14. wave motion
DESCRIPTION
14. Wave Motion. Waves & their Properties Wave Math Waves on a String Sound Waves Interference Reflection & Refraction Standing Waves The Doppler Effect & Shock Waves. Other kinds of waves : Sound Light Radio Ultrasound Microwave Earthquake / Tsunami. - PowerPoint PPT PresentationTRANSCRIPT
14. Wave Motion
1. Waves & their Properties2. Wave Math3. Waves on a String4. Sound Waves5. Interference6. Reflection & Refraction7. Standing Waves8. The Doppler Effect & Shock Waves
Ocean waves travel thousands of kilometers
across the open sea before breaking on shore.
How much water moves with the waves?
Other kinds of waves:
• Sound
• Light
• Radio
• Ultrasound
• Microwave
• Earthquake / Tsunami
Wave:
Traveling disturbance that
transport energy but not matter.
None
14.1. Waves & their Properties
Mechanical waves: mechanical disturbances in material medium.
E.g., air, water, violin string, Earth’s interior, ….
Electromagnetic waves: EM disturbances anywhere (including vacuum)
E.g., Visible, infrared, & ultraviolet light, radio waves, X ray, …
Longitudinal & Transverse Waves
Longitudinal wavesTransverse waves
Water waves
LongitudinalTransverse
mixed
1-D Vibration
Water Waves
Wave Amplitude
Wave amplitude = maximum value of the disturbance.
( w.r.t. undisturbed state )
Water wave: max height above undisturbed level.
Sound wave: max excess pressure.
Wave in coupled springs: max displacement from equilibrium position.
Wave Shape
Waveform = shape of waves.
Pulse = isolated disturbance.
Continuous wave
= ongoing periodic disturbance.
Wave train
= periodic disturbance of finite duration.
Wavelength, Period, & Frequency
A continuous wave is periodic in both time & space.
Wavelength : distance over which the wave pattern repeats. ( length of 1 cycle )
Period T : duration over which the wave pattern repeats. ( time for 1 cycle )
Frequency f : number of wave cycles per unit time. ( f = 1 / T )
Wave Speed
Speed of wave depends only on the medium.
Sound in air 340 m/s 1220 km/h. in water 1450 m/s in granite 5000 m/s
Small ripples on water 20 cm/s.
Earthquake 5 km/s.
vT
fWave speed
GOT IT? 14.1.
A boat bobs up & down on a water wave, moving a vertical distance of 2 m in 1 s.
A wave crest moves a horizontal distance of 10 m in 2 s.
Is the wave speed
(a) 2 m/s, or
(b) 5 m/s ?
Explain.
( Speed of disturbance )
14.2. Wave Math
At t = 0, ,0y x f x
At t , y(0) is displaced to the right by v t.
,y x t f x v t
For a wave moving to the left : ,y x t f x v t
For a SHW (sinusoidal):
,0 cosy x A k x2k
= wave number
SHW moving to the right :
, cosy x t A k x t 2T
k x t = phase
vT k
= wave speed
k x v t
pk @ x = 0 pk @ x = v t
Waves
Example 14.1. Surfing
A surfer paddles to where the waves are sinusoidal with crests 14 m apart.
He bobs a vertical distance 3.6 m from trough to crest, which takes 1.5 s.
Find the wave speed, & describe the wave.
, cosy x t A k x t
1 3.62
A m 1.8m
14 m 2 1.5T s 3.0 s
12 0.449k m
12 2.09 s
T
4.7 /v m sT
, 1.8 cos 0.449 2.09y x t x t
GOT IT? 14.2.
Figure shows two waves propagating with the same speed.
Which has the greater
(a) amplitude, (b) wavelength, (c) period, (d) wave number, (e) frequency ?
U LL U U
v = / T
The Wave Equation
1-D waves in many media can be described by the partial differential equation
,y x t f x v t
2 2
2 2 2
y yx v t
Wave Equation
whose solutions are of the form
v = velocity of wave.
E.g., •water wave ( y = wave height )•sound wave ( y = pressure )•…
, cosy x t A k x t vk
( towards x )
14.3. Waves on a String
= mass per unit length [ kg/m ]
A pulse travels to the right.
In the frame moving with the pulse, the entire string
moves to the left.
Top of pulse is in circular motion with speed v & radius
R.Centripedal accel:
2
ˆm vmR
a y
Tension force F is cancelled out in the x direction:
2 sinyF F 2F ( small segment )
2
2 m vFR
22 R v
R
Fv
2F v
Example 14.2. Rock Climbing
A 43-m-long rope of mass 5.0 kg joins two climbers.
One climber strikes the rope, and 1.4 s later, the 2nd one feels the effect.
