Chapter 13 Mechanical Waves

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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 Examples are ripples in water, sound Electromagnetic waves No medium required Examples are light, radio waves, x-rays

4 General Features of Waves In wave motion, energy is transferred over a distance Matter is not transferred over a distance A disturbance is transferred through space without an accompanying transfer of matter All waves carry energy The amount of energy and the mechanism responsible for the transport of the energy differ

5 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 This requirement ensures that the disturbance will, in fact, propagate through the medium

6 Pulse on a Rope The wave is generated by a flick on one end of the rope The rope is under tension A single bump is formed and travels along the rope The bump is called a pulse Fig 13.1

7 Pulse on a Rope The rope is the medium through which the pulse travels The pulse has a definite height The pulse has a definite speed of propagation along the medium A continuous flicking of the rope would produce a periodic disturbance which would form a wave

8 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 Fig 13.2

9 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 Fig 13.3

10 Traveling Pulse The shape of the pulse at t = 0 is shown The shape can be represented by y = f (x) This describes the transverse position y of the element of the string located at each value of x at t = 0 Fig 13.4

11 Traveling Pulse, 2 The speed of the pulse is v At some time, t, the pulse has traveled a distance vt The shape of the pulse does not change Simplification model Its position is now y = f (x – vt) Fig 13.4

12 Traveling Pulse, 3 For a pulse traveling to the right y (x, t) = f (x – vt) For a pulse traveling to the left y (x, t) = f (x + vt) The function y is also called the wave function: y (x, t) The wave function represents the y coordinate of any element located at position x at any time t The y coordinate is the transverse position

13 Traveling Pulse, final If t is fixed then the wave function is called the waveform It defines a curve representing the actual geometric shape of the pulse at that time

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Sinusoidal Waves A continuous wave can be created by shaking the end of the string in simple harmonic motion The shape of the wave is called sinusoidal since the waveform is that of a sine curve The shape remains the same but moves Toward the right in the text diagrams

22 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 point at the negative amplitude is called the trough The wavelength,, is the distance from one crest to the next Fig 13.6

23 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.6

24 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 The period of the wave is the same as the period of the simple harmonic oscillation of one element of the medium

25 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 The frequency of the wave is the same as the frequency of the simple harmonic motion of one element of the medium

26 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

27 Fig 13.7

28 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.7

Traveling Wave The brown curve represents a snapshot of the curve at t = 0 The blue curve represents the wave at some later time, t Fig 13.8

30 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.8

31 Speed of Waves Waves travel with a specific speed The speed depends on the properties of the medium being disturbed The wave function is given by This is for a wave moving to the right For a wave moving to the left, replace x – vt with x + vt

32 Wave Function, Another Form Since speed is distance divided by time, v = / T The wave function can then be expressed as This form shows the periodic nature of y of y in both space and time

33 Wave Equations We can also define the angular wave number (or just wave number), k The angular frequency can also be defined

34 Wave Equations, cont The wave function can be expressed as y = A sin (k x –  t) The speed of the wave becomes v = ƒ If x  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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36 Fig 13.9

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40 Linear Wave Equation The maximum values of the transverse speed and transverse acceleration are v y, max =  A a y, max =  2 A The transverse speed and acceleration do not reach their maximum values simultaneously v is a maximum at y = 0 a is a maximum at y =  A

41 The Linear Wave Equation, cont. The wave functions y (x, t) represent solutions of an equation called the linear wave equation This equation gives a complete description of the wave motion From it you can determine the wave speed The linear wave equation is basic to many forms of wave motion

42 Linear Wave Equation, General The equation can be written as This applies in general to various types of traveling waves y represents various positions For a string, it is the vertical displacement of the elements of the string For a sound wave, it is the longitudinal position of the elements from the equilibrium position For em waves, it is the electric or magnetic field components

43 Linear Wave Equation, General cont The linear wave equation is satisfied by any wave function having the form y (x, t) = f (x  vt) Nonlinear waves are more difficult to analyze A nonlinear wave is one in which the amplitude is not small compared to the wavelength

