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
Solution 1: (b) i, ii & iii only
To fly, sound needed a medium. Sound cannot move in a vacuum since there is no medium to propagate sound waves.
Question 2: The loudness of a sound depends on:
(a) its amplitude.
(b) its frequency.
(c) its time period.
(d) its speed.
Solution 2: (a) its amplitude.
The amplitude of sound waves is directly proportional to its loudness. If the amplitude is high, the sound will be louder, and if the amplitude is smaller, the sound will be weak.
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
Solution 3: (d) i, ii & iv only
Since light and sound are two distinct entities, statement iii) is incorrect.
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
Solution 4: (a) 0.02 s
If an object is vibrates at 50 hertz
50 = 1/T
T = 1/50
T = 0.02s
It will have a period of 0.02 seconds.
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.
Solution 5: (c) decrease its amplitude of vibration of the sound.
The loudness of a sound is directly proportional to its volume. As a result, as the amplitude of the sound decreases, the loudness of the sound decreases.
Question 6: The loudness of sound is measured in units of
(a) Decibel (dB)
(b) Hertz (Hz)
(c) Metre (m)
(d) Metre/Second (m/s)
Solution 6: (a) Decibel (dB)
The loudness of sound is measured in Decibel (dB) units.
Question 7: The loudness of sound is determined by the
(a) amplitude of vibration
(b) the ratio of amplitude and frequency of vibration
(c) frequency of vibration
(d) product of amplitude and frequency of vibration
Solution 7: (a) amplitude of vibration
The amplitude of sound waves is directly proportional to its loudness. If the amplitude is high, the sound will be louder, and if the amplitude is smaller, the sound will be weak.
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
Solution 8: (c) 60 vibrations per minute
1 hertz is equal to 1 vibration/second
1 minute consists 60 seconds
Hence, 1 hertz is equal to 60 vibrations per minute.
Question 9: Pitch of sound is determined by its
(a) frequency
(b) amplitude
(c) speed
(d) loudness
Solution 9: (a) frequency
By frequency pitch of sound is determined.
Question 10: Ultrasound has a 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
Solution 10: (c) above 20,000 Hz
Ultrasound has a frequency of vibration above 20,000 Hz.
Very Short Answer Questions.................
Question 11: Lightning can be seen as 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).
Solution 11: Paheli observes lightning in her area. She hears the sound 5 s after she observed lightning.
Distance is equals to Speed x Time
Distance will be 330 m/s x 5 s = 1650 m
Paheli must be 1650 meters far from the place where lightening occurs.
Question 12: Does any part of our body vibrate when we speak? Name the part.
Solution 12: When we talk, our larynx, or vocal cords, vibrate.
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 the reason for the delay in hearing the sound.
Solution 13: Since light travels faster than sound, Boojho heard the cracker's sound after seeing the cracker burst a little later.
Question 14: When we hear a sound, does any part of our body vibrate? Name the part.
Solution 14: Our eardrum vibrates when we hear a sound.
Question 15: Name two musical instruments which produce sound by vibrating strings?
Solution 15: Two musical instruments which produce sound by vibrating strings are Violin and Guitar.
Short Answer Questions.................
Question 16: A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?
Solution 16: In 20 seconds, a pendulum produces 10 oscillations.
So it makes 1 oscillation in every 2 seconds.
Time period = 2 seconds
Frequency= 1/Time = ½ = 0.5 Hz
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?
Solution 17: Each has their own frequency spectrum. However, we can only detect frequencies between 20 and 2000 Hz, so we won't be able to hear the sound of a few vibrating bodies.
Question 18: Suppose a stick is struck against a frying pan in a vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.
Solution 18: Vibrations can be felt in the frying pan. Since sound cannot move in a vacuum, we would be unable to hear the sound of vibration.
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.
Solution 19: There would be no medium for sound waves to pass through in space, just the vacuum. Since sound cannot pass through vacuum, it cannot be heard without a special unit.
Question 20: List three sources of noise pollution in your locality.
Solution 20: Vehicle honking, industrial sirens, and crackers exploding.
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?
Solution 21: The loudness of a sound is determined by its amplitude of vibration. When a string is plucked with more energy, the amplitude of the string increases. This amplifies the vibration.
Question 22: How is the sound produced and how is it transmitted and heard by us?
Solution 22: When the body vibrates due to some force, sound is made. This vibration induces waves that pass through a medium to reach their destination (our ear) and are heard.
Our ears pick up the echo. The outer portion of the ear is shaped like a funnel. When sound enters, it flows down a canal to a thin stretched membrane at the other end. The eardrum is what it's called. The eardrum resembles a rolled-up rubber mat. The eardrum vibrates as a result of sound waves. The inner ear receives vibrations from the eardrum. The signal then travels to the brain. That's how we hear.
Question 23: An alarm bell is kept inside a vessel as shown in given figure. A person standing close to it can distinctly hear the sound of the alarm. Now if the air inside the vessel is removed completely how will the loudness of alarm get affected for the same person?
Solution 23: To be distributed, air needs a medium. We may gradually extract the air from the container. The vacuum would be the only means for sound to pass through. Due to the vacuum, the volume of the sound steadily decreases as the amount of air in the container increases.
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.
Solution 24: When sound is created, vibration is used to create speed, pitch, and loudness. During the day, we are surrounded by a variety of vibrations. The amplitude of vibrations decreases as a result of this. There will be no such noises throughout the night, making the sound clearer.
Question 25: Suggest three measures to limit noise pollution in your locality.
Solution 25:
i) Horns can be used judiciously.
i) Along the roads and around the houses, trees must be planted.
iii) Transport vehicles and industrial equipment must have silencers mounted.