What’s the rope’s tension?
mL
Lvt
110 N2
m LFt
2
5.0 43
1.4
kg m
s
2F v
Wave Power
SHO :
Segment of length x at fixed x : 2 212
E x A
2 212
xP At
2 21
2v A
v = phase velocity of wave
2 212
E m A
Wave Intensity
Wave front = surface of constant phase.
Plane wave : planar wave front.
Spherical wave : spherical wave front.
Intensity = power per unit area direction of propagation [ W / m2 ]
Plane wave : I const
Spherical wave :24
PIr
Example 14.3. Reading Light
A book 1.9 m from a 75-watt light bulb is barely readable.
How far from a 40-W bulb the book should be to provide the same intensity at the page.
24PIr
75 402 2
75 40
P Pr r
4040 75
75
Pr r
P 401.9
75WmW
1.4 m
GOT IT? 14.3.
The intensity of light from the more distant one of two identical stars is only 1% that
of the closer one. Is the more distant star
(a) twice
(b) 100 times
(c) 10 times
(d) 10 times
as far away.
14.4. Sound Waves
Sound waves = longitudinal mechanical waves through matter.
Speed of sound in air :Pv
P = background pressure.
= mass density.
= 7/5 for air & diatomic gases.
= 5/3 for monatomic gases, e.g.,
He.
P, = max , x = 0
P, = min , x = 0
P, = eqm , |x| = max
Sound & the Human Ear
Audible freq:20 Hz ~ 20 kHz
Bats: 100 kHz
Ultrasound: 10 MHz
db = 0 :Hearing Threshold @ 1k Hz
Decibels
Sound intensity level :
100
10 log II
12 20 10 /I W m Threshold of hearing at 1
kHz.
[ ] = decibel (dB)/10
0 10I I
22 1 10
1
10 log II
2 1 / 102
1
10II
2 110I I2 1 10 dB
3/102 110I I2 1 3 dB 12 I
Nonlinear behavior: Above 40dB, the ear percieves = 10 dB as a doubling of loudness.
Example 14.4. TV
A TV blasts at 75 dB.
If it’s then turned down to 60 dB, by what factor has the power dropped ?
60 75 / 1010
22 1 10
1
10 log II
210
1
10 log PP
24PIr
2 1 / 102
1
10PP
3 / 210 0.032130
1
10 10
10 db drop ½ in loudness
15 db drop between ½ & ¼ in loudness
14.5. Interference
constructive interference
destructive interference
Principle of superposition: tot = 1 + 2 .
Interference
Fourier Analysis
Fourier analysis:
Periodic wave = sum of SHWs.
E note from electric guitar
0
1 sin2 1n
square wave A n tn
Fourier Series
Dispersion
Non-dispersive medium
Dispersive medium
Dispersion:wave speed is wavelength (or freq) dependent
Surface wave on deep water:
2gv
long wavelength waves reaches shore 1st.
Dispersion of square wave pulses determines max
length of wires or optical fibres in computer networks.
Dispersion
Conceptual Example 14.1. Storm Brewing
It’s a lovely, sunny day at the coast,but large waves, their crests far apart, are crashing on the beach.
How do these waves tell of a storm at sea that may affect you later?
crests far apart long wavelength
v = ( g / 2 ) large
storm that generates the waves are not far behind
Note: tsunamis generate shallow-water waves that do not obey2gv
Making the Connection
A storm develops 600 km offshore & starts moving towards you at 40 km/h.
Large waves with crests 250 m apart are your 1st hint of the storm.
How long after you observe these waves will the storm hit?
Time for storm to reach you = 600 15
40 /km h
km h
Speed of wave =2g
2250 9.8 /2
m m s
19.7 /m s 71.0 /km h
Time for wave to reach you = 600 8.45
71.0 /km hkm h
The storm is 15 8.45 = 6.55 h 6.6 h away.
Beats
Beats: interference between 2 waves of nearly equal freq.
1 2cos cosy t A t A t
1 2 1 21 12 cos cos2 2
A t t
Freq of envelope = 1 2 .
smaller freq diff longer period between beats
Applications:
Synchronize airplane engines (beat freq 0).
Tune musical instruments.
High precision measurements (EM waves).
ConstructiveDestructive
Interference in 2-D
Water waves from two sources with separation
Nodal lines:amplitude 0
path difference = ½ n
Destructive Constructive
Interference
Example 14.5. Calm Water
Ocean waves pass through two small openings, 20 m apart, in a breakwater.
75 m from the breakwater & midway between the openings, water is rough.
33 m parallel to the breakwater away, the water is calm.
What’s the wavelength of the waves?
2 275 33 10AP m m m
2AP BP
86.5m
2 275 33 10BP m m m 78.4 m
2 86.5 78.4m m 16 m
GOT IT? 14.4.
