wave basics or, how to destroy a slinky without even trying… d taylor © 2008
TRANSCRIPT
Wave Basics
Or, How to Destroy a Slinky Without Even Trying…
D Taylor
© 2008
2nd Semester Shtuff
• Topics left• Waves & Sound (this week)• Heat & Thermo (Throw Away Topic)• Electricity (3 wks)• Magnetism (0.1 wks)• Electromagnetism (1 week)• Optics (2 wks)• Atomic/Nuclear Physics (2 wks)• Lions & Tigers, & Bears! Oh, MY!
Basic Terminology
• Wavelength – DUH!• Displacement of a sinusoidal periodic wave along
the independent coordinate.
• i.e. The length of the wave. ‘’.
Tesladownunder Diversion
Basic Terminology
• Pulse• Single disturbance
• ½
Pulse
Can you say, “Idiot”?
Basic Terminology
• Period• Time required for one complete wavelength.
• ‘T’
• Unit - Seconds
Basic Terminology
• Frequency• # wavelengths per second
• f = 1/T
• Unit – 1/sec, sec-1, Hz (Hertz)
Tech-TV Blunder!
Basic Terminology
• Amplitude• Disturbance along the vertical.
• Height of wave as measured from x-axis
A
Basic Terminology
• Velocity of waves?
fvT
v
t
dv
Wave Motion gifsExt LINK
Wave Combo ExamplesLAB 25 stuff
Weird Wave WorldDoppler & Co.
Waves-Advanced Stuff
Tighten your seatbelt…It’s getting’ bumpy!
Video of Invisibility Cloak
Advanced Wave Stuff• Sound Intensity
• Logarithmic Function (HUH?)
=10 log I/Io
• Io = 1 x 10-12 W/m2
• Chart pg 475
Intensity I
(W/m2)
Intensity (dB)
Threshold of human hearing
1 x 10-12 0
Leaves rustling 1 x 10-11 10
Whisper 1 x 10-10 20Normal Conversation @ 1 m
3 x 10-16 65
Inside car 1 x 10-4 80
Car w/o muffler 1 x 10-2 100Live rock concert 1 120
Pain 10 130
Taylor Mad 1 x 10488 5000
A log excursion…
LAB 25Ext Link
Advanced Wave Stuff• Sound Intensity Problems?
• One sound produces a 1 = 90dB and another is 2 = 93dB. What is the ratio I2/I1?
13
1129
1219
121
121
1
11
101
)101)(10(
10110
1019
1011090
10
Ix
Ix
x
Ix
ILog
x
ILog
I
ILog
o
27.2
2123.9
1223.9
122
122
2
21
101
)101)(10(
10110
1013.9
1011093
10
Ix
Ix
x
Ix
ILog
x
ILog
I
ILog
o
21010
10 3.03
7.2
1
2
I
I
Advanced Wave Stuff• Doppler Effect?
• Occurs when a sound or an observer moves in the direction of the sound! (Say WHAT?)•Sirens, Moving cars, Planes…
•Resonance tube Demo•Tuning Fork Demo
Ugly Doppler Math
vvvv
ffs
o
1
1'
Ugly Doppler Math
vvvv
ffs
o
1
1'
v
v
v
f
f
s
o
'= Apparent Frequency
= Actual Frequency
= Object’s Velocity
= Source Velocity
= Sound VelocityTOP Sign = TOWARD
Apps & Probs• A little pot-bellied kid is
standing in the waves unattended. He notices 10 waves crash against his little chubby legs in 2 minutes. What is the frequency of these ocean waves?
Hzf
Seconds
Wavesf
f
12
1120
10sec
#
Apps & Probs• Two boats are hanging out 10
meters from each other. As the waves go by, one boat is at max up while the other is max down and no crests are between them. What’s the wavelength of the water waves?
m20
Apps & Probs• A boat is anchored. Over the side is
a rope ladder with rungs one foot apart and there’s 8 of them visible above the waterline. The tide comes in and rises at a rate of ½ foot per hour. After 5 hours, how much of the rope ladder is still visible above the waterline?
!8 ftDUH!
Apps & Probs• The wavelength of a sound in
air is 2.74m @ 20oC. What is its wavelength if it enters fresh water at 20oC?
m
Hzs
m
f
v
Hzms
mvf
water
water
waterwater
air
69.11
088.124
1450
088.12474.2
340
Wave Motion & Interference
• Reflection
• Refraction
• Diffraction
• Resonance
Wave Motion & Interference
• Reflection• i=r (duh…)
Wave Motion & Interference
• Refraction• Bending of a wave entering or
exiting media. (mediums?)
sin sini rn i n r
EM Spectrum
Less Basic Wave Stuff
i
r Why?
Wave Fronts:“crests”
Direction of wave travel
•What happens to ?•What happens to f?•What happens to v?
Wave Motion & Interference
• Diffraction• Bending of a wave around an
obstacle OR thru an opening