Case based Questions: Sound | Science Class 9 PDF Download

Q1: Read the following passage and answer the questions that follow:

i. What does this experiment demonstrate about the nature of sound production? (1 mark)
ii. Why does the ball move when touched by the vibrating tuning fork? (2 mark)
iii. Can sound be produced without vibration? Justify your answer. (1 mark)

Ans:
i. Sound is produced by vibrations. The tuning fork vibrates when struck, creating sound waves.
ii. The vibrating tuning fork transfers energy to the ball, making it move. This shows that sound waves carry energy.
iii. No, sound cannot be produced without vibration. All sound sources involve vibrating objects, such as vocal cords, musical instruments, and tuning forks.

Q2: Read the following passage and answer the questions that follow:

A science teacher explains to students that sound waves travel in the form of compressions and rarefactions in the air. She uses a slinky to demonstrate how sound propagates.

i. What kind of wave is a sound wave, and how does it propagate? (1 mark)
ii. How does the motion of particles in a sound wave differ from a water wave? (2 mark)
iii. Explain why sound cannot travel in a vacuum. (1 mark)

Ans:
i. Sound is a longitudinal wave, meaning it travels by compressions (high pressure) and rarefactions (low pressure) in the medium.
ii. In sound waves, particles move parallel to the wave direction, while in water waves, particles move perpendicularly to the wave direction.
iii. Sound requires a medium (solid, liquid, or gas) for propagation. In a vacuum, there are no particles to transmit the vibrations.

Q3: Read the following passage and answer the questions that follow:

i. Why are they unable to hear each other on the Moon? (1 mark)
ii. How does the absence of a medium affect sound transmission? (2 mark)
iii. What method can they use to communicate instead? (1 mark)

Ans:
i. The Moon has no atmosphere, meaning there is no medium (air) for sound waves to travel through.
ii. Sound waves need particles to vibrate and carry the wave. Without a medium, the vibrations cannot propagate.
iii. They can use radio waves, which are electromagnetic waves and can travel through a vacuum.

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Q4: Read the following passage and answer the questions that follow:

i. Which property of sound is responsible for the difference in sounds produced by different strings? (1 mark)
ii. How does the length of the string affect the pitch of the sound? (2 mark)
iii. Why do musical instruments have different tones even when playing the same note? (1 mark)

Ans:
i. The pitch of the sound, which depends on the frequency of vibration.
ii. A shorter string vibrates faster, producing a higher pitch, while a longer string vibrates slower, producing a lower pitch.
iii. Different instruments produce different overtones and harmonics, which affect the quality (timbre) of the sound.

Q5: Read the following passage and answer the questions that follow:

i. Why does the flash of lightning appear before the sound of thunder? (1 mark)
ii. Which factor determines the time delay between the two events? (2 mark)
iii. If the person is closer to the storm, how will this affect the delay? (1 mark)

Ans:
i. Light travels much faster than sound, so we see the flash almost instantly, while sound takes time to reach us.
ii. The distance of the thunderstorm and the speed of sound in air determine the delay.
iii. The delay will be shorter because the sound will have less distance to travel.

Q6: Read the following passage and answer the questions that follow:

Ans:
i. An echo is the reflection of sound that is heard after bouncing off a surface.
ii. Distance, d = (speed x time) / 2d = (340 x 2) / 2 = 340 m
iii. The speed of sound increases with temperature, so the echo would return faster.

Q7: Read the following passage and answer the questions that follow:

i. What is the formula for calculating the wavelength of a wave? (1 mark)
ii. Calculate the wavelength of the sound wave. (2 mark)
iii. If the sound wave travels through water where the speed of sound is 1500 m/s, what will be the new wave length? (1 mark)

Ans:
i. Wave length, λ = speed / frequency
ii. λ = 340 / 500 = 0.68 m
iii. λ = 1500 / 500 = 3 m

Q8: Read the following passage and answer the questions that follow:

i. What is the relationship between speed, frequency, and wave length? (1 mark)
ii. Calculate the wavelength of the sound wave. (2 mark)
iii. If the frequency is doubled, what happens to the wave length? (1 mark)

Ans:
i. v = λ x f
ii. λ = 340 / 256 = 1.33 m
iii. If f = 512 Hz, then λ = 340 / 512 = 0.664 m (wave length is halved).