Light shines through two small holes onto a screen in a dark room.
The holes spacing is comparable to the wavelength of the light.
Looking at the screen, will you see
(a) two bright spots
(b) a pattern of light & dark patches?
Explain.
14.6. Reflection & Refraction
Fixed end
Free end
Partial Reflection
A = 0;reflected wave inverted
A = max;reflected wave not inverted
light + heavy ropes
Rope
Partial reflection + oblique incidence
refraction
Partial reflection + normal incidence
Application: Probing the Earth
P wave = longitudinal
S wave = transverse
S wave shadow
liquid outer core
P wave partial reflection
solid inner core
Explosive thumps
oil / gas deposits
14.7. Standing Waves
String with both ends fixed:
2L n
, cos cosy x t A k x t B k x t
Superposition of right- travelling & reflected waves:
, 2 sin siny x t A k x t
1 1cos cos 2 sin sin2 2
A
standing wave
sin 0kL 1,2,3,n
Allowed waves = modes or harmonics
n = mode numbern = 1 fundamental moden > 1 overtones
y = 0 node y = max antinode
2 L n
0, 0y t B = A
Standing Waves
1 end fixed node,
1 end free antinode.
2 14
L n
cos 0kL
1,2,3,n
2 2 12
L n
, cos cosy x t A k x t B k x t
0x L
dydx
B A
sin sin 0kA kL t kA kL t
cos sin 0kL t
Standing Waves
Standing Wave Resonance
vf
v = const fundamental mode ~ lowest freq
overtones ~ multiples of fund. freq
Skyscraper ~ string with 1 free end & 1 fixed end.
Tacoma bridge: resonance of torsional standing waves.
Other Standing Waves:
• Water waves in confined spaces (waves in lake).
• EM waves in cavity (microwave oven).
• Sound wave in the sun.
• Electrons in atom.
Musical Instruments
Standing waves on a violin, imaged using holographic interference of laser light waves.
Standing waves in wind instruments:
(a)open at one end L = (2n1) / 4
(b) open at both ends L = n / 2
Example 14.6. Double Bassoon
Double bassoon is the lowest pitched instrument in most orchestra.
It’s “folded” to achieve an effective open-ended column of 5.5 m long.
What is the fundamental freq, assuming sound speed is 343 m/s.
vf
343 /2 5.5
m sm
31Hz ~ B0
/ 2
GOT IT? 14.5.
A string 1 m long is clamped tight at one end & free to slide up & down at the other.
Which of the following are possible wavelengths for standing waves on it:
4/5 m, 1 m, 4/3 m, 3/2 m, 2 m, 3 m, 4 m, 5 m, 6 m, 7 m, 8 m ?
2 14
L n
14.8. The Doppler Effect & Shock Waves
Point source at rest in medium radiates uniformly in all directions.
When source moves, wave crests bunch up in the direction of motion ( ).
Wave speed v is a property of the medium & hence independent of source motion.
vf
f Doppler effectApproaching source:
.
t = T
u T
t = 2T 2 uT = uT
t = 0
approach u T
u = speed of source
uv 1 u
v
recede u T 1 uv
1 /recede
ffu v
T = period of wave
Moving Source
1 /approachapproach
v ffu v
Application of the Doppler effect:
• Ultrasound: measures blood flow & fetal heartbeat.
• High freq radio wave: speeding detector.
• Starlight: stellar motion.
• Light from galaxies: expanding universe.
Example 14.7. Wrong Note
A car speeds down the highway with its stereo blasting.
An observer with perfect pitch stands by the roadside, & as the car approach,
notices that a musical note that should be G ( f = 392 Hz ) sounds like A ( 440 Hz ).
How fast is the car moving?
392343 / 1440
Hzm sHz
37.4 /m s
1app
ff uv
1app
fu vf
134 /km h
Moving Observers
An observer moving towards a point source at rest in medium sees a faster moving wave.
Since is unchanged, observed f increases.
1towarduf fv
1awayuf fv
Prob. 76
For u/v << 1:
1app
ffuv
1 ufv
towardf
Waves from a stationary source that reflect from a moving object undergo 2 Doppler effects.
1.A f toward shift at the object.
2.A f approach shift when received at source.
Doppler Effect for Light
Doppler shift for EM waves is the same whether the source or the observer moves.
1appuc
correct to 1st order in u/c
1appufc
Shock Waves
1appuv
0app if u v Shock wave: u > v
Mach number = u / v
Mach angle = sin1(v/u)
E.g.,
Bow wave of boat.
Sonic booms.
Solar wind at ionosphere
Shock wave front
Source, 1 period ago
Moving Source