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