Questions & Discussion 9th class (lec 5)

 video lecture for Questions and Discussion

Question 1: A body traveling with a speed of more than the velocity of sound in air is said to travel with
(a) supersonic speed
(b) hypersonic speed
(c) ultrasonic speed
(d) infrasonic speed

Question 2: If wind blows in a direction opposite to the sound propagation, then the velocity of the sound
(a) increases
(b) decreases
(c) remains constant
(d) Cannot be determined 

Question 3: Infrasound can be heard by
(a) dog
(b) bat
(c) rhinoceros
(d) human beings

Question 4: Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the string suitably. By doing so, he is adjusting
(a) intensity of sound only
(b) amplitude of sound only
(c) frequency of the sitar string with the frequency of other musical instruments
(d) loudness of sound

Question 5: Which of the two graphs (a) and (b) representing the human voice is likely to be the male voice? Give a reason for your answer.

Question 6: A girl is sitting in, the middle of a park of dimension `12 m\times12 m`. On the left side of it there is a building adjoining the park and on right side of the park, there is a road adjoining the park. A sound is produced on the road by a cracker Is it possible for the girl to hear the echo of this sound? Explain your answer.

Question 7: Why do we hear the sound produced by the humming bees while the sound of vibrations of pendulum is not heard?

Question 8:  Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also define wavelengths and time period using this curve.

Question 9: Two boys are at the two ends of an aluminum pipe. A boy hits one end of the pipe with a stone. Find the ratio of the time taken by the sound waves to travel through air and through aluminum to reach the child at the other end.

a) `3 : 7`

b) `2: 1`

c) `11.22 : 1`

d) `18.55 : 1`

Question 10: What is resonance? How can it be reduced?

Question 11: What is meant by loudness of sound? On what factors does it depend?

Question 12: What is the reason that the sound of moving the hands back and forth is not heard? 

Question 13: What is the reason that the sound of a train coming from a distance is not heard but we can hear its sound by keeping our ears on the track?

Question 14: Give a simple experiment on the basis of which you say that the velocity of sound is more in solids than in air. 

Question 15: Why is it that we cannot hear the waves produced by an earthquake while bats or dogs can?

Question 16: What is the difference between the sound of a mosquito and the sound of a lion?

Question 17: Mentioning the names of substances that attenuate sound, write how they attenuate sound.

 Question 18: What are the crest and troughs of the wave in the sound graph?

Question 19: What is wavelength?

Question 20: By which letter is the wavelength denoted?

Question 21: What is the SI unit of wavelength?

Question 22: What is known by frequency?

Question 23: When is one oscillation complete?

Question 24: What is the frequency of sound waves?

Question 25: How is the frequency of sound waves expressed?

Question 26: What is the SI unit of a sound wave?

Question 27: A sender is standing in front of a rock `200` meters away. He whistles. If sound travels in air with a velocity of `332 ms^{-1}`, then after how long will it be able to hear the echo of the whistle?

a) `1.2 `s

b) `1s`

c) `2s`

d) `1.5s`

Question 28: A sound signal was sent from a boat. This signal returned to the boat after `0.8` seconds of reflection from the ocean floor. Find the velocity of sound in water if the depth of the sea is `600 m`.

a) `600 ms^{-1}`

b) `1200ms^{-1}`

c) `1500ms^{-1}`

d) `1700ms^{-1}`

Question 29: A sound signal was sent from a boat. This signal returned to the boat after `0.8` seconds of reflection from the ocean floor. Find the depth of the ocean if the velocity of sound in water is `1500` meters per second.

a) `1200m`

b) `600 m`

c) `300m`

d) `1800m`

Question 30: A messenger standing in front of a rock `166` meters away blows a whistle. If sound travels with a velocity of `332 ms^{-1}`, after how long will he be able to hear the echo of the whistle?

a) `1s`

b) `0.5s`

c) `0.2s`

d) `1.2s`

Question 31: A ripple tank produces `10` complete waves per second. The distance between neighboring troughs and crests is `12 cm`. Find the frequency, wavelength, and velocity of the wave.

a) `220 ms^{-1}`

b) `230 ms^{-1}`

c) `240 ms^{-1}`

d) `210 ms^{-1}`

Question 32: What will be the frequency of the wave whose time period is `0.05`?

a) `25 Hz`

d) `15  Hz`

c) `24 Hz`

d) `20 Hz`

Question 33: The wavelength of sound produced by a source is `1.7\times10^{2}m`. If its velocity is `343.4 ms^{-1}`, then find the frequency of the sound.

a) `2.02\times10^{4}  Hz`

b) `2.7\times10^{4}  Hz`

c) `1.2\times10^{4}  Hz`

d) `1.7\times10^{4} Hz`

Question 34: An ultrasonic wave is sent from a ship towards the ocean floor. There is a difference of `1.55s` between sending and receiving messages. If the velocity of sound in seawater is `1400 ms^{-1}`, then find the depth of the sea. 

a) `1250m`

b) `1200m`

c) `1400m`

d) `1050m`

Question 35: The velocity of sound waves is `330 ms^{-1}`. If the frequency of the sound wave is `550 Hz`, calculate its wavelength.

