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Page No 170:

Question 1:

Which of the following is true for an oscillating pendulum of fixed length?
(a) Frequency depends on amplitude of oscillation.
(b) Frequency and time-period are not related.
(c) Time-period depends on amplitude of oscillation.
(d) Frequency and time-period do not depend on amplitude of oscillation.

Answer:

(d) Frequency and time-period do not depend on amplitude of oscillation.

Frequency is inversely related to time-period and time-period does not depend on amplitude.

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Question 2:

The quality of sound produced by a tuning fork is different from that produced by a musical instrument because of difference in
(a) amplitude
(b) frequency
(c) harmonics
(d) none of these

Answer:

(c) harmonics

Tuning fork and musical instruments will produce sounds of different harmonics and loudness.

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Question 3:

Sound can travel through
(a) solids only
(b) liquids only
(c) gases only
(d) all of these

Answer:

(d) all of these

Sound travels through the vibrations of molecules and molecules are present in all solids, liquids and gases.

Page No 170:

Question 4:

An object vibrates with a frequency of 15 hertz. Which of the following is true?
(a) It produces sound that we can hear.
(b) It does not produce sound.
(c) It produces sound that we cannot hear.
(d) It produces sound that we can hear if we strain our ears.

Answer:

(c) It produces sound that we cannot hear.

Humans can hear sounds in the frequency range 20Hz - 20000Hz.

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Question 5:

In which medium does sound travel fastest?
(a) air
(b) water
(c) steel
(d) vacuum

Answer:

(c) steel

The molecules of steel are much more closely packed compared to that of air, water and vacuum. So, molecular vibrations pass quite quickly in steel. Consequently, sound travels fastest in steel.

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Question 6:

Which of the following characteristics of sound depends on the amplitude of vibrations?
(a) speed
(b) pitch
(c) loudness
(d) quality

Answer:

(c) loudness

Loudness of sound depends on the amplitude of vibrations.

Page No 170:

Question 7:

Sound of frequency 320 Hz is of lower pitch than sound of frequency
(a) 10 Hz
(b) 180 Hz
(c) 256 Hz
(d) 512 Hz

Answer:

(d) 512 Hz

Higher the frequency, higher is the pitch of sound.

Page No 170:

Question 8:

Which of the following frequencies produces sound that is audible?
(a) 5 Hz
(b) 15 Hz
(c) 500 Hz
(d) 50,000 Hz

Answer:

(c) 500 Hz

We can hear sound in the frequency range of 20 - 20000 Hz.

Page No 170:

Question 9:

Which of these is expected to be the best absorber of sound?
(a) iron
(b) wood
(c) wool
(d) cemented wall

Answer:


Wool is the best absorber of sound among the given options.

Page No 170:

Question 1:

Sound is produced by __________.

Answer:

Sound is produced by vibrations.

Page No 170:

Question 2:

Name the instrument used in the laboratory to produce sound of a fixed frequency.

Answer:

Tuning fork is used in the laboratory to produce sound of a fixed frequency.

Page No 170:

Question 3:

What does loudness of sound depend on?

Answer:

Loudness of sound depends on the amplitude of vibration. Higher the amplitude of vibration, louder is the sound produced.

Page No 170:

Question 4:

Give the term for 'number of vibrations per second'.

Answer:

'Frequency' is the term for 'number of vibrations per second'.

Page No 170:

Question 5:

Frequency is measured in __________.

Answer:

Frequency is measured in hertz (Hz).

Page No 170:

Question 6:

The pitch of sound depends on the time period of vibration. Is it true or false?

Answer:

Yes, the pitch of sound depends on the time period of vibration.

Page No 170:

Question 7:

In general, which sound has greater frequency−a bird singing or a man singing?

Answer:

The sound of a singing bird has higher frequency than that of a singing man due to higher pitch of  the bird's song.

Page No 170:

Question 8:

Most vibrating objects generate sound of several frequencies simultaneously. Is it true or false?

Answer:

It's true that most vibrating objects generate sound of several frequencies simultaneously.

Page No 170:

Question 9:

The quality of two sounds with the same fundamental frequency differs because of the number and relative loudness of the __________

Answer:

The quality of two sounds with the same fundamental frequency differs because of the number and relative loudness of the harmonics produced.



