13. Wave Motionphome.postech.ac.kr/user/genphys/download/phy101-13.pdf · Wave Motion 13-1. Three...
Transcript of 13. Wave Motionphome.postech.ac.kr/user/genphys/download/phy101-13.pdf · Wave Motion 13-1. Three...
13. Wave Motion13. Wave Motion13-1. Three Wave Characteristics
1. Wave Length λ2. Period T or Frequency f = 1/T3. Speed of wave v
13-2. Types of Waves
Transverse WaveLongitudinal Wave
13-3. One-Dimensional Transverse Traveling Wave
y = f (x) at t = 0
ymax : Amplitude
• General form of wave, y (x,t)
y = f (x – vt) : traveling to the right
y = f (x + vt) : traveling to the left
Same phase ( ) vtxt,x −=φ( ) ( )ttvxxtt,xx δ+−δ+=δ+δ+φ
tvxvtx δ⋅−δ++=
( ) vtxt,x +=φ=
dtdxv =⇒ : wave velocity,
phase speed (velocity)
• Wave- Energy ⇒ Propagate - Media
13-4. Sinusoidal Traveling Waves
( )
−
λπ
= vtxsinAy 2
Wave length λWave velocity v Period T
vT = λ ⇒ T = λ / v , v = λ / T
λ
−λπ
= tT
xsinAy 2
π
−λπ
= tT
xsinA 22
Convenient to define
fT
π=π
≡ω 22λπ
≡2k
( )tkxsinAy ω−=⇒
fk
v λ=ω
=
Generalized wave function form
( )φ+ω−= tkxsinAy
Phase constant
Sinusoidal Waves on Strings
( )tkxsinAy ω−=
Transverse speed
( )tkxcosAty
dtdyv
ttanconsxy ω−ω−=
∂∂
≡=
=
Transverse acceleration
( )tkxsinAt
vdt
dva y
ttanconsx
yy ω−ω−=
∂
∂=
=
=
2
( ) Avmaxy ω=
( ) Aamaxy
2ω=
13-5. Superposition and Interference of waves
• if two or more traveling waves are moving through a medium, the resultant wave function at any point is the sum of the wave functions of the individual waves.
: Superposition principle
• Two traveling waves can pass through each other without being destroyed or even altered.
⇒ Interference Phenomena (간섭현상)
One of the wave characteristics
13-6. The Speed of Transverse Waves on Strings
θ⋅≅θ=∑ FsinFF 22r
Rmv2
=
µ: the mass per unit length (선밀도)
θ⋅µ=∆⋅µ= 2Rsm
22
222 vR
vRF µθ=⋅θ⋅µ
=θ⋅
µ=
Fv : speed of transverse waves on strings
F = Mg
M
Mgµ
=µ
=MgFv
13-7. Reflection and Transmission of Waves
Reflection Rigid End Free End
The reflected pulse remains same without inversion.
Inverted reflected pulsebut the shape remains same.
Transmission
vA
vB
AA
Fvµ
=
BB
Fvµ
=
BA vv >
Inverted Reflected pulse but no inversion in the Transmitted pulse.
vBvA BA vv <
No inversion in both the Reflected and Transmitted pulse.
13-8. Energy Transmitted by Sinusoidal Waves on Strings
( )tkxsinAy ω−=
∆m : Simple Harmonic Motion
Total Energy 22212
21 AmkAE ω==
Total Energy for ∆m 22212
21 AmkAE ω∆==∆
( ) 2221 AxE ω∆µ=∆
2221 A
dtdx
dtdEP ω
µ==Power P
vAP 2221 µω=
13-9. Sound WavesLongitudinal Wave
λPressure
( ) ( )tkxcosst,xs max ω−=
Pressure relative to its equilibrium
( )tkxsinPP max ω−∆=∆
maxmax svP ωρ=∆
ρ : density of mediumv : the wave speedωsmax : the maximum longuitudinal
speed of the medium
( )2π−ω−ωρ=∆ tkxcossvP max
90° phase off
AtmvP s
⋅∆∝
Axm ⋅∆ρ=
( )tkxsinsdtdsv maxs ω−ω==
( )tkxsinstxP max ω−ω
∆∆
ρ∝
( )tkxsinsv max ω−ωρ=
13-10. Doppler EffectRelative motion between the source and the observer causes a higher or lower frequency than the original frequency.
Moving Observerfrequency
λ=
vf
vvvfvvf 00 +
=λ+
=′
Observer moving toward source
Moving Source
λ′=′ vf
vT=λ
( )fvvTvv s
s−
=−=λ′
−
=λ′
=′svv
vfvf
Source moving toward observerIn general
±=′
s
o
vvvvff
m