Introduction

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
All Rights Reserved -- Fair Use Encouraged

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Discussion

examples, examples, examples.

basic properties

classed by medium

mechanical
electromagnetic
gravitational
matter waves

classed by orientation of change

transverse
- synonyms
  - shear
- examples:
  - light and all electromagnetic waves
  - musical instruments
    - chordophones - vibrating strings (violin, guitar, piano)
    - membranophones - vibrating membranes (drums, vocal chords, kazoo?)
    - idiophones - vibrating solid object (bells, glockenspiel, xylophone, mbira)
  - seismic
    - secondary (S) shear
  - animal locomotion
    - vertebrates (snakes, fish)
- parts
  - crest: the high, the greatest positive displacement
  - trough: the low part, the lowest negative displacement
longitudinal,
- synonyms
  - pressure, compression, density
- examples
  - sound
  - musical instruments
    - aerophones - vibrating columns of air (horns, whistles, organ pipes )
  - seismic
    - primary (P) pressure
  - animal locomotion
    - invertebrates (worms)
  - traffic jams
  - density waves in spiral galaxies generate the arms
- parts
  - compressions (a.k.a. condensation): the pressed part, the greatest positive pressure change, a region where the medium is under compression
  - rarefactions (a.k.a. dilations): the stretched part, the lowest negative pressure change, a region where the medium is under tension
[magnify]
surface, interface, complex
- ocean
  
  [magnify]
- seismic waves come in two basic families
  - body waves, which have already been discussed
    - Primary (longitudinal, compression, Pressure)
    - Secondary (transverse, Shear), can't propagate through liquids
  - surface waves, which are what this discussion is a part of
    - Love, (Lateral shear)
    - Rayleigh, (elliptical, plate waves, ground Roll), something like ocean waves, but elliptical instead of circular
torsional
- ribbons, plates, bars
- bridges, skyscrapers
unclassifiable
- animals
  - arthropod locomotion (centipedes, millipedes, caterpillars)
  - peristalsis
  - nerve impulses
  - heart contractions
- dominoes

classified by duration

episodic
periodic

classified by appearance

traveling
standing

wave characteristics like period, frequency, wavelength, wave speed and phase are in the next section

Summary

Basic Properties of Waves
- A wave is a disturbance that propagates through a medium.
- Propagation describes the spreading of a disturbance
- Waves transfer energy, momentum, and information, but not mass.
Classifying Waves by Medium
- Mechanical Waves: matter is the medium
  - Sound is a mechanical wave
- Electromagnetic Waves: electric and magnetic fields are the media
  - Light is an electromagnetic wave
- Gravitational Waves: the gravitational field is the medium.
  (The existence of gravitational waves has not yet been confirmed.)
Classifying Waves by Orientation
- Transverse Waves: disturbance is perpendicular to the direction of propagation
  - All electromagnetic waves are transverse. This includes light.
  - Crest: a point of maximal displacement in the positive direction
  - Trough: a point of maximal displacement in the negative direction
- Longitudinal Waves: disturbance is parallel to the direction of propagation
  - Sound is a longitudinal wave
  - Compression or Condensation: a region where the medium is under compression
  - Rarefaction or Dilation: a region where the medium is under tension
- Surface Waves or Complex Waves: a combination transverse-longitudinal wave, forms near the surface of some media
- Torsional Waves: disturbance causes the medium to twist
Classifying Waves by Duration
- adj. episodic; noun pulse: disturbance is momentary and sudden
- adj. periodic, harmonic; noun wave train: disturbance repeats at regular intervals
Classifying Waves by Appearance
- Traveling Waves: appear to move
- Standing Waves: do not appear to move
Waves propagate with a finite speed (sometimes called the wave speed) that depends upon …
- the type of wave,
- the composition of the medium, and
- the state of the medium

Problems

practice

Write something.
- Answer it.
Write something else.
- Answer it.
Write something different.
- Answer it.
Write something completely different.
- Answer it.

conceptual

Does the medium in which a wave travels move along with the wave itself? Describe a situation that could be used to verify your claim.
A freight train at rest starts moving forward. There is enough slack in the couplings between cars that, after the first car starts moving, there is a slight delay before the second car starts to move. Then, after the second car starts moving, there is a slight delay before the third car starts to move. Likewise, after the third car starts moving (keep repeating this procedure over and over again) until finally the last car starts to move. This phenomenon satisfies the definition of a wave. Is the wave I just described a …
1. mechanical or electromagnetic wave?
2. transverse, longitudinal, complex, or torsional wave?
3. pulse or a periodic wave?
4. standing or traveling wave?

Resources

general
- The Mechanical Universe and Beyond (video on demand, login required)
  - Waves, With an analysis of simple harmonic motion and a stroke of genius, Newton extended mechanics to the propagation of sound.
traffic waves
- Ben Coifman, Ohio State University -- University of California Berkeley
  - Shocking (html)
  - Time Space Diagrams for Thirteen Shock Waves (pdf)
- Dynamics of Traffic Jams (Shock Waves in a 2D Granular Flow), S. Hørlück and P. Dimon, Niels Bohr Institute
- Kinematics of Driving: Some "Real" Traffic Considerations, DC Tech
- Microsimulation of road traffic, Martin Treiber, Dresden University of Technology
- Starlogo, Media Lab, Massachusetts Institute of Technology
- Stop-and-Go Science, Peter Weiss, Science News, 3 July 1999
- Traffic Waves, William Beaty, Science Hobbyist
ocean waves
- Seabased Energy AB, using oceanwaves to generate electricity
seismic waves
- Oklahoma Geological Survey Nuclear Explosion Catalog

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