Physical Science
Grade 10
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SESSION 5: LONGITUDINAL WAVES
KEY CONCEPTS:
In this session we will consider:


Longitudinal waves
Sound
TERMINOLOGY
Longitudinal waves: A longitudinal wave is a wave where the particles in the
medium move parallel to the direction of propagation of the wave
Compression: A compression is a region in a longitudinal wave where the particles
are closest together.
Rarefaction: A rarefaction is a region in a longitudinal wave where the particles are
furthest apart.
Wavelength: The wavelength in a longitudinal wave is the distance between two
consecutive points that are in phase
Amplitude: The amplitude is the maximum displacement from equilibrium. This
would be maximum increase (or decrease) in pressure from the equilibrium pressure
that is caused when a compression (or rarefaction) passes a point.
Period: The period of a wave is the time taken by the wave to move one wavelength
Frequency: The frequency of a wave is the number of wavelengths per second.
SONAR:
Sound Navigation and Ranging.
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X-PLANATION
Calculations for longitudinal waves
and
Where:
f = frequency of the wave measured in Hertz (Hz)
T = period measured in seconds (s)
So if a longitudinal wave has wavelength, amplitude, frequency and period, just as a
transverse wave does, it is possible to calculate the speed of the longitudinal wave in
the same way as for a transverse wave.
The equation we use is
Where: v = speed in m.s-1
λ = wavelength in m
f = frequency in Hz
SOUND
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The speed of sound in different materials:
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Echolocation:
Range of Frequencies:
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X-AMPLE QUESTIONS
Question 1:
Which of the following is not a longitudinal wave?
a.
b.
c.
light
sound
ultrasound
Question 2:
Which of the following media can a longitudinal wave like sound not travel through?
a.
b.
c.
d.
solid
liquid
gas
vacuum
Question 3:
A longitudinal wave has a compression to compression distance of 10 m. It takes
the wave 5 s to pass a point.
a.
b.
What is the wavelength of the longitudinal wave?
What is the speed of the wave?
Question 4:
A flute produces a musical sound travelling at a speed of 320m s -1. The frequency of
the note is 256 Hz. Calculate:
a.
b.
the period of the note
the wavelength of the note
Question 5:
Study the following diagram representing a musical note. Redraw the diagram for a
note:
1. with a higher pitch
2. that is louder
3. that is softer
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Question 6:
A lightning storm creates both lightning and thunder. You see the lightning almost
immediately since light travels at 3 x 108 m s -1. After seeing the lightning, you count
5 s and then you hear the thunder. Calculate the distance to the location of the
storm.
Question 7:
Sound cannot travel in space. Discuss what other modes of communication
astronauts can use when they are outside the space shuttle?
Question 8:
An automatic focus camera uses an ultrasonic sound wave to focus on objects. The
camera sends out sound waves which are reflected off distant objects and return to
the camera. A sensor detects the time it takes for the waves to return and then
determines the distance an object is from the camera. If a sound wave (speed = 344
m s -1) returns to the camera 0,150 s after leaving the camera, how far away is the
object?
Question 9:
Calculate the frequency (in Hz) and wavelength of the annoying sound made by a
mosquito when it beats its wings at the average rate of 600 wing beats per second.
Assume the speed of the sound waves is 344 m s -1.
Question 10:
A person shouts at a cliff and hears an echo from the cliff 1 s later. If the speed of
sound is 344 m s -1, how far away is the cliff?
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Question 11:
Select a word from Column B that best fits the description in Column A:
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