Corrections for 13A-13C

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Here are the worksheets : Note that some of the question are missing , please copy the answers to these questions from your friends. Thanks .
Worksheet 13A
Sound


For topics13.1What is Sound?
13.2Transmission of Sound
Build your understanding!

Attempt the following questions on your own or in a group setting.

1. Sound is an example of a longitudinal wave and comprises a series of compressions and
rarefactions in the medium.
(a) What are compressions and rarefactions?
Compressions are regions where the air pressure is slightly higher than the surrounding air

pressure. Rarefactions are regions where the air pressure is slightly lower than the surrounding

air pressure.

(b) What is the distance, in terms of wavelength, between
(i) two consecutive compressions or two consecutive rarefactions? one wavelength
half
(ii) the centre of a compression and the centre of the nearest rarefaction? wavelength
2. State one example of a common occurrence when sound is not heard immediately after
it is produced.
Seeing the flash of a gun fired at a distance but not hearing the sound immediately until after a short

time later.

Check your understanding
• Can you describe the production of sound?
• Can you describe the longitudinal nature of sound waves in terms of compressions and
rarefactions?
2007 Marshall Cavendish International (S) Pte Ltd Discover Physics Workbook 101


3. Figure 13.1 shows a large diameter steel pipe 80 m long (not drawn to scale). An
experimenter at E bangs the pipe and his assistant at O listens for the sound reaching
him.
..Figure 13.1steel pipe
OE
(a) Explain why the assistant will hear two sounds, one arriving before the other.
One sound travels through air to reach the assistant. The other sound reaches him through the pipe
metal. The time difference is due to the difference of the speed of sound in the two media.

(b) In an experiment to measure the time needed for the sound to travel through the
air from E to O, fi ve values were recorded: 0.20 s, 0.28 s, 0.25 s, 0.27 s, 0.23 s.
Hence fi nd the

(i) mean time,
Total time = 0.20 + 0.28 + 0.25 + 0.27 + 0.23
= 1.23


1.23

Mean time=

5
= 0.25 s


(ii) mean speed of sound in air.
Total distance travelled by the sound = 80 m
Mean time = 0.246 s
distance 80


.. speed of sound in air = = = 325 m s–1

time0.246

(c) Suggest how you would attempt to fi nd the time needed for the sound to travel from
E to O through air.
Using a stopwatch or a suitable data logger: Start recording the time when the experimenter
is seen banging the pipe and stop recording when the second sound is heard.

(Phy/Nov85/P2/Q6a–c)


102 Sound 2007 Marshall Cavendish International (S) Pte Ltd


4.   (a) State the approximate values of the speed of sound in air, liquids and solids
at room temperature.
Approximately 300 m s–1 (in air), 1500 m s–1 (in liquids), 5000 m s–1 (in solids).

(b) Before World War II, the people in Singapore relied on trains to transport food
to them from Malaya. When the Japanese invaded Malaya, train services were
disrupted, causing grave concern that possible food shortages would arise. Many
people would gather and place their ears close to the railway tracks during the time
of the day when trains transporting food were scheduled to set off. Why did people
do this?
Sound travels much faster through solids than through air. By placing their ears onto the metallic

tracks, the people could hear the train coming before seeing it or hearing it through air.

Check your understanding
• Do you know that sound waves require a medium to be transmitted, and that the speed
at which they travel differs in air, liquids and solids?
2007 Marshall Cavendish International (S) Pte Ltd Discover Physics Workbook 103


Challenge yourself!

Attempt the following questions on your own. You are advised to spend no more than the
time indicated.

1. Which of the following correctly describes the natures of sound, light and radio
waves?
Sound Waves Light Waves Radio Waves
A Longitudinal Transverse Longitudinal
B Longitudinal Transverse Transverse
C Transverse Longitudinal Longitudinal
D Transverse Longitudinal Transverse ( B )

2. Table 13.1 shows how the speed of sound varies with substances of different densities.
..Table 13.1SubstanceSpeed of sound insubstance/m s–1Density of
substance/kg m–3Air (gas)
Oxygen (gas)
Aluminium (metal)
Iron (metal)
Lead (metal)
3303205100500012001.291.432710787011 300
From this information, what conclusion can you draw about the speed of sound?

