Question 1

Q1: How does the sound produced by a vibrating object in a medium reach your ear?

Accepted Answer

A vibrating object produces sound by setting the particles of the medium around it into vibration.  1.  The vibrating object displaces the particles of the medium immediately in contact with it. These particles then displace the adjacent particles. 2.  This process continues as a chain reaction, and...

Question 2

Q2: Explain how sound is produced by your school bell.

Accepted Answer

Sound is produced by the school bell through vibrations. 1.  When the bell is struck with a hammer, the metal of the bell starts to vibrate rapidly back and forth. 2.  These vibrations disturb the surrounding air particles. 3.  As the bell's surface moves forward, it pushes the air particles togethe...

Question 3

Q3: Why are sound waves called mechanical waves?

Accepted Answer

Sound waves are called mechanical waves because they require a material medium (such as a solid, liquid, or gas) for their propagation. They travel by causing the particles of the medium to vibrate. Without a medium, like in a vacuum, there are no particles to vibrate, and therefore, sound cannot tr...

Question 4

Q4: Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Accepted Answer

No, you will not be able to hear any sound produced by your friend on the moon.  Reason: Sound waves are mechanical waves, which means they need a medium (like air, water, or a solid) to travel. The moon has no atmosphere; it is essentially a vacuum. Since there are no particles to carry the vibrati...

Question 5

Q1: Which wave property determines (a) loudness, (b) pitch?

Accepted Answer

(a) Loudness: The loudness of a sound is determined by the amplitude of the sound wave. A larger amplitude corresponds to a louder sound, while a smaller amplitude corresponds to a softer sound. (b) Pitch: The pitch of a sound is determined by the frequency of the sound wave. A higher frequency corr...

Question 6

Q2: Guess which sound has a higher pitch: guitar or car horn?

Accepted Answer

A guitar generally has a higher pitch than a car horn. Reason: Pitch is determined by frequency. A car horn is designed to produce a loud, low-frequency sound to be easily heard over traffic noise, so it has a low pitch. A guitar, on the other hand, can produce a wide range of frequencies. The thinn...

Question 7

Q1: What are wavelength, frequency, time period and amplitude of a sound wave?

Accepted Answer

The characteristics of a sound wave are:    Wavelength ($\lambda$): It is the distance between two consecutive compressions (C) or two consecutive rarefactions (R). It is the spatial period of the wave. Its SI unit is the metre (m).    Frequency ($
u$): It is the number of complete oscillations (or...

Question 8

Q2: How are the wavelength and frequency of a sound wave related to its speed?

Accepted Answer

The speed of a sound wave is related to its wavelength and frequency by the following equation: Speed = Wavelength $	imes$ Frequency Mathematically, this is expressed as: $$v = \lambda 
u$$ Where:    $v$ is the speed of the sound wave.    $\lambda$ (lambda) is the wavelength of the sound wave. *...

Question 9

Q3: Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is $440 \mathrm{m} / \mathrm{s}$ in a given medium.

Accepted Answer

Given: Frequency, $
u = 220 	ext{ Hz}$ Speed of sound, $v = 440 	ext{ m/s}$ To Find: Wavelength, $\lambda$ Formula: The relationship between speed, frequency, and wavelength is: $$v = \lambda 
u$$ Solution: Rearranging the formula to solve for wavelength: $$\lambda = \frac{v}{
u}$$ Substituting...

Question 10

Q4: A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive co...

Accepted Answer

Given: Frequency of the tone, $
u = 500 	ext{ Hz}$ Distance from the source, $d = 450 	ext{ m}$ (This information is not needed for the calculation). To Find: The time interval between successive compressions. Concept: The time interval between two successive compressions is equal to the time per...

Question 11

Q1: Distinguish between loudness and intensity of sound.

Accepted Answer

Loudness and intensity are related but distinct properties of sound. | Feature | Intensity | Loudness | |---|---|---| | Definition | Intensity is a physical quantity defined as the amount of sound energy passing per unit time through a unit area. | Loudness is the physiological sensation or the resp...

Question 12

Q1: In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

Accepted Answer

At a particular temperature, sound travels fastest in iron. Reason: The speed of sound depends on the properties of the medium, specifically its elasticity and density. Sound travels fastest in solids, slower in liquids, and slowest in gases. This is because the particles in a solid are much closer...

Question 13

Q1: An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is $342 \mathrm{~m} \mathrm{~s}...

Accepted Answer

Given: Time taken to hear the echo, $t = 3 	ext{ s}$ Speed of sound, $v = 342 	ext{ m/s}$ To Find: The distance of the reflecting surface from the source, $d$. Concept: In the time taken to hear an echo, the sound travels from the source to the reflecting surface and back to the source. Therefore,...

Question 14

Q1: Why are the ceilings of concert halls curved?

Accepted Answer

The ceilings of concert halls are curved for acoustic reasons, based on the principle of reflection of sound. 1.  Even Distribution of Sound: A curved (concave) ceiling reflects sound waves and spreads them evenly across the hall. This ensures that the sound reaches all corners of the auditorium and...

Question 15

Q1: What is the audible range of the average human ear?

Accepted Answer

The audible range of frequency for the average human ear is from about 20 Hz to 20,000 Hz (or 20 kHz).

Question 16

Q2: What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?

Accepted Answer

(a) Infrasound: The range of frequencies for infrasound is below 20 Hz. These sounds are of too low a frequency for humans to hear. (b) Ultrasound: The range of frequencies for ultrasound is above 20,000 Hz (or 20 kHz). These sounds are of too high a frequency for humans to hear.

Question 17

Q1: What is sound and how is it produced?

Accepted Answer

Sound is a form of energy that travels as a wave and produces the sensation of hearing in our ears. Production of Sound: Sound is produced when an object vibrates. A vibration is a rapid back-and-forth motion of an object. This vibration causes the particles of the surrounding medium (like air) to a...

Question 18

Q10: When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound producti...

Accepted Answer

Whether you hear the echo depends on whether the time interval between the original sound and the reflected sound is at least 0.1 seconds. 1.  The speed of sound increases as the temperature of the air increases. So, on a hotter day, the speed of sound ($v$) is greater than on a cooler day. 2.  The...

Question 19

Q11: Give two practical applications of reflection of sound waves.

Accepted Answer

Two practical applications of the reflection of sound waves are: 1.  Megaphones and Horns: Instruments like megaphones, loudhailers, and musical instruments like trumpets are designed with a conical shape. This shape causes the sound waves from the source to undergo multiple reflections off the inne...

Question 20

Q12: A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, $g=10...

Accepted Answer

This problem has two parts: the time for the stone to fall and the time for the sound of the splash to travel back up. Part 1: Calculate the time for the stone to fall ($t_1$) Given: Initial velocity, $u = 0$ (since it is dropped) Distance (height), $s = 500 	ext{ m}$ Acceleration due to gravity, $...

NCERT Solutions