Q9: Read the following passage and answer the questions that follow:

i. Why do bats use ultrasonic waves? (1 mark)
ii. Calculate the wavelength of the emitted wave. (2 mark)
iii. If the bat is in water where the speed of sound is 1500 m/s, what would be the new wave length? (1 mark)

Ans:
i. Bats use ultrasound for echolocation as it has a short wavelength and high precision.
ii.  λ = 340 / 40000 = 0.0085 m
iii.  λ = 1500 / 40000 = 0.0375 m

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Q10: Read the following passage and answer the questions that follow:

i. What causes the delay between seeing and hearing the horn? (1 mark)
ii. Calculate the distance of the train. (2 mark)
iii. If the person was underwater, would the delay be shorter or longer? (1 mark)

Ans:
i. Light travels much faster than sound, so we see the train instantly, while sound takes time to reach our ears.
ii. Distance, d = speed x time
d = 343 x 1.5
d = 514.5 meters
iii. Shorter because the speed of sound in water is much higher (around 1500 m/s), meaning sound would reach the person faster.

Q11: Read the following passage and answer the questions that follow:

i. What is the phenomenon responsible for the sound continuing in the empty room? (1 Mark)
ii. How do curtains and carpets affect the sound in the second case? (2 Mark)
iii. Why is a concert hall designed with materials that absorb sound? (1 Mark)

Ans:
i. The phenomenon responsible for the sound continuing in the empty room is reverberation. It occurs due to multiple reflections of sound waves from walls, ceilings, and floors.
ii. Curtains and carpets absorb sound waves instead of reflecting them, reducing reverberation and making the sound disappear quickly.
iii. A concert hall is designed with sound-absorbing materials to prevent excessive reverberation, ensuring that the audience hears clear and undistorted sound.

Q12: Read the following passage and answer the questions that follow:

A ship in the ocean sends ultrasonic waves toward the sea floor to measure its depth. The waves reflect back after hitting the ocean bed, and the time taken for the echo to return is recorded.

i. What is the name of this technique used for measuring depth? (1 mark)
ii. Why are ultrasonic waves used instead of normal sound waves? (2 mark)
iii. How does increasing the speed of sound in water affect the measurement? (1 mark)

Ans:
i. This technique is called SONAR (Sound Navigation and Ranging). It is used to measure the depth of water bodies and locate underwater objects.
ii. Ultrasonic waves are used instead of normal sound waves because they have high frequency and short wavelength, allowing them to travel long distances and provide precise measurements.
iii. If the speed of sound in water increases, the depth measurement will change because the time taken for the echo to return will be shorter.

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Q13: Read the following passage and answer the questions that follow:

i. Which property of sound is responsible for this difference? (1 week)
ii. What factors determine the quality of sound produced by an instrument? (2 week)
iii. How does the waveform of sound affect the perception of different instruments? (1 week)

Ans:
i. The property of sound responsible for this difference is timbre or quality of sound.
ii. The quality of sound depends on factors such as the waveform, overtones, harmonics, and the material of the instrument.
iii. The waveform of sound determines its distinct shape, which affects how the human ear perceives different instruments. Even if two instruments produce the same frequency, their overtones make them sound different.

Q14: Read the following passage and answer the questions that follow:

i. Why does the sound become fainter as air is removed from the chamber? (1 week)
ii. What does this experiment demonstrate about the propagation of sound? (2 week)
iii. If the mobile phone were placed inside a water tank instead of a vacuum chamber, would the sound still be heard? Why? (1 week)

Ans:
i. The sound becomes fainter because sound waves require a medium (air) to propagate. As air is removed, fewer particles are available to carry the sound.
ii. This experiment demonstrates that sound cannot travel in a vacuum because it needs a material medium (solid, liquid, or gas) for transmission.
iii. If the mobile phone were placed inside a water tank, the sound would still be heard but might be distorted due to the difference in wave propagation in liquids. Sound travels faster in water than in air, but the human ear is less sensitive to underwater sound waves.

Q15: Read the following passage and answer the questions that follow:

i. Why is the sound of the train heard earlier through the railway track than through the air? (1 week)
ii. How does the speed of sound vary in solids, liquids, and gases? (2 week)
iii. Why do earthquake waves travel faster through the Earth's crust than through the air? (1 week)

Ans:
i. The sound of the train is heard earlier through the railway track because sound travels faster in solids than in air. The particles in a solid medium are more tightly packed, allowing sound waves to travel more efficiently.
ii. The speed of sound varies as follows:
Fastest in solids (due to closely packed particles)
Slower in liquids
Slowest in gases (due to widely spaced particles)
iii. Earthquake waves travel faster through the Earth's crust than through the air because solid rock is a much better conductor of sound than air. This is why seismic waves travel quickly and can be detected over long distances.