Wave motion chapter 1
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Transcript of Wave motion chapter 1
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Chapter 1
Wave Motion
1
Dr Mohamed Saudy
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Why learn about waves?
Waves carry useful
information and energy.
Waves are all around
us:
light from the stoplight
electricity flowing in wires
radio and television and
cell phone transmissions 2
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Definition: A wave is a traveling disturbance in some physical system.
Wave Motion : Basic Concept
Alternatively, a periodic disturbance that travels through space and time
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Wavelength visualized Parameters of a Wave
–Wavelength l (e.g., meters)
–Frequency f (cycles per sec Hertz)
–Propagation speed c (e.g., meters / sec)
Characteristics:
C= f l
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Wave Motion
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Wave Motion : Classification
Traveling waves – Disturbance
moves along the direction of wave propagation
Standing waves - Disturbance
oscillates about a fixed point.
WAVE MOTION
Standing/Stationary Wave
E. M. WAVES
Longitudinal Wave
Transverse Wave
Mechanical Waves
Traveling Wave
Waves can be characterized as
Transverse or Longitudinal.
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Types of Waves
There are two main types of waves
Mechanical waves
Some physical medium is being disturbed
The wave is the propagation of a disturbance through
a medium (such as water, air and rock)
Examples: water waves, and sound waves
Electromagnetic waves (E.M Waves)
No medium required
Examples are light, radio waves, x-rays
All e.m waves travel through the vacuum at the same
speed. 7
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General Features of Waves
In wave motion, energy is transferred over a
distance
Matter is not transferred over a distance
All waves carry energy
The amount of energy and the mechanism
responsible for the transport of the energy differ
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Mechanical Wave
Requirements
Some source of disturbance
A medium that can be disturbed
Some physical mechanism through which
elements of the medium can influence each
other
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Transverse Wave
A traveling wave or pulse
that causes the elements of
the disturbed medium to
move perpendicular to the
direction of propagation is
called a transverse wave
The particle motion is
shown by the blue arrow
The direction of propagation
is shown by the red arrow
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Longitudinal Wave
A traveling wave or pulse that causes the elements of the disturbed medium to move parallel to the direction of propagation is called a longitudinal wave
The displacement of the coils is parallel to the propagation
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Complex Waves
Some waves exhibit a combination of transverse
and longitudinal waves
Surface water waves are an example
Use the active figure to observe the displacements
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Example: Earthquake Waves
(Complex wave)
P waves “P” stands for primary
Fastest, at 7 – 8 km / s
Longitudinal
S waves “S” stands for secondary
Slower, at 4 – 5 km/s
Transverse
A seismograph records the waves and allows determination of information about the earthquake’s place of origin
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• An ocean wave is a
combination of transverse and
longitudinal.
• The individual particles move
in ellipses as the wave
disturbance moves toward the
shore.
Water Waves
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Sinusoidal Waves
The wave represented by
the curve shown is a
sinusoidal wave
It is the same curve as sin q
plotted against q
This is the simplest
example of a periodic
continuous wave
It can be used to build more
complex waves
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Terminology: Amplitude and
Wavelength
The crest of the wave is the location of the maximum displacement of the element from its normal position This distance is called
the amplitude, A
The wavelength, l, is the distance from one crest to the next
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Terminology: Wavelength and
Period
More generally, the wavelength is the
minimum distance between any two identical
points on adjacent waves
The period, T , is the time interval required for
two identical points of adjacent waves to pass
by a point
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Terminology: Frequency
The frequency, ƒ, is the number of crests (or
any point on the wave) that pass a given
point in a unit time interval
The time interval is most commonly the second
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Terminology: Frequency, cont
The frequency and the period are related
When the time interval is the second, the
units of frequency are s-1 = Hz
Hz is a hertz
1ƒ
T
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period ( T ) units - time
frequency ( f ) units - 1/time
time per wave
waves per time
T = 1
f f =
1
T v = = f
T
l l
if f = 10
sec , then T =
10 sec
1
sec
1 =
sec
cycle = hz
Wave Motion : Properties
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Terminology, Example
The wavelength, l, is
40.0 cm
The amplitude, A, is
15.0 cm
The wave function can
be written in the form
y = A cos(kx – t)
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Wave Equations
We can also define the angular wave number
(or just wave number), k
The angular frequency can also be defined
2k
l
22 ƒ
T
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Wave Equations, cont
The wave function can be expressed as
y = A sin (k x – t).
The speed of the wave becomes v = l ƒ.
If x 0 at t = 0, the wave function can be
generalized to
y = A sin (k x – t + )
where is called the phase constant.
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Speed of a Wave on a String
The speed of the wave depends on the physical characteristics of the string and the tension to which the string is subjected
This assumes that the tension is not affected by the pulse
This does not assume any particular shape for the pulse
tension
mass/length
Tv
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Example 1: A wave pulse on a string moves a distance of 10 m
in 0.05 s.
(a) What is the velocity of the pulse?
(b) What is the frequency of a periodic wave on the same
string if its wavelength l is 0.8 m?