A The speed increases as the density of the substance increases.
B The speed is greater in less dense substances.
C The speed is greater in metals than in gases.
D The speed is greatest in the densest metal. ( C )


3.   What is the correct order for the speed of sound in water, air and steel?
slowest fastestABCDwaterwaterairairairsteelsteelwatersteelairwatersteel
( D )
104 Sound 2007 Marshall Cavendish International (S) Pte Ltd


4. Figure 13.2 illustrates an apparatus which can be used to demonstrate that the transmission
of sound requires a material medium.
glass bell jarSd.c. powersupplyhookelectric bellvacuum seal on base platebase platepressure gaugetap
..Figure 13.2
vacuum pump

The stages in the demonstration are set out below. Initially, the air inside the bell jar

is at atmospheric pressure. State what is seen and heard at each of the following stages
and what deductions can be made.

Stage 1. The bell circuit is completed by closing switch S.

Seen and heard: The striker is seen vibrating against the bell and a sound is heard.

Deduction: Sound can be transmitted in air.

Stage 2. The tap is closed and the vacuum pump is switched on.

The striker is seen vibrating against the bell but a fainter sound is
Seen and heard: heard.

Deduction: Sound cannot be transmitted in the absence of air.

Stage 3. The vacuum pump is switched off and the tap opened.

The striker is seen vibrating against the bell and a sound is heard,
Seen and heard: which increases in loudness.

Deduction: Sound can be transmitted in air.
(Phy/Jun88/P2/4a)

5.   (a) Explain how sound travels in air.
Sound travels in air as a series of compressions and rarefactions.

(b) Given that the speed of sound in air is 330 m s–1, what is the wavelength of a sound
produced by a source vibrating at a frequency of 10 kHz?
v = 330 m s–1
f = 10 × 103 Hz


..= v 330

= = 0.033 m

f104

2007 Marshall Cavendish International (S) Pte Ltd Discover Physics Workbook 105


Sound


For topic13.3Reflection of Sound
Worksheet 13B
Build your understanding!

Attempt the following questions on your own or in a group setting.

1.   (a) Explain how an echo is formed.
An echo is a sound heard after the reflection of sound from hard, flat surfaces such as a wall.

(b) A pulse of sound is sent vertically down into the sea and an echo from the seabed
is received 0.3 s after the pulse is sent. If the speed of sound in water is 1500 m s–1,
what is the depth of the sea?
Given:   Time for sound to travel to-and-fro the sea bed, t = 0.3 s
Speed of sound in water, v = 1500 m s–1


2d vt 0.3

v =   where d = depth of the sea ..d = = 1500 ×

t22
= 225 m

2. A man is standing between two tall vertical walls 990 m apart. He is 330 m away from
one wall. He fires a starting pistol into the air and subsequently hears a few echoes.
Taking the speed of sound in air to be 330 m s–1, calculate the interval of time between
the fi rst two echoes he hears.
Speed of sound in air = 330 m s–1
Distance from first wall = 330 m


dis tan ce 330 ×2

Time for sound to travel to-and-fro the first wall = = = 2 s

speed330

Distance from second wall = 990 – 330 = 660 m

660 ×2

Time for sound to travel to-and-fro the second wall = = 4 s

330

. Time interval between first two echoes = 4 – 2 = 2 s

Check your understanding
• Do you know what an echo is? Can you describe a method of measuring distances by
the use of echoes?
106 Sound 2007 Marshall Cavendish International (S) Pte Ltd


Challenge yourself!


Attempt the following questions on your own. You are advised to spend no more than the
time indicated.