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Linear Wave Equation Applied to a Wave on a String The string is under tension T Consider one small string element of length  s The net force acting in the y direction is This uses the small-angle approximation Fig 13.10

48 Linear Wave Equation and Waves on a String, cont  s is the mass of the element Applying the sinusoidal wave function to the linear wave equation and following the derivatives, we find that This is the speed of a wave on a string It applies to any shape pulse

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50 Fig 13.11

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Reflection of a Wave, Fixed End When the pulse reaches the support, the pulse moves back along the string in the opposite direction This is the reflection of the pulse The pulse is inverted when it is reflected from a fixed boundary Fig 13.12

57 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.12

58 Reflection of a Wave, Free End With a free end, the string is free to move vertically The pulse is reflected The pulse is not inverted when reflected from a free end Fig 13.13

59 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.13

60 Transmission of a Wave When the boundary is intermediate between the last two extremes Part of the energy in the incident pulse is reflected and part undergoes transmission Some energy passes through the boundary Fig 13.14

61 Transmission of a Wave, 2 Assume a light string is attached to a heavier string The pulse travels through the light string and reaches the boundary The part of the pulse that is reflected is inverted The reflected pulse has a smaller amplitude

62 Transmission of a Wave, 3 Assume a heavier string is attached to a light string Part of the pulse is reflected and part is transmitted The reflected part is not inverted Fig 13.14

63 Transmission of a Wave, 4 Conservation of energy governs the pulse When a pulse is broken up into reflected and transmitted parts at a boundary, the sum of the energies of the two pulses must equal the energy of the original pulse

64 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.14

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

66 Fig 13.16

67 Fig 13.17

68 Energy, cont. Each element can be considered to have a mass of  m Its kinetic energy is  K = 1/2 (  m) v y 2 The mass  m is also equal to  x and  K = 1/2 (  x) v y 2 As the length of the element of the string shrinks to zero, the equation becomes a differential equation: dK =1/2 (  x) v y 2 = 1/2  2 A 2 cos 2 (kx –  t) dx

69 Energy, final Integrating over all the elements, the total kinetic energy in one wavelength is K = 1/4  2 A 2 The total potential energy in one wavelength is U = 1/4  2 A 2 This gives a total energy of E = K + U = 1/2  2 A 2

70 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 Square of the frequency Square of the amplitude Speed of the wave

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Introduction to Sound Waves Sound waves are longitudinal waves They travel through any material medium The speed of the wave depends on the properties of the medium The mathematical description of sinusoidal sound waves is very similar to sinusoidal waves on a string

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75 Speed of Sound Waves Use a compressible gas as an example with a setup as shown at right Before the piston is moved, the gas has uniform density When the piston is suddenly moved to the right, the gas just in front of it is compressed Darker region in the diagram Fig 13.18

76 Speed of Sound Waves, cont When the piston comes to rest, the compression region of the gas continues to move This corresponds to a longitudinal pulse traveling through the tube with speed v The speed of the piston is not the same as the speed of the wave The light areas are rarefactions Fig 13.18

77 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.18

78 Description of a Sound Wave The distance between two successive compressions (or two successive rarefactions) is the wavelength, As these regions travel along the tube, each element oscillates back and forth in simple harmonic motion Their oscillation is parallel to the direction of the wave

79 Displacement Wave Equation The displacement of a small element is s(x,t) = s max sin (kx –  t) s max is the maximum position relative to equilibrium This is the equation of a displacement wave k is the wave number  is the angular frequency of the piston

80 Pressure Wave Equation The variation  P in the pressure of the gas as measured from its equilibrium value is also sinusoidal  P =  P max cos (kx –  t) The pressure amplitude,  P max is the maximum change in pressure from the equilibrium value The pressure amplitude is proportional to the displacement amplitude  P max =  v  s max

81 Sound Waves as Displacement or Pressure Wave A sound wave may be considered either a displacement wave or a pressure wave The pressure wave is 90 o out of phase with the displacement wave Fig 13.19

