Sound Book Back Questions 8th Science Lesson 6
8th Science Lesson 6
6] Sound
Book Back Questions with Answer and Do You Know Box Content
Do You Know?
Thomas Alva Edison, in 1877 invented the phonograph, a device that played the recorded sound.
Wavelength is the distance between two consecutive particles, which are in the same phase of vibration. It is denoted by the Greek letter ‘λ’. The unit of wavelength is metre (m). Frequency is the number of vibrations of a particle in the medium, in one second. It is denoted by ‘n’. The unit of frequency is hertz (Hz).
Problem 1: A sound has a frequency of 50 Hz and a wavelength of 10 m. What is the speed of the sound?
Solution: Given, n= 50 Hz, λ = 10 m
v = n λ
v = 50 x 10 = 500 ms-1
Problem 2: A sound has a frequency of 5 Hz and a speed of 25 ms-1. What is the wavelength of the sound?
Solution: Given, n = 5 Hz, v = 25 ms-1
v = nλ
λ = v/n = 25/5 = 5 m.
The amount of water vapour present in the air is known as humidity. It is less during winter and more during summer. The speed of sound increases with increase in humidity. This is because the density of air decreases with increase in humidity.
How do astronauts communicate with each other? The astronauts have devices in their helmets which transfer the sound waves from their voices into radio waves and transmit it to the ground (or other astronauts in space). This is exactly the same as how radio at your home works.
Amplitude is the maximum displacement of a vibrating particle from its mean position. It is denoted by ‘A’. The unit of amplitude is ‘metre’ (m).
The seismic wave formed during earthquake is an example for a longitudinal wave. Wave travelling through the layers of the Earth due to explosions, earthquakes and volcanic explosions are called seismic waves. Using a hydrophone and seismometer one can study these waves and record them. Seismology is the branch of science that deals with the study of seismic waves.
A bat can hear the sounds of frequencies higher than 20,000 Hz. Bats produce ultrasonic sound during screaming. These ultrasonic waves help them to locate their way and the prey.
Choose the best answers:
1. Sound waves travel very fast in
(a) Air
(b) Metals
(c) Vacuum
(d) Liquids
2. Which of the following are the characteristics of vibrations?
i. Frequency ii. Time period iii. Pitch iv. Loudness
(a) i and ii
(b) ii and iii
(c) iii and iv
(d) i and iv
3. The amplitude of the sound wave decides its
(a) Speed
(b) Pitch
(c) Loudness
(d) Frequency
4. What kind of musical instrument is a sitar?
(a) String instrument
(b) Percussion instrument
(c) Wind instrument
(d) None of these
5. Find the odd one out.
(a) Harmonium
(b) Flute
(c) Nadaswaram
(d) Violin
6. Noise is produced by
(a) Vibrations with high frequency
(b) regular vibrations
(c) Regular and periodic vibrations
(d) irregular and non-periodic vibrations
7. The range of audible frequency for the human ear is
(a) 2 Hz to 2000 Hz
(b) 20 Hz to 2000 Hz
(c) 20 Hz to 20000 Hz
(d) 200 Hz to 20000 Hz
8. If the amplitude and frequency of a sound wave are increased, which of the following is true?
(a) Loudness increases and pitch is higher
(b) Loudness increases and pitch is unchanged
(c) Loudness increases and pitch is lower
(d) Loudness decreases and pitch is lower
9. Which of the following may be caused by noise?
(a) Irritation
(b) Stress
(c) Nervousness
(d) All the above
Fill in the blanks:
1. Sound is produced by ____________________.
2. The vibrations of a simple pendulum are also known as _____________.
3. Sound travels in the form of ___________.
4. High frequency sounds that cannot be heard by you are called _____________.
5. Pitch of a sound depends on the ________________ vibration.
6. If the thickness of a vibrating string is increased, its pitch __________________.
Match the following:
1. Ultrasonics – frequency below 20 Hz
2. Speed of sound in air – needs material medium
3. Infrasonics – 330 ms-1
4. Sound propagation – frequency more than 20000 Hz
Consider the statements given below and choose the correct option:
1. Assertion: When lightning strikes, the sound is heard a little after the flash is seen.
Reason: The velocity of light is greater than that of the sound.
(a) Both assertion and reason are true and reason is the correct explanation of the assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Assertion is false but reason is true.
(e) Both assertion and reason are false.
2. Assertion: Two persons on the surface of moon cannot talk to each other.
Reason: There is no atmosphere on moon.
(a) Both assertion and reason are true and reason is the correct explanation of the assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Assertion is false but reason is true.
(e) Both assertion and reason are false.
Answers:
Choose the best answers:
1. Metals 2. Pitch and Loudness 3. Loudness 4. String instrument 5. Violin
6. irregular and non-periodic vibrations 7. 20 Hz to 20000 Hz 8. Loudness increases and pitch is higher.
9. all the above
Fill in the blanks:
1. Vibrating bodies 2. Oscillation 3. Mechanical waves 4. Ultrasonic waves 5. Frequency 6. Decreases
Match the following:
1.Ultrasonics- Frequency more than 20000 Hz
2.Speed of sound in air – 330ms-1
3.Infrasonics- Frequency below 20Hz
4.Sound propagation- Needs material medium
Consider the statements given below and choose the correct option:
1.Both assertion and reason are true and reason is the correct explanation of assertion.
2.Both assertion and reason are true and reason is the correct explanation of assertion.