1. A pulse of high frequency sound is emitted from a ship to the seabed and the time taken
for the reflected pulse to return to the ship is measured to be 0.20 s. The wavelength of
the emitted sound wave is 0.030 m and its frequency is 50 kHz.
Find
(a) the speed of the sound in seawater,
v = fh
= 50 × 103 × 0.030
= 1500 m s–1


(b) the distance to the seabed.
2d

v =

t

1500 ×0.2

d = = 150 m

2

2. In a determination of v, the speed of sound in air on a windless day, one person P fi red
a starting pistol whilst another person Q started a digital stopwatch when he saw the
fl ash and stopped it when he heard the sound. P and Q then exchanged positions and
repeated the procedure.
The results obtained were: Experiment 1—1.36 s; Experiment 2—1.29 s.

(a) Given that the distance between P and Q was 450 m in both experiments, calculate
the value of v.
1.36 + 1.29
Mean time, t = = 1.325 s

2
450


v = = 340 m s–1

t

(b) Why is it better, in such a determination, to use a distance of 450 m between P and
Q, rather than 100 m?
This is because the time interval between seeing the flash and hearing the sound may be too

small such that the two events cannot be distinguished.

(Phy/Jun88/P2/Q4b)


2007 Marshall Cavendish International (S) Pte Ltd Discover Physics Workbook 107


3. To investigate a layer of rock underground, an explosion is made on the surface of the
Earth. Figure 13.3 shows the arrangement.
detector

explosion made heresurface of Earthearthair123
..Figure 13.3
layer of rock

Sound from the explosion may travel to the detector through air (path 1), through earth
(path 2), or by refl ection from a layer of rock (path 3). The time taken for sound to reach
the detector is shown in Table 13.2.

..Table 13.2Time taken for sound to travel fromsource to detector/spath1path2path30.1000.0200.300
(a) Explain why sound arrives first at the detector along path 2.

(b) Given that the speed of sound in air is 320 m s–1, calculate the distance between the
source of sound and the detector.



(d) Assume the depth of the layer of rock is much longer than the distance between the
source and detector such that path 3 is vertical.


2d

t

1600 × 0.3

""E. "E"#K$".

2

(Phy/Jun01/P2/Q11modifi ed)

Sound


For topic13.4Pitch and Loudness
Worksheet 13C
Build your understanding!

Attempt the following questions on your own or in a group setting.

1.   (a) Two properties that are used to distinguish one musical sound from another are
pitch and loudness. For each term in italics, state the physical characteristics of the
sound waves to which they are related.




(b) A student tries to produce notes of higher frequency by blowing a trumpet harder.
Discuss whether he will succeed.

2. Using a microphone and a cathode-ray oscilloscope, the waveforms of sounds produced
by two different tuning forks can be displayed on the screen of the oscilloscope. Figures
13.4 and 13.5 show the two waveforms of displacement against time for a fi xed setting
of the time-based knob.
sameheight
tuning fork A tuning fork B

..Figure 13.4 ..Figure 13.5

(b) In Table 13.3 below, draw the expected waveforms if
(i) tuning fork A was struck twice as hard, and
(ii) tuning fork B was struck half as hard.
..Table 13.3

(i) Tuning fork A(ii) Tuning fork Boriginal waveformnew waveformoriginal waveformnew waveformCheck your understanding
!Do you know that the loudness and pitch of a sound are related to the amplitude and
frequency of the sound wave respectively?
110 Sound 2007 Marshall Cavendish International (S) Pte Ltd


Challenge yourself!

Attempt the following questions on your own. You are advised to spend no more than the
time indicated.

1. Two students, P and Q, play a note on a recorder. The note played by P is louder and
has a lower pitch than the note played by Q. How do the amplitude and frequency of
the note played by P compare to that of the note played by Q?
Amplitude of P’s note Amplitude of P’s note

A Smaller Lower
B Larger Lower
C Smaller Higher
D Larger Higher ( B )

2. A tuning fork produces a sound wave in air when the prongs of the fork vibrate with a
frequency of 500 Hz.
(a) What is the frequency of the sound wave? 500 Hz
(b)   (i) How does the vibration of the fork change as the sound becomes less loud?
The amplitude of the vibration decreases.

(ii) State one feature of this sound wave which remains constant as the sound
becomes less loud.
frequency

(Jun99/P2/Q4)


2007 Marshall Cavendish International (S) Pte Ltd Discover Physics Workbook 111