82 Speed of Sound Waves, General The speed of sound waves in air depends only on the temperature of the air v = 331 m/s + (0.6 m/s. o C) T C T C is the temperature in Celsius The speed of sound in air at 0 o C is 331 m/s

83 Speed of Sound in Gases, Example Values Note: temperatures given, speeds are in m/s

84 Speed of Sound in Liquids, Example Values Speeds are in m/s

85 Speed of Sound in Solids, Example Values Speeds are in m/s; values are for bulk solids

The Doppler Effect The Doppler effect is the apparent change in frequency (or wavelength) that occurs because of motion of the source or observer of a wave When the motion of the source or the observer is toward the other, the frequency appears to increase When the motion of the source or observer is away from the other, the frequency appears to decrease

87 Fig 13.20

88 Doppler Effect, Observer Moving The observer moves with a speed of v o Assume a point source that remains stationary relative to the air It is convenient to represent the waves with a series of circular arcs concentric to the source These surfaces are called a wave front Fig 13.21

89 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.21

90 Doppler Effect, Observer Moving, cont The distance between adjacent wave fronts is the wavelength The speed of the sound is v, the frequency is ƒ, and the wavelength is When the observer moves toward the source, the speed of the waves relative to the observer is v rel = v + v o The wavelength is unchanged

91 Doppler Effect, Observer Moving, final The frequency heard by the observer, ƒ ’, appears higher when the observer approaches the source The frequency heard by the observer, ƒ ’, appears lower when the observer moves away from the source

92 Doppler Effect, Source Moving Consider the source being in motion while the observer is at rest As the source moves toward the observer, the wavelength appears shorter As the source moves away, the wavelength appears longer Fig 13.22

93 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.22

94 Doppler Effect, Source Moving, cont When the source is moving toward the observer, the apparent frequency is higher When the source is moving away from the observer, the apparent frequency is lower

95 Doppler Effect, General Combining the motions of the observer and the source The signs depend on the direction of the velocity A positive value is used for motion of the observer or the source toward the other A negative sign is used for motion of one away from the other

96 Doppler Effect, final Convenient rule for signs The word toward is associated with an increase in the observed frequency The words away from are associated with a decrease in the observed frequency The Doppler effect is common to all waves The Doppler effect does not depend on distance

97 Shock Wave The speed of the source can exceed the speed of the wave The concentration of energy in front of the source results in a shock wave

98 Doppler Effect, Submarine Example Sub A (source) travels at 8.00 m/s emitting at a frequency of 1400 Hz The speed of sound in water is 1533 m/s Sub B (observer) travels at 9.00 m/s What is the apparent frequency heard by the observer as the subs approach each other? Then as they recede from each other?

99 Doppler Effect, Submarine Example cont Approaching each other: Receding from each other:

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Speed of Sound Waves, General The speed of sound waves in a medium depends on the compressibility and the density of the medium The compressibility can sometimes be expressed in terms of the elastic modulus of the material The speed of all mechanical waves follows a general form:

106 Speed of Transverse Wave in a Bulk Solid The shear modulus of the material is S The density of the material is  The speed of sound in that medium is

107 Speed of Sound in Liquid or Gas The bulk modulus of the material is B The density of the material is  The speed of sound in that medium is

108 Speed of a Longitudinal Wave in a Bulk Solid The bulk modulus of the material is B The shear modulus of the material is S The density of the material is  The speed of sound in that medium is

109 Seismic Waves When an earthquake occurs, a sudden release of energy takes place at its focus The epicenter is the point on the surface of the earth radially above the focus The released energy will propagate away from the focus by means of seismic waves

110 Types of Seismic Waves P waves P stands for primary They are longitudinal waves They arrive first at a seismograph S waves S stands for secondary They are transverse waves They arrive next at the seismograph

111 Seismograph Trace Fig 13.23

112 Fig 13.24

113 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 13.24

114 Fig 13.25