel in vacuum because the sound needs mediums to travel.

Question 19: Two astronauts are floating close to each other in space. Can they talk to each other without using any special device? Give reasons.
Answer:
No, the two astronauts can’t talk to each other without using any special device, because in the space there is vacuum which doesn’t allow sound to pass through it.

Question 20: List three sources of noise pollution in your locality.
Answer:
The three sources of noise pollution in our locality
• Car horns
• Bursting crackers
• Loud music playing during the party or festival.

Long Answer Questions
Question 21: We have a stringed musical instrument. The string is plucked in the middle first with a force of greater magnitude and then with a force of smaller magnitude. In which case would the instrument produce a louder sound?
Answer:
The string that is plucked in the middle first with a force of greater magnitude will have louder sound because the loudness of sound depends upon the amplitude of vibration.

Question 22: How is sound produced and how is it transmitted and heard by us?
Answer:
Sound is a form of energy that enables us to hear and it travels in longitudinal waves. When matter vibrates or moves back and forth very quickly a sound is made. Sound waves travels through solids, liquids and gases. Sound can’t travel in vacuum because no particles are present. Sound travels fastest in solids because of the molecules are closer while the sound travels slowest in gases because of the molecules are far.
Sound waves are sent. The outer ear catches the sound waves. The middle ear takes the sound waves and vibrate the eardrum. The inner ear sends the messages to the brain. The brain puts in together and we can understand sound.

Question 23: An alarm bell is kept inside a vessel as shown in Fig. 13.1. A person standing close to it can distinctly hear the sound of alarm. Now if the air inside the vessel is removed completely how will the loudness of alarm get affected for the same person?

Fig 13.1

Answer: The loudness of the sound will decrease as the air is removed slowly from the plastic bottle. If the air in the plastic bottle is removed completely, there is vacuum in the bottle. The sound cannot travel through vacuum and we cannot hear the sound of the alarm clock at all.

Question 24: The town hall building is situated close to Boojho’s house. There is a clock on the top of the town hall building which rings the bell every hour. Boojho has noticed that the sound of the clock appears to be much clearer at night. Explain.
Answer:
When sound is produced, speed, pitch, loudness all are initiated with vibration. During the day, there are many vibrations around us. This results in a decrease of the amplitude of vibrations. During the night, there will be no such disturbances which will make the sound clearer. And therefore, Boojho noticed the sound of the clock clear during night.

Question 25: Suggest three measures to limit noise pollution in your locality.
Answer:
The measures to limit noise pollution in our locality are
→ Trees must be planted along the roads and around the buildings because trees help to reduce the loudness of sound.
→ Use of horns should be minimised.
→ We shouldn’t burst crackers.

NCERT Class 10 Science Chapter 4 - Carbon and its Compounds Activity 4.1

Chapter 4 - Carbon and its Compounds Activity 4.1

Activity 4.1
• Make a list of ten things you have used or consumed since the morning.
• Compile this list with the lists made by your classmates and then sort the items into the adjacent Table.
• If there are items which are made up of more than one material, put them into both the relevant columns of the table.
Answer:

• Ten things that we have used since morning are
→ Pen
→ Pencil
→ Bucket
→ Cloth
→ Needle
→  Scissors
→ Utensils
→ Book
→ Paper
→ Toys

• Ten things that have been used by our friend are
→ Vessels
→ Clay flower pot
→ Washing machine
→ Clothes
→ Scissors
→ Pen
→ Metal Study Table
→ Mobile
→ Oil
→ Switch


Activity 4.1

NCERT Class 8 Science Chapter 9 - Reproduction in Animal Notes

Chapter 9 - Reproduction in Animal Notes

1. What is Reproduction?
Reproduction is a process by which living things give birth to a new generation of organism of same kind.

2. Importance of Reproduction
Reproduction is important because without it, a species would not survive over time. Individuals must reproduce to ensure that they will continue on over generations of time.

3. Types of Reproduction
• Asexual Reproduction: The reproduction in which offspring are produced by a single parent, without the union of sex cells, is called asexual reproduction.
• Sexual Reproduction: The reproduction in organisms by the union of male and female sex cells (gametes) is called sexual reproduction.