a) `0.6m`

b) `1.6m`

c) `3.06m`

d) `6m`

Question 36: Delhi-C station of All India Radio broadcasts programs on `219 m` and `1370 kHz`. the velocity of radio waves.

a) `3103 ms^{-1}`

b) `2108 ms^{-1}`

c) `3108 ms^{-1}`

d) `3106 ms^{-1}`

Question 37: A man fires a gun at a place between two parallel hills and hears the first echo after 2.5 seconds and the second after `3` seconds. If the speed of sound is `332 ms^{-1}`, find the distance between the hills.

a) `415 m`

b) `498m`

c) `913 m`

d) `984m`

Question 38:  A radar signal is reflected from an airplane and is received after `2\times10^{-5}` seconds. If the speed of these waves is `3\times10^{8} ms^{-1}`, how far will the airplane

a) `3km`

b) `5km`

c) `2km`

d) `1km`

Question 39: A boy hears the echo of his voice after 1 second from a distant hill. The velocity of sound in air is `350 ms^{-1}`. What is the distance of the hill from the boy?

a) `224m`

b) `350m`

c) `175 m`

d) `124m`

Question 40: If the frequency of a tuning fork is `400 Hz` and the speed of sound in air is `320 ms^{-1}`, find the distance traveled by the sound when the tuning fork completes `20` vibrations.

a) `16 m`

b) `20m`

c) `8m`

d) `24 m`

Question 41: An object makes `6000` vibrations in `1` minute. If the velocity of sound produced in the air is `330 ms^{-1}`, find the following.

(a) the frequency of the vibrations 

(b) the wavelength of the waves produced.

a) `200 Hz, 2.3m`

b) `100 Hz, 2.3 m`

c) `300 Hz, 3.3m`

d) `100 Hz, 3.3m`

Question 42: A sound signal was sent from a boat. This signal returned to the boat after `0.8` seconds of reflection from the ocean floor. If the velocity of sound in water is `1500` meters per second, then find the depth of the sea.

a) `1200m`

b) `600m`

c) `200m`

d) `1500m`

Question 43: The velocity of a light wave is `3\times10^{5} kms^{-1}` and its wavelength is `6\times10^{-7} m`. Find its frequency.

a) `0.5\times10^{15} Hz`

b) `1.5\times10^{10} Hz`

c) `2\times10^{15} Hz`

d) `0.5\times10^{10} Hz`

Question 44: The frequency of a sound wave is `2 kHz` and its wavelength is `35 cm`. How long will it take to cover a distance of `1.5 km`?

a) `2.2s`

b) `1.2s`

c) `1.1s`

d) `2.1s`

Question 45: A man claps his hands near a sheer cliff and its echo is heard after `55 s`. If the speed of sound is taken as `346ms^{-1}`, then what will be the distance between the rock and the man?

a) `865 m`

b) `765m`

c) `443m`

d) `289m`

Question 46: A ship emits ultrasound which is detected after `3.42s` by reflection from the sea floor. If the speed of ultrasound in seawater is `1531 ms^{-1}`, then what will be the distance of the ship from the sea level?

a) `2.62 km`

b) `2.2 km`

c) `1.26 km`

d) `2 km`

Question 47: The human heart, on average, beats `75` times in a minute. Calculate its frequency.

a) `1.5`

b) `2.5`

c) `5.5`

d) `5.9`

Question 48: The audible range of a person is `20 Hz` to `20` kife. Find the propagation range of sound waves in the air corresponding to these two frequencies. The velocity of sound in air is `344 ms^{-1}`.

a) `17m`

b) `0.17m`

c) `0.1m`

d) `0.017m`

Question 49: Two boys are at two opposite ends of an iron. A boy hits one end of the tube with a stone. Find the ratio of time taken by the sound to travel through iron and air to reach the other child when the velocity of sound in air is `32 ms^{-1}` and the velocity of sound in iron is `5130ms^{-1}`.

a) `12 :1`

b) `15.45 : 1`

c) `13 : 3`

d) `14.7 : 3`

Question 50: Water waves are continuously hitting an anchor (a stationary boat). If the distance between two successive crests of these waves is `100 m` and the velocity of the waves in the water is `20 ms^{-1}`, then what is the frequency of the waves hitting the boat?

a) `2 Hz`

b) `1 Hz`

c) `2.4 Hz`

d) `1.2 Hz`

Question 51: A longitudinal wave is produced on a slinky. This wave travels with a velocity of `30 ms^{-1}` and its frequency is `20 Hz`. What is the minimum distance between two successive compressions of the slinky?

a) `1.5 cm`

b) `2 cm`

c) `2.5 cm`

d) `0.5 cm`

Question 52: A wave source produces `40` crests and `40` troughs in `0.4 sec`. Find the frequency of the wave.

a) `80 Hz`

b) `75 Hz`

c) `100 Hz`

d) `40 Hz`

Question 53: Sound pulses are emitted on the water's surface by sonar. These pulses are detected after reflection from the bottom of the water. If the time interval between emission and detection is `2` seconds, what is the depth of the water?

a) `3000m`

b) `750m`

c) `1500m`

d) `2250m`

Question 54: A child hears the echo due to a vertical rock `4` seconds after the bursting of a powerful firecracker. What is the distance of the rock from the child?

a) `344m`

b) `1376m`

c) `244 m`

d) `688 m`

Answer Key

Q1:   (a),  Q2:   (b),

Q3:  (c) Infrasound can be heard by rhinoceros.