Solution: (a) The velocity of the pulse is C=x/t, where
x= 10 m, t=0.05 s, C= 200 m/s
(b) The periodic wave has the same velocity 200 m/s,
f=C/l=250 Hz= 250 s-1
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Example 2: The tension on the longest string of
a grand piano is 1098 N, and the mass per unit
length is 0.065 Kg/m. What is the velocity of a
wave on this string?
Solution
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Example 3: An electromagnetic vibrator sends waves down a
string. The vibrator makes 600 complete cycles in 5 s. For one
complete vibration, the wave moves a distance of 20 cm. What
are the frequency, wavelength, and velocity of the wave?
Solution
The distance moved during a time of one cycle is the
wavelength; therefore: l=0.2 m
The velocity of wave
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120 0.2 24 /v f Hz m m sl
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Energy in Waves in a String
Waves transport energy when they propagate
through a medium
We can model each element of a string as a
simple harmonic oscillator
The oscillation will be in the y-direction
Every element has the same total energy
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Energy, final
the total kinetic energy in one wavelength is
Kl = ¼2A 2l
The total potential energy in one wavelength
is Ul = ¼2A 2l
This gives a total energy of
El = Kl + Ul = ½2A 2l
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Power Associated with a Wave
The power is the rate at which the energy is being transferred:
The power transfer by a sinusoidal wave on a string is proportional to the Frequency squared
Square of the amplitude
Wave speed
l
2 2
2 2
1122
AEnergy E
A vTime t T
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Example 4: A 2 m string has a mass of 300 g and
vibrates with a frequency of 20 Hz and an amplitude
of 50 mm. If the tension in the rope is 48 N, how
much power must be delivered to the string?
Solution
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The Superposition Principle • When two or more waves (blue and green) exist in
the same medium, each wave moves as though the other were absent.
• The resultant displacement of these waves at any point is the algebraic sum (yellow) wave of the two displacements.
Constructive Interference Destructive Interference
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Formation of a
Standing
Wave:
Incident and reflected
waves traveling in
opposite directions
produce nodes N and
antinodes A.
The distance between
alternate nodes or anti-nodes
is one wavelength.
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Possible Wavelengths for Standing
Waves
Fundamental, n = 1
1st overtone, n = 2
2nd overtone, n = 3
3rd overtone, n = 4
n = harmonics
2 1, 2, 3, . . .n
Ln
nl
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Possible Frequencies f = v/l :
Fundamental, n = 1
1st overtone, n = 2
2nd overtone, n = 3
3rd overtone, n = 4
n = harmonics
f = 1/2L
f = 2/2L
f = 3/2L
f = 4/2L
f = n/2L
1, 2, 3, . . .2
n
nvf n
L
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Characteristic Frequencies
Now, for a string under
tension, we have:
and 2
F FL nvv f
m L
Characteristic frequencies:
; 1, 2, 3, . . .2
n
nf n
L
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Example 5: A uniform cord has a mass of 0.3 kg and a length of 6
m. The cord passes over a pulley and supports a 2 kg object. Find
the speed of a pulse traveling along this cord?
Solution: The tension in the cord is equal to the weight of the
suspended M=2 kg object:
=Mg= (2 Kg)(9.8 m/s2)=19.6 N
The mass per unit length of the cord is
Therefore, the wave speed is
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Example 6: A 9-g steel wire is 2 m long and is under a tension of
400 N. If the string vibrates in three loops, what is the
frequency of the wave?
Solution: For three loops: n = 3
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Summary for Wave Motion:
Lv
m
1f
T
; 1, 2, 3, . . .2
n
nf n
L
2 21, 2
2E A f l
v f l
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Multiple Choice
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1. The wavelength of light visible to the human eye is on the
order of 5 10–7 m. If the speed of light in air is 3 108 m/s, find
the frequency of the light wave.
a. 3 107 Hz
b. 4 109 Hz
c. 5 1011 Hz
d. 6 1014 Hz
e. 4 1015 Hz
2. The speed of a 10-kHz sound wave in seawater is
approximately 1500 m/s. What is its wavelength in sea water?
a. 5.0 cm
b. 10 cm
c. 15 cm
d. 20 cm
e. 29 cm
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3. If y = 0.02 sin (30x – 400t) (SI units), the frequency of the wave is
a. 30 Hz
b. 15/ Hz
c. 200/ Hz
d. 400 Hz
e. 800 Hz
4. If y = 0.02 sin (30x – 400t) (SI units), the wavelength of the wave
is
a. /15 m
b. 15/ m
c. 60 m
d. 4.2 m
e. 30 m
5. If y = 0.02 sin (30x – 400t) (SI units), the velocity of the wave is
a. 3/40 m/s
b. 40/3 m/s
c. 60/400 m/s
d. 400/60 m/s
e. 400 m/s
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6. A piano string of density 0.005 kg/m is under a tension of 1350 N. Find
the velocity with which a wave travels on the string.
a. 260 m/s
b. 520 m/s
c. 1040 m/s
d. 2080 m/s
e. 4160 m/s
7. A 100-m long transmission cable is suspended between two towers. If
the mass density is 2.01 kg/m and the tension in the cable is 3 104 N,
what is the speed of transverse waves on the cable?
a. 60 m/s
b. 122 m/s
c. 244 m/s
d. 310 m/s
e. 1500 m/s