4. Stages of Sexual Reproduction
• Stage 1: Fertilization
→ The fusion of the genetic material contained in the gametes to form a zygote is known as fertilization.
→ Zygote is the fertilized egg or ovum. It is a single cell, thus we begin our life as a single cell.
→ Types of fertilization
♦ Internal fertilization which takes place inside the body of the female. Example: humans, dogs, cows, birds etc.
♦ External fertilization which takes place outside the body of the female. Example: fish, frogs etc.

• Stage 2: Development of Zygote
→ Zygote divides repeatedly to give rise to a ball of cells. As it passes from the fallopian tube into the uterus it divides further and develops into an embryo.
→ When the embryo gets embedded in the wall of the uterus it is known as implantation. The female is then said to be pregnant or to have conceived.
→ The embryo gradually develops body parts and is called as foetus.
→ When the development of the foetus is complete the mother gives birth to a baby.

• Stage 3: Birth of Young Ones
→ Animals which give birth to young ones directly are called viviparous animals. The embryo obtain nourishment from mother.
→ Animals which lay eggs are called oviparous animals. The embryo obtains nourishment from the yolk and albumin present in the egg itself.
→ The time between fertilization and birth of young one is called gestation period.

• Stage 4: Young Ones to Adults
→ The natural process by which an animal, after birth or hatching, undergoes abrupt changes in its body structure until it reaches an adult stage is called metamorphosis.

5. Asexual Reproduction
• Budding in Hydra
→ In each hydra, there may be one or more bulges. These bulges are the developing new individuals and they are called buds.
→ In hydra, the new individuals 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 hydra, this type of asexual reproduction is called budding.

• Binary Fission in Amoeba
→ Amoeba is a single-celled organism. It begins the process of reproduction by the division of its nucleus into two nuclei.
→ This is followed by division of its body into two, each part receiving a nucleus.
→ Finally, two amoebae are produced from one parent amoeba. This type of asexual reproduction in which an animal reproduces by dividing into two individuals is called binary fission.

6. The male reproductive organs include a pair of testes (singular, testis), two sperm ducts and a penis. The testes produce the male gametes called sperms. Millions of sperms are produced by the testes.

7. Each sperm has a head, middle piece and tail.

8. The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes) and the uterus. The ovary produces female gametes called ova (eggs). In human beings, a single 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.

9. The zygote is the beginning of new individual.

10. Fertilization in Frogs and Toads
During spring or rainy season, frogs and toads move to ponds and slow-flowing streams. When the male and female come together in water, the female lays hundreds of eggs. Frog’s egg is not covered by a shell and it is comparatively very delicate. A layer of jelly holds the eggs together and provides protection to the eggs. As the eggs are laid, the male deposits sperms over them. Each sperm swims randomly in water with the help of its long tail. The sperms come in contact with the eggs. This results in fertilisation.

11. How are the chicks born?
After fertilization, the zygote divides repeatedly and travels down the oviduct. As it travels down, many protective layers are formed around it. The hard shell in hen’s egg is one such protective layers. After the hard shell is formed around the developing embryo, the hen finally lays egg. The embryo takes about 3 weeks to develop into chicks. The development of the chicks takes place inside the egg shell during this period. After the chick is completely developed it bursts open the egg shell.

12. What purpose does the tail in a sperm serve?
The main function of the tail in sperm is to provide motility to the sperm.

13. What are test tube babies?
In some women oviducts are blocked. These women 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.

14. Why do fishes and frogs lay eggs in hundreds whereas a hen lays only one egg at a time?
Though these animals lay hundreds of eggs and release millions of sperms, all the eggs do not get fertilised and develop into new individuals. This is because the eggs and sperms get exposed to water movement, wind and rainfall. Also, there are other animals in the pond which may feed on eggs. Thus, production of large number of eggs and sperms is necessary to ensure fertilisation of at least a few of them.

15. What is karyokinesis and cytokinesis?
→ Cytokinesis is the process by which the cytoplasm of the parent cell divides into two daughter cells.
→ Karyokinesis is a process where the nucleus of the parent cell divides into two daughter nuclei.

16. Which animal has the highest gestation period?
African elephants have the highest gestation period of 22 months.

17. What is mitosis and meiosis?
→ Mitosis is the division of a cell into two daughter cells that are genetically identical to the parent cell.
→ Meiosis is the division of a germ cell into four sex cells (e.g. egg or sperm), each with half the number of chromosomes of the parent cell.

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