CBSE Class 7 Science Advanced Physics Sound Short Answer Questions Set 02

Short Answer Questions

Question 1. Explain how sound is produced by animals.
Answer:
1. Mammals produce sound by the vibration of their vocal cords.
2. Birds use the syrinx, a ring of cartilage at the beginning of the windpipe. Some birds have a two-part syrinx and can sing two notes at the same time.
3. Insects such as mosquitoes and houseflies produce sound by flapping their wings; it is audible because the wings beat more than 20 times a second.
In simple words: Mammals use vocal cords, birds use a syrinx, insects use their wings.

Teacher's Note:
1. Take one line for each group of animals.
2. The wing beat has to be above 20 Hz to be heard.

Question 2. A body vibrates with a frequency of 600 Hz (a tuning fork). What is the number of vibrations made by the tuning fork, when the sound produced by it travels a distance of 110 m? (Velocity of sound in air is 330 m s⁻¹)
Answer:
Time taken by the sound to travel 110 m = 110 ÷ 330 = 1/3 s.
Number of vibrations in 1/3 s = 600 × 1/3 = 200 vibrations.
In simple words: Find the travel time and multiply by the frequency.

Teacher's Note:
1. 600 Hz means 600 vibrations every second.
2. Time first, then count.

Question 3. Dinesh, who is fond of music, vibrates a glass with a spoon. If he fills the glass with water, will there be any change in any of the characteristics of the sound produced? Explain.
Answer:
Yes. Water inside the glass reduces the amplitude of the vibration of the glass, so the loudness decreases.
(The added mass of water also makes the glass vibrate more slowly, so the note is lower as well.)
In simple words: A glass with water rings more softly.

Teacher's Note:
1. The book's answer is that the loudness decreases.
2. Try it with a glass of water to hear the change.

Question 4. A sound wave is travelling from a metallic rod to water. What changes will occur in the velocity and frequency of the sound?
Answer:
The frequency stays the same because it is fixed by the source of the sound.
The velocity decreases because sound is faster in a solid (the rod) than in a liquid (water).
In simple words: Speed changes with the medium, frequency does not.

Teacher's Note:
1. Frequency is decided by the source only.
2. Solid to liquid means a slower speed.

Question 5. Two astronauts who land on the moon cannot talk with each other. What could be the reason for this? Suggest a method for them to talk to each other.
Answer:
The moon has no atmosphere, so there is no medium to carry sound waves.
They can talk through a medium such as a toy-telephone type string or a metal rod touching both of them, or by using radio waves, which do not need a medium.
In simple words: No air means no sound; use radio or a solid link.

Teacher's Note:
1. This is the bell jar idea on a large scale.
2. Radio waves are not sound waves.

Question 6. The time period of a simple pendulum is given by the formula, \( T = 2\pi\sqrt{\dfrac{\ell}{g}} \), where T = time period, ℓ = length of pendulum and g = acceleration due to gravity. If the length of the pendulum is decreased to 1/4 of its initial value, then what happens to its frequency of oscillations?
Answer:
New length ℓ′ = ℓ/4.
\( T' = 2\pi\sqrt{\dfrac{\ell/4}{g}} = \dfrac{1}{2}\times 2\pi\sqrt{\dfrac{\ell}{g}} = \dfrac{T}{2} \)
The time period becomes half, and since f = 1/T, the frequency becomes double the initial value.
In simple words: A quarter of the length gives half the period and double the frequency.

Teacher's Note:
1. The square root of 1/4 is 1/2.
2. Frequency is the reciprocal of the time period.

Question 7. A police targets a thief, who is at rest at point Q. He fires a bullet from a point P which is at a distance of 990 m from Q and it reaches the point Q in 3 s. When the bullet is fired at P it produced a sound, which was heard by the thief. What is the difference in time taken by the sound and the bullet to reach point Q? Will the thief be able to escape from the bullet fired by police? Take the velocity of sound in air as 330 m s⁻¹.

Answer:
Speed of the bullet = 990 ÷ 3 = 330 m s⁻¹.
Time taken by the sound = 990 ÷ 330 = 3 s.
The sound and the bullet reach Q together, so the time difference is zero.
The thief hears no warning before the bullet arrives, so he cannot escape.
In simple words: Both arrive at the same moment, so there is no warning.

Teacher's Note:
1. The bullet's speed is exactly the speed of sound here.
2. Compare the two times before answering.

Question 8. Why is the sound made to undergo multiple reflections in a mega phone?
Answer:
The horn-shaped tube stops the sound from spreading in all directions.
Because of the multiple reflections the sound comes out in one direction with high directionality, so it travels a longer distance.
In simple words: Repeated reflections push the sound forward as a narrow beam.

Teacher's Note:
1. Loudspeakers and hearing aids work in a similar way.
2. Name the effect: directed sound travels farther.

Question 9. The flash of lightning, produced at a distance of 10 km above the ground level is seen much before hearing the thunder bolt. Find the time gap between the two events observed by a person on the ground if the time taken by the lightning to reach the surface of earth is negligible. (velocity of sound = 330 m s⁻¹ and velocity of light = 3 × 10⁸ m s⁻¹.)
Answer:
Distance = 10 km = 10 000 m.
Time taken by the flash to reach the ground is negligible, so t₁ ≈ 0.
Time for the thunder: t₂ = 10 000 ÷ 330 = 1000/33 = 30.30 s.
Time gap = t₂ − t₁ = 30.30 s.
In simple words: The gap is just the travel time of the sound, about 30 seconds.

Teacher's Note:
1. Light takes only about 0.00003 s, which is ignored.
2. Convert kilometres to metres first.

Question 10. Why is the voice of children more shrill than that of adults? Explain.
Answer:
The vocal cords of a man are about 20 mm long and about 5 mm shorter in women.
Children have very short vocal cords, so they vibrate with a higher frequency.
A higher frequency means a higher pitch, so the voice of a child is more shrill.
In simple words: Shorter vocal cords vibrate faster and give a higher pitch.

Teacher's Note:
1. Link length to frequency and frequency to pitch.
2. Shrillness means high pitch.

Question 11. A ship at rest is sending ultrasonic sound waves to detect a shoal of fish. If the time gap between the signal sent from the ship and received (reflected) signal is 0.5 s, then find the distance of the shoal of fish from the ship. (Velocity of sound in water is 1400 m s⁻¹)
Answer:
The signal goes to the fish and back, so 2d = v × t.
2d = 1400 × 0.5 = 700 m
d = 700 ÷ 2 = 350 m
In simple words: Half of the round-trip distance is the distance of the fish.

Teacher's Note:
1. Sonar uses d = vt/2.
2. The same working applies to ocean depth.

Question 12. Why are the walls, roof, seats and steps, of a cinema theatre or auditorium covered with special material?
Answer:
Sound reflected from the walls, floor and ceiling would make the speech or music confusing and unclear.
To avoid this, the walls and seats are covered with sound absorbing materials, and windows and doors have thick curtains, so that the reflected sound is reduced.
In simple words: Soft covering soaks up sound so that it is not reflected back.

Teacher's Note:
1. Examples of absorbers: carpets, curtains, cork.
2. Hard walls would cause echoes.

Question 13. Why are concave reflectors used in big auditoria?
Answer:
Concave reflectors spread the sound evenly in the hall. When the speaker is placed at the focus of the concave reflector, the reflected sound waves travel parallel to each other and reach all parts of the hall evenly.
In simple words: A curved reflector behind the speaker sends sound to every seat.

Teacher's Note:
1. The idea is reflection of sound from a curved surface.
2. Remember the source is placed at the focus.