Page No 171:

Question 1:

Explain quality of sound. How is it determined?

Answer:

Quality of sound is the characteristic that allows us to distinguish sounds from two different sources even though the sound produced by them has same frequency and loudness. This happens because with every single frequency, called fundamental frequency, many other frequencies, known as harmonics, are also generated with differing loudness.

The quality of sound is determined by the frequencies present in it and their relative loudness.

Page No 171:

Question 2:

Explain how sound produced by a vibrating object reaches your ears. What happens after it enters the ears?

Answer:

A vibrating object causes air molecules to vibrate. When these vibrations reach our ears, they are collected by the pinnae and funnelled into the ear tube. These vibrations strike the eardrum, which starts to vibrate at the same frequency. This causes the delicate bones of the middle ear to vibrate. This stimulates tiny hair in the hearing organ that sends a signal to the auditory nerve of our nervous system. The auditory nerve takes the signal to the brain and we can hear the sound.

Page No 171:

Question 3:

Describe an experiment to show that sound can travel through liquids.

Answer:

Hold a bell in one hand and dip it in water. Now bend till your ear touches the surface of water (caution: water should not enter the ear). On ringing the bell you will hear a clear sound. This shows that sound can travel in liquids.

Page No 171:

Question 4:

Describe an experiment to show that sound cannot travel through vacuum.

Answer:

Take a container with a tightly held lid. Make a hole at the bottom of the container. Now, connect a vacuum pump to this hole with a rubber tube. Make sure that there is no gap between hole and tube. The vacuum pump is used to extract air from the container.
Put a cell phone inside the container and close it with the lid. Now call the number of the cell phone so that it rings. We can hear the sound clearly. Extract the air from the container by switching on the vacuum pump. Again, call the number of the cell phone. This time no sound is heard because there is no air (medium) to help the propagation of sound. This experiment shows that sound cannot travel through vacuum.

Page No 171:

Question 10:

The sound produced by a tuning fork has no harmonics. Is it true or false?

Answer:

False, a tuning fork produces harmonics that are very soft and so, barely audible.

Page No 171:

Question 11:

Flute is a __________ instrument.

Answer:

Flute is a wind instrument.

Page No 171:

Question 12:

The pitch of the sound produced by a guitar can be altered by reducing the vibrating length of the guitar string. Is it true or false?

Answer:

It's true that the pitch of the sound produced by a guitar can be altered by reducing the vibrating length of the guitar string.

Page No 171:

Question 13:

What is the name given to the sound produced by irregular vibrations?

Answer:

The sound produced by irregular vibrations is called noise.

Page No 171:

Question 14:

The vibrations in air are picked up by the __________ ear.

Answer:

The vibrations in air are picked up by the outer ear.

Page No 171:

Question 15:

What travels faster−light or sound?

Answer:

Light travels faster than sound.

Page No 171:

Question 16:

The two kinds of sound that we cannot hear are __________ and __________.

Answer:

The two kinds of sound that we cannot hear are infrasonic and ultrasonic.

Page No 171:

Question 17:

A soft board will ___________ (reflect/absorb) most of the sound falling on it.

Answer:

A soft board will absorb most of the sound falling on it.

Page No 171:

Question 18:

What is the unit in which loudness of sound is measured?

Answer:

Loudness of sound is measured in decibel (dB).

Page No 171:

Question 19:

Prolonged exposure to noise louder than __________ dB can lead to permanent hearing damage.

Answer:

Prolonged exposure to noise louder than 70 dB can lead to permanent hearing damage.

Page No 171:

Question 20:

Which surfaces reflect sound better−hard or soft?

Answer:

Hard surfaces reflect sound better than soft surfaces.

Page No 171:

Question 1:

How is sound produced?

Answer:

Sound is produced by vibration. For example, when a guitar string is plucked, sound is produced due to vibrations produced in the string.

Page No 171:

Question 2:

Distinguish between infrasonics and ultrasonics.

Answer:

Infrasonics Ultrasonics
Sound waves with frequency less than 20 Hz are called infrasonics. Sound waves with frequency greater than 20000 Hz are called ultrasonics.

Page No 171:

Question 3:

What do you mean by time period of a vibrating object?

Answer:

The time taken by an object to complete one oscillation is called time period of the vibrating object.

Page No 171:

Question 4:

What is the 'amplitude' of a vibrating object?

Answer:

Amplitude is the distance covered by a vibrating object from its mean/zero position.

Page No 171:

Question 5:

What is the 'frequency' of a vibrating object? How is it related to shrillness of sound?

Answer:

Number of oscillations completed by a vibrating object per second is called frequency.
Shrillness of sound is directly dependent on frequency. Higher the frequency of vibration more is the shrillness of sound.

Page No 171:

Question 6:

Why do women have shriller voices than men?

Answer:

Women have shriller voices than men because the sound produced by women is of higher frequency than that produced by men.

Page No 171:

Question 1:

How does loudness of sound depend on amplitude? If the amplitude is tripled, by how much does loudness increase?

Answer:

Loudness of sound is proportional to the square of amplitude.
If the amplitude is tripled, the loudness increases by nine times.

Page No 171:

Question 2:

Ramesh measured the time gap between lightning and thunder to be 3 seconds. Assuming light covers the distance instantly, what is the distance of the clouds from Ramesh?

Answer:

As light covers the distance instantly, the time gap is the time taken by sound to reach Ramesh from its point of origin.
Now, speed of sound in air = 340 m/s
Time gap = time taken to travel = 3 s
Distance between Ramesh and the clouds = (340 m/s) × (3 s)
                                                              = 1020 m.

Page No 171:

Question 3:

Why can an echo not be heard if you stand 10 m away from a wall and shout facing it?

Answer:

We cannot hear the echo because the time difference between the original sound and reflected sound will be less than 1/10th of a second. So, we are not able to distinguish between the two sounds.

Page No 171:

Question 4:

How is sound produced in our throat?

Answer:

Sound, in our throat is produced in the larynx by the vibration of two vocal cords when air passes through the opening between the vocal cords.

Page No 171:

Question 5:

Name the three types of musical instruments. How is sound produced in each type?

Answer:

The three types of musical instruments are:

  1. Stringed instruments: They produce sounds using vibrations generated in metal wires fixed at both ends. For example: guitar.
  2. Wind instruments: They produce sounds using vibrations in an air column trapped in a hollow structure. For example: flute.
  3. Percussion instruments: They produce sounds using vibrations in a stretched membrane to produce sound. For example: drum.

Page No 171:

Question 6:

What do you mean by noise pollution? What are the main causes of noise pollution?

Answer:

Any sound produced by irregular vibrations is called noise. Too much noise in our surroundings is harmful to us and is called noise pollution. Loud music, traffic on roads, machines in factories, mixers and grinders, etc. are major causes of noise pollution.

Page No 171:

Question 7:

List four ill effects of noise pollution.

Answer:

Four ill effects of noise pollution are:

  1. Exposure to sudden high noise level (e.g. explosion) can cause permanent hearing loss due to rupture of the ear drum.
  2. Prolonged exposure to noise of volume higher than 70 dB can cause permanent hearing loss.
  3. High noise can lead to nervous tension and increase in blood pressure.
  4. Noise pollution can lead to sleep deprivation and headaches.

Page No 171:

Question 8:

List three methods of controlling noise pollution.

Answer:

Three methods of controlling noise pollution are:

  1. Reducing noise emissions by developing low-noise products, such as better silencers for automobiles.
  2. Controlling recreational noise, such as those of loudspeakers.
  3. Making screens or enclosures around machinery to obstruct the path of noise.



Page No 172:

Question 1:

An explosion occurs on moon. Will it be
(a) seen
(b) heard on earth instantly or after some time?

Answer:

(a) It will be seen instantly on earth, as light travels at a great speed of 300000000 m/s.

(b) It will not be heard on earth, as sound needs a medium to travel and there is no medium between the moon and earth.

Page No 172:

Question 2:

In a game of hide-and-seek, how can the blindfolded person guess which player is closest to him?

Answer:

In a game of hide-and-seek, the blindfolded person guesses the player closest to him by following the sound generated by the movement and speech of the closest player.



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