Question 1

Question 1A boy fires a gun and another boy at a distance of 1020 m hears the sound of firing the gun 3 s after seeing its smoke. Find the speed of sound.

Accepted Answer

Distance (d) = 1020 mtime (t) = 3 sSpeed of sound = distancetime\dfrac{	ext{distance}}{	ext{time}}timedistance​ = 10203\dfrac{	ext{1020}}{	ext{3}}31020​ = 340 m s-1.So, speed of sound = 340 m s-1.

Question 2

Question 2A boy on a hill A fires a gun. Another boy on hill B hears the sound after 4 s. If the speed of sound is 330 m s-1, find the distance between the two hills.

Accepted Answer

Speed of sound = 330 m s-1time (t) = 4 sSpeed of sound = distancetime\dfrac{	ext{distance}}{	ext{time}}timedistance​distance = speed x time = 330 x 4 = 1320 mSo, distance between two hills = 1320 m .

Question 3

Question 3Match the following:Column AColumn B(a) Vibrations cause(i) absorb sound(b) A shriller sound is(ii) second(c) Unit of frequency(iii) sound(d) Unit of time period(iv) of high pitch(e) Curtains(v) hertz

Accepted Answer

Column AColumn B(a) Vibrations cause(iii) sound(b) A shriller sound is(iv) of high pitch(c) Unit of frequency(v) hertz(d) Unit of time period(ii) second(e) Curtains(i) absorb sound

Question 4

Question 4bWe can hear sound of frequency:10 Hz500 Hz100,000 Hz50,000 Hz

Accepted Answer

500 HzReason — We can hear sounds of frequency in the range of 20 Hz to 20,000 Hz.

Question 5

Question 4cSound cannot travel in:gasesliquidssolidsvacuum

Accepted Answer

vacuumReason — Sound needs a medium for its propagation. It cannot travel in vacuum.

Question 6

Question 4dThe minimum distance required between the source and the reflector so as to hear the echo in air is:10 m17 m34 m50 m

Accepted Answer

17 mReason — To hear an echo clearly in air the reflecting surface should be at a minimum distance of 16.5 m from the source of sound.

Question 7

Question 4Describe an experiment to show that each source of sound is a vibrating body.

Accepted Answer

The following experiment shows that each source of sound is a vibrating body:Take a ruler.Press its one end on the table with the left hand as shown in the figure below.Pull down the other end of the ruler with the right hand and then leave it.We will notice that the ruler vibrates i.e., the ruler moves to and fro and a humming sound is heard. After sometime the ruler stops vibrating. No sound is then heard.This shows that the humming sound is produced only because of the vibrations of the ruler. Hence, we can say that each source of sound is a vibrating body.

Question 8

Question 8Can sound travel through a vacuum? Describe an experiment to explain your answer.

Accepted Answer

No, sound cannot travel through a vacuum.It is shown by the following experiment:Take an electric bell and an air-tight glass jar connected to a vacuum pump. Suspend the electric bell inside the jar.Connect the bell to a battery through a switch.On pressing the switch bell starts ringing and a sound is heard.Now start the vacuum pump. It withdraws the air from the jar.We will notice that as the air from the jar is evacuated the sound gradually becomes more and more feeble. After sometime when no air is left within the jar, no sound is heard. But we can notice the hammer of the electric bell is striking the gong.This shows that sound cannot travel in vacuum because when there is air in the jar, we can hear the sound but when air is evacuated from the jar through vacuum pump, we cannot hear the sound although the bell is vibrating.

Question 9

Question 9Describe an experiment to show that sound can travel in water.

Accepted Answer

The following experiment shows that sound can travel in water:Fill a balloon with water and hold it near your ear.Now keep a watch gently to the other side of the balloon.We will hear the ticking sound of the watch.This shows that sound can travel in water.

Question 10

Question 10Describe an experiment to show that sound can travel in a solid.

Accepted Answer

The following experiment show that sound can travel in a solid:Take two empty ice-cream cups. Make a small hole at the bottom of each cup and pass a long thread through them.Tie a knot or match stick at each end of the thread so that the thread does not slip out through the holes. This makes a toy telephone.Now use the telephone as shown in the below figure and talk to your friend.You will be able to hear the sound of your friend.This shows that sound can travel through solid as sound travels through the thread and reaches the ear.

Question 11

Question 11Can two persons hear each other on moon's surface? Give reason to support your answer.

Accepted Answer

No, two persons cannot hear each other on moon's surface because sound needs a medium for its propagation but on moon's surface there is vacuum, so sound cannot propagates through it.

Question 12

Question 12What is a longitudinal wave?

Accepted Answer

The wave in which the particles of the medium vibrate about their mean positions, in the direction of propagation of sound is called longitudinal wave.

Question 13

Question 13Define the following terms:AmplitudeTime periodFrequency

Accepted Answer

Amplitude (a) — The maximum displacement of a wave on either side of its mean position, is called the amplitude of wave.Time period (T) — The time taken by a wave to complete its one vibration is called the time period of the wave.Frequency (f or n) — The number of vibrations produced by a source of sound wave in one second is called the frequency of the wave.

Question 14

Question 14Write the audible range of frequency for the normal human ear.

Accepted Answer

The audible range of frequency for the normal human ear is 20 Hz to 20,000 Hz.

Question 15

Question 15What are ultrasonics? Can you hear the ultrasonic sound?

Accepted Answer

Sounds of frequencies higher than 20,000 Hz are called ultrasonic sounds. We cannot hear ultrasonic sound.

Question 16

Question 16What are infrasonics? Can you hear them?

Accepted Answer

Sounds of frequency lower than 20 Hz are called infrasonic sounds. We cannot hear the infrasonic sounds.

Question 17

Question 17How does a bat make use of ultrasonic waves to find its way?

Accepted Answer

Bat produces ultrasonic sound while flying. Ultrasonic waves reflect back after striking any obstacles in their way and bats hear it. Thus, they detect the obstacles in their path and change their path without colliding with the obstacle.

Question 18

Question 18Name the two characteristics of sound which differentiate any two sounds from each other.

Accepted Answer

Loudness and pitch

Question 19

Question 19On what factor does the loudness of a sound depend?

Accepted Answer

The loudness of a sound depends on:The amplitude of vibration of the vibrating body producing the sound.The area of the vibrating body.

Question 20

Question 20How does the loudness of sound produced depend on the vibrating area of the body?

Accepted Answer

The loudness of sound produced is directly proportional to the vibrating area of the body. Greater the area of the vibrating body, louder is the sound produced.

Sound

Numericals

Objective Questions

Short/Long Answer Questions