Q4:  (c) Frequency of the sitar depends upon tension in string, so, sitarist is adjusting frequency of the sitar string with the frequency of other instruments.

Q5:  Solution:

Usually, the male voice has less pitch (or frequency) as compared to female voice.

As the time period of the wave represented by a graph (a) is more than that of graph (b), so, graph (a) represents a lower frequency wave than graph (b) and hence is likely to be the male voice.

Q6:  Solution:

If the time gap between the original sound and reflected sound received by the listener is around 0.1 s, only then the echo can be heard.

Speed of sound in air, `v=344 ms^{-1}`

Time gap between the original sound and reflected sound received by the girl

`=\frac{18}{344}s-\frac{6}{344}s=0.035s`

As `0.035 s < 0.1 s`, therefore no echo will be heard by the girl.

Q7:  Solution:

Sound produced by humming bees by vibrating their wings, lies in the audible range of hearing for average human being which is `20 Hz` to `20 kHz`, so it can be heard.

While the sound of vibrations of pendulum is less than `20 Hz`, so it can not be heard.

Q8:  Solution:

The points of maximum density (or pressure) and minimum density (or pressure) are also called crests and troughs respectively.

Wavelength (A): The distance between two consecutive compressions or two consecutive rarefactions is called wavelength.

Time period (T): Time period of a wave is defined as the time taken by its two consecutive compressions or rarefactions to cross a fixed point.

Q9:  (d),

Q10:  Answer: Sound reflects. If there is repeated reflection in a large hall, the sound is restricted, which is called reverberation. This is undesirable as it does not allow clear hearing. To reduce it, sound-absorbing materials are applied on the roofs and walls of the building. This can be controlled by applying compressed fiberboards, rough plaster, or curtains.

Q11:  Ans- The 'loudness' of a sound is a measure of the sensitivity of the ears. It is not the sound energy passing through a unit area in one second in the same way as 'intensity'. Two sounds can be of equal intensity but still, one of them can be heard as louder than the other because our ears are more sensitive to it.

Q12:  Answer- When the hands are shaken, the frequency of vibration of the hand is less than `20`. Humans can hear sound with a frequency between `20` and `20,000` vibrations per second. That's why the sound of moving the hands back and forth is not heard.

Q13:  Ans-  Sound travels easily from one place to another in metals etc. at a faster speed than in air. Hence the sound of the train reaches our ears through the track.

Q14:  Answer- When a train is far away from us, we cannot hear its sound. But by listening with ear on the track of the train, we can hear the sound. The only reason for this is that the sound travels faster in iron than in air.

Q15:  Answer- Our ear can hear waves with frequencies ranging from `20` Hz to `20,000` Hz. This range of frequency is called the auditory range. The frequency of the waves generated by the earthquake does not lie in this range. So we cannot hear these waves but the ears of bats and dogs are sensitive to the waves produced by earthquakes. So bats and dogs can easily hear these waves.

Q16:  Answer- The loudness of the mosquito's sound is less but the frequency is more. On the other hand, the loudness of the lion's voice is more but the frequency is less, so the voice of a mosquito is louder than the voice of a lion.

Q17:  Answer- Wool, cotton, cardboard, wood etc. materials attenuate sound. They are not dense and flexible. Therefore, there is no vibration in them or the vibration is very less, as a result of which the sound becomes weak.

Q18:  Answer – The peak is called the crest of the wave and the valley is called the trough. 

Q19:  Answer- The distance between two successive compressions (C) or two successive rarefactions (R) is called wavelength.

Q20:  Answer – From the Greek letter Lambda (1).

Q21:  Answer - Meter `(m)`.

Q22:  Answer- It is known how quickly an event is happening.

Q23:  Answer – One oscillation is complete in the change of density from a maximum value to a minimum value and again to a maximum value.

Q24:  Answer- The total number of these oscillations in unit time is called the frequency of the sound wave.

Q25:  Answer- It is represented by the Greek letter 'new' (v).

Q26:  Answer – Hertz `(Hz)`.

Q27:   (a),  Q28:   (c),  Q29:   (b),  Q30:   (a),  Q31:   (c),  Q32:   (d),  Q33:   (a),  Q34:   (d),     

Q35:   (a),  Q36:   (c),  Q37:   (c),  Q38:   (a),  Q39:   (c),  Q40:   (a),  Q41:   (d),  Q42:   (b),

Q43:   (a),  Q44:   (d),  Q45:   (a),  Q46:   (a),  Q47:   (c),  Q48:   (d),  Q49:   (b),  Q50:   (a),

Q51:   (a),  Q52:   (c),  Q53:   (c),  Q54:   (d).