Chapter – 8 Sound, Science, physics.

CHAPTER 8, Sound, Science, physics. 9th std, tamilnadu board,

I. Choose the correct answer:

Question 1.

Which of the following vibrates when a musical note is produced by the cymbals in a orchestra?

(a) stretched strings

(b) stretched membranes

(c) air columns

(d) metal plates

Answer:

(a) stretched strings

Question 2.

Sound travels in air:

(a) if there is no moisture in the atmosphere.

(b) if particles of medium travel from one place to another.

(c) if both particles as well as disturbance move from one place to another.

(d) if disturbance moves.

Answer:

(b) if particles of medium travel from one place to another.

Question 3.

A musical instrument is producing continuous note. This note cannot be heard by a person having a normal hearing range. This note must then be passing through ………….

(a) wax

(b) vacuum

(c) water

(d) empty vessel

Answer:

(d) empty vessel

Question 4.

The maximum speed of vibrations which produces audible sound will be in ……………..

(a) seawater

(b) ground glass

(c) dry air

(d) Human blood

Answer:

(a) seawater

Question 5.

The sound waves travel faster

(a) in liquids

(b) in gases

(c) in solids

(d) in vacuum

Answer:

(c) in solids

---

II. Fill in the blanks.

1. Sound is a ………… wave and needs a material medium to travel.
2. Number of vibrations produced in one second is …………..
3. The velocity of sound in solid is …………. than the velocity of sound in air.
4. Vibration of object produces …………..
5. Loudness is proportional to the square of the …………..
6. …………… is a medical instrument used for listening to sounds produced in the body.
7. The repeated reflection that results in persistence of sound is called ……………..

Answer:

1. longitudinal
2. frequency of wave
3. faster
4. Sound
5. amplitude
6. ECG
7. reverberation

---

III. Match the following.

Column – I	Column – II
(a) Tuning fork	(i) The point where density of air is maximum
(b) Sound	(ii) Maximum displacement from the equilibrium position
(c) Compressions	(iii) The sound whose frequency is greater than 20,000 Hz
(d) Amplitude	(iv) Longitudinal wave
(e) Ultrasonics	(v) Production of sound

Answer:

(a) (v)

(b) (i)

(c) (iv)

(d) (ii)

(e) (iii)

---

IV. Answer in brief.

Question 1.

Through which medium sound travels faster, iron or water? Give reason.

Answer:

Sound travels faster through iron as solids are packed together tighter than liquids and gases.

Question 2.

Name the physical quantity whose SI unit is ‘hertz’. Define.

Answer:

The SI unit of frequency is hertz (Hz). The number of vibrations (complete waves or cycles) produced in one second is called frequency of the wave.

Question 3.

What is meant by supersonic speed?

Answer:

When the speed of any object exceeds the speed of sound in air (330 m s-1) it is said to be travelling at supersonic speed.

Question 4.

How does the sound produced by a vibrating object in a medium reach your ears?

Answer:

When an object vibrates, it forces the neighbouring particles of the medium to vibrate. These vibrating particles then force the particles adjacent to them to vibrate. In this way vibrations produced by an object are transferred till it reaches the ear.

Question 5.

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

Answer:

No, as there is no medium on moon for the sound to travel.

---

V. Answer in detail.

Question 1.

Describe with diagram, how compressions and rarefactions are produced.

Answer:

Sound is also a longitudinal wave. Sound can travel only when there are particles which can be compressed and rarefied. Compressions are the regions where particles are crowded together. Rarefactions are the regions of low pressure where particles are spread apart. A sound wave is an example of a longitudinal mechanical wave. Below figure represents the longitudinal nature of sound wave in the medium.

---

Question 2.

Verify experimentally the laws of reflection of sound.

Answer:

The laws of reflection are:

• The angle in which the sound is incident is equal to the angle in which sound is reflected.
• Direction of incident sound, direction of the reflected sound and the normal are in the same plane.

Take two metal tubes A and B. Keep one end of each tube on a metal plate as shown in figure. Place a wrist watch c at the open end of the tube A and interpose a cardboard between A and B. Now at a particular inclination of the tube B with the cardboard, ticking of the watch is clearly heard. The angle of reflection made by the tube B with the cardboard is equal to the angle of incidence made by the tube A with the cardboard.

For example, if the angle of incidence is 20° on the left side of a protractor (see figure arrangement), the angle of reflection at which we are able to hear the sound clearly will also be at 20° on the right side of the protractor.

∴ ∠ i = ∠ r    ➝ verifying Law I

We will also further observe that pipe 1, pipe 2 i.e, the incident ray, reflected ray and the normal lie on the same plane, verifying law II.

---

Question 3.

List the applications of sound.

Applications of ultrasound

Answer:

• Ultra sound can be used in cleaning technology. Minute foreign particles can be removed from objects placed in a liquid bath through which ultrasound is passed.
• Ultrasounds can also be use d to detect cracks and flaws in metal blocks.
• Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘echo cardiography’.
• Ultrasound may be employed to break small ‘stones’ formed in the kidney into fine grains. These grains later get flushed out with urine.

---

Question 4.

Explain how does SONAR work?

Answer:

SONAR stands for Sound Navigation And Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. Sonar consists of Science – 9 (Physics)

a transmitter and a detector and is installed at the bottom of boats and ships. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector.

The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound. Sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs etc.

---

VI. Numerical problem.

Question 1.

The frequency of a source of sound is 600 Hz. How many times does it vibrate in a minute?

Answer:

The number of vibrations (complete waves or cycles) produced in one second is called frequency of the wave.

Hence, in one minute 600 x 60 vibrations are produced = 36000 (Need clarification)

---

Question 2.

A stone is dropped from the top of a tower 750 m high into a pond of water at the base of the tower. When is the splash heard at the top?

(Given g = 10 m s^{– 2} and speed of sound = 340 ms^{– 1})

Height = 750m

h = ut + 0.5 gt²

The initial velocity is 0

750 = 0.5 × 10 × t²

t = 10sec

Speed of sound is 340m/sec

So, time taken to travel 750m upwards is,

750

340

 = 2.20s

time taken = 10 + 2.20 = 12.